Network Working Group Carsten Bormann (ed.), TZI/Uni Bremen INTERNET-DRAFT Expires: April 2000 Carsten Burmeister, Matsushita Christopher Clanton, Nokia Mikael Degermark, U of Arizona Hideaki Fukushima, Matsushita Hans Hannu, Ericsson Lars-Erik Jonsson, Ericsson Rolf Hakenberg, Matsushita Tmima Koren, Cisco Khiem Le, Nokia Zhigang Liu, Nokia Anton Martensson, Ericsson Akihiro Miyazaki, Matsushita Krister Svanbro, Ericsson Thomas Wiebke, Matsushita Haihong Zheng, Nokia October 03, 2000 RObust Header Compression (ROHC) Status of this memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/lid-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This document is a product of the IETF ROHC WG. Comments should be directed to its mailing list, rohc@cdt.luth.se. Bormann (ed.) [Page 1] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Abstract Existing header compression schemes do not work well when used over links with significant error rates and long round-trip times. For many bandwidth limited links where header compression is essential, such characteristics are common. A highly robust and efficient header compression scheme for RTP/UDP/IP and UDP/IP is specified in this document. The scheme can adapt to the characteristics of the link over which it is used and also to the properties of the packet streams whose headers it compresses. Revision History -03: Redo packet formats to allow new ROHC framework. Fill in gaps. -02: Major changes after 48th IETF -01: Minor editorial changes for 48th IETF -00: Document created from ROHC submissions Bormann (ed.) [Page 2] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Table of contents Status of this memo.................................................1 Abstract............................................................2 Revision History....................................................2 Table of contents...................................................3 0. ROHC WG internal short-term time plan...........................8 1. Introduction....................................................9 2. Terminology....................................................11 3. Background.....................................................15 3.1. Header compression fundamentals..............................15 3.2. Existing header compression schemes..........................15 3.3. Requirements on a new header compression scheme..............17 3.4. Classification of header fields..............................17 4. Header compression framework...................................19 4.1. Operating assumptions .......................................19 4.2. Dynamicity...................................................20 4.3. Compression and decompression states.........................21 4.3.1. Compressor states..........................................22 4.3.1.1. Initialization and Refresh (IR) State....................22 4.3.1.2. First Order (FO) State...................................22 4.3.1.3. Second Order (SO) State..................................23 4.3.2. Decompressor states........................................23 4.4. Modes of operation...........................................24 4.4.1. Unidirectional mode - U-mode...............................24 4.4.2. Bi-directional optimistic mode - O-mode....................25 4.4.3. Bi-directional reliable mode - R-mode......................25 4.5. Encoding methods.............................................26 4.5.1. Least Significant Bits (LSB) encoding .....................26 4.5.2. Window-based LSB encoding (W-LSB encoding).................28 4.5.3 . Scaled RTP Timestamp encoding .............................29 4.5.4 . Timer-Based Compression of RTP Timestamp .................30 4.5.5. Offset IP-ID encoding......................................33 4.5.6. Self-describing variable-length values. ...................34 4.5.7. Encoded values across several fields in compressed headers.35 5. The protocol...................................................36 5.1. Data structures..............................................36 5.1.1. Per-channel parameters.....................................36 5.1.2. Per-context parameters, profiles...........................36 5.1.3. Contexts and context identifiers ..........................36 5.2. Packet types.................................................37 5.2.1. Packet formats from compressor to decompressor ............37 5.2.2. Feedback from decompressor to compressor...................38 5.2.3. Parameters needed for mode transition......................40 5.3. Operation in unidirectional mode.............................41 5.3.1. Compressor states and logic (U-mode).......................41 5.3.1.1. State transition logic (U-mode)..........................41 5.3.1.1.1. Optimistic approach, upwards transition................41 5.3.1.1.2. Timeouts, downward transition..........................42 5.3.1.1.3. Need for updates, downward transition..................42 5.3.1.2. Compression logic and packets used (U-mode)..............42 Bormann (ed.) [Page 3] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.3.1.3. Feedback in unidirectional mode..........................42 5.3.2. Decompressor states and logic (U-mode).....................42 5.3.2.1. State transition logic (U-mode)..........................43 5.3.2.2. Decompression logic (U-mode).............................43 5.3.2.2.1. Decide whether decompression is allowed................43 5.3.2.2.2. Reconstruct and verify the header......................43 5.3.2.2.3. Actions upon CRC failure...............................44 5.3.2.2.3.1. Correction of SN wrap-around.........................45 5.3.2.2.3.2. Repair of incorrect SN updates.......................45 5.3.2.2.3.3. Judge whether bit error or context damage............46 5.3.2.3. Feedback in unidirectional mode..........................47 5.4. Operation in bi-directional optimistic mode..................48 5.4.1. Compressor states and logic (O-mode).......................48 5.4.1.1. State transition logic...................................48 5.4.1.1.1. Negative acknowledgements (NACKs), downward transition.48 5.4.1.1.2. Optional acknowledgements, upwards transition..........48 5.4.1.2. Compression logic and packets used.......................49 5.4.2. Decompressor states and logic (O-mode).....................49 5.4.2.1. Decompression logic, timer-based timestamp decompression.49 5.4.2.2. Feedback logic (O-mode)..................................49 5.5. Operation in bi-directional reliable mode....................51 5.5.1. Compressor states and logic (R-mode).......................51 5.3.1.1. State transition logic (R-mode)..........................51 5.5.2. Decompressor states and logic (R-mode).....................51 5.5.2.1. Decompression logic (R-mode).............................51 5.5.2.2. Feedback logic (R-mode)..................................51 5.6. Mode transitions.............................................52 5.6.1. Compression and decompression during mode transitions......52 5.6.2. Transition from Unidirectional to Optimistic mode..........53 5.6.3. From Optimistic to Reliable mode...........................54 5.6.4. From Unidirectional to Reliable mode.......................54 5.6.5. From Reliable to Optimistic mode...........................54 5.6.6. Transition to Unidirectional mode..........................56 5.7. Packet formats ..............................................57 5.7.1. Packet type 0: UO-0, R-0, R-0-CRC .........................61 5.7.2. Packet type 1 (R-mode): R-1, R-1-TS, R-1-ID ...............61 5.7.3. Packet type 1 (UO-modes): UO-1, UO-1-ID, UO-1-TS ..........63 5.7.4. Packet type 2: UOR-2 ......................................64 5.7.5. Extension formats .........................................65 5.7.5.1 RND flags and packet types ................................69 5.7.5.2 Flags/Fields in context ...................................70 5.7.6. Feedback packets and formats ..............................71 5.7.6.1 Feedback packet type for ROHC RTP..........................71 5.7.6.2 Feedback formats for ROHC RTP..............................72 5.7.6.3 RTP Feedback options ......................................74 5.7.6.4 The CRC option ............................................74 5.7.6.5 The REJECT option .........................................74 5.7.6.6 The SN option .............................................75 5.7.6.7 The CLOCK option ..........................................75 5.7.6.8 The JITTER option .........................................75 5.7.6.9 The LOSS option ...........................................76 Bormann (ed.) [Page 4] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.7.6.10 Unknown option types .....................................76 5.7.6.11 RTP feedback example .....................................77 5.7.7 IR and IR-DYN packets.......................................77 5.7.7.2. Basic structure of the IR-DYN packet......................79 5.7.7.3. Initialization of IPv6 Header [IPv6].....................80 5.7.7.4. Initialization of IPv4 Header [IPv4, section 3.1].........81 5.7.7.5. Initialization of UDP Header [RFC-768]....................82 5.7.7.6. Initialization of RTP Header [RTP]........................82 5.7.7.7. Minimal Encapsulation header [RFC-2004, section 3.1].....83 5.8 List-Based Compression.........................................84 5.8.1. CSRC compression...........................................84 5.8.1.1. Transformation Classification for CSRC List..............84 5.8.1.2. Encoding Schemes.........................................85 5.8.1.3. Format of compressed CSRC list............................85 5.8.1.3.1 Insertion Only Scheme....................................85 5.8.1.3.1.1 R-mode.................................................85 5.8.1.3.1.2 UO-modes...............................................86 5.8.1.3.2 Removal Only Scheme......................................87 5.8.1.3.2.1 R-mode.................................................87 5.8.1.3.2.2 UO-mode................................................87 5.8.1.3.3 Generic Scheme...........................................88 5.8.1.3.3.1 R-mode.................................................88 5.8.1.3.3.2 UO-mode................................................89 5.8.2. Header Compression for IPv6 Extension Headers..............89 5.8.2.1. Terminology...............................................90 5.8.2.2. Transformation Classification and Encoding Schemes........90 5.8.2.2.1 Transformation Classification............................90 5.8.2.2.2 Encoding Schemes.........................................91 5.8.2.2.3 Special Handling.........................................91 5.8.2.2.3.1 Special Handling of AH.................................91 5.8.2.2.3.2 Encapsulating Security Payload Header..................92 5.8.2.2.3.3 Special Handling of Next Header Field..................92 5.8.2.3. Packet Format.............................................94 5.8.2.3.1. Format in extension "11" header.........................94 5.8.2.3.2 Format of compressed IPv6 extension header...............95 5.8.2.3.2.1 Insertion Only Scheme..................................95 5.8.2.3.2.1.1 R-mode...............................................95 5.8.2.3.2.2 Removal Only Scheme....................................96 5.8.2.3.2.2.1 R-mode...............................................96 5.8.2.3.2.3 Content Change Only Scheme.............................97 5.8.2.3.2.3.1 R-mode...............................................97 5.8.2.3.2.3.2 UO-mode..............................................98 5.8.2.3.2.4 Uncompressed Scheme (common to R-mode and UO-modes)....98 5.9. Header compression CRCs, coverage and polynomials............99 5.9.1. IR & IR-DYN packet CRCs....................................99 5.9.2. CRCs in compressed packets.................................99 5.10. ROHC UDP _ non-RTP UDP/IP compression (Profile 1)...........100 5.10.1 Initialization.............................................100 5.10.2 States and Modes...........................................101 5.10.3 Packet types...............................................101 5.10.4 Extensions.................................................101 Bormann (ed.) [Page 5] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.10.5 IP-ID......................................................102 5.10.6. Feedback..................................................102 6. Implementation issues.........................................104 6.1. Reverse decompression.......................................104 6.2. RTCP........................................................105 6.3. Implementation parameters and signals.......................105 6.3.1. ROHC implementation parameters at compressor:.............106 6.3.2. ROHC implementation parameters at decompressor:...........106 6.5. Handling of resource limitations at the decompressor........107 7. Further work..................................................108 7.3. Tunneling...................................................108 7.3.1. Header Compression for IPv4 Tunneling Header..............108 7.3.1.1. Mobile IPv4 Tunneling Header Fields Type................108 7.3.1.2. Compression of Tunneling Headers in MIPv4...............109 7.3.1.2.1. IP in IP Encapsulation in IPv4........................109 7.3.1.2.2. Minimum Encapsulation in IPv4.........................109 7.3.1.2.3. Generic Routing Encapsulation in IPv4.................109 8. Section 8 has been removed....................................110 9. Security considerations.......................................110 10. Acknowledgements.............................................110 11. Intellectual property considerations.........................112 12. References...................................................113 13. Authors' addresses...........................................114 Appendix A. Detailed classification of header fields.............115 A.1. General classification......................................115 A.1.1. IPv6 header fields........................................116 A.1.2. IPv4 header fields........................................117 A.1.3. UDP header fields.........................................119 A.1.4. RTP header fields.........................................120 A.1.5. Summary for IP/UDP/RTP....................................121 A.2. Analysis of change patterns of header fields................121 A.2.1. IPv4 Identification.......................................123 A.2.2. IP Traffic-Class / Type-Of-Service........................124 A.2.3. IP Hop-Limit / Time-To-Live...............................125 A.2.4. UDP Checksum..............................................125 A.2.5. RTP CSRC Counter..........................................125 A.2.6. RTP Marker................................................125 A.2.7. RTP Payload Type..........................................125 A.2.8. RTP Sequence Number.......................................125 A.2.9. RTP Timestamp.............................................125 A.2.10. RTP Contributing Sources (CSRC)..........................126 A.3. Header compression strategies...............................126 A.3.1. Do not send at all........................................126 A.3.2. Transmit only initially...................................127 A.3.3. Transmit initially, but be prepared to update.............127 A.3.4. Be prepared to update or send as-is frequently............127 A.3.5. Guarantee continuous robustness...........................127 A.3.6. Transmit as-is in all packets.............................128 A.3.7. Establish and be prepared to update delta.................128 Appendix E - Encoding Examples....................................129 E.1. Basic VLE...................................................129 Bormann (ed.) [Page 6] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 E.2. Timer-Based VLE.............................................130 (Editor's note: The TOC has not necessarily been updated.) Bormann (ed.) [Page 7] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 0. ROHC WG internal short-term time plan This document captures the state of the ROHC RTP specification as on October 03, 2000. For information, the ROHC WG internal short-term time plan that led to this document is as follows: 18 September ROHC-02 completed Draft review and discussion Complementary contributions created 29 September Cutoff for draft review and discussion 02 October ROHC-03 completed (this document) 04 October Judge whether draft ready for last call As of Oct 04, the WG chairs believe this draft is not yet ready for last call on Oct 04. At least the following points are identified as to be completed: Complete section 5.5. Merge section 5.8 and 7.3. Align terminology in informational Appendix E. Time plan for continued work is as follows. 10 October ROHC-04 Completed. 13 October Cutoff for draft review and discussion 16 October ROHC-05 Completed WG Last call at completion of ROHC-05 on October 16. Bormann (ed.) [Page 8] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 1. Introduction During the last five years, two communication technologies in particular have become commonly used by the general public: cellular telephony and the Internet. Cellular telephony has provided its users with the revolutionary possibility of always being reachable with reasonable service quality no matter where they are. The main service provided by the dedicated terminals has been speech. The Internet, on the other hand, has from the beginning been designed for multiple services and its flexibility for all kinds of usage has been one of its strengths. Internet terminals have usually been general-purpose and have been attached over fixed connections. The experienced quality of some services (such as Internet telephony) has sometimes been low. Today, IP telephony is gaining momentum thanks to improved technical solutions. It seems reasonable to believe that in the years to come, IP will become a commonly used way to carry telephony. Some future cellular telephony links might also be based on IP and IP telephony. Cellular phones may have become more general-purpose, and may have IP stacks supporting not only audio and video, but also web browsing, email, gaming, etc. One of the scenarios we are envisioning might then be the one in Figure 1.1, where two mobile terminals are communicating with each other. Both are connected to base stations over cellular links, and the base stations are connected to each other through a wired (or possibly wireless) network. Instead of two mobile terminals, there could of course be one mobile and one wired terminal, but the case with two cellular links is technically more demanding. Mobile Base Base Mobile Terminal Station Station Terminal | ~ ~ ~ \ / \ / ~ ~ ~ ~ | | | | | +--+ | | +--+ | | | | | | | | | | | | +--+ | | +--+ | | |=========================| Cellular Wired Cellular Link Network Link Figure 1.1 : Scenario for IP telephony over cellular links Bormann (ed.) [Page 9] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 It is obvious that the wired network can be IP-based. With the cellular links, the situation is less clear. IP could be terminated in the fixed network, and special solutions implemented for each supported service over the cellular link. However, this would limit the flexibility of the services supported. If technically and economically feasible, a solution with pure IP all the way from terminal to terminal would have certain advantages. However, to make this a viable alternative, a number of problems have to be addressed, in particular problems regarding bandwidth efficiency. For cellular phone systems, it is of vital importance to use the scarce radio resources in an efficient way. A sufficient number of users per cell is crucial, otherwise deployment costs will be prohibitive [CELL]. The quality of the voice service should also be as good as in today's cellular systems. It is likely that even with support for new services, lower quality of the voice service is acceptable only if costs are significantly reduced. A problem with IP over cellular links when used for interactive voice conversations is the large header overhead. Speech data for IP telephony will most likely be carried by RTP [RTP]. A packet will then, in addition to link layer framing, have an IP [IPv4] header (20 octets), a UDP [UDP] header (8 octets), and an RTP header (12 octets) for a total of 40 octets. With IPv6 [IPv6], the IP header is 40 octets for a total of 60 octets. The size of the payload depends on the speech coding and frame sizes being used and may be as low as 15- 20 octets. From these numbers, the need for reducing header sizes for efficiency reasons is obvious. However, cellular links have characteristics that make header compression as defined in [IPHC,CRTP,PPPHC] perform less than well. The most important characteristic is the lossy behavior of cellular links, where a bit error rate (BER) as high as 1e-3 must be accepted to keep the radio resources efficiently utilized [CELL]. In severe operating situations, the BER can be as high as 1e-2. The other problematic characteristic is the long round-trip time (RTT) of the cellular link, which can be as high as 100-200 milliseconds [CELL]. An additional problem is that the residual BER is nontrivial, i.e., lower layers can sometimes deliver frames containing undetected errors. A viable header compression scheme for cellular links must be able to handle loss on the link between the compression and decompression point as well as loss before the compression point. Bandwidth is the most costly resource in cellular links. Processing power is very cheap in comparison. Implementation or computational simplicity of a header compression scheme is therefore of less importance than its compression ratio and robustness. Bormann (ed.) [Page 10] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. BER Bit Error Rate. Cellular radio links can have a rather high BER. In this document BER is usually given as a probability, but one also needs to consider the error distribution as bit errors are not independent. Cellular links Wireless links between mobile terminals and base stations. The BER and the RTT are rather high in order to achieve an efficient system overall. Compression efficiency The performance of a header compression scheme can be described with three parameters, compression efficiency, robustness and compression transparency. The compression efficiency is determined by how much the header sizes are reduced by the compression scheme. Compression transparency The performance of a header compression scheme can be described with three parameters, compression efficiency, robustness and compression transparency. The compression transparency is a measure for how well the scheme ensures that the decompressed headers are semantically identical to the original headers. If all decompressed headers are semantically identical to the corresponding original headers, the transparency is 100 per cent. Compression transparency is high when damage propagation is low. Context The context is the state which the compressor uses to compress a header and which the decompressor uses to decompress a header. The context basically contains the uncompressed version of the last header sent (compressor) or received (decompressor) over the link, except for fields in the header that are included "as-is" in compressed headers or can be inferred from, e.g., the size of the link-level frame. The context can also contain additional information describing the packet stream, for example the typical inter-packet increase in sequence numbers or timestamps. Context damage Bormann (ed.) [Page 11] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 When the context of the decompressor is not consistent with the context of the compressor, header decompression may fail to reproduce the original header. This situation can occur when the context of the decompressor has not been initialized properly or when packets have been lost or damaged between compressor and decompressor. Packets for which the decompressor detects they cannot be decompressed due to inconsistent contexts are said to be lost due to context damage. Context repair mechanism To avoid excessive context damage, a context repair mechanism is needed. Context repair mechanisms can be based on explicit requests for context updates, periodic updates sent by the compressor, or methods for local repair at the decompressor side. CRC-DYNAMIC Opposite of CRC-STATIC. CRC-STATIC A CRC over the original header is the primary mechanism of ROHC to detect incorrect decompression. In order to decrease computational complexity, the fields of the header are conceptually rearranged when computing the CRC, such that it is first computed over octets which are static (called CRC-STATIC in this document) and then over octets whose values are expected to change between packets (CRC- DYNAMIC). In this manner, the intermediate result of the CRC computation, after it has covered the CRC-STATIC fields, can be reused for several packets. The restarted CRC computation only covers the CRC-DYNAMIC octets. See also section 5.9. Damage propagation Generation of incorrect decompressed headers due to damage to previous packet(s). Loss propagation Failure to decompress headers due to loss of previous frame(s). Error detection Detection of errors. If error detection is not perfect, there will be residual errors. Error propagation Damage propagation or loss propagation. Bormann (ed.) [Page 12] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 FLR Frame Loss Rate, given as a probability that a frame is lost on the channel between compressor and decompressor. (In contrast, frames lost due to context damage contribute to the packet loss rate.) Frame Packet emitted by the compressor/received by the decompressor. Note that, in this document, there is no relationship to other (e.g. physical layer) frame concepts such as radio frames. Header compression profile A header compression profile is a specification of how to compress the headers of a certain kind of packet stream over a certain kind of link. Compression profiles provide the details of the header compression framework introduced in this document. The profile concept makes use of profile identifiers to separate different profiles which are used when setting up the compression scheme. All variations and parameters of the header compression scheme that are not part of the context state are handled by different profile identifiers. Packet Generally, a unit of transmission and reception (protocol data unit). Specifically, when contrasted to "frame", the packet compressed and then decompressed by ROHC. Also called "uncompressed packet". Pre-HC links Pre-HC links are all links a packet has traversed before the header compression point. If we consider a path with cellular links as first and last hops, the Pre-HC links for the compressor at the last link are the first cellular link plus the wired links in between. Residual error Error introduced during transmission and not detected by lower- layer error detection schemes. Robustness The performance of a header compression scheme can be described with three parameters, compression efficiency, robustness and compression transparency. A robust scheme tolerates errors on the link over which header compression takes place (including both Bormann (ed.) [Page 13] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 frame losses and residual bit errors) without losing additional packets, introducing additional errors, or using more bandwidth. RTT Round-trip time -- The time it takes to send a packet from compressor to decompressor and back again from decompressor to compressor. Simplex link A simplex (or unidirectional) link is a point to point link without a return channel. Over simplex links, header compression must rely on periodic refreshes since feedback from the decompressor can not be sent to the compressor. Spectrum efficiency Radio resources are limited and expensive. Therefore they must be used efficiently to make the system economically feasible. In cellular systems this is achieved by maximizing the number of users served within each cell, while the quality of the provided services is kept at an acceptable level. A consequence of efficient spectrum use is a high rate of errors (frame loss and residual bit errors), even after channel coding with error correction. Timestamp stride The timestamp stride (TS STRIDE) is the expected increase in the timestamp value between two RTP packets with consecutive sequence numbers. Bormann (ed.) [Page 14] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 3. Background This chapter provides a background to the subject of header compression. The fundamental ideas are described together with descriptions of existing header compression schemes, their drawbacks and requirements and motivation for new header compression solutions. 3.1. Header compression fundamentals The main reason why header compression can be done at all is the fact that there is significant redundancy between header fields, both within the same packet header but in particular between consecutive packets belonging to the same packet stream. By sending static field information only initially and utilizing dependencies and predictability for other fields, the header size can be significantly reduced for most packets. In general, header compression methods maintain a context, which is essentially the uncompressed version of the last header sent over the link, plus some additional information, at both compressor and decompressor. Compression and decompression are done relative to the context. When compressed headers carry differences from the previous header, each compressed header will update the context of the decompressor. In this case, when a packet is lost between compressor and decompressor, the context of the decompressor will be brought out of sync since it is not updated correctly. A header compression method must have a way to repair the context, i.e., bring it into sync, after such events. 3.2. Existing header compression schemes The original header compression scheme, CTCP [VJHC], was invented by Van Jacobson. CTCP compresses the 40 octet IP+TCP header to 4 octets. The CTCP compressor detects transport-level retransmissions and sends a header that updates the context completely when they occur. This repair mechanism does not require any explicit signaling between compressor and decompressor. A general IP header compression scheme, IP header compression [IPHC], improves somewhat on CTCP and can compress arbitrary IP, TCP, and UDP headers. When compressing non-TCP headers, IPHC does not use delta encoding and is robust. When compressing TCP, the repair mechanism of CTCP is augmented with a link-level nacking scheme which speeds up the repair. IPHC does not compress RTP headers. CRTP [CRTP, IPHC] by Casner and Jacobson is a header compression scheme that compresses 40 octets of IPv4/UDP/RTP headers to a minimum of 2 octets when no UDP checksum is present. If the UDP checksum is present, the minimum CRTP header is 4 octets. CRTP cannot use the Bormann (ed.) [Page 15] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 same repair mechanism as CTCP since UDP/RTP does not retransmit. Instead, CRTP uses explicit signaling messages from decompressor to compressor, called CONTEXT_STATE messages, to indicate that the context is out of sync. The link roundtrip time will thus limit the speed of this context repair mechanism. On lossy links with long roundtrip times, such as most cellular links, CRTP does not perform well. Each lost packet over the link causes several subsequent packets to be lost since the context is out of sync during at least one link roundtrip time. This behavior is documented in [CRTPC]. For voice conversations such long loss events will degrade the voice quality. Moreover, bandwidth is wasted by the large headers sent by CRTP when updating the context. [CRTPC] found that CRTP did not perform well enough for a lossy cellular link. It is clear that CRTP alone is not a viable header compression scheme for IP telephony over cellular links. To avoid losing packets due to the context being out of sync, CRTP decompressors can attempt to repair the context locally by using a mechanism known as TWICE. Each CRTP packet contains a counter which is incremented by one for each packet sent out by the CRTP compressor. If the counter increases by more than one, at least one packet was lost over the link. The decompressor then attempts to repair the context by guessing how the lost packet(s) would have updated it. The guess is then verified by decompressing the packet and checking the UDP checksum - if it succeeds, the repair is deemed successful and the packet can be forwarded or delivered. TWICE has got its name from the observation that when the compressed packet stream is regular, the correct guess is to apply the update in the current packet twice. [CRTPC] found that even with TWICE, CRTP doubled the number of lost packets. TWICE improves CRTP performance significantly. However, there are several problems with using TWICE: 1) It becomes mandatory to use the UDP checksum: - the minimal compressed header size increases by 100% to 4 octets. - most speech codecs developed for cellular links tolerate errors in the encoded data. Such codecs will not want to enable the UDP checksum, since they do want damaged packets to be delivered. - errors in the payload will make the UDP checksum fail when the guess is correct (and might make it succeed when it is wrong). 2) Loss in an RTP stream that occurs before the compression point will make updates in CRTP headers less regular. Simple-minded versions of TWICE will then perform badly. More sophisticated versions would need more repair attempts to succeed. Bormann (ed.) [Page 16] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 3.3. Requirements on a new header compression scheme The major problem with CRTP is that it is not sufficiently robust against packets being damaged between compressor and decompressor. A viable header compression scheme must be less fragile. This increased robustness must be obtained without increasing the compressed header size; a larger header would make IP telephony over cellular links economically unattractive. A major cause of the bad performance of CRTP over cellular links is the long link roundtrip time, during which many packets are lost when the context is out of sync. This problem can be attacked directly by finding ways to reduce the link roundtrip time. Future generations of cellular technologies may indeed achieve lower link roundtrip times. However, these will probably always be rather high [CELL]. The benefits in terms of lower loss and smaller bandwidth demands if the context can be repaired locally will be present even if the link roundtrip time is decreased. A reliable way to detect a successful context repair is then needed. One might argue that a better way to solve the problem is to improve the cellular link so that packet loss is less likely to occur. Such modifications do not appear to come for free, however. If links were made (almost) error free, the system might not be able to support a sufficiently large number of users per cell and might thus be economically infeasible [CELL]. One might also argue that the speech codecs should be able to deal with the kind of packet loss induced by CRTP, in particular since the speech codecs probably must be able to deal with packet loss anyway if the RTP stream crosses the Internet. While the latter is true, the kind of loss induced by CRTP is difficult to deal with. It is usually not possible to completely hide a loss event where well over 100 ms worth of sound is completely lost. If such loss occurs frequently at both ends of the end-to-end path, the speech quality will suffer. A detailed description of the requirements specified for ROHC may be found in [REQ]. 3.4. Classification of header fields As mentioned earlier, header compression is possible due to the fact that there is much redundancy between header field values within packets, but especially between consecutive packets. To utilize these properties for header compression, it is important to understand the change patterns of the various header fields. Bormann (ed.) [Page 17] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 All header fields have been classified in detail in appendix A. The fields are first classified on a high level and then some of them are studied more in detail. Finally, the appendix concludes with recommendations about how the various fields should be handled by header compression algorithms. The main conclusion that can be drawn is that most of the header fields can easily be compressed away since they never or seldom change. Only 5 fields, with a combined size of about 10 octets, need more sophisticated mechanisms. Those fields are: - IPv4 Identification (16 bits) - IP-ID - UDP Checksum (16 bits) - RTP Marker (1 bit) - M-bit - RTP Sequence Number (16 bits) - SN - RTP Timestamp (32 bits) - TS The analysis in Appendix A reveals that the values of the TS and IP- ID fields can usually be predicted from the RTP Sequence Number, which increments by one for each packet emitted by an RTP source. M- bit is also usually the same, but needs to be communicated explicitly occasionally. The UDP checksum should not be predicted and is sent as-is when enabled. The way ROHC RTP compression operates, then, is to first establish functions from SN to the other fields, and then reliably communicate the SN. Whenever a function from SN to another field changes, i.e., the existing function gives a result which is different from the field in the header to be compressed, additional information is sent to update the parameters of that function. Bormann (ed.) [Page 18] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 4. Header compression framework 4.1. Operating assumptions The cellular links which are a primary target for ROHC have a number of characteristics that are briefly described here. ROHC requires functionality from lower layers which is outlined here and more thoroughly described in the lower layer guidelines document [LLG]. Channels ROHC header-compressed packets flow on channels. Unlike many fixed links, some cellular radio links can have several channels connecting the same pair of nodes. Each channel can have different characteristics in terms of error rate, bandwidth, etc. Context identifiers On some channels, the ability to transport multiple packet streams is required. It can also be feasible to have channels dedicated to individual packet streams. Therefore, ROHC uses a distinct context identifier space per channel and can eliminate context identifiers completely in channels where only a single packet stream is compressed. Packet type indication Packet type indication is done in the header compression scheme itself. Unless the link already has a way of indicating packet types which can be used, such as PPP, this provides smaller compressed headers, overall. It may also be less difficult to allocate a single packet type, rather than many, in order to run ROHC over links such as PPP. Reordering The channel between compressor and decompressor is not assumed to reorder packets, i.e., the decompressor receives packets in the same order as the compressor sends them. (Reordering before the compression point, however, is dealt with, i.e., there is no assumption that the compressor will only receive packets in sequence.) Packet length ROHC is designed under the assumption that lower layers indicate the length of a compressed packet. ROHC packets do not contain length information for the payload. Bormann (ed.) [Page 19] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Framing The link layer must provide framing, which makes it possible to distinguish frame boundaries and individual frames. Error detection/protection The ROHC scheme has been designed to cope with residual errors in the headers delivered to the decompressor. CRCs and sanity checks are used to prevent or reduce damage propagation. However, it is RECOMMENDED that lower layers deploy error detection for ROHC headers and do not deliver ROHC headers with high residual error rates. Without giving a hard limit on the residual error rate acceptable to ROHC, it is noted that for a residual bit error rate of at most 1E-5, the ROHC scheme has been designed not to increase the number of damaged headers, i.e., the number of damaged headers due to damage propagation is designed to be less than the number of damaged headers caught by the ROHC error detection scheme. Negotiation In addition to the packet handling mechanisms above, the link layer MUST provide a way to negotiate header compression parameters. (For unidirectional links, this negotiation may be performed out- of-band or even a-priori.) 4.2. Dynamicity The ROHC protocol achieves its compression gain by establishing state at both ends of the link, i.e., at the compressor and at the decompressor. Different parts of the state are established at different times and with different frequency; hence, it can be said some state is more dynamic that other state. Some state is established at the time a channel is established; ROHC assumes the existence of an out-of-band negotiation protocol (such as PPP), or pre-defined channel state (most useful for uni-directional links). In both cases, we speak of "negotiated channel state". ROHC does not assume that this state can change dynamically during the channel lifetime (and does not explicitly support such changes, although some changes may be innocuous from a protocol point of view). An example of negotiated channel state is the maximum size of packet headers that can be compressed (MAX_HEADER). Other state is associated with the individual packet streams in the channel; this state is said to be part of the context. Using context identifiers (CIDs), multiple packet streams with different contexts Bormann (ed.) [Page 20] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 can share a channel. The highest context identifier to be used is one of the members of the negotiated channel state, as well as the selection of one of two ways to indicate the CID in the compressed header. It is up to the compressor to decide which packets to associate with a context (or, equivalently, which packets constitute a single stream); however, ROHC is efficient only when all packets of a stream share certain properties, such as having the same values for fields that are described as "static" in this document (e.g., the IP addresses, port numbers, and RTP parameters such as the payload type). The efficiency of ROHC RTP also depends on the compressor seeing most RTP sequence numbers. Streams need not share all characteristics important for compression. ROHC has a notion of compression profiles: a compression profile denotes a pre-defined set of such characteristics. To provide for extensibility, the negotiated channel state includes the set of profiles acceptable to the decompressor. The context state includes the profile currently in use for the context. Other elements of the context state may include: The current value of all header fields (from this one can deduce whether an IPv4 header is present in the header chain, and whether UDP checksums are enabled), as well as additional compression context that is not part of an uncompressed header: e.g., TS STRIDE, IP-ID characteristics (in network byte order? random?), a number of old reference headers, and the compressor/decompressor state machines (see next section). 4.3. Compression and decompression states Header compression with ROHC can be characterized as an interaction between two state machines, one compressor machine and one decompressor machine, each instantiated once per context. The compressor and the decompressor have three states each, which in many ways are related to each other even if the meaning of the states are slightly different for the two parties. Both machines start in the lowest compression state and transits gradually to higher states. Transitions need not be synchronized between the two machines. In normal operation it is only the compressor that temporarily transits back to lower states. Subsequent sections present an overview of the state machines and their corresponding states respectively, starting with the compressor. Bormann (ed.) [Page 21] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 4.3.1. Compressor states For ROHC compression, the three compressor states are the Initialization and Refresh (IR), First Order (FO), and Second Order (SO) states. The compressor starts in the lowest compression state (IR) and transits gradually to higher compression states. The compressor will always operate in the highest possible compression state, under the constraint that the compressor is sufficiently confident that the decompressor has the information necessary to decompress a header compressed according to that state. +----------+ +----------+ +----------+ | IR State | <--------> | FO State | <--------> | SO State | +----------+ +----------+ +----------+ Decisions about transitions between the various compression states are taken by the compressor based on: - variations in packet headers. - positive feedback from decompressor (Acknowledgements - ACKs) - negative feedback from decompressor (Negative ACKs - NACKs) - periodic timeouts (when no feedback is used) How transitions are performed is explained in detail in chapter 5 for each mode of operation. 4.3.1.1. Initialization and Refresh (IR) State The purpose of the IR state is to initialize the static parts of the context at the decompressor or to recover after failure. In this state, the compressor sends complete header information. This includes all static and non-static fields in uncompressed form plus some additional information. Refreshes may also be performed on non- static information only. The compressor stays in IR state until it is rather confident that the decompressor has received the static information correctly. 4.3.1.2. First Order (FO) State The purpose of the FO state is to efficiently communicate irregularities in the packet stream. When operating in this state, the compressor rarely sends complete information and the information sent is usually compressed at least partially. The difference between IR and FO should therefore be clear. The compressor enters this state after the IR state and when the headers of the packet stream does not conform to their previous pattern. It stays in FO state until it is confident that the Bormann (ed.) [Page 22] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 decompressor has acquired all the parameters of the new pattern. Changes in fields that are always irregular are communicated in all packets and is therefore part of what is a uniform pattern. Since packets sent in the FO state usually carry context updating information, successful transmission of this information may be of vital importance for successful decompression of subsequent packets. The decompression process is sensitive to loss of, or damage to, such packets. 4.3.1.3. Second Order (SO) State This is the state where compression is optimal. The compressor enters SO state when the header to be compressed is completely predictable given the SN, and the compressor is sufficiently confident that the decompressor has acquired all parameters of the functions from SN to other fields. Packets sent in SO state are almost independent of each other and error sensitivity is therefore low. However, successful decompression of packets sent in SO state requires that the information sent in preceding FO state operations has been successfully received by the decompressor. The compressor leaves this state and goes back to the FO state when the header no longer conforms to the uniform pattern and can not be independently compressed based on previous context information. 4.3.2. Decompressor states The decompressor starts in its lowest compression state, "No Context" and gradually transits to higher states. The decompressor state machine normally never leaves the "Full Context" state when it once has started to work in that state. +--------------+ +----------------+ +--------------+ | No Context | <---> | Static Context | <---> | Full Context | +--------------+ +----------------+ +--------------+ When initially working in the "No Context" state, the decompressor has never successfully decompressed any packet. When a packet once has been decompressed correctly (upon reception of an initialization packet with static and dynamic information, for example), the decompressor can transit all the way to "Full Context" state, and only upon repeated failures will it transit back to lower states. However, when that happens it first transits back to "Static Context" state. There, reception of any packet sent in FO state is normally sufficient to enable transition to "Full Context" state again. Only when decompression of several packets sent in FO state fails in "Static Context" state will the decompressor go all the way back to the "No Context" state. Bormann (ed.) [Page 23] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 When state transitions are performed is explained in detail in chapter 5. 4.4. Modes of operation. The ROHC scheme has three modes of operation, called Unidirectional, Bi-directional Optimistic, and Bi-directional Reliable mode. It is important to understand the difference between states, as described in previous chapter, and modes. These abstractions are orthogonal to each other. The state abstraction is the same for all modes of operation, while the mode controls the logic of state transitions and what actions to perform in each state. +----------------------+ | Unidirectional Mode | | +--+ +--+ +--+ | | |IR| |FO| |SO| | | +--+ +--+ +--+ | +----------------------+ ^ ^ / \ / \ v v +----------------------+ +----------------------+ | Optimistic Mode | | Reliable Mode | | +--+ +--+ +--+ | | +--+ +--+ +--+ | | |IR| |FO| |SO| | <--------------> | |IR| |FO| |SO| | | +--+ +--+ +--+ | | +--+ +--+ +--+ | +----------------------+ +----------------------+ The optimal mode to operate in depends on the characteristics of the environment of the compression protocol, such as feedback abilities, error probabilities and distributions, header size variation effects, etc. All ROHC implementations MUST implement and support all three modes of operation. The three modes are briefly described in the following subsections. Detailed descriptions of the three modes of operation regarding compression and decompression logic are given in chapter 5. The mode transition mechanisms are also described in chapter 5. 4.4.1. Unidirectional mode - U-mode When in the unidirectional mode of operation, packets are sent in one direction only; from compressor to decompressor. This mode therefore makes ROHC usable over links where a return path from decompressor to compressor is not available or is undesirable. Bormann (ed.) [Page 24] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 In U-mode, transitions between compressor states are performed based only on periodic timeouts and irregularities in the header field change patterns in the compressed packet stream. Due to the periodic refreshes and the lack of feedback for initiation of error recovery, compression in the unidirectional mode will be less efficient and have a slightly higher probability for loss propagation compared to any of the bi-directional modes. Compression with ROHC MUST start in the unidirectional mode. Transition to any of the bi-directional modes can be performed as soon as a packet has reached the decompressor and it has replied with a feedback packet indicating that a mode transition is desired (see chapter 5). 4.4.2. Bi-directional optimistic mode - O-mode The bi-directional optimistic mode is similar to the unidirectional mode. The difference is that a feedback channel is used to send error recovery requests and (optionally) acknowledgments of significant context updates from decompressor to compressor (not for sequence number updates only). Periodic refreshes are not used in the bi- directional optimistic mode. O-mode is designed for good compression efficiency and sparse usage of the return channel while maintaining reasonable robustness. Loss of compressor-decompressor synchronization and introduction of loss propagation is rare even under high probabilistic error rates, but can occur with longer consecutive loss events. When loss propagation does occur, the amount is limited by the CRC check and NACKing scheme used. Nevertheless, this mode is not completely robust against loss propagation. 4.4.3. Bi-directional reliable mode - R-mode The bi-directional reliable mode differs in many ways from the previous two. The most important differences are a more intensive usage of the feedback channel and a stricter logic at both the compressor and the decompressor that prevents loss of context synchronization between compressor and decompressor except for very high residual bit error rates. Feedback is sent to acknowledge all context updates, including updates of the sequence number field. However, not every packet updates the context in reliable mode. The chief advantage of R-mode is almost complete robustness against packet loss between compressor and decompressor. Loss propagation can never occur due to header compression when operating in this mode. For reasonably well-behaving links, the price is slightly higher overhead in some cases and additional feedback traffic. Bormann (ed.) [Page 25] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 4.5. Encoding methods This chapter describes the encoding methods that are used for different header fields. How the methods are applied to each field (e.g., values of associated parameters) is specified in section 5.7. 4.5.1. Least Significant Bits (LSB) encoding Least Significant Bits (LSB) encoding is used for header fields whose values are usually subject to small changes. With LSB encoding, the k least significant bits of the field value are transmitted instead of the original field value, where k is a positive integer. After receiving k bits, the decompressor derives the original value using a previously received value as reference (v_ref). The scheme is guaranteed to be correct if the compressor and the decompressor agree on an interpretation interval 1) in which the original value resides, and 2) in which the original value is the only value that has the exact same k least significant bits as those transmitted. The interpretation interval can be described as a function f(v_ref, k). Let f(v_ref, k) = [v_ref - p, v_ref + (2^k - 1) - p] where p is an integer. <------- interpretation interval (size is 2^k) -------> |-------------+---------------------------------------| v_ref - p v_ref v_ref + (2^k-1) - p The function f has the following property: for any value k, k least significant bits will uniquely identify a value in f(v_ref, k). The parameter p is introduced so that the interpretation interval can be shifted with respect to v_ref. Choosing a good value for p will yield more efficient encoding for fields with certain characteristics. Examples of appropriate values of p are a) for field values that are expected to always increase, p can be set to _1. The interpretation interval becomes [v_ref + 1, v_ref + 2^k]. b) for field values that stay the same or increase, p can be set to 0. The interpretation interval becomes [v_ref, v_ref + 2^k - 1]. Bormann (ed.) [Page 26] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 c) for field values that are expected to have small deviations around a constant value, p can be set to 2^(k-1)-1. The interpretation interval becomes [v_ref _ 2^(k-1) + 1, v_ref + 2^(k-1)]. d) for field values that are expected to have small negative changes and larger positive changes, such as the RTP TS for video, or RTP SN when there is misordering, p can be set to 2^(k-2). The interval becomes [v_ref _ 2^(k-2) + 1, v_ref + 3 * 2^(k-2)], i.e., 3/4 of the interval is used for positive changes. The following is a simplified procedure for LSB compression and decompression, it is modified for robustness and damage propagation protection in the next subsection: 1) The compressor (decompressor) always uses v_ref_c (v_ref_d), the last value that has been compressed (decompressed), as v_ref; 2) When compressing a value v, the compressor finds the minimal value of k such that v falls into the interval f(v_ref_c, k). Call this function k = g(v_ref_c, v). When only a few distinct values of k are possible, for example due to limitations imposed by packet formats (see section 5.7), the compressor will instead pick the smallest k of those possible such that v is in the interval f(v_ref_c, k). 3) When receiving m LSBs, the decompressor uses the interpretation interval f(v_ref_d, m), called interval d. It picks as the decompressed value the one in interval_d whose LSBs match the received m bits. The scheme is complicated by two factors: packet loss between the compressor and decompressor, and transmission errors undetected by the lower layer. In the former case, the compressor and decompressor will lose the synchronization of v_ref, and thus also of the interpretation interval. If v is still covered by the intersection(interval_c, interval_d), decompression will be correct. Otherwise, incorrect decompression will happen. The next section will address this issue further. In the case of undetected transmission errors, the corrupted LSBs will give an incorrectly decompressed value that will later be used as v_ref_d, which in turn is likely to lead to damage propagation. This problem is addressed by using a secure reference, i.e., a reference value whose correctness is verified by a protecting CRC. Consequently, the procedure 1) above is modified as follows: 1) a) the compressor always uses as v_ref_c the last value that has been compressed and sent with a protecting CRC. b) the decompressor always uses as v_ref_d the last correct value, as verified by a succeeding CRC. Bormann (ed.) [Page 27] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Note that in U/O mode, 1) b) is modified so that if decompression of the SN fails using the last verified SN reference, another decompression attempt is made using the last but one verified SN reference. This procedure dampens damage propagation when a small CRC fails to detect a damaged value. 4.5.2. Window-based LSB encoding (W-LSB encoding) This section describes how to modify the simplified algorithm in 4.5.1 to achieve robustness. The compressor may not be able to determine the exact value of v_ref_d that will be used by the decompressor for a particular value v, since some candidates for v_ref_d may have been lost or damaged. However, by using feedback or by making reasonable assumptions the compressor can limit the candidate set. The compressor then calculates k such that no matter which v_ref_d in the candidate set the decompressor uses, v is covered by the resulting interval_d. Since the decompressor always uses the last received value where the CRC succeeded as the reference, the compressor maintains a sliding window (VSW) containing the candidates for v_ref_d. VSW is initially empty. The following operations are performed on VSW by the compressor: 1) After sending a value v (compressed or uncompressed) protected by a CRC, the compressor adds v to the VSW; 2) For each value v being compressed, the compressor chooses k = max(g(v, v_min), g(v, v_max)), where v_min and v_max are the minimal and maximal values in VSW, and g is the function defined in the previous section; 3) When the compressor has sufficient confidence that a certain value v will not be used as a reference by the decompressor, the window is advanced by removing v and all values older than v. The confidence may be obtained by various means. In R mode an ACK from the decompressor implies that values older than the ACKed one can be removed from VSW. In U/O mode there is always a CRC to verify correct decompression, and a VSW with a limited maximum width is used. The window width is an implementation dependent optimization parameter. Note that the decompressor follows the procedure described in the previous section, except that in R mode it MUST ACK each value received with a CRC (see also section 5.5). Bormann (ed.) [Page 28] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 4.5.3. Scaled RTP Timestamp encoding The RTP Timestamp (TS) will usually not increase by an arbitrary number from packet to packet. Instead, the increase is normally an integral multiple of some unit (TS_STRIDE). For example, in the case of audio, the sample rate is normally 8Khz and one voice frame may cover 20 ms. Furthermore, each voice frame is usually carried in one RTP packet. In this case, the RTP increment is always n * 160 (= 8000 * 0.02), for some integer n. Note that silence periods has no impact on this as the sample clock at the source normally keeps running without changing either frame rate or frame boundaries. For the case of video, there is usually a TS_STRIDE as well when we consider the video frame level. The sample rate for most video codecs is 90Khz. If the video frame rate is fixed, say to 30 frames/second, the TS will increase by n * 3000 (= n * 90000 / 30) between video frames. Note that a video frame is often divided into several RTP packets to achieve robustness against packet loss. In this case several RTP packets will carry the same TS. When using scaled RTP Timestamp encoding, the TS is downscaled by a factor of TS_STRIDE before compression. This saves floor(log2(TS_STRIDE)) bits for each compressed TS. The following equality holds between TS and TS SCALED: TS = TS SCALED * TS STRIDE + TS OFFSET TS STRIDE is explicitly, and TS OFFSET implicitly, communicated to the decompressor. The following algorithm is used: 1. Initialization: The compressor sends to the decompressor the value of TS_STRIDE (e.g., via in-band signaling, see packet format section) and the absolute value of one or several TS. The latter are used by the decompressor to initialize TS_OFFSET to (absolute value) modulo TS_STRIDE. Note that TS OFFSET is the same regardless of which absolute value is used, as long as the unscaled TS value does not wrap around, see 4) below. 2. Compression: After initialization, the compressor no longer compresses the original TS values. Instead, it compresses the down-scaled values: TS_SCALED = TS / TS_STRIDE. The compression method could be either W-LSB encoding or the timer-based encoding described in the next section. 3. Decompression: When receiving the compressed value of TS_SCALED, the decompressor first derives the value of the original TS_SCALED. The original RTP TS is then calculated as TS = TS_SCALED * TS_STRIDE + TS_OFFSET. Bormann (ed.) [Page 29] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 4. Wrap around: Wrap around of the unscaled 32-bit TS will invalidate the current value of TS_OFFSET used in the equation above. For example, let's assume TS_STRIDE = 160 = 0xA0 and the current TS = 0xFFFFFFF0. TS_OFFSET is then 0x50 = 80. Then if the next RTP TS = 0x00000130 (i.e., the increment is 160 * 2 = 320), the new TS_OFFSET should be 0x00000130 modulo 0xA0 = 0x90 = 144. The compressor is not required to re-initialize TS OFFSET at wrap around. Instead, the decompressor MUST detect wrap around of the unscaled TS (which is trivial) and update TS_OFFSET to TS OFFSET = (Wrapped around unscaled TS) modulo TS STRIDE This scaling method can be applied to many frame-based codecs. However, the value of TS_STRIDE might change during a session, for example due to adaptation strategies. If that happens, the unscaled TS is compressed until re-initialization of the new TS_STRIDE and TS_OFFSET is completed. 4.5.4. Timer-Based Compression of RTP Timestamp The RTP timestamp [RFC 1889] is defined to identify the number of the first sample used to generate the payload. When RTP packets carry payloads corresponding to a fixed sampling interval, the sampling is done at a constant rate, and packets are generated in lock-step with sampling, the timestamp will closely follow a linear pattern as a function of the time of day. This is the case for conversational media, such as interactive speech. The linear ratio is determined by the source sample rate. The linear pattern can be complicated by packetization (e.g., in the case of video where a video frame usually corresponds to several RTP packets) or frame re-arrangement (e.g., MPEG B-frames are sent out-of-order by some video codecs). With a fixed sample rate of 8kHz, 20 ms in time domain is equivalent to an increment of 160 in the unscaled TS domain, and to an increment of 1 in the scaled TS domain with TS_STRIDE=160. As a consequence, the (scaled) TS of headers coming to the decompressor will follow a linear pattern as a function of time of day, with some deviation due to the delay jitter between the source and the decompressor. In normal operation, i.e., no crashes or failures, the delay jitter will be bounded to meet the requirements of conversational real-time traffic. Hence, by using a local clock the decompressor can obtain an approximation of the (scaled) TS in the header to be decompressed by considering its arrival time. The approximation can then be refined with the k LSBs of the (scaled) TS carried in the header. The required value of k to ensure correct decompression is a function of the jitter between the source and decompressor. Bormann (ed.) [Page 30] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 If the compressor knows the potential jitter introduced between compressor and decompressor, it can determine k by using a local clock to estimate jitter in packet arrival times, or alternatively it can use a fixed k and discard packets arriving too much out of time. The advantages of this scheme include: a) The size of the compressed TS is constant and small. In particular, it does NOT depend on the length of silence intervals. This is in contrast to other TS compression techniques, which at the beginning of a talk-spurt requires sending a number of bits dependent on the duration of the preceding silence interval. b) No synchronization is required between the clock local to the compressor and the clock local to the decompressor. Note that although this scheme can be made to work using both scaled and unscaled TS, in practice it is always combined with scaled TS encoding because of the less demanding requirement on the clock resolution, e.g., 20 ms instead of 1/8 ms. Therefore, the algorithm described below assumes that the clock-based encoding scheme operates on the scaled TS. The case of unscaled TS would be similar, with changes to scale factors. Compressor: its major task is to determine the value of k. Its sliding window, TSW, now contains not only potential reference values for the TS, but also their times of arrival at the compressor. 1) The compressor maintains a sliding window TSW = {(T_j, a_j), for each header j that can be used as a reference}, where T_j is the scaled TS for header j, and a_j is the arrival time of header j. The TSW fills the same purpose as the VSW of section 4.5.2. 2) When a new header n arrives with T_n as the scaled TS, the compressor notes the arrival time a_n. It then calculates Max_Jitter_BC = max {|(T_n - T_j) - ((a_n - a_j) / TIME_STRIDE)|, for all headers j in TSW}, where TIME_STRIDE is the time interval equivalent to one TS_STRIDE, e.g., 20 ms. Max_Jitter_BC is the maximum observed jitter before the compressor, in units of TS_STRIDE, for the headers in TSW. 3) k is calculated as: k = ceiling(log2(2 * J + 1), where J = Max_Jitter_BC + Max_Jitter_CD + 2. Bormann (ed.) [Page 31] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Max_Jitter_CD is the upper bound of jitter expected on the communication channel between compressor and decompressor (CD- CC). It depends only on the characteristics of CD-CC. The factor 2 accounts for the quantization error introduced by the clocks at the compressor and decompressor, which can be +/- 1. Note that the calculation of k follows the compression algorithm described in section 4.5.1, with p = 2^(k-1) - 1. 4) TSW is subject to the same window operations as in section 4.5.2, 1) and 3), except that the values added and removed are paired with their arrival times. Decompressor: 1) The decompressor uses as its reference header the last correctly (as verified by CRC) decompressed header. It maintains the pair (T_ref, a_ref), where T_ref is the scaled TS of the reference header, and a_ref is the arrival time of the reference header. 2) When receiving a compressed header n at time a_n, the approximation of the original scaled TS is calculated as: T_approx = T_ref + (a_n - a_ref) / TIME_STRIDE. 3) The approximation is then refined by the k least significant bits carried in header n, following the decompression algorithm of section 4.5.1, with p = 2^(k-1) - 1. Note: the algorithm does not assume any particular pattern in the packets arriving to the compressor, i.e., it tolerates reordering before the compressor and non-increasing RTP timestamp behavior. Note: Integer arithmetic is used in all equations above. If TIME_STRIDE is not equal to an integral number of clock ticks, time must be normalized such that TIME_STRIDE is an integral number of clock ticks. For example, if a clock tick is 20 ms and TIME STRIDE is 30 ms, (a_n _ a_ref) in 2) can be multiplied by 3 and TIME_STRIDE can have the value 2. Note: the clock resolution of compressor or decompressor can be worse than TIME_STRIDE, in which case the difference, i.e., actual resolution _ TIME_STRIDE, is treated as additional jitter in the calculation of k. Note: the clock resolution of the decompressor may be communicated to the compressor using the CLOCK feedback option. Bormann (ed.) [Page 32] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Note: the decompressor may observe the jitter and report this to the compressor using the JITTER feedback option. The compressor may use this information to refine its estimate of Max_Jitter_CD. 4.5.5. Offset IP-ID encoding As all IPv4 packets have an IP Identifier to allow for fragmentation, ROHC provides for transparent compression of this ID. There is no explicit support in ROHC for the IPv6 fragmentation header, so there is never a need to discuss IP IDs outside the context of IPv4. This section assumes (initially) that the IPv4 stack at the source host assigns IP-ID to the value of a 2-byte counter which is increased by one after each assignment to an outgoing packet. Therefore, the IP-ID field of a particular IPv4 packet flow will increment by 1 from packet to packet except when the source has emitted intermediate packets not belonging to that flow. For such IPv4 stacks, the RTP SN will increase by 1 for each packet emitted and the IP-ID will increase by at least the same amount. Thus, it is more efficient to compress the offset, i.e., (IP-ID - RTP SN), instead of IP-ID itself. The following text describes how to compress/decompress the sequence of offsets using W-LSB encoding/decoding, with p = 0 (see section 4.5.1). Compressor: The compressor uses W-LSB encoding to compress a sequence of offsets Offset_i = ID_i _ SN_i, where ID_i and SN_i are the values of the IP-ID and RTP SN of header i. The sliding window contains such offsets and not the values of header fields, but the rules for adding and deleting offsets from the window otherwise follow section 4.5.2. Decompressor: The reference header is the last correctly (as verified by CRC) decompressed header. When receiving a compressed packet m, the decompressor calculates Offset_ref = ID_ref - SN_ref, where ID_ref and SN_ref are the values of IP-ID and RTP SN in the reference header, respectively. Then W-LSB decoding is used to decompress Offset_m, using the received LSBs in packet m and Offset_ref. Note that m may contain zero LSBs for Offset_m, in which case Offset_m = Offset_ref. Bormann (ed.) [Page 33] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Finally, the IP-ID for packet m is regenerated as IP-ID for m = decompressed SN of packet m + Offset_m Network byte order: Some IPv4 stacks do use a counter to generate IP ID values as described, but do not transmit the contents of this counter in network byte order, but instead send the two octets reversed. In this case, the compressor can compress the IP-ID field after swapping the bytes. Consequently, the decompressor also swaps the bytes of the IP- ID after decompression to regenerate the original IP-ID. This requires that the compressor and the decompressor synchronize on the byte order of the IP-ID field using the NBO or NBO2 flag (see section 5.7.) Random IP Identifier: Some IPv4 stacks generate the IP Identifier values using a pseudo- random number generator. While this may provide some security benefits, it makes it pointless to attempt compressing the field. Therefore, the compressor should detect such random behavior of the field. After detection and synchronization with the decompressor using the RND or RND2 flag, the field is sent as-is in its entirety as additional octets after the compressed header. 4.5.6. Self-describing variable-length values. The values of TS STRIDE and a few other compression parameters can vary widely. TS STRIDE can be 160 for voice and 90 000 for 1 f/s video. To optimize the transfer of such values, a variable number of octets is used to encode them. The first few bits of the encoded value determines its length. 1 octet: first bit is zero. 7 bits transferred. Up to 127 decimal. Encoded octets in hexadecimal: 00 to 7F 2 octets: first bits 10. 14 bits. Up to 16 383 decimal. Encoded octets in hexadecimal: 80 00 to BF FF 3 octets: first bits 110. 21 bits. Up to 2 097 151 decimal. Encoded octets in hexadecimal: C0 00 00 to DF FF FF 4 octets: first bits 111. 29 bits. Up to 536 870 911 decimal. Encoded octets in hexadecimal: E0 00 00 00 to FF FF FF FF Bormann (ed.) [Page 34] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 4.5.7. Encoded values across several fields in compressed headers. When a compressed header has an extension, pieces of an encoded value can be present in more than one field. When an encoded value is split over several fields in this manner, the more significant bits of the value are closer to the beginning of the header. If the number of bits available in compressed header fields exceeds the number of bits of the value, the most significant field is padded with zeroes in its most significant bits. For example, an unscaled TS value can be transferred using an UOR-2 header (see section 5.7) with an extension of type 3. The Tsc bit of the extension is then unset and the variable length TS field of the extension is 4 octets (see section 4.5.6). The UOR-2 TS field will contain the most significant three bits of the unscaled TS, and the 4-octet TS field in the extension contains the remaining 29 bits. Bormann (ed.) [Page 35] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5. The protocol 5.1. Data structures The ROHC protocol is based on a number of parameters that form part of the negotiated channel state and the per-context state. This section describes some of this state in an abstract way. Implementations can use a different structure and representation of this state. In particular, negotiation protocols that set up per- channel state need to establish the information that constitutes the negotiated channel state, but not necessarily have to exchange it in the form it is described here. 5.1.1. Per-channel parameters MAX_CID: non-negative integer; highest context ID number to be used by the compressor (note that this parameter is not coupled to, but in effect further constrained by, LARGE_CIDS). LARGE_CIDS: Boolean; if false, the short CID representation (0 bytes or 1 prefix byte, covering CID 0 to 15) is used, if true, the embedded CID representation (1 or 2 embedded CID bytes covering CID 0 to 16383) is used. PROFILES: Set of non-negative integers, indicating which profiles are supported by the decompressor. FEEDBACK_FOR: Optional reference to a channel in the reverse direction. If set, this indicates which channel any feedback sent on this channel refers to (see 5.7.6.1). 5.1.2. Per-context parameters, profiles Per-context parameters are established in the I/R headers (see section 5.7). Note that the profile parameter influences the semantics of the packet type identifiers and packet types of the profile used for a specific context. This document describes profiles 0 and 1; further profiles may be defined when ROHC is extended in the future. Profile 0 is for RTP/UDP/IP compression. Profile 1 is for UDP/IP compression, i.e., compression of the first 12 octets of the UDP payload is not attempted. See section 5.10. 5.1.3. Contexts and context identifiers Associated with each compressed flow is a context, which is the state compressor and decompressor maintains in order to correctly compress or decompress the headers of the flow. Contexts are identified by a Bormann (ed.) [Page 36] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 context identifier, CID, which is sent along with compressed headers and feedback information. The CID space is distinct for each channel, i.e., CID 3 over channel A and CID 3 over channel B do not refer to the same context, even if the endpoints of A and B are the same nodes. In particular, CIDs for any pairs of forward and reverse channels are not related (forward and reverse channels need not even have the same size CID space). Context information is conceptually kept in a table. The context table is indexed using the CID which is sent along with compressed headers and feedback information. The CID space can be negotiated to be either small, which means that CIDs can take the values 0 through 15, or large, which means that CIDs take values between 0 and 2^14-1 = 16383. Whether the CID space is large or small is negotiated no later than when a channel is established. A small CID is either CID 0, which is represented using zero bits, or CID 1 to 15, which are represented using a packet type plus four bits. A large CID is represented using the encoding scheme of section 4.5.6., limited to two octets. 5.2. Packet types A ROHC RTP packet always starts with a packet type. The packet type indicates the format of the (first part of) the packet. Packet types allow distinguishing compressed packets from feedback packets, and also allows compressed ROHC RTP packets to carry piggybacked feedback information. The format of a compressed packets can depend on the mode. Therefore a naming scheme of the form -- is used to uniquely identify the format when necessary. E.g, UOR-2, R-1. For exact formats of the packet types, see section 5.7. 5.2.1. Packet formats from compressor to decompressor Three packet types are used to identify compressed headers, and two for initialization/refresh. Context identifiers of the appropriate length is sent along with these headers. Exact header formats are found in section 5.7. Packet type zero: R-0, R-0-CRC, UO-0. This, the minimal, packet type is used when parameters of all SN- functions are known by the decompressor, and the header to be Bormann (ed.) [Page 37] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 compressed adheres to those functions. Thus, only the W-LSB encoded RTP SN needs to be communicated. R-mode: Only if a CRC is present (packet type R-0-CRC) may the header be used as a reference for subsequent decompression. U-mode and O-mode: A small CRC is present in the UO-0 packet. Packet type 1: R-1, R-1-ID, R-1-TS, UO-1, UO-1-ID, UO-1-TS. This packet type is used when the number of bits needed for the SN exceeds those available in packet type zero, or when the parameters of the SN-functions for RTP TS or IP-ID change. R-mode: R-1-* packets are not used as references for subsequent decompression. Values for other fields than the RTP TS or IP-ID can be communicated using an extension, but they do not update the context. U-mode and O-mode: Only the values of RTP SN, RTP TS and IP-ID can be used as references for future compression. Non-updating values can be provided for other fields using an extension (UO-1-ID). Packet type 2: UOR-2, UOR-2-ID, UOR-2-TS This packet type can be used to change the parameters of any SN- function, except for those for most static fields. Headers of packets transferred using packet type 2 can be used as references for subsequent decompression. Packet type 5: IR This packet type communicates the static part of the context, i.e., the value of the constant SN-functions. It can optionally also communicate the dynamic part of the context, i.e., the parameters of the non-constant SN-functions. Packet type 6: IR-DYN This packet type communicates the dynamic part of the context, i.e., the parameters of non-constant SN-functions. 5.2.2. Feedback from decompressor to compressor Feedback carry information from decompressor to compressor. The following principal kinds of feedback is supported. In addition to these, feedback can carry options which provide the compressor with various information. ACK : Acknowledges successful decompression of a packet, Bormann (ed.) [Page 38] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 which means that the context is up to date with a high probability. NACK : Indicates that the dynamic context of the decompressor is out of sync. STATIC-NACK : Indicates that the static context of the decompressor is not valid or has not been established. It is anticipated that feedback to the compressor can be realized in many ways, depending on the properties of the particular lower layer. The exact details of how feedback is realized is specified in a "ROHC over X" document, for each lower layer X in question. As examples, feedback might be realized using 1) lower-layer specific mechanisms, or 2) a dedicated feedback-only channel, realized for example by the lower layer providing a way to indicate that a packet is a feedback packet, or 3) a dedicated feedback-only channel, where the timing of the feedback provides information about which compressed packet caused the feedback, or 4) interleaving of feedback packets among normal compressed packets going in the same direction as the feedback (lower layers do not indicate feedback), or 5) piggybacking of feedback information in compressed packets going in the same direction as the feedback (this technique may reduce the per-feedback overhead). Alternatives 1-3 do not place any particular requirements on the ROHC packet type scheme. Alternatives 4 and 5 do, however. The ROHC RTP packet types have been designed to allow alternatives 4 and 5, who may be used for example over PPP. A) The ROHC RTP profile provides a feedback packet type. The packet type is able to carry variable-length feedback information. B) The feedback information sent in a particular channel is passed to, and interpreted by, the compressor associated with feedback on that channel. Thus, the feedback information must contain CID information if the associated compressor can use more than one context. How a compressor is associated with feedback on a particular channel needs to be defined in a "ROHC over X" document. Bormann (ed.) [Page 39] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 C) The ROHC feedback information format is octet-aligned, i.e., starts at an octet boundary, to allow using the format over a dedicated feedback channel, 2). D) To allow piggybacking, 5), it is possible to explicitly provide the length of the feedback information. This allows the decompressor to pass piggybacked feedback information to the associated compressor without understanding its format. The explicit length decouples the decompressor from the compressor in the sense that the decompressor can process the compressed header immediately without waiting for the compressor to hand it back after parsing the feedback information. 5.2.3. Parameters needed for mode transition The packet type UOR-2 is common for all modes. It can carry an extension with a mode parameter which can take the values U = Unidirectional, O = Bi-directional Optimistic, and R= Bi-directional Reliable. Feedback of types ACK, NACK, and STATIC-NACK carry sequence numbers and feedback packets can also carry a mode parameter indicating the desired compression mode: U, O, or R. As a shorthand, the notation PACKET(mode) is used to indicate which mode value a packet carries. For example, an ACK with mode parameter R is written ACK(R), and an UOR-2 with mode parameter O is written UOR-2(O). Bormann (ed.) [Page 40] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.3. Operation in unidirectional mode 5.3.1. Compressor states and logic (U-mode) Below is the state machine for the compressor in unidirectional mode. Details of the transitions between states and compression logic are given subsequent to the figure. Optimistic approach +------>------>------>------>------>------>------>------>------+ | | | Optimistic approach Optimistic approach | | +------>------>------+ +------>------>------+ | | | | | | | | | v | v v +----------+ +----------+ +----------+ | IR State | | FO State | | SO State | +----------+ +----------+ +----------+ ^ ^ | ^ | | | | Timeout | | Timeout / Update | | | +------<------<------+ +------<------<------+ | | | | Timeout | +------<------<------<------<------<------<------<------<------+ 5.3.1.1. State transition logic (U-mode) The transition logic for compression states in unidirectional mode is based on three principles; the optimistic approach principle, timeouts, and the need for updates. 5.3.1.1.1. Optimistic approach, upwards transition Transition to higher compression state in unidirectional mode is carried out according to the optimistic approach principle. This means that the compressor transits to a higher compression state when it is rather confident that the decompressor has received enough information to correctly decompress packets sent according to the higher compression state. When the compressor is in IR state, it will stay there until it assumes that the decompressor has correctly received the static context information. For transition from FO to SO state, the compressor should be confident that the decompressor has all parameters needed to decompress according to a fixed pattern. The compressor normally gets its confidence about decompressor status by sending several packets with the same information according to the lower compression state. If the decompressor receives any of these Bormann (ed.) [Page 41] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 packets, it will be in sync with the compressor. The number of consecutive packets to send for confidence is not defined in this document. 5.3.1.1.2. Timeouts, downward transition By using the optimistic approach described above, there will always be a possibility for failure since the decompressor may not have received sufficient information for correct decompression. Therefore, the compressor MUST periodically transit to lower compression states. Periodic transition to IR state SHOULD be carried out less often than transition to FO state. Two different timeouts SHOULD therefore be used for these transitions. For an example of how to implement periodic refreshes, see [IPHC] chapter 3.3.1-3.3.2. 5.3.1.1.3. Need for updates, downward transition In addition to the downward state transitions carried out due to periodic timeouts, the compressor must also immediately transit back to FO state when the header to be compressed does not confirm to the established pattern. 5.3.1.2. Compression logic and packets used (U-mode) The compressor chooses the smallest possible packet format that can communicate the desired changes, and has the required bits for W-LSB encoded values. Sliding windows used in W-LSB encoding have a fixed width, not defined in this document. 5.3.1.3. Feedback in unidirectional mode The unidirectional mode of operation is designed to operate over links where a feedback channel is not available. If a feedback channel is available, however, the decompressor MAY send an acknowledgment of successful decompression with the mode parameter set to U (send an ACK(U)). When the compressor receives such a message, it MAY disable (or increase the interval between) periodic IR refreshes. 5.3.2. Decompressor states and logic (U-mode) Below is the state machine for the decompressor in unidirectional mode. Details of the transitions between states and decompression logic are given subsequent to the figure. Success Bormann (ed.) [Page 42] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 +-->------>------>------>------>------>--+ | | No Static | No Dynamic Success | Success +-->--+ | +-->--+ +--->----->---+ +-->--+ | | | | | | | | | | v | | v | v | v +--------------+ +----------------+ +--------------+ | No Context | | Static Context | | Full Context | +--------------+ +----------------+ +--------------+ ^ | ^ | | Repeated failure | | Repeated failure | +-----<------<------<-----+ +-----<------<------<-----+ 5.3.2.1. State transition logic (U-mode) The state transition logic of the decompressor is much simpler than for the compressor side. It is also common for all the three modes of operation. Successful decompression will always move the decompressor to the Full Context state. Repeated failed decompression will force the decompressor to transit downwards to a lower state. The decompressor does not attempt to decompress headers at all in the No Context and Static Context states unless sufficient information is included in the packet itself. 5.3.2.2. Decompression logic (U-mode) Decompression in unidirectional mode is carried out following three steps which are described in subsequent sections. 5.3.2.2.1. Decide whether decompression is allowed In Full Context state, decompression may be attempted regardless of what kind of packet is received. However, for the other states decompression is not always allowed. In the No Context state, only IR packets, which carry the static information fields may be decompressed. Further, when in the Static Context state, only packets carrying a strong CRC can be decompressed (i.e., IR, IR-DYN, or UOR-2 packets). If decompression may not be performed the packet is discarded, unless the optional delayed decompression mechanism is used, see section 6.1. 5.3.2.2.2. Reconstruct and verify the header When reconstructing the header, the decompressor takes the header information already stored in the context, and updates it with the information received in current header. Bormann (ed.) [Page 43] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 The sequence number is reconstructing by replacing the sequence number LSBs in the context with those received in the header. The resulting value is then verified to be within the expected interpretation interval based on previously decompressed value v_ref, see chapter 4.5.1. If not within this interval, an interpretation adjustment should be applied by adding or subtracting Nxinterval_size to/from the reconstructed value. If the interpretation is adjusted, the old value MUST be stored together with a flag telling that this happened. First when the next packet has been correctly decompressed, this flag can be reset and the old value discarded. If RTP Timestamp and IP Identification fields are not included in the received header, they are supposed to be calculated based on the sequence number. The IP Identifier is usually increasing with the same delta as the sequence number and the timestamp with the same delta times a fixed value. See chapters 4.5.3 and 4.5.5 for details about how these fields are encoded in compressed headers. When working in unidirectional mode, all decompressed headers carry a CRC which MUST be used to verify decompression. 5.3.2.2.3. Actions upon CRC failure A mismatch in the 3-bit CRC can be caused by one or more of: 1. residual bit errors in the current header, 2. a damaged context due to residual bit errors in previous headers, or 3. loss of many consecutive packets which causes the 4-bit SN LSB to wrap around without the compressor noticing this (which is, in essence, another kind of context damage). The 3-bit CRC present in some header formats will reliably detect context damage eventually, since the probability of undetected context damage decreases exponentially with each new header processed. However, residual bit errors in the current header are only detected with good probability, not reliably. When a CRC mismatch is caused by residual bit errors in the current header (case 1 above), the decompressor should stay in its current state to avoid unnecessary loss of subsequent packets. On the other hand, when the mismatch is caused by a damaged context, the decompressor can attempt to repair the context but if that fails, it must move to a lower state to avoid delivering incorrect headers. When the mismatch is caused by long loss, the decompressor might attempt additional decompression attempts. Bormann (ed.) [Page 44] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 In the following sections the various actions to take in case of CRC failure are described in detail. If these mechanisms fail in finding the reason for the CRC mismatch, additional decompression attempts SHOULD NOT be performed. 5.3.2.2.3.1. Correction of SN wrap-around If many consecutive packets are lost there will be a risk of sequence number LSB wrap around. The decompressor might be able to detect this situation and avoiding context damage by using a local clock. The following algorithm can be used: a.The decompressor notes the arrival time, a(i), of each incoming packet i. Arrival times of packets where decompression fails are discarded. b.When decompression fails, the decompressor computes INTERVAL = a(i) - a(i-1), i.e., the time elapsed between the arrival of the previous correctly decompressed packet and the current packet. c.If wrap-around has occurred, INTERVAL will correspond to at least 16 inter-packet times. Based on an estimate of the packet inter-arrival time, obtained for example using a moving average of arrival times, TS STRIDE, or TS TIME, the decompressor judges if INTERVAL can correspond to 16 or more inter-packet times. d.If INTERVAL is judged to be at least 16 packet inter-arrival times, the decompressor adds 16 to the SN of the context and attempts to decompress the packet using the new context. e.If this decompression succeeds, the decompressor updates the context but SHOULD NOT deliver the packet to upper layers. The following packet is also decompressed and updates the context if its CRC succeeds, but SHOULD be discarded. If decompression of the third packet using the new context also succeeds, the context repair is deemed successful and this and subsequent decompressed packets are delivered to the upper layers. f.If any of the three decompression attempts in d. and e. fails, the decompressor discards the packets and moves to the Static Context state. Using this mechanism, the decompressor may be able to repair the context after excessive loss, at the expense of discarding two packets. 5.3.2.2.3.2. Repair of incorrect SN updates The CRC can fail to detect residual errors in the compressed header because of its limited length, i.e., the incorrectly decompressed Bormann (ed.) [Page 45] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 packet can happen to have the same CRC as the original uncompressed packet, causing the incorrectly decompressed packet to be accepted and the context being updated. This can lead to an erroneous reference SN being used in W-LSB decoding, as the reference SN is updated for each successfully decompressed header of certain types. If this happens, the decompressor will detect the incorrect decompression of the following packet with high probability, but it does not know the reason for the failure. The following mechanism allows the decompressor to judge if the context was updated incorrectly by an earlier packet. 1) The decompressor maintains two decompressed SN: the last one (ref 0) and the one before that (ref -1). 2) When receiving a compressed SN: SN curr = decompressed SN using ref 0 as reference value decompress the rest of the header using SN curr IF (the header passes the CRC test) ref -1 = ref 0 ref 0 = SN curr 3) ELSE SN curr = decompressed SN using ref -1 as reference value decompress rest of header using SN curr IF (header passes the CRC test) ref0 = SN curr; // note ref-1 unchanged 4) ELSE // one of two error situations: // 1) bit error in current header, // 2) decompression context corrupted. Use 5.3.2.2.4 to // distinguish, (e.g., assume context corrupted // only after k out of n errors) The purpose of this algorithm is to repair the context. If the header generated in 3) passes the CRC test, two more headers MUST also be successfully decompressed before the repair is deemed successful. Of the three successful headers, the first two SHOULD be discarded and only the third delivered to upper layers. If decompression of any of the three headers fail, the decompressor MUST discard that header and the previously generated headers, and move to the Static Context state. 5.3.2.2.3.3. Judge whether bit error or context damage When the CRC check fails, the first step should be to check whether a SN wrap-around has occurred, as described in 5.3.2.2.3.1. If that attempt fails, repair of a possible incorrect SN update should be carried out, as described in previous section. Bormann (ed.) [Page 46] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 However, if both these attempts fail there are two possible reasons for the CRC failure either bit errors that have not been detected by the link layer or an unrecoverable context damage. It is impossible to know which of these is the cause for failure but there are some simple rules for how to act when this happens. 1: For the first CRC failures, assume bit error and simply discard the packet. A context request MAY be sent at this time. 2: When several consecutive decompressed packets have been erroneous, context damage SHOULD be assumed and a context request be sent. The decompressor MUST also move to Static Context state and discard all packets until an update is received. 5.3.2.3. Feedback in unidirectional mode To improve performance for the unidirectional mode over a link that does have a feedback channel, the decompressor MAY send an acknowledgment when decompression succeeds. Setting the mode parameter in the ACK packet to U indicates that the compressor is to stay in unidirectional mode. When receiving an ACK(U), the compressor should reduce the frequency of IR packets since the static information has been correctly received, but it is not required to stop sending IR packets. If IR packets continue to arrive, the decompressor MAY repeat the ACK(U), but it SHOULD not repeat the ACK(U) continuously. Bormann (ed.) [Page 47] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.4. Operation in bi-directional optimistic mode 5.4.1. Compressor states and logic (O-mode) Below is the state machine for the compressor in bi-directional optimistic mode. Details of each state, the transitions between states and compression logic are given subsequent to the figure. Optimistic approach / ACK +------>------>------>------>------>------>------>------>------+ | | | Optimistic appr. / ACK Optimistic appr. / ACK | | +------>------>------+ +------>------>------+ | | | | | | | | | v | v v +----------+ +----------+ +----------+ | IR State | | FO State | | SO State | +----------+ +----------+ +----------+ ^ ^ | ^ | | | | STATIC-NACK | | NACK / Update | | | +------<------<------+ +------<------<------+ | | | | STATIC-NACK | +------<------<------<------<------<------<------<------<------+ 5.4.1.1. State transition logic The transition logic for compression states in bi-directional optimistic mode has much in common with the logic of the unidirectional mode. The optimistic approach principle and transitions because of the need for updates works in the same way as described in chapter 5.3.1. However, in optimistic mode there are no timeouts. Instead, the optimistic mode makes use of feedback from decompressor to compressor both for transitions in the backward direction and for improved forward transition. 5.4.1.1.1. Negative acknowledgements (NACKs), downward transition Negative acknowledgements (NACKs), also called context requests, replaces the periodic updates needed in unidirectional mode. Upon reception of a NACK the compressor transits back to the FO state and send context updates to the decompressor. Similarly, reception of a STATIC-NACK packet makes the compressor transit back to IR state. 5.4.1.1.2. Optional acknowledgements, upwards transition In addition to the NACKs, also positive feedback (ACKs) MAY be used in the bi-directional optimistic mode. Upon reception of an ACK, the compressor knows that the decompressor has received the acknowledged packet and transition to a higher compression state can be carried Bormann (ed.) [Page 48] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 out without any doubts. Since this functionality is optional, a compressor MUST NOT expect to get such ACKs initially. First after reception of an acknowledgement for a non-IR packet, the compressor can expect to get ACKs also after that. 5.4.1.2. Compression logic and packets used The compression logic is the same for bi-directional optimistic mode as for the unidirectional mode (see section 5.3.1.2) 5.4.2. Decompressor states and logic (O-mode) The decompression states and the state transition logic are the same as for the unidirectional case (see section 5.3.2). What differs is the decompression and feedback logic. 5.4.2.1. Decompression logic, timer-based timestamp decompression In bi-directional mode, timer-based timestamp decompression may be used to improve compression efficiency if the RTP timestamp increases linearly. The mechanisms used are those described in 4.5.4. 5.4.2.2. Feedback logic (O-mode) The feedback logic defines what feedback to send due to different events when operating in the various states. As mentioned above, there are three principal kinds of feedback that can be used; ACK, NACK and STATIC-NACK. Further, the logic described below will refer to three kind of packets that can be received by the decompressor; Initiation and Refresh (IR or IR-DYN) packets, type 1 or 2 (UO-1 or UOR-2) packets and UO-0 packets. An UO-0 packet carries a packet header compressed according to a fixed pattern while type 1 and 2 packets are used when this patterns is broken. Below, rules for which feedback to use when are defined. For the optional feedback, it is REQUIRED that if used once, the decompressor MUST continue to send optional feedback for the lifetime of the packet stream. In NC state: - When an IR packet is correctly decompressed, send an ACK with the mode parameter set to O - When type 1 or 2 packet, or UO-0 packet is received or decompression of an IR packet has failed, send a STATIC-NACK with the mode parameter set to O In SC state: - When an IR packet is correctly decompressed, send an ACK with the mode parameter set to O Bormann (ed.) [Page 49] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 - When a type 1 or 2 packet is correctly decompressed, optionally send an ACK with the mode parameter set to O - When an UO-0 packet is received, send a NACK with the mode parameter set to O - When decompression of a type 1 or 2, or an IR packet has failed, send a STATIC-NACK with the mode parameter set to O In FC state: - When an IR packet is correctly decompressed, send an ACK with the mode parameter set to O - When a type 1 or 2 packet is correctly decompressed, optionally send an ACK with the mode parameter set to O - When an UO-0 packet is correctly decompressed, no feedback is sent - When decompression of any packet has failed, send a NACK with the mode parameter set to O Bormann (ed.) [Page 50] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.5. Operation in bi-directional reliable mode 5.5.1. Compressor states and logic (R-mode) Below is the state machine for the compressor in bi-directional reliable mode. Details of each state, the transitions between states and compression logic are given subsequent to the figure. ACK +------>------>------>------>------>------>------>------+ | | | ACK ACK | ACK | +------>------>------+ +------>------>------+ +->-+ | | | | | | | | | v | v | v +----------+ +----------+ +----------+ | IR State | | FO State | | SO State | +----------+ +----------+ +----------+ ^ ^ | ^ | | | | STATIC-NACK | | NACK / Update | | | +------<------<------+ +------<------<------+ | | | | STATIC-NACK | +------<------<------<------<------<------<------<------<------+ 5.3.1.1. State transition logic (R-mode) The transition logic for compression states in reliable mode is similar to the logic of the unidirectional mode, but completely relies on ACKs for forward transitions. 5.5.2. Decompressor states and logic (R-mode) The decompression states and the state transition logic are the same as for the unidirectional case (see section 5.3.2). What differs is the decompression and feedback logic. 5.5.2.1. Decompression logic (R-mode) TBW. 5.5.2.2. Feedback logic (R-mode) The feedback logic for the bi-directional reliable mode is identical to the one in optimistic mode (see chapter 5.4.2.2) with two important exceptions: - All feedback is mandatory, also the feedback defined as optional for the optimistic mode. Bormann (ed.) [Page 51] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 - Feedback MUST NOT be sent for packets not updating the context (i.e. packets that do not carry a CRC). 5.6. Mode transitions The decision to move from one compression mode to another is taken by the decompressor and the possible mode transitions are shown in the figure below. Subsequent chapters describe how the transitions are performed together with exceptions for the compression and decompression functionality during transitions. +-------------------------+ | Unidirectional (U) mode | +-------------------------+ / ^ \ ^ / /Feedback(U) \ \Feedback(U) / / \ \ / / \ \ Feedback(O)/ / Feedback(R)\ \ v / v \ +---------------------+ Feedback(R) +-------------------+ | Optimistic (O) mode | ----------------> | Reliable (R) mode | | | <---------------- | | +---------------------+ Feedback(O) +-------------------+ 5.6.1. Compression and decompression during mode transitions The following sections assume that compressor and decompressor for each context maintains a variable, whose value is the current compression mode for that context. The value of the variable controls which packet types to use, actions taken, etc, for that context. Subsequent chapters define exactly when to change the value of the MODE variable. When ROHC transits between compression modes, there are several cases where the behavior of compressor or decompressor must be restricted during the transition phase. These restrictions are defined by exception parameters that specifies which restrictions to apply. The transition descriptions in subsequent chapters refer to these exception parameters and defines when and to what values they are set. All mode related parameters are listed below together with their possible values, with explanations and restrictions: Parameters for the compressor side: - C_MODE Possible values for the C_MODE parameter are (U)NIDIRECTIONAL, (O)PTIMISTIC and (R)ELIABLE. C MODE MUST be initialized to U. Bormann (ed.) [Page 52] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 - C_TRANS Possible values for the C_TRANS parameter are (P)ENDING and (D)ONE. C TRANS MUST be initialized to D. When C TRANS is P, it is REQUIRED that 1) the compressor only uses packet formats common to all modes, 2) it does not transit to SO state. 3) New mode transition requests are ignored. Parameters for the decompressor side: - D_MODE Possible values for the D_MODE parameter are (U)NIDIRECTIONAL, (O)PTIMISTIC and (R)ELIABLE. C MODE MUST be initialized to U. - D_TRANS Possible values for the D_TRANS parameter are (I)NITIATED, (P)ENDING and (D)ONE. D TRANS MUST be initialized to D. A mode transition can be initiated only when D TRANS is D. While D TRANS is I, the decompressor sends a NACK or ACK for all received packets. 5.6.2. Transition from Unidirectional to Optimistic mode As long as there is a feedback channel available, the decompressor can at any moment decide to initiate transition from unidirectional to bi-directional Optimistic mode. All feedback packets can be used with the mode parameter set to O and the decompressor can then directly start working in Optimistic mode. The compressor transits from unidirectional to optimistic mode as soon as it receives any feedback packet with the mode parameter set to O. The transition procedure is described below: Comnpressor Decompressor ---------------------------------------------- | | | ACK(R)/NACK(R) +-<-<-<-| D_MODE = O | +-<-<-<-<-<-<-<-+ | C_MODE = O |-<-<-<-+ | | | If the feedback packet is lost, the compressor will continue to work in unidirectional mode, but as soon as any feedback packet reaches the compressor it will transit to Optimistic mode. Bormann (ed.) [Page 53] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.6.3. From Optimistic to Reliable mode Transition from Optimistic to Reliable mode is permitted only after at least one packet has been correctly decompressed, which means that at least the static part of the context is established. An ACK(R) or a NACK(R) feedback packet is sent to initiate the mode transition. The compressor MUST NOT use packet types 0 or 1 during transition. The transition procedure is described below: Comnpressor Decompressor ---------------------------------------------- | | | ACK(R)/NACK(R) +-<-<-<-| D_TRANS = I | +-<-<-<-<-<-<-<-+ | C_TRANS = P |-<-<-<-+ | C_MODE = R | | |->->->-+ FO(SN0,R) | | +->->->->->->->-+ | |->-.. +->->->-| D TRANS = P |->-.. | D_MODE = R | ACK(SN0,R) +-<-<-<-| | +-<-<-<-<-<-<-<-+ | C_TRANS = D |-<-<-<-+ | | | |->->->-+ SO (Reliable mode) | | +->->->->->->->-+ | | +->->->-| D_TRANS = D | | As long as the decompressor has not received an FO packet with the mode transition parameter set to R, it must stay in Optimistic mode. The compressor must stay in FO state until it has received an ACK for an FO packet sent with the mode transition parameter set to R (indicated by the sequence number). 5.6.4. From Unidirectional to Reliable mode Since transition from Unidirectional to Optimistic mode do not require any handshakes, it is possible to transit directly from Unidirectional to Reliable mode, following the same transition procedure in 5.6.3 above. 5.6.5. From Reliable to Optimistic mode Either the ACK(O) or the NACK(O) feedback packet is used to initiate the transition from Reliable to Optimistic mode and the compressor MUST always run in FO state during transition. The transition procedure is described below: Bormann (ed.) [Page 54] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Comnpressor Decompressor ---------------------------------------------- | | | ACK(O)/NACK(O) +-<-<-<-| D_TRANS = I | +-<-<-<-<-<-<-<-+ | C_TRANS = P |-<-<-<-+ | C_MODE = O | | |->->->-+ FO(SN0,O) | | +->->->->->->->-+ | |->-.. +->->->-| D_MODE = O |->-.. | | ACK(SN0,O) +-<-<-<-| | +-<-<-<-<-<-<-<-+ | C_TRANS = D |-<-<-<-+ | | | |->->->-+ SO (Optimistic mode) | | +->->->->->->->-+ | | +->->->-| D_TRANS = D | | As long as the decompressor has not received an FO packet with the mode transition parameter set to O, it must stay in Reliable mode. The compressor must stay in FO state until it has received an ACK for a FO packet sent with the mode transition parameter set to O (indicated by the sequence number). Bormann (ed.) [Page 55] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.6.6. Transition to Unidirectional mode It is possible to force transition back to unidirectional mode if the decompressor desires to do so. Independent of which mode it starts from, a three way handshake MUST be carried out to ensure correct transition on the compressor side. The transition procedure is described below: Compressor Decompressor ---------------------------------------------- | | | ACK(U)/NACK(U) +-<-<-<-| D_TRANS = I | +-<-<-<-<-<-<-<-+ | C_TRANS = P |-<-<-<-+ | C_MODE = U | | |->->->-+ FO(SN0,U) | | +->->->->->->->-+ | |->-.. +->->->-| |->-.. | | ACK(SN0,U) +-<-<-<-| | +-<-<-<-<-<-<-<-+ | C_TRANS = D |-<-<-<-+ | | | |->->->-+ SO (Uni. mode) | | +->->->->->->->-+ | | +->->->-| D_TRANS = D | | D_MODE = U The decompressor must continue to send feedback until it knows that the compressor is ready with the transition. Bormann (ed.) [Page 56] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.7. Packet formats A ROHC RTP packet has the following general format. --- --- --- --- --- --- --- --- / Padding / --- --- --- --- --- --- --- --- / Feedback / --- --- --- --- --- --- --- --- / Header / --- --- --- --- --- --- --- --- / Payload / --- --- --- --- --- --- --- --- Padding is any number (zero or more) of padding octets. Padding Octet 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 1 1 0 0 0 0 0 | +---+---+---+---+---+---+---+---+ (Note that the Padding Octet looks like, and in many implementations can be processed as, an Add-CID Octet for CID 0.) Either of Feedback or Header must be present. Header is either a compressed header or an IR or IR-DYN header. Feedback and Header either 1) do not carry any CID information (indicating CID zero), or 2) start with one Add-CID Octet (see below), or 3) contain embedded CID information of length one or two octets. Alternatives 1) and 2) apply to compressed headers in channels where the CID space is small, and feedback related to such headers. Alternative 3 applies only to compressed headers in channels where the CID space is large, and feedback related to such headers. Add-CID Octet 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 1 1 0 | CID | +---+---+---+---+---+---+---+---+ CID: 0x1 through 0xF indicates CIDs 1 through 15. (0x0 indicates a padding octet.) Bormann (ed.) [Page 57] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 All remaining types have the following general format: 0 x-1 x 7 --- --- --- --- --- --- --- --- | Add-CID octet | if for CID 1-15 and small CIDs +---+--- --- --- ---+--- --- ---+ | type indication | body | 1 octet (8-x bits of body) +---+--- ---+---+---+--- --- ---+ | | / 0, 1, or 2 octets of CID / 1 or 2 octets if large CIDs | | +---+---+---+---+---+---+---+---+ / body / variable +---+---+---+---+---+---+---+---+ The additional CID octets, if present, are encoded according to section 4.5.6. Sections 5.7.1-7 describe the formats more fully. Similar fields occur in several formats, therefore it is explained here how these fields are used. The following notation is used: bits(X) = the number of bits for field X present in the compressed header (including extension). field(X) = the value of field X in the compressed header. context(X) = the value of field X as established in the context. value(X) = field(X) if X is present in the compressed header, else context(X). hdr(X) = the value of field X in the uncompressed or decompressed header. The following fields occur in several headers and extensions. SN The compressed RTP sequence number. Compressed with W-LSB. The interpretation intervals, see section 4.5.1, are defined as follows: p = -1 if bits(SN) <= 4 p = 2^(bits(SN)-2) - 1 if bits(SN) > 4 Bormann (ed.) [Page 58] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 IP-ID A compressed IP-ID field. IP-ID fields in compressed base headers carry the compressed IP-ID of the innermost IPv4 header whose corresponding RND flag is not 1. The rules below assume the IP-ID is for the innermost IP header. If it is for an outer IP header, the RND2 and NBO2 flags should be used instead of RND and NBO. If value(RND)=0, hdr(IP-ID) is compressed using Offset IP-ID encoding, see section 4.5.5., using p=0 and default-slope(IP-ID offset)=0. If value(RND)=1, IP-ID is the uncompressed hdr(IP-ID). IP-ID is then passed as additional octets at the end of the compressed header, after any Extensions. If value(NBO)=1, the octets of hdr(IP-ID) are swapped before compression and after decompression. It is pointless but not harmful to set both NBO and RND. TS The compressed RTP Timestamp value. If value(TIME_STRIDE) > 0, Timer-based compression of RTP Timestamp, see section 4.5.4., is used. If value(Tsc)=1, Scaled RTP Timestamp encoding is used before compression, see section 4.5.3., and default-slope(TS) = 1. If value(Tsc)=0, the Timestamp value is compressed as-is, and default-slope(TS) = value(TS_STRIDE). CRC CRC over the original, uncompressed, header. For 3-bit CRCs, the polynomial of section 5.9.2 is used. For 7-bit CRCs, the polynomial of section 5.9.2 is used. For 8-bit CRCs, the polynomial of section 5.9.1 is used. M RTP Marker bit. Context(M) is initially zero and is never updated. Value(M)=1 only when field(M)=1. Bormann (ed.) [Page 59] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 The general format for a compressed header is as follows 0 1 2 3 4 5 6 7 --- --- --- --- --- --- --- --- | Add-CID octet | if for CID 1-15 and small CIDs +---+---+---+---+---+---+---+---+ | first octet of base header | (with type indication) +---+---+---+---+---+---+---+---+ | | / 0, 1, or 2 octets of CID / 1-2 octets if large CIDs | | +---+---+---+---+---+---+---+---+ / remainder of base header / variable number of bits +---+---+---+---+---+---+---+---+ / Extension (see 5.7.5) / extension, if X=1 in base header --- --- --- --- --- --- --- --- / IP-ID of inner Ipv4 header / 2 octets, if value(RND)=1 --- --- --- --- --- --- --- --- / IP-ID of outer IPv4 header / 2 octets, if value(RND2)=1 --- --- --- --- --- --- --- --- / UDP Checksum / 2 oct, if context(UDP Checksum)!= 0 --- --- --- --- --- --- --- --- In subsequent sections, the position of the large CID is indicated using the notation +===+===+===+===+===+===+===+===+ Whether the UDP Checksum field is present or not is controlled by the value of the UDP Checksum in the context. If non-zero, the UDP checksum is enabled and sent along with each packet. If zero, the UDP Checksum is disabled and not sent. Should hdr(UDP checksum) be non- zero when context(UDP Checksum) is non-zero, the header cannot be compressed. It must be sent uncompressed or the context reinitialized using an IR-packet. When an IPv4 header is present in the static context, for which the corresponding RND flag has not been established to be 1, packet types R-1 and UO-1 MUST NOT be used. When no IPv4 header is present in the static context, or the RND flags for all IPv4 headers in the context have been established to be 1, packet types R-1-ID, R-1-TS, UO-1-ID, and UO-1-TS MUST NOT be used. During the transient state when an RND flag is being established, packet types R-1-ID, R-1-TS, UO-1-ID, or UO-1-TS, MUST NOT be used. This implies that the RND flag(s) of the Extension 3 may have to be consulted before the format of a base header carrying an Extension 3 can be determined. Bormann (ed.) [Page 60] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.7.1. Packet type 0: UO-0, R-0, R-0-CRC Packet type 0 is indicated by the first bit being 0: R-0 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 0 | SN | +===+===+===+===+===+===+===+===+ Updating properties: R-0 packets do not update any part of the context. R-0-CRC 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | 1 | SN | +===+===+===+===+===+===+===+===+ |SN | CRC | +---+---+---+---+---+---+---+---+ Updating properties: R-0-CRC packets updates context(RTP sequence number). Note: the SN field straddles the CID field. UO-0 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | SN | CRC | +===+===+===+===+===+===+===+===+ Updating properties: The UO-0 packet updates the current value of context(RTP sequence number). 5.7.2. Packet type 1 (R-mode): R-1, R-1-TS, R-1-ID Packet type 1 is indicated by the first bits being 10 R-1 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 0 | SN | +===+===+===+===+===+===+===+===+ Bormann (ed.) [Page 61] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 | M | X | TS | +---+---+---+---+---+---+---+---+ Note: R-1 cannot be used if the context contains at least one IPv4 header with value(RND) = 0. This disambiguates it from R-1-ID and R-1-TS. R-1-ID 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 0 | SN | +===+===+===+===+===+===+===+===+ | M | X |T=0| IP-ID | +---+---+---+---+---+---+---+---+ Note: R-1-ID cannot be used if there is no IPv4 header in the context or if value(RND) and value(RND2) are both 1. R-1-TS 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 0 | SN | +===+===+===+===+===+===+===+===+ | M | X |T=1| TS | +---+---+---+---+---+---+---+---+ Note: R-1-TS cannot be used if there is no IPv4 header in the context or if value(RND) and value(RND2) are both 1. X: X=0 indicates that no Extension is present, X=1 indicates that an Extension is present. T: T=0 indicates format R-1-ID, T=1 indicates format R-1-TS. Updating properties: R-1* headers do not update any part of the context. Bormann (ed.) [Page 62] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.7.3. Packet type 1 (UO-modes): UO-1, UO-1-ID, UO-1-TS UO-1 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 0 | TS | +===+===+===+===+===+===+===+===+ | M | SN | CRC | +---+---+---+---+---+---+---+---+ Note: UO-1 cannot be used if the context contains at least one IPv4 header with value(RND) = 0. This disambiguates it from UO-1-ID and UO-1-TS. UO-1-ID 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 0 |T=0| IP-ID | +===+===+===+===+===+===+===+===+ | X | SN | CRC | +---+---+---+---+---+---+---+---+ Note: UO-1-ID cannot be used if there is no IPv4 header in the context or if value(RND) and value(RND2) are both 1. UO-1-TS 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 0 |T=1| TS | +===+===+===+===+===+===+===+===+ | M | SN | CRC | +---+---+---+---+---+---+---+---+ Note: UO-1-TS cannot be used if there is no IPv4 header in the context or if value(RND) and value(RND2) are both 1. X: X=0 indicates that no Extension is present, X=1 indicates that an Extension is present. T: T=0 indicates format UO-1-ID, T=1 indicates format UO-1-TS. Updating properties: UO-1* packets update context(RTP sequence number). UO-1 and UO-1-TS packets update context(RTP Timestamp). UO-1-ID packets update context(IP-ID). Values provided in Extensions, other than in other SN, TS, or IP-ID fields, do not update the context. Bormann (ed.) [Page 63] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.7.4. Packet type 2: UOR-2 Packet type 2 is indicated by the first bits being 110 UOR-2 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 1 0 | TS | +===+===+===+===+===+===+===+===+ |TS | M | SN | +---+---+---+---+---+---+---+---+ | X | CRC | +---+---+---+---+---+---+---+---+ UOR-2-ID 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 1 0 | IP-ID | +===+===+===+===+===+===+===+===+ |T=0| M | SN | +---+---+---+---+---+---+---+---+ | X | CRC | +---+---+---+---+---+---+---+---+ UOR-2-TS 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 1 0 | TS | +===+===+===+===+===+===+===+===+ |T=1| M | SN | +---+---+---+---+---+---+---+---+ | X | CRC | +---+---+---+---+---+---+---+---+ X: X=0 indicates that no Extension is present, X=1 indicates that an Extension is present. T: T=0 indicates format UOR-2-ID, T=1 indicates format UOR-2-TS. Updating properties: All values provided in UOR-2* packets update the context, unless explicitly stated otherwise. Bormann (ed.) [Page 64] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.7.5. Extension formats Fields in extensions are concatenated with the corresponding field in the base compressed header, if any. Bits in an extension are more significant than bits in the base compressed header. The TS field is scaled in all extensions, as it is in the base header, except optionally when using extension 3 where the Tsc flag can indicate that the TS field is not scaled. Value(TS_STRIDE) is used as the scale factor when scaling the TS field. In the following three extensions, the interpretation of the fields depend on whether there is a T-bit in the base compressed header, and if so, on the value of that field. When there is no T-bit, +T and -T both mean TS. This is the case when there are no IPv4 headers in the static context, and when all IPv4 headers in the static context have their corresponding RND flag set (i.e., RND=1). If there is a T-bit, T = 1 indicates that +T is TS, and -T is IP-ID. T = 0 indicates that +T is IP-ID, and -T is TS. Extension 0: 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 0 | SN | +T | +---+---+---+---+---+---+---+---+ Extension 1: +---+---+---+---+---+---+---+---+ | 0 1 | SN | +T | +---+---+---+---+---+---+---+---+ | -T | +---+---+---+---+---+---+---+---+ Extension 2: +---+---+---+---+---+---+---+---+ | 1 0 | SN | +T | +---+---+---+---+---+---+---+---+ | +T | +---+---+---+---+---+---+---+---+ | -T | +---+---+---+---+---+---+---+---+ Bormann (ed.) [Page 65] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Extension 3 is a more elaborate extension which can give values for fields other than SN, TS, and IP-ID. Three optional flag octets indicates changes to IP header(s) and RTP header, respectively. Extension 3: 0 1 2 3 4 5 6 7 +-----+-----+-----+-----+-----+-----+-----+-----+ | 1 1 | S |R-TS | Tsc | I | ip | rtp | (FLAGS) +-----+-----+-----+-----+-----+-----+-----+-----+ | Inner IP hdr flags | ip2 | if ip=1 ..... ..... ..... ..... ..... ..... ..... ..... | Outer IP hdr flags | if ip2=1 ..... ..... ..... ..... ..... ..... ..... ..... | SN | if S=1 ..... ..... ..... ..... ..... ..... ..... ..... / TS (encoded as in section 4.5.6.) / 1-4 oct, if R-TS=1 ..... ..... ..... ..... ..... ..... ..... ..... | | / Inner IP header fields / variable, if ip=1 | | ..... ..... ..... ..... ..... ..... ..... ..... | IP-ID | 2 octets if I=1 ..... ..... ..... ..... ..... ..... ..... ..... | | / Outer IP header fields / variable, if ip2=1 | | ..... ..... ..... ..... ..... ..... ..... ..... | | / RTP header flags and fields / variable, if rtp=1 | | ..... ..... ..... ..... ..... ..... ..... ..... S, R-TS, I, ip, rtp, ip2: Indicate presence of fields as shown to the right of each field above. Tsc: Tsc=0 indicates that TS is not scaled. Tsc=1 indicates that TS is scaled according to section 4.5.3, using value(TS STRIDE). Context(Tsc) is always 1. If scaling is not desired, the compressor will establish TS_STRIDE = 1 SN: see beginning of section 5.7. TS: Variable number of bits of TS, encoded according to section 4.5.6. See beginning of section 5.7. IP-ID: see beginning of section 5.7. Bormann (ed.) [Page 66] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Inner IP header flags These correspond to the inner IP header if there are two, and the single IP header otherwise. 0 1 2 3 4 5 6 7 ..... ..... ..... ..... ..... ..... ..... ..... | TOS | TTL | DF | PR | IPX | NBO | RND | ip2 | if ip=1 ..... ..... ..... ..... ..... ..... ..... ..... TOS, TTL, PR, IPX: Indicates presence of fields as shown to the right of the field in question below. DF: Don't Fragment bit of IP header. NBO: Indicates whether the octets of hdr(IP identifier) of this IP header are swapped before compression and after decompression. NBO=1 indicates that the octets need not be swapped. NBO=0 indicates that the octets are to be swapped. See section 4.5.5. RND: Indicates whether hdr(IP identifier) is not to be compressed but instead sent as-is in compressed headers. IP2: Indicates presence of Outer IP header fields. Unless the static context contains two IP headers, IP2 is always zero. Inner IP header fields ..... ..... ..... ..... ..... ..... ..... ..... | Type of Service/ Traffic Class | if TOS=1 ..... ..... ..... ..... ..... ..... ..... ..... | Time to live/ Hop Limit | if TTL=1 ..... ..... ..... ..... ..... ..... ..... ..... | Protocol/ Next Header | if PR=1 ..... ..... ..... ..... ..... ..... ..... ..... / Compressed IP extension hdr(s) / variable, if IPX=1 ..... ..... ..... ..... ..... ..... ..... ..... Type of Service/Traffic Class: That field in the IP header, absolute value. Time to live/ Hop Limit: That field in the IP header. Protocol/ Next Header: That field in the IP header. Compressed IP extension hdr(s): According to section 5.8.2. Bormann (ed.) [Page 67] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Outer IP header flags The fields in this part of the Extension 3 header refers to the outermost IP header. 0 1 2 3 4 5 6 7 ..... ..... ..... ..... ..... ..... ..... ..... | TOS2| TTL2| DF2 | PR2 |IPX2 |NBO2 |RND2 | I2 | if ip2=1 ..... ..... ..... ..... ..... ..... ..... ..... These flags are the same as the Inner IP header flags, but refer to the outer IP header instead of the inner IP header. The following flag, however, has no counterpart in the Inner IP header flags: I2: indicates presence of the IP-ID field. Outer IP header fields ..... ..... ..... ..... ..... ..... ..... ..... | Type of Service/ Traffic Class | if TOS2=1 ..... ..... ..... ..... ..... ..... ..... ..... | Time to live/ Hop Limit | if TTL2=1 ..... ..... ..... ..... ..... ..... ..... ..... | Protocol/ Next Header | if PR2=1 ..... ..... ..... ..... ..... ..... ..... ..... / compressed IP extension hdr(s) / variable, if IPX2=1 ..... ..... ..... ..... ..... ..... ..... ..... | IP-ID | 2 octets if I2=1 ..... ..... ..... ..... ..... ..... ..... ..... The fields in this part of the Extension 3 are as for the Inner IP header fields, but refers to the outer IP header instead of the inner IP header. The following field, however, has no counterpart among the Inner IP header fields: IP-ID: the compressed Identifier field of the outer IP header, unless the inner header is an IPv6 header, in which case I2 is always zero. Bormann (ed.) [Page 68] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 RTP header flags and fields 0 1 2 3 4 5 6 7 ..... ..... ..... ..... ..... ..... ..... ..... | Mode |R-PT |R-P | R-X |CSRC | TSS | IPT | if rtp=1 ..... ..... ..... ..... ..... ..... ..... ..... | M | RTP PT | if R-PT=1 ..... ..... ..... ..... ..... ..... ..... ..... / Compressed CSRC-list / if CSRC=1 ..... ..... ..... ..... ..... ..... ..... ..... / TS STRIDE / 1-4 oct if TSS=1 ..... ..... ..... ..... ..... ..... ..... .... / TIME STRIDE (milliseconds) / 1-4 oct if IPT=1 ..... ..... ..... ..... ..... ..... ..... ..... Mode: Compression mode. 0 = Reserved, 1 = Unidirectional, 2 = Bi-directional Optimistic, 3 = Bi-directional Reliable. R-PT, CSRC, TSS, IPT: Indicate presence of fields as shown to the right of each field below. R-P: RTP Padding bit, absolute value. R-X: RTP eXtension bit, absolute value. M: see beginning of section 5.7. RTP PT: Absolute value of RTP Payload type field. Compressed CSRC-list: see section 5.8.1. TS STRIDE: Predicted increment/decrement of the RTP Timestamp field, when it changes. Encoded as in section 4.5.6. TIME STRIDE: Predicted time interval between changes in RTP Timestamp, in milliseconds. Also indication that compressor desires to do Timer-based compression of the RTP Timestamp field, see section 4.5.4. Encoded as in section 4.5.6. 5.7.5.1 RND flags and packet types The values of the RND flag or RND2 flag is changed by sending UOR-2 packets with an Extension 3 where the flag(s) have the new value. In U mode, the UOR-2 with Extension 3 is sent several times in order to ensure that the decompressor receives at least one. In O mode and R mode, the UOR-2 with Extension 3 is repeated until an acknowledgment for a packet with the new RND flag value(s) is received. Bormann (ed.) [Page 69] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 The decompressor updates the value of its RND and RND2 flags whenever it receives an UOR-2 with an Extension 3 carrying values for RND or RND2, and the UOR-2 CRC verifies successful decompression. When an IPv4 header is present in the static context, for which the corresponding RND flag has not been established to be 1, packet types R-1 and UO-1 MUST NOT be used. When no IPv4 header is present in the static context, or the RND flags for all IPv4 headers in the context have been established to be 1, packet types R-1-ID, R-1-TS, UO-1-ID, and UO-1-TS MUST NOT be used. During the transient state when an RND flag is being established, packet types R-1-ID, R-1-TS, UO-1-ID, or UO-1-TS, MUST NOT be used. This implies that the RND flag(s) of the Extension 3 may have to be consulted before the exact format of a base header carrying an Extension 3 can be determined, i.e., whether a T-bit is present or not. 5.7.5.2 Flags/Fields in context Some flags and fields in Extension 3 need to be maintained in the context of the decompressor. Their values are established using the mechanism appropriate to the compression mode, unless otherwise indicated in the table below and in referred sections. Flag/Field Initial value comment --------------------------------------------------------------------- Mode Unidirectional See section 5.x.x NBO 1 RND 0 NBO2 1 RND2 0 TS STRIDE 1 TIME STRIDE 0 Tsc 0 Tsc is always 0 in context. Can be 1 only when an Ext 3 is present. Bormann (ed.) [Page 70] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.7.6. Feedback packets and formats When the roundtrip time between compressor and decompressor is large, several packets can be in flight. Therefore, several packets can be received by the decompressor after feedback has been sent and before the compressor has reacted to feedback. When alternatives 4 or 5 are used, the decompressor SHOULD NOT send feedback for each successful decompression, unsuccessful decompression, or packet received in a rejected packet stream. Instead, a compressor SHOULD limit the rate at which feedback is sent. 5.7.6.1 Feedback packet type for ROHC RTP Feedback has the following format: 0 1 2 3 4 5 6 7 --- --- --- --- --- --- --- --- | Add-CID octet | if for CID!=0 and small CIDs +---+---+---+---+---+---+---+---+ | 1 1 1 1 1 0 | Code | +---+---+---+---+---+---+---+---+ | | / 0-2 octets of CID info / 1-2 octets if for large CIDs | | +---+---+---+---+---+---+---+---+ : Feedback Length : if Code=1 +---+---+---+---+---+---+---+---+ / feedback / FEEDBACK-1 or FEEDBACK-2 +---+---+---+---+---+---+---+---+ Code: 0 indicates FEEDBACK-2 format, no Feedback Length octet 1 indicates FEEDBACK-2 format, with Feedback Length octet 2 indicates FEEDBACK-1 format, R-mode (length 1 octet) 3 indicates FEEDBACK-1 format, O-mode (length 1 octet) Feedback Length: indicates size of feedback field Feedback: According to section 5.7.6.2 The CID identifies the context the feedback refers to. The total size of the feedback information is determinable as the decompressor must be informed about whether the CID space of the associated compressor is large or small. The feedback information is passed to the associated compressor upon reception. Bormann (ed.) [Page 71] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Interleaved feedback For interleaved feedback, alternative 4) in 5.7.6, Code SHOULD not have the value one (1). Piggybacked feedback For piggybacked feedback, alternative 5) in 5.7.6, Code MUST NOT have the value zero (0), as the decompressor must be able to determine the size of the piggybacked feedback information without parsing it. Dedicated feedback channel Alternative 2) in 5.7.6. To achieve an octet-aligned format, the packet type indication and the code field is eliminated. Packet types are not needed in a dedicated channel. FEEDBACK-1 is always for R- mode. If the feedback is for large CIDs, the packet will always start with a CID. If the feedback is for small CIDs, the following procedure can determine the format: 1) If the packet length is one octet, it is a FEEDBACK-1 (for R- mode). 2) Otherwise, if the packet starts with the bits 11, and its length is 2 octets, it consists of an Add-CID octet followed by FEEDBACK-1. 3) Otherwise, if the packet starts with the bits 11, and its length is larger than 2 octets, it consists of an Add-CID octet followed by FEEDBACK-2. 4) Otherwise, the packet consists of a FEEDBACK-2. 5.7.6.2 Feedback formats for ROHC RTP The following is the format for feedback information for ROHC RTP. It is to be used when feedback information is piggybacked or interleaved (alternatives 4 and 5 in 5.7.6). Several feedback formats carry a field labeled SN. The SN field contains LSBs of an RTP sequence number. The sequence number to use is the sequence number of the header which caused the FEEDBACK packet to be sent. If that sequence number cannot be determined, for example Bormann (ed.) [Page 72] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 when decompression fails, the sequence number to use is that of the last successfully decompressed header. If no sequence number is available, the SN field is zero. FEEDBACK-1 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | SN | +---+---+---+---+---+---+---+---+ A FEEDBACK-1 is an ACK. In order to send a NACK or a STATIC-NACK, FEEDBACK-2 must be used. To indicate Unidirectional mode, FEEDBACK- 2 must be used. FEEDBACK-2 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ |Acktype| Mode | SN | +---+---+---+---+---+---+---+---+ | SN | +---+---+---+---+---+---+---+---+ / Feedback options / +---+---+---+---+---+---+---+---+ Acktype: 0 = ACK 1 = NACK 2 = STATIC-NACK 3 is reserved Mode: 0 is reserved 1 = Unidirectional mode 2 = Bi-directional Optimistic mode 3 = Bi-directional Reliable mode Feedback options: A variable number of feedback options, see section 5.7.6.3. Options may appear in any order. Bormann (ed.) [Page 73] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.7.6.3 RTP Feedback options An RTP Feedback option has variable length and the following general format 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | Opt Type | Opt Len | +---+---+---+---+---+---+---+---+ / option data / Opt Len octets +---+---+---+---+---+---+---+---+ Sections 5.7.6.4-9 describe the currently defined RTP feedback options. 5.7.6.4 The CRC option The CRC option contains an 8-bit CRC computed over the entire feedback packet, including any CID fields, using the polynomial of section 5.9.1. For purposes of computing the CRC, the CRC fields of all CRC options are zero. 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | Opt Type = 1 | Opt Len = 1 | +---+---+---+---+---+---+---+---+ | CRC | +---+---+---+---+---+---+---+---+ When receiving a FEEDBACK packet with a CRC option, the compressor MUST verify the header by computing the CRC and comparing the result with the CRC carried in the CRC option. If the two are not identical, all information in the FEEDBACK packet MUST be ignored. 5.7.6.5 The REJECT option The REJECT option informs the compressor that the decompressor does not have sufficient resources to handle the flow. 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | Opt Type = 2 | Opt Len = 0 | +---+---+---+---+---+---+---+---+ When receiving a REJECT option, the compressor stops compressing the packet stream, and should refrain from attempting to increase the number of compressed packet streams for some time. Any FEEDBACK packet carrying a REJECT option MUST also carry a CRC option. Bormann (ed.) [Page 74] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.7.6.6 The SN option The SN option provides 8 additional bits of SN. 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | Opt Type = 3 | Opt Len = 1 | +---+---+---+---+---+---+---+---+ | SN | +---+---+---+---+---+---+---+---+ 5.7.6.7 The CLOCK option The CLOCK option informs the compressor of the clock resolution of the decompressor. This is needed to allow the compressor to estimate the jitter introduced by the clock of the decompressor when doing Timer-based compression of the RTP timestamp. 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | Opt Type = 4 | Opt Len = 1 | +---+---+---+---+---+---+---+---+ | clock resolution (ms) | +---+---+---+---+---+---+---+---+ The smallest clock resolution which can be indicated is 1 millisecond. The value zero has a special meaning: it indicates that the decompressor cannot do Timer-based compression of the RTP timestamp. Any FEEDBACK packet carrying a CLOCK option SHOULD also carry a CRC option. 5.7.6.8 The JITTER option The JITTER option allows the decompressor to report the maximum jitter it has observed lately, using the following formula which is very similar to the formula for Max_Jitter BC in section 4.5.4. Let Observation window i contain the decompressor's best approximation of the TSW of the compressor (see section 4.5.4) when header i is received. Max_Jitter i_= max {|(T_i _ T_j) _ ((a_i _ a_j) / TIME_STRIDE)|, for all headers j in Observation window i} Max Jitter = max { Max Jitter i, for a large number of recent headers i } Bormann (ed.) [Page 75] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 This information may be used by the compressor to refine the formula for determining k when doing Timer-based compression of the RTP Timestamp. 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | Opt Type = 5 | Opt Len = 1 | +---+---+---+---+---+---+---+---+ | Max Jitter | +---+---+---+---+---+---+---+---+ The decompressor MAY ignore the oldest observed values of Max Jitter i. Thus, the reported Max Jitter may decrease. Robustness will be reduced if the compressor uses a jitter estimate which is too small. Therefore, a FEEDBACK packet carrying a JITTER option SHOULD also carry a CRC option. Moreover, the compressor MAY ignore decreasing Max Jitter values. 5.7.6.9The LOSS option The LOSS option allows the decompressor to report the largest observed number of packets lost in sequence. This information MAY be used by the compressor to adjust the size of the reference window used in U and O mode. 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | Opt Type = 6 | Opt Len = 1 | +---+---+---+---+---+---+---+---+ | longest loss event (packets) | +---+---+---+---+---+---+---+---+ The decompressor MAY choose to ignore the oldest loss events. Thus, the value reported may decrease. Since setting the reference window too small can reduce robustness, a FEEDBACK packet carrying a LOSS option SHOULD also carry a CRC option. The compressor MAY choose to ignore decreasing loss values. 5.7.6.10 Unknown option types If an option type unknown to the compressor is encountered, it must continue parsing the rest of the FEEDBACK packet, which is possible since the length of the option is explicit, but MUST otherwise ignore the unknown option. Bormann (ed.) [Page 76] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.7.6.11 RTP feedback example Feedback for CID 8 indicating an ACK for SN 17 and Bi-directional Reliable mode can have the following format. 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 1 1 0 | 0 1 1 1 | (Assuming add-on CID format) +---+---+---+---+---+---+---+---+ | 1 1 1 1 1 0 | 0 | feedback packet type, Code=0 +---+---+---+---+---+---+---+---+ | 0 1 | 1 1 | SN MSB = 0 | AckType = ACK, Mode = Reliable +---+---+---+---+---+---+---+---+ | SN LSB=17 | +---+---+---+---+---+---+---+---+ The FEEDBACK-3 format may also be used. 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 1 1 0 | 0 1 1 1 | (Assuming add-on CID format) +---+---+---+---+---+---+---+---+ | 1 1 1 1 1 0 | 2 | feedback packet type, Code=2 +---+---+---+---+---+---+---+---+ | SN=17 | +---+---+---+---+---+---+---+---+ 5.7.7 IR and IR-DYN packets The subheaders which are compressible are split into a STATIC part and a DYNAMIC part. These parts are defined in sections 5.7.6.3-*. The structure of a chain of subheaders is determined by each header having a Next Header, or Protocol, field. This field identifies the type of the following header. Each Static part below contains that field and allows parsing of the Static chain. Bormann (ed.) [Page 77] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.7.7.1 Basic structure of the IR packet This packet type communicates the static part of the context, i.e., the value of the constant SN-functions. It can optionally also communicate the dynamic part of the context, i.e., the parameters of non-constant SN-functions. It can also optionally communicate the payload of an original packet, if any. 0 1 2 3 4 5 6 7 --- --- --- --- --- --- --- --- | Add-CID octet | if for CID!=0 for small CIDs +---+---+---+---+---+---+---+---+ | 1 1 1 1 1 1 0 | D | +---+---+---+---+---+---+---+---+ | | / 0-2 octets of CID info / 1-2 octets if for large CIDs | | +---+---+---+---+---+---+---+---+ | Profile | 1 octet +---+---+---+---+---+---+---+---+ | CRC | 1 octet +---+---+---+---+---+---+---+---+ | | | Static chain | variable length | | +---+---+---+---+---+---+---+---+ | | | Dynamic chain | present if D=1, variable length | | - - - - - - - - - - - - - - - - | | | Payload | variable length | | - - - - - - - - - - - - - - - - D: D=1 indicates that the dynamic chain is present. Profile: Indicates transport/application of this stream. CRC: 8-bit CRC, computed according to section 5.9.1. Static chain: A chain of static subheader information. Dynamic chain: A chain of dynamic subheader information. What dynamic information is present is inferred from the Static chain. Payload: payload of corresponding original packet, if any. Presence inferred from packet size. Bormann (ed.) [Page 78] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.7.7.2. Basic structure of the IR-DYN packet This packet type communicates the dynamic part of the context, i.e., the parameters of non-constant SN-functions. 0 1 2 3 4 5 6 7 --- --- --- --- --- --- --- --- | Add-CID octet | if for CID!=0 for small CIDs +---+---+---+---+---+---+---+---+ | 1 1 1 1 1 1 1 0 | +---+---+---+---+---+---+---+---+ | | / 0-2 octets of CID info / 1-2 octets if for large CIDs | | +---+---+---+---+---+---+---+---+ | Profile | 1 octet +---+---+---+---+---+---+---+---+ | CRC | 1 octet +---+---+---+---+---+---+---+---+ | | / Dynamic chain / variable length | | +---+---+---+---+---+---+---+---+ | | / Payload / variable length | | - - - - - - - - - - - - - - - - Profile: Indicates transport/application of this stream. CRC: 8-bit checksum covering original header, computed according to section 5.9.1. Dynamic chain: A chain of dynamic subheader information. What dynamic information is present is inferred from the Static chain of the context. Payload: payload of corresponding original packet, if any. (Presence of payload is inferred from length of packet.) NOTE: The static and dynamic chains of IR or IR-DYN packets for profile 0 MUST end with static and dynamic parts for an RTP header. If not, the packet MUST be discarded and the context must not be updated. NOTE: The static or dynamic chains of IR or IR-DYN packets for profile 1 MUST end with static and dynamic parts for a UDP header. If not, the packet MUST be discarded and the packet must not be updated. Bormann (ed.) [Page 79] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.7.7.3. Initialization of IPv6 Header [IPv6] Static part: +---+---+---+---+---+---+---+---+ | Version = 6 |Flow Label(msb)| 1 octet +---+---+---+---+---+---+---+---+ / Flow Label (lsb) / 2 octets +---+---+---+---+---+---+---+---+ | Next Header | 1 octet +---+---+---+---+---+---+---+---+ | Hop Limit | 1 octet +---+---+---+---+---+---+---+---+ / Source Address / 16 octets +---+---+---+---+---+---+---+---+ / Destination Address / 16 octets +---+---+---+---+---+---+---+---+ Dynamic part: +---+---+---+---+---+---+---+---+ | Traffic Class | 1 octet +---+---+---+---+---+---+---+---+ Eliminated: Payload Length CRC-DYNAMIC: Payload Length field (octets 5-6). CRC-STATIC: All other fields (octets 1-4, 7-40). Bormann (ed.) [Page 80] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.7.7.4. Initialization of IPv4 Header [IPv4, section 3.1]. Static part: Version, Flags, Time to Live, Protocol, Source Address, Destination Address. +---+---+---+---+---+---+---+---+ | Version = 4 | 0 | DF| 0 | +---+---+---+---+---+---+---+---+ | Time to Live | +---+---+---+---+---+---+---+---+ | Protocol | +---+---+---+---+---+---+---+---+ / Source Address / 4 octets +---+---+---+---+---+---+---+---+ / Destination Address / 4 octets +---+---+---+---+---+---+---+---+ Dynamic part: Type of Service, Identification +---+---+---+---+---+---+---+---+ | Type of Service | +---+---+---+---+---+---+---+---+ / Identification / 2 octets +---+---+---+---+---+---+---+---+ Eliminated: IHL (must be 5) Total Length (inferred in decompressed packets) MF flag (More Fragments flag, must be 0) Fragment Offset (must be 0) Header Checksum (inferred in decompressed packets) Options, Padding (must not be present) CRC-DYNAMIC: Total Length, Identification, Header Checksum (octets 3-4, 5-6, 11-12). CRC-STATIC: All other fields (octets 1-2, 7-10, 13-20). Bormann (ed.) [Page 81] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.7.7.5. Initialization of UDP Header [RFC-768]. Static part: +---+---+---+---+---+---+---+---+ / Source Port / 2 octets +---+---+---+---+---+---+---+---+ / Destination Port / 2 octets +---+---+---+---+---+---+---+---+ Dynamic part: +---+---+---+---+---+---+---+---+ / Checksum / 2 octets +---+---+---+---+---+---+---+---+ Eliminated: Length The Length field of the UDP header MUST match the Length field(s) of preceding subheaders, i.e., there must not be any padding after the UDP payload that is covered by the IP Length. CRC-DYNAMIC: Length field, Checksum (octets 5-8). CRC-STATIC: All other fields (octets 1-4). 5.7.7.6. Initialization of RTP Header [RTP]. Static part: P, X, CC, PT, SSRC, CSRC identifiers. 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | V=2 | P | X | CC | +---+---+---+---+---+---+---+---+ | M | PT | +---+---+---+---+---+---+---+---+ / SSRC / 4 octets +---+---+---+---+---+---+---+---+ : : : CSRC ids : multiple of 4 octets : : +---+---+---+---+---+---+---+---+ Bormann (ed.) [Page 82] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Dynamic part: M, sequence number, timestamp, timestamp delta. 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ / RTP sequence number / 2 octets +---+---+---+---+---+---+---+---+ | RTP timestamp (absolute) | 4 octets +---+---+---+---+---+---+---+---+ | M | TS-Stride (msb) | +---+---+---+---+---+---+---+---+ | TS-Stride (lsb) | +---+---+---+---+---+---+---+---+ Eliminated: Nothing. CRC-DYNAMIC: Octet containing M-bit, sequence number field, timestamp (octets 2-8). CRC-STATIC: All other fields (octets 1, 9-12, CSRC list). 5.7.7.7. Minimal Encapsulation header [RFC-2004, section 3.1] STATIC Part: Entire header. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Protocol |S| reserved | Header Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Original Destination Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : (if present) Original Source Address : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ DYNAMIC Part: Empty. Eliminated: Nothing. Bormann (ed.) [Page 83] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 CRC-DYNAMIC: Empty. CRC-STATIC: Entire header. 5.8 List-Based Compression For CSRC lists in RTP packet, and extension header chains in IP headers, the information to be compressed can be described as an ordered list, which is largely constant between packets, but to which additions and deletions are made occasionally during the life-time of the packet stream. This section describes the compression scheme for such information [[Editor's note: This section still is under active discussion in the mailing list. A somewhat simplified version has emerged and will be integrated in the next revision.]] 5.8.1. CSRC compression The Contributing Source (CSRC) List in a RTP header contains the Synchronization Source (SSRC) identifiers of the contributing sources for the payload in the current packet. A CSRC list contains at most 15 identifiers, due to the 4-bit size of CSRC Count (CC) field in RTP header. Each 32-bit identifier is chosen randomly by the original synchronization source so that it is globally unique within an RTP session. The compression scheme introduced here will utilize the facts mentioned above. To maintain transparency, the order of identifiers is preserved during compression. In other words, the CSRC list is really compressed as a list, not as a set. 5.8.1.1. Transformation Classification for CSRC List A given CSRC list (curr_list) can be classified as belonging to one of the following transformation cases when compared with a reference CSRC list (ref_list). - Transformation Case A: curr_list can be derived from ref_list just by adding some CSRCs; the relative positions of the CSRCs common to curr_list and ref_list are the same. - Transformation Case B: curr_list can be derived from ref_list just by deleting some CSRCs; the relative positions of the CSRCs common to curr_list and ref_list are the same Bormann (ed.) [Page 84] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 - Transformation Case C: All the other transformation cases that are not covered by transformation case A and B. 5.8.1.2. Encoding Schemes To address aforementioned 3 transformation cases, four encoding schemes are used. Each scheme addresses one or more transformation cases mentioned above. The four encoding schemes and the transformation cases they address is listed in the following table. +----------------------------+--------------------+ | Encoding Scheme | Transformaton Case | +----------------------------+--------------------+ | Insertion Only | A | +----------------------------+--------------------+ | Removal Only | B | +----------------------------+--------------------+ | Generic | mixture of A,B,C | +----------------------------+--------------------+ | Uncompressed | mixture of A,B,C | +----------------------------+--------------------+ - In the insertion only scheme, compared with CSRC list in the ref_list, the newly added CSRCs in the curr_list are sent along with the positions of the CSRCs in the ref_list, before which the new CSRCs will be inserted. - In the removal only scheme, the positions of the CSRCs, which are in the ref_list, but not in the curr_list, are sent. - In the generic scheme, for a given CSRC in the curr_list, it is sent compressed only with a position field if the CSRC is also in the ref_list, or it is sent uncompressed. All the aforementioned 3 schemes generate compressed format of the CSRC list. The CSRC list can also be sent in an uncompressed format. 5.8.1.3. Format of compressed CSRC list The format of the compressed CSRC list using the four schemes is as follows. 5.8.1.3.1 Insertion Only Scheme 5.8.1.3.1.1 R-mode 2 6 or 14 8 or 16 +---------+--------+--------------------+ | ET = 00 | ref_id | insertion bit mask | +---------+--------+--------------------+ Bormann (ed.) [Page 85] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 +--------+----+--------+ | CSRC 1 |....| CSRC m | +--------+----+--------+ * ref_id - the LSB of the RTP sequence number of the ref_list 6 bit: "0" + 5-bit LSB of RTP sequence number 14 bit: "1" + 13-bit LSB of RTP sequence number * insertion bit mask: the bit mask indicating the position of the following CSRCs to be inserted into the ref_list in order to reconstruct the curr_list. The length of insertion bit mask can be 8 bits or 16 bits. 8 bits: "0" + 7-bit insertion bit mask 16 bits: "1" + 15-bit insertion bit mask To construct the insertion bit mask and the following inserted CSRC list, the following steps are taken. ** A list of '0' and an empty inserted CSRC list are generated as the starting point. The number of '0's in the '0' list equals the number of CSRCs in the ref_list. The i-th '0' in the '0' list corresponds to the i-th CSRC in the ref_list. ** Comparing the curr_list with the ref_list, if a new CSRC is inserted between the i-th item and the (i+1)-th item in the ref_list, a '1' is inserted between the i-th '0' and (i+1)-th '0' in the original '0' list. The new CSRC should be added to the end of inserted CSRC list. This procedure is repeated until all the m new CSRCs have been processed. If the length of new insertion bit mask is less than 7 bits or 15 bits, additional '0' should be added at the end until it reaches 7 bits or 15 bits. When the decompressor receives the insertion bit mask, it scans from left to right. When a '0' is observed, the decompressor copies the corresponding CSRC in the ref_list into the curr_list; when a '1' is observed, the decompressor adds the correspondent CSRC into the curr_list. * CSRC i (i=1..m): the CSRCs to be inserted; assuming that the number of CSRC to be inserted is m. 5.8.1.3.1.2 UO-modes 2 6 8 8 8 or 16 +---------+------+--------+--------+--------------------+ | ET = 00 | resv | Gen_id | ref_id | insertion bit mask | +---------+------+--------+--------+--------------------+ Bormann (ed.) [Page 86] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 +--------+----+--------+ | CSRC 1 |....| CSRC m | +--------+----+--------+ * resv: reserved * Gen_id: is used to identify a set of packets that belongs to the same generation * ref_id: the Gen_id carried in the ref_list * insertion bit mask and CSRC i : the same as defined for R-mode 5.8.1.3.2 Removal Only Scheme 5.8.1.3.2.1 R-mode 2 6 or 14 8 or 16 +---------+--------+------------------+ | ET = 01 | ref_id | removal bit mask | +---------+--------+------------------+ * ref_id: the same as defined in section 3.1.1. * removal bit mask: the bit mask indicating the CSRC in the ref_list to be removed in order to reconstruct the curr_list. A '1' in the i-th bit in the removal bit mask means that the i-th CSRC in the ref_list is not in the curr_list, while a '0' means it is still present in the curr_list. The length of insertion bit mask can be 8 bits or 16 bits. 8 bits: "0" + 7-bit insertion bit mask 16 bits: "1" + 15-bit insertion bit mask Which format to be used depends on the number of CSRCs in the ref_list. If it is less than 8, then 8-bit removal bit mask can be used; otherwise the 16-bit format is required. 5.8.1.3.2.2 UO-mode 2 6 8 8 8 or 16 +---------+------+--------+--------+------------------+ | ET = 01 | resv | gen_id | ref_id | removal bit mask | +---------+------+--------+--------+------------------+ * Gen_id, resv and ref_id: the same as defined in section 3.1.2. * removal bit mask: the same as defined for R mode Bormann (ed.) [Page 87] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.8.1.3.3 Generic Scheme 5.8.1.3.3.1 R-mode 2 6 or 14 4 +---------+--------+------------+--------------------------+ | ET = 10 | ref_id | gcount = m | g_CSRC 1 |....| g_CSRC m | +---------+--------+------------+--------------------------+ +:::::::::+ | padding | +:::::::::+ * ref_id: the same as defined in section 3.1.1. * gcount: the number of following g_CSRC field; the length is 4 bits. * g_CSRC i (i=1..m): corresponds to the i-th CSRC in the curr_list. The order of g_CSRC represents the order of CSRCs in the curr_list. Two types of g_CSRC is defined. ** g_CSRC type 0 is used to compress a CSRC that is in both ref_list and curr_list. ** g_CSRC type 1 is used to carry a CSRC that is in the curr_list but not in the ref_list. The format of g_CSRC is defined as follows. 1 ceiling(log2(k)) +--------+-----+ g_CSRC type 0: | GC = 0 | pos | +--------+-----+ - pos: the position of a CSRC in the ref_list; the length is ceiling(log2(k)) where k is the number of CSRC in the ref_list. Since the value of k is known to both the compressor and decompressor, the length of the pos field doesn't need to be carried in the compressed list 1 +--------+------+ g_CSRC type 1: | GC = 1 | CSRC | +--------+------+ * padding: padding bits may be required to maintain the byte alignment Bormann (ed.) [Page 88] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.8.1.3.3.2 UO-mode 8 2 8 4 +--------+---------+--------+------------+--------------------------+ | gen_id | ET = 10 | ref_id | gcount = m | g_CSRC 1 |....| g_CSRC m | +--------+---------+--------+------------+--------------------------+ +:::::::::+ | padding | +:::::::::+ * Gen_id and ref_id: the same as defined in section 3.2.1.2. * gcount, g_CSRC and padding: the same as defined for R mode 3.4 Uncompressed (common to R-mode and UO-modes) 2 1 1 8 4 +---------+------+---+::::::::+------------+----------------------+ | ET = 11 | resv | G | gen_id | ccount = m | CSRC 1 |....| CSRC m | +---------+------+---+::::::::+------------+----------------------+ * resv: reserved * G: indicating the presence of gen_id field * Gen_id: the same as defined in section 3.1.2. - In R-mode, gen_id is not present. - During the transition from any mode to U-mode or O-mode, gen_id should not be sent. The list without gen_id should not be used as a reference to compress and decompress a CSRC list. - In UO-modes, gen_id is present if it may be used as a reference to compress or decompress the subsequent CSRC list. * ccount: the number of CSRCs in the CSRC list * CSRC i (i=1..m): the original CSRC list in the curr_list 5.8.2. Header Compression for IPv6 Extension Headers The IPv6 extension headers are encoded as a list of items. Each item is one of the extension headers. The length of each extension header may vary from each other. When more than one extension header is used in the same packet, the order of these extension headers is recommended in RFC 2460, but not mandatory. Thus, although it is unlikely to happen, the order of the extension headers may vary during the same session. In addition, one or more extension header Bormann (ed.) [Page 89] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 may be added or removed during the session and the content of each extension header may change. Therefore, the IPv6 extension headers are classified as a list of items and the item list compression mechanism can be applied. The compression of IPv6 extension headers at the list level is similar to that of the CSRC entries (section 5.8.1). The compressed value of the extension header list is referred to as a compressed extension header list. The compression of IPv6 extension headers at the item level, i.e., the compression scheme used for each type of extension header, is defined in this subsection. The reference extension header used to compress a given extension header is the extension header in the reference list that has the same type. The compressed value of an extension header is referred to as a compressed extension header. 5.8.2.1. Terminology * extension header list: the list of IPv6 extension headers * compressed extension header: the compressed value of an IPv6 extension header * compressed extension header list: the list of compressed extension headers * reference extension header list: the extension header list which is used as the reference to compress an extension header list * reference extension header: an extension header in the reference list that has the same type and is used as the reference to compress a given extension header 5.8.2.2. Transformation Classification and Encoding Schemes 5.8.2.2.1 Transformation Classification A given extension header list (curr_list) can be classified as belonging to one of the following transformation cases when compared with a reference extension header list (ref_list). - Transformation Case A: curr_list can be derived from ref_list just by adding (inserting) some extension headers; the relative positions of the extension headers common to curr_list and ref_list are the same. - Transformation Case B: curr_list can be derived from ref_list just by deleting some extension headers; the relative positions of the extension headers common to curr_list and ref_list are the same Bormann (ed.) [Page 90] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 - Transformation Case C: curr_list can be derived from ref_list just by modifying the content of some extension headers; the relative positions of the extension headers remain the same. - Transformation Case D: All the other transformation cases that are not covered by transformation case A, B and C. 5.8.2.2.2 Encoding Schemes To address aforementioned 4 transformation cases, four encoding schemes are used. Each scheme addresses one or more transformation cases mentioned above. The four encoding schemes and the transformation cases they address is listed in the following table. +---------------------+----------+--------------------+ | Encoding Scheme | ET Value | Transformaton Case | +---------------------+----------+--------------------+ | Insertion Only | 00 | A | +---------------------+----------+--------------------+ | Removal Only | 01 | B | +---------------------+----------+--------------------+ | Content Change Only | 10 | C | +---------------------+----------+--------------------+ | Uncompressed | 11 | mixture of A,B,C,D | +---------------------+----------+--------------------+ * In the insertion only scheme, compared with ref_list, the uncompressed value of the newly added extension headers in the curr_list are sent along with the positions of the extension headers in the ref_list, before which the new extension headers will be inserted. * In the removal only scheme, the positions of the extension headers, which are in the ref_list, but not in the curr_list, are sent. * In the change only scheme, the positions of the extension header whose content is changed as well as its uncompressed or compressed value are sent. (Note that in the current stage, only uncompressed extension header is used. ) The aforementioned 3 schemes generate compressed format of the extension header list. The curr_list can also be sent in an uncompressed format. 5.8.2.2.3 Special Handling 5.8.2.2.3.1 Special Handling of AH Bormann (ed.) [Page 91] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 The sequence number field in the AH contains a monotonically increasing counter value for a security association. Therefore, when comparing curr_list with ref_list, if the sequence number in AH changes and SPI field doesn't change, the AH is not necessary to be classified as changed. If the sequence number in the AH linearly increases as RTP sequence number increases, it doesn't need to be sent. The decompressor applies linear extrapolation to reconstruct the sequence number in AH. Otherwise, a compressed sequence number should be included in the IPv6 Extension Headers compression element field in PT2 extension "11" header. The authentication data field in AH changes from packet to packet and should be sent in every packet. If the uncompressed AH is sent, the authentication data field is sent inside the uncompressed AH; otherwise, it is sent after the compressed IP/UDP/RTP and IPv6 extension headers and before the payload. 5.8.2.2.3.2 Encapsulating Security Payload Header If Encapsulating Security Payload Header (ESP) is used, the UDP and RTP headers are both encrypted and cannot be compressed. In this case, special compressed packet format needs to be defined in ROHC. In ESP, the only fields that can be compressed are the SPI and the sequence number. * In the case that the SPI field changes, the uncompressed ESP is sent. * In the case that no change happens to the SPI field, the ESP is not considered as changed. The sequence number in ESP has the same behavior as the same field in AH. If it linearly increases, it doesn't need to be sent. Otherwise, a compressed sequence number should be sent in the IPv6 Extension Headers compression element field in PT2 extension "11" header. 5.8.2.2.3.3 Special Handling of Next Header Field The next header field in an extension header changes whenever the type of the immediately following header changes, e.g., a new extension header is inserted after it, the immediate subsequent extension header is removed from the list, or the order of several extension headers is changed. Thus, in particular, it may not be uncommon that for a given extension header, only the next header field changes but the remaining fields don't change. Therefore, the next header field in each extension header needs to be treated Bormann (ed.) [Page 92] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 in a special way. In the case that only the next header field changes, the extension header should be considered as unchanged. The special treatment of the change of the next header field is defined as follows. * In the case that a subsequent extension header is removed from the list, the new value of the next header field can be obtained from the reference extension header list. For example, assume that the reference extension header list (ref_list) consists of extension header A, B and C (ref_ext_hdr A, B, C), and the current extension header list (curr_list) only consists of extension headers A and C (curr_ext_hdr A, C). The order and value of the next header field of these extension headers are as follows. ref_list: +--------+-----+ +--------+-----+ +--------+-----+ | type B | | | type C | | | type D | | +--------+ | +--------+ | +--------+ | | | | | | | +--------------+ +--------------+ +--------------+ ref_ext_hdr A ref_ext_hdr B ref_ext_hdr C curr_list: +--------+-----+ +--------+-----+ | type C | | | type D | | +--------+ | +--------+ | | | | | +--------------+ +--------------+ curr_ext_hdr A curr_ext_hdr C Comparing the curr_ext_hdr A in curr_list and the ref_ext_hdr A in ref_list, the value of next header field is changed from "type B" to "type C" because of removal of extension header B. The new value of the next header field in curr_ext_hdr A, i.e., "type C" doesn't need to be sent to the decompressor, because when the decompressor detects (by observing the list level encoding) that the immediate following extension header B is removed from the list, it retrieves the next header field in ref_ext_hdr B and use it to replace the next header field in the curr_ext_hdr A. * In the case that a new extension header is inserted after an existing extension header, the next header field in the new extension header carries the type of itself, instead of the type of extension header that follows. For example, assume that the reference extension header list (ref_list) consists of extension header A and C (ref_ext_hdr A, C), and the current extension header list (curr_list) consists of extension header A, B and C (curr_ext_hdr A, B, C). The order and the value of the next header field of these extension headers are as follows. Bormann (ed.) [Page 93] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 ref_list: +--------+-----+ +--------+-----+ | type C | | | type D | | +--------+ | +--------+ | | | | | +--------------+ +--------------+ ref_ext_hdr A ref_ext_hdr C curr_list: +--------+-----+ +--------+-----+ +--------+-----+ | type B | | | type C | | | type D | | +--------+ | +--------+ | +--------+ | | | | | | | +--------------+ +--------------+ +--------------+ curr_ext_hdr A curr_ext_hdr B curr_ext_hdr C Comparing the curr_list and the ref_list, the value of the next header field in extension header A is changed from "type C" to "type B". In the compressed extension header list, the uncompressed curr_ext_hdr B is carried in the uncompressed data field in c_item or u_item depending on the list encoding scheme used. However, instead of carrying the type of the next header (type C) in the next header field, the type of curr_ext_hdr B (type B) should be carried. When the decompressor detects (by observing the list level encoding) that a new extension is inserted after curr_ext_hdr A, it will replace the old next header field in ref_ext_hdr A with the type of the inserted extension header, i.e., type B, which is carried in the next header field in the c_item or u_item for extension header B. At the same time, the decompressor also replace the next header field in curr_ext_hdr B with the old value of the next header field in ref_ext_hdr A, i.e., type C. 5.8.2.3. Packet Format 5.8.2.3.1. Format in extension "11" header 1 1 1 5 +------+------+------+-------+ | ASeq | ESeq | CExt | resv | +------+------+------+-------+ 8 or 32 bits 8 or 32 bits +::::::::::::::::::::::::::+:::::::::::::::::::::::::::+ | compressed AH Seq Number | compressed ESP Seq Number | +::::::::::::::::::::::::::+:::::::::::::::::::::::::::+ variable length +:::::::::::::::::::::::::::::::+ | compressed IPv6 extension hdr | Bormann (ed.) [Page 94] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 +:::::::::::::::::::::::::::::::+ * ASeq: presence of compressed AH Seq Number * ESeq: presence of compressed ESP Seq Number * CExt: presence of compressed IPv6 extension hdr * compressed AH Seq Number and compressed ESP Seq Number formats: "0" + 7-bit LSB "11" + 30-bit LSB 5.8.2.3.2 Format of compressed IPv6 extension header 5.8.2.3.2.1 Insertion Only Scheme 5.8.2.3.2.1.1 R-mode 2 6 or 14 8 +---------+--------+--------------------+ | ET = 00 | ref_id | insertion bit mask | +---------+--------+--------------------+ +----------+----+----------+ | u_ehdr 1 |....| u_ehdr m | +----------+----+----------+ * ref_id - the LSB of the RTP sequence number of the ref_list 6 bit: "0" + 5-bit LSB 14 bit: "1" + 13-bit LSB * u_ehdr i (i=1..m) : the uncompressed value of the new extension header in the curr_list; m is the number of new extension headers to be added. * insertion bit mask: the bit mask indicating the position of the following u_ehdrs to be inserted into the ref_list in order to reconstruct the curr_list. The length of insertion bit mask is 8 bits. To construct the insertion bit mask and the following inserted u_ehdr list, the following steps are taken. ** A list of '0' and an empty inserted u_ehdr list are generated as the starting point. The number of '0's in the '0' list equals the number of extension headers in the ref_list. The i-th '0' in the '0' list corresponds to the i-th extension header in the ref_list. ** Comparing the curr_list with the ref_list, if a new extension header is inserted between the i-th item and the (i+1)-th item in the ref_list, a '1' is inserted between the i-th '0' and Bormann (ed.) [Page 95] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 (i+1)-th '0' in the original '0' list. The new extension header should be added to the end of inserted CSRC list. This procedure is repeated until all the m new extension headers have been processed. If the length of new insertion bit mask is less than 7 bits, additional '0' should be added at the end until it reaches 7 bits. When the decompressor receives the insertion bit mask, it scans from left to right. When a '0' is observed, the decompressor copies the corresponding extension header in the ref_list into the curr_list; when a '1' is observed, the decompressor adds the correspondent u_ehdr into the curr_list. 3.2.1.2 UO-modes 2 6 8 8 8 +---------+------+--------+--------+--------------------+ | ET = 00 | resv | Gen_id | ref_id | insertion bit mask | +---------+------+--------+--------+--------------------+ +----------+----+----------+ | u_ehdr 1 |....| u_ehdr m | +----------+----+----------+ * resv: reserved * Gen_id: is used to identify a set of packets that belongs to the same generation * ref_id: the Gen_id carried in the ref_list * insertion bit mask and u_ehdr i: the same as defined for R-mode 5.8.2.3.2.2 Removal Only Scheme 5.8.2.3.2.2.1 R-mode 2 6 or 14 8 +---------+--------+------------------+ | ET = 01 | ref_id | removal bit mask | +---------+--------+------------------+ * ref_id: the same as defined in section 3.2.1.1. * removal bit mask: the bit mask indicating the extension header in the ref_list to be removed in order to reconstruct the curr_list. A '1' in the i-th bit in the removal bit mask means that the i-th extension header in the ref_list is not in the curr_list, while a '0' means it is still present in the curr_list. Bormann (ed.) [Page 96] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 The length of insertion bit mask can be 8 bits. 3.2.2.2 UO-mode 2 6 8 8 8 +---------+------+--------+--------+------------------+ | ET = 01 | resv | gen_id | ref_id | removal bit mask | +---------+------+--------+--------+------------------+ * Gen_id, resv and ref_id: the same as defined in section 3.2.1.2. * removal bit mask: the same as defined for R mode 5.8.2.3.2.3 Content Change Only Scheme 5.8.2.3.2.3.1 R-mode 2 6 or 14 8 +---------+--------+-----------------+ | ET = 10 | ref_id | change bit mask | +---------+--------+-----------------+ +-----------+----+-----------+ | uc_ehdr 1 |....| uc_ehdr m | +-----------+----+-----------+ * ref_id: the same as defined in section 2.3.1.1 * change bit mask: a bit mask indicating the postion of the extension header that is changed. A '1' in the i-th bit in the change bit mask means that the i-th extension header in the ref_list is not the same as the i-th extension in the curr_list, while a '0' means they are the same. * uc_ehdr: corresponds to the extension header whose content is changed when comparing it with the extension header at the same position in the ref_list. Their positions in curr_list is indicated in the change bit mask field. uc_ehdr: 1 +-------+-------------------+ | C = 0 | uncompressed ehdr | +-------+-------------------+ +-------+-----------------+ | C = 1 | compressed ehdr | +-------+-----------------+ - uncompressed ehdr: uncompressed extension header Bormann (ed.) [Page 97] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 - compressed ehdr: compressed extension header using the extension header at the same position in ref_list as the reference. The compression mechanisms for different types of extension headers are different from each other and FFS. 5.8.2.3.2.3.2 UO-mode 2 6 8 8 8 +---------+------+--------+--------+-----------------+ | ET = 00 | resv | gen_id | ref_id | change bit mask | +---------+------+--------+--------+-----------------+ +-----------+----+-----------+ | uc_ehdr 1 |....| uc_ehdr m | +-----------+----+-----------+ * Gen_id, resv, ref_id: the same as defined in section 3.2.1.2. * change bit mask and uc_ehdr i: the same as defined for R mode 5.8.2.3.2.4 Uncompressed Scheme (common to R-mode and UO-modes) 2 5 1 8 +---------+------+---+::::::::+------------------------------------+ | ET = 11 | resv | G | Gen_id | uncompressed IPv6 Extension Header | +---------+------+---+::::::::+------------------------------------+ * resv: reserved * G: indicating the presence of Gen_id * Gen_id: the same as defined in section 3.2.1.2. - In R-mode, gen_id is not present. - During the transition from any mode to U-mode or O-mode, gen_id should not be sent. The list without gen_id should not be used as a reference to compress and decompress an extension header list. - In UO-modes, gen_id is present if it may be used as a reference to compress or decompress the subsequent extension header list. Bormann (ed.) [Page 98] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.9. Header compression CRCs, coverage and polynomials This chapter describes how to calculate the CRCs used in packet headers defined in this document. 5.9.1. IR & IR-DYN packet CRCs The CRC in the IR and IR-DYN packet is calculated over the entire IR or IR-DYN packet, excluding Payload and including CID. For purposes of computing the CRC, the CRC field in the header is set to zero. The initial content of the CRC register be preset to all 1's. The CRC polynomial to be used is: C(x) = 1 + x + x^2 + x^8 5.9.2. CRCs in compressed packets The CRC in compressed headers is calculated over all octets of the entire original header, before compression, in the following manner. The octets of the header are classified as either CRC-STATIC or CRC- DYNAMIC, and the CRC is calculated over: 1) the concatenated CRC-STATIC octets of the original header, placed in the same order as they appear in the original header, followed by 2) the concatenated CRC-DYNAMIC octets of the original header, placed in the same order as they appear in the original header. The intent is that the state of the CRC computation after 1) will be saved. As long as the CRC-STATIC octets do not change, the CRC calculation will then only need to process the CRC-DYNAMIC octets. For a typical RTP/UDP/IPv4 header, 25 octets are CRC-STATIC and 15 are CRC-DYNAMIC. For a typical RTP/UDP/IPv6 header, 49 octets are CRC-STATIC and 11 are CRC-DYNAMIC. This technique will thus reduce the computational complexity of the CRC calculation by roughly 60% for RTP/UDP/IPv4 and by roughly 80% for RTP/UDP/IPv6. Note: whenever the CRC-STATIC fields change, the new saved CRC state after 1) is compared with the old state. If the states are identical, the CRC cannot catch the error that the decompressor has not updated the static context. The compressor should then require an ACK in O mode and send the information for a longer time in U mode. Optimistic transition from IR or FO state should not be attempted in this case. Bormann (ed.) [Page 99] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 The initial content of the CRC register is preset to all 1's. The polynomial to be used for the 3 bit CRC is: C(x) = 1 + x + x^3 The polynomial to be used for the 7 bit CRC is: C(x) = 1 + x + x^2 + x^3 + x^6 + x^7 The CRC in compressed packets is calculated over the entire original header, before compression. 5.10. ROHC UDP _ non-RTP UDP/IP compression (Profile 1) UDP/IP headers do not have a sequence number which is as well-behaved as the RTP sequence number. Therefore the compressed packet formats for RTP are not appropriate, and neither are the feedback formats. For UDP/IPv4, there is an IP-ID field which may be echoed in feedback information, but when no IPv4 header is present feedback identification becomes problematic. A context generation number is used for this purpose in profile 1. 5.10.1 Initialization The static context for UDP/IP streams can be initialized in either of two ways; 1) By using an IR packet (section 7.7.1), where the profile is one (1) and the static chain ends with the static part of an UDP packet. 2) reusing an existing context, where the existing static chain contains the static part of a UDP packet, e.g., the context of a stream compressed using RTP/UDP/IP (profile 0). This is done with an IR-DYN packet (section 7.7.2) with profile = 1, where the dynamic chain corresponds to the prefix of the existing static chain that ends with the UDP header. Method 2) can be used for packet streams where the initial assumption was that they were RTP streams, so compression started with profile zero, but where it later becomes evident that they are not RTP streams. The context generation number is re-initialized to zero whenever an IR packet is sent or received. It is incremented by one whenever a packet which changes the static context is sent or received. Bormann (ed.) [Page 100] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 5.10.2 States and Modes ROHC UDP uses the same states and modes as ROHC RTP. Mode transitions and state logic is the same except when explicitly stated otherwise. 5.10.3 Packet types The general format of a ROHC UDP packet is the same as for ROHC RTP (see beginning of section 5.7.). Padding and CIDs are the same, as is the feedback packet type (5.7.6.1) (but not the feedback), and the IR and IR-DYN packets (5.7.7). The general format of compressed packets is also the same, but there are differences in specific formats and extensions as follows. IP-ID and IP-ID2 (in Extension 0) are encoded using W-LSB encoding (section 4.5.2) with p = 16. UDP-0 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 0 | CRC | +===+===+===+===+===+===+===+===+ UDP-1 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 0 | IP-ID | +===+===+===+===+===+===+===+===+ | X | CRC | +---+---+---+---+---+---+---+---+ 5.10.4 Extensions Extensions are as in 5.7.5, with the following exceptions: Extension 0: +---+---+---+---+---+---+---+---+ | 0 | IP-ID2 | +---+---+---+---+---+---+---+---+ Provides bits for a second IP Identifier. Extension 1: Bormann (ed.) [Page 101] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Extension 1 is the same as Extension 3 in section 5.7.5, with the following exceptions. 1) The initial flag octet has the following format: 0 1 2 3 4 5 6 7 +-----+-----+-----+-----+-----+-----+-----+-----+ | 1 | Generation | I | ip | ip2 | +-----+-----+-----+-----+-----+-----+-----+-----+ Generation: Flags S, R-TS, Tsc, and rtp are eliminated (i.e., always zero). Instead, a context generation identifies the changes and allows accurate feedback to be sent. Flag ip2 is moved here from the Inner IP hdr flags. 2) In the position of the old ip2 flag in the Inner IP hdr flags, there is a new flag Mod, which indicates whether an octet with the following format occurs in the position of the old RTP header flags and fields. +---+---+---+---+---+---+---+---+ | 0 0 0 0 0 0 | Mode | if Mod=1 +---+---+---+---+---+---+---+---+ Mode: Compression mode. Values as for Mode field in 5.7.5. 5.10.5 IP-ID The IP-ID field is the compressed IP Identifier of the innermost IPv4 header whose RND flag is not set. The IP identifier is compressed according to section 4.5.2, using p = 8. The IP-ID2 field in extension 0 is the compressed outer IP-ID, if there is one. The RND and NBO flags are used as in profile zero, i.e., as described in 5.7.5.1. (replace UOR-2 with UDP-1, Extension 3 with Extension 1). 5.10.6. Feedback The feedback packet type and general format of feedback is the same as for profile zero (5.7.6.1.). The feedback formats of section 5.7.6.2 are replaced by the following: FEEDBACK-1 +---+---+---+---+---+---+---+---+ | Generation | IP-ID | +---+---+---+---+---+---+---+---+ Bormann (ed.) [Page 102] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Generation: generation of current static context. IP-ID: LSBs of innermost IP Identifier of packet which caused Feedback to be sent. If there is no IP identifier, zero. A FEEDBACK-1 is an ACK. FEEDBACK-2 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ |Acktype| Mode | Generation | +---+---+---+---+---+---+---+---+ | IP-ID | +---+---+---+---+---+---+---+---+ / Feedback options / +---+---+---+---+---+---+---+---+ Acktype: 0 = ACK 1 = NACK 2 = STATIC-NACK 3 is reserved Mode: 0 is reserved 1 = Unidirectional mode 2 = Bi-directional Optimistic mode 3 = Bi-directional Reliable mode Feedback options: A variable number of feedback options, see section 5.7.6.3. Options may appear in any order. Bormann (ed.) [Page 103] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 6. Implementation issues This document specifies mechanisms for the protocol, while much of the usage of these mechanisms is left to the implementers to decide upon. This chapter is aimed to give guidelines, ideas and suggestions for implementing the scheme. 6.1. Reverse decompression This chapter describes an OPTIONAL decompressor operation to reduce discarded packets due to an invalid context. Once a context becomes invalid (e.g., in the case when more consecutive packet losses than expected has occurred), subsequent compressed packets cannot be decompressed correctly immediately. Reverse decompression aims at decompressing such packets later instead of discarding them, by storing them until the context has been updated and validated and then attempting decompression. Let the sequence of stored packets be i, i+1, ..., i+k, where i is the first packet and I+k is the packet before the context was updated. The decompressor will attempt to recover the stored packets in reverse order, i.e., starting with i+k, and working towards i. When a stored packet has been reconstructed, its correctness is verified using its CRC. Packets not carrying a CRC must not be delivered to upper layers. Packets where the CRC succeeds, are delivered to upper layers in the original order, i.e., i, ..., i+k. Note that this reverse decompression introduces buffering while waiting for the context to be validated and thereby introduces additional delay. Thus, it should be used only when some amount of delay is acceptable. For example, for video packets belonging to the same video frame, the delay of packet arrival time does not cause presentation time delay. Delay-insensitive streaming applications can also be tolerant to such delay. If the decompressor cannot determine if the application can tolerate delay, it should not do reverse decompression. The following illustrates the decompression procedure in some detail: 1. The decompressor stores compressed packets that cannot be decompressed correctly due to an invalid context. 2. When the decompressor has received a context updating packet and the context has been validated, it starts to recover the stored packets in reverse order. Decompression is carried out followed by the last decompressed packet to its previous packet as if the two packets were reordered. After that, the decompressor checks the correctness of the reconstructed header using the CRC. Bormann (ed.) [Page 104] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 3. If the CRC indicates successful decompression, the decompressor stores the complete packet and attempts to decompress the preceeding packet. In this way, the stored packets are recovered until no compressed packets are left. For each packet, the decompressor checks the correctness of the decompressed headers using header compression CRC. 4. If the CRC indicates an incorrectly decompressed packet, the reverse decompression attempt must be terminated and all remaining uncompressed packets must be discarded. 5. Finally, the decompressor forwards all the correctly decompressed packets to upper layers in the original order. 6.2. RTCP RTCP is the RTP Control Protocol, [RTP]. RTCP is based on periodic transmission of control packets to all participants in a session, using the same distribution mechanism as for data packets. Its primary function is to provide feedback from the data receivers on the quality of the data distribution. The feedback information may be used for issues related to congestion control functions, and directly useful for control of adaptive encodings. In an RTP session there will be two types of packet streams; one with the RTP-header and application data, and a second stream with the RTCP control information. The difference between the streams at the transport level is the UDP port numbers, which is plus one for RTCP. The ROHC header compressor implementation has several ways at hand to handle the RTCP stream. 1. One compressor/decompressor entity for both streams and carried on the same channel using CIDs to distinguish between them. On the RTCP stream, basically only IP/UDP compression will be utilized. 2. Two compressor/decompressor entities, one for RTP and another one for RTCP, and the streams carried on their own channel. This means that they will not share the same CID number space. 6.3. Implementation parameters and signals A ROHC implementation may have two kinds of parameters; configuration parameters that are mandatory and must be negotiated between compressor and decompressor peers, and also implementation parameters that are optional and when used, mandate how a ROHC implementation is to operate. Configuration parameters are mandatory and must be negotiated between compressor and decompressor, so they have the same values at Bormann (ed.) [Page 105] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 compressor and decompressor. An example of a configuration parameter is whether context identification bits (CID) should be used or not. Implementation parameters make it possible to mandate how an implementation of a ROHC compressor or decompressor should operate. Implementation parameters have local significance, are optional to use and are thus not necessary to negotiate between compressor and decompressor. Note: this does not preclude that implementation parameters may be signaled or negotiated using lower layer functionality in order to set the way a ROHC implementation operate. Some implementation parameters are valid only at either of compressor or decompressor. Implementation parameters may further be divided into parameters that describe the way an implementation operates and into parameters that trigger a specific event, i.e., signals. 6.3.1. ROHC implementation parameters at compressor: CONTEXT_REFRESH - signal This parameters triggers a complete refresh of the context at the decompressor, both static and dynamic part. The compressor MUST, when CONTEXT_REFRESH is triggered, fully refresh the context by sending IR/DYN headers until it is reasonably confident that the decompressor contexts are refreshed. The context refresh MUST be done for all contexts at the compressor. This parameter may for instance be used to do context relocation at e.g. a cellular handover that result in a change of compression point in the radio access network. NO_OF_HEADER_SIZES - values: positive integer value This parameter should only be used when HEADER_SIZES_USED is used. In that case, this parameter mandates the number of header sizes a ROHC implementation can use. With this number is set, the ROHC implementation at the compressor MUST NOT use more different header sizes than the value of this parameter mandates. The ROHC implementation should be able to determine itself which header sizes to use. Note: one header size may be used for several header formats. HEADER_SIZES_USED - values: list of positive integer values This parameter should only be used when NO_OF_HEADER_SIZES is used. In that case, this parameter tells which header sizes in bytes that may be used by the ROHC implementation. The information about used header sizes in this parameter, may then be used to configure lower layers. For the two parameters above the following MUST apply: NO_OF_HEADER_SIZES must be equal to the number of list elements in HEADER_SIZES_USED 6.3.2. ROHC implementation parameters at decompressor: MODE - values: [U_mode, O_mode, R_mode] Bormann (ed.) [Page 106] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 This parameter triggers a mode transition using the mechanism described in chapter 5 when the parameter changes value, i.e. to U_mode (Unidirectional mode), O_mode (bidirectional Optimistic mode) or R_mode (bidirectional Reliable mode). The mode transition is made from the current mode to the new mode as signaled in the implementation parameter. For example, if the current mode is bidirectional optimistic mode, MODE should have the value: O_mode. If the MODE is changed to R_mode, a mode transition MUST be made from bidirectional optimistic mode to bidirectional reliable mode. MODE should not only serve as a trigger for mode transitions, but also make it visible which mode ROHC operates in. TIMER_BASED_TS_COMPRESSION - values: [YES, NO] This parameter determines whether timer-based time stamp compression as described in chapter X.X should be used or not. It SHOULD be used when the parameter has the value YES, and it MUST NOT be used when the parameter has the value NO. REVERSE_DECOMPRESSION - values: [YES, NO] This parameter determines whether reverse decompression as described in chapter X.X should be used or not. It MUST be used when the parameter has the value YES, and it MUST NOT be used when the parameter has the value NO. 6.5. Handling of resource limitations at the decompressor In a point to point link the two nodes can agree on the number of compressed sessions they are prepared to support for this link. It may, however, not be possible for the decompressor to accurately predict when it will run out of resources. ROHC allows the negotiated number of contexts to be larger than could be accommodated in the worst case. Then, as context resources are consumed, an attempt to set up a new context may be rejected by the decompressor, using the REJECT option of the feedback payload. Upon reception of a REJECT option, the compressor SHOULD wait for a while before attempting to compress additional streams destined to the rejecting host. Bormann (ed.) [Page 107] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 7. Further work (Editor: This section is _further work_ in particular as it needs to be integrated into the rest of the document. It will go away in the next version of this document, -04.) 7.3. Tunneling 7.3.1. Header Compression for IPv4 Tunneling Header In order to route the packets to the mobile node that is on a foreign link, the home agent of the mobile node may encapsulate the original packet into an IP header and tunnel the packet to the care- of address of the mobile node. In the case of foreign agent care-of address in Mobile IPv4, the tunneling header in each tunneled packet will be removed by the foreign agent before transferring it to the mobile node through the air interface; therefore there is no need for compression of tunneling header. In the case that mobile node uses collocated care-of address, the tunneled packet will be sent to mobile station through air interface, and compression needs to be applied to the tunneling header. 7.3.1.1. Mobile IPv4 Tunneling Header Fields Type The table below summarizes classification of the various fields defined in different tunneling headers used in Mobile IPv4. In the column of Encapsulation Scheme (Enc. Scheme), three encapsulation methods are included - IP in IP Encapsulation (IIE), Minimum Encapsulation (ME), Generic Routing Encapsulation (GRE). (Editor's note: Harmonize with the way this is described in ROHC document) +------+------+---------+---------------------------------+ |Enc. |Header| Static | Non-static | |Scheme|type | +-----------+---------------------+ | | | | Essential | Non-Essential | +------+------+---------+-----------+---------------------+ | IIE |inner | same as in the table in Appendix A | | |header| in ACE Internet Draft | | +------+-------------------------------------------+ | |outer | same as in the table in Appendix A | | |header| in ACE Internet Draft | +------+------+-------------------------------------------+ | ME |IP | same as in the table in Appendix A | | |header| in ACE Internet Draft | | +------+----------+----------+---------------------+ | |Mini. | Protocol | | S bit | | |Fw. | | | Header Checksum | | |header| | | Original Dest. Addr.| | | | | | Original Src. Addr. | Bormann (ed.) [Page 108] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 +------+------+----------+----------+---------------------+ | GRE |inner | same as in the table in Appendix A | | |header| in ACE Internet Draft | | +------+-------------------------------------------+ | |outer | same as in the table in Appendix A | | |header| in ACE Internet Draft | | +------+----------+----------+---------------------+ | |GRE | Protocol | Sequence | C, R, K, S, s bits | | |header| Ver | number | Recur | | | | | | Flags | | | | | | Checksum | | | | | | Offset | | | | | | Key | | | | | | Routing | +------+------+----------+----------+---------------------+ 7.3.1.2. Compression of Tunneling Headers in MIPv4 Three encapsulation schemes have been specified in MIPv4. For different encapsulation scheme, the compression methods are different from each other. 7.3.1.2.1. IP in IP Encapsulation in IPv4 Using IP in IP Encapsulation, the original inner IP header is not modified at all and therefore can be compressed as if it is not encapsulated. The outer header is compressed at the IP level, while the inner header is compressed as defined in ROHC. 7.3.1.2.2. Minimum Encapsulation in IPv4 With Minimum Encapsulation, the original IP header is modified and the Minimal Forwarding Header is inserted between the modified IP header and the original IP payload. The modified IP header plus the the UDP/RTP headers is compressed as defined in ROHC. The compression scheme for the Minimal Forwarding Header is similar to the scheme applied to the IP header. The static and changing non- essential fields in the Minimum Forwarding Header are sent in the Full Header and Refresh state. When any change happens to any non-essential field in the Minimum Forwarding Header, a compressed header with a bit mask indicating the change should be sent. 7.3.1.2.3. Generic Routing Encapsulation in IPv4 With Generic Routing Encapsulation, the original IP packet is encapsulated in an outer IP header. A GRE header is inserted between the inner header and the outer header. The original IP/UDP/RTP header is compressed as if there is no encapsulation. The outer IP header is compressed at the IP level. Bormann (ed.) [Page 109] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 The compression scheme for the GRE header is similar to the scheme applied to the IP header. All the static and changing non-essential fields in the GRE header are sent in the Full Header and refresh state. When any change happens to any non-essential field in the GRE header, a compressed header with a bit mask indicating the change should be sent. If the sequence number in the GRE header is present, the scheme to compress sequence number could be VLE, as defined in ACE draft. 8. Section 8 has been removed. 9. Security considerations Because encryption eliminates the redundancy that header compression schemes try to exploit, there is some inducement to forego encryption of headers in order to enable operation over low-bandwidth links. However, for those cases where encryption of data (and not headers) is sufficient, RTP does specify an alternative encryption method in which only the RTP payload is encrypted and the headers are left in the clear. That would still allow header compression to be applied. ROHC compression is transparent with regards to the RTP sequence number and RTP timestamp fields, so the values of those fields can be trusted by payload encryption schemes. A malfunctioning or malicious header compressor could cause the header decompressor to reconstitute packets that do not match the original packets but still have valid IP, UDP and RTP headers and possibly also valid UDP checksums. Such corruption may be detected with end-to-end authentication and integrity mechanisms which will not be affected by the compression. Moreover, this header compression scheme uses an internal checksum for verification of re-constructed headers. This reduces the probability of producing decompressed headers not matching the original ones without this being noticed. Denial-of-service attacks are possible if an intruder can introduce (for example) bogus STATIC, DYNAMIC or FEEDBACK packets onto the link and thereby cause compression efficiency to be reduced. However, an intruder having the ability to inject arbitrary packets at the link layer in this manner raises additional security issues that dwarf those related to the use of header compression. 10. Acknowledgements When designing this protocol, earlier header compression ideas described in [CJHC], [IPHC] and [CRTP] have been important sources of knowledge. Bormann (ed.) [Page 110] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Thanks to Takeshi Yoshimura at NTT DoCoMo for providing the reverse decompression section (6.1). Thanks also to Andreas Jonsson (Lulea University), who made a great job supporting this work in his study of header field change patterns. Thanks also to all others who have given comments. Bormann (ed.) [Page 111] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 11. Intellectual property considerations (Editor's note: this section will go to www.ietf.org/ipr and be replaced by the standard reference to that, but for now it is left in the draft to simplify working on it.) This proposal in is conformity with RFC 2026. Telefonaktiebolaget LM Ericsson and its subsidiaries, in accordance with corporate policy, will for submissions rightfully made by its employees which are adopted or recommended as a standard by the IETF offer patent licensing as follows: If part(s) of a submission by Ericsson employees is (are) included in a standard and Ericsson has patents and/or patent application(s) that are essential to implementation of such included part(s) in said standard, Ericsson is prepared to grant - on the basis of reciprocity (grant-back) - a license on such included part(s) on reasonable, non- discriminatory terms and conditions. For the avoidance of doubt this general patent licensing undertaking applies to this proposal. Nokia has filed patent applications that might possibly have technical relation to this contribution. Matsushita has filed patent applications that might possibly have technical relation to this contribution. If part(s) of the contribution by Matsushita employee is (are) included in a standard and Matsushita has patents and/or patent application(s) that are essential to implementation of such included part(s) in said standard, Matsushita is prepared to grant - on the basis of reciprocity (grantback) - a license on such included part(s) on reasonable, non-discriminatory terms and conditions (in according with paragraph 10.3.3 of the RFC 2026). NTT DoCoMo, Inc. also declares this text may relevant to their patent, and offer patent licensing as follows: If part(s) of this text provided by NTT DoCoMo employees is (are) included in a standard and NTT DoCoMo has patents and/or patent application(s) that are essential to implementation of such included part(s) in said standard, NTT DoCoMo is prepared to grant - on the basis of reciprocity (grant-back) - a license on such included part(s) on reasonable, non-discriminatory terms and conditions. Bormann (ed.) [Page 112] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 12. References [UDP] Jon Postel, "User Datagram Protocol", RFC 768, August 1980. [IPv4] Jon Postel, "Internet Protocol", RFC 791, September 1981. [IPv6] Steven Deering, Robert Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RTP] Henning Schulzrinne, Stephen Casner, Ron Frederick, Van Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 1889, January 1996. [HDLC] William Simpson, "PPP in HDLC-like framing", RFC 1662, 1994. [VJHC] Van Jacobson, "Compressing TCP/IP Headers for Low-Speed Serial Links", RFC 1144, February 1990. [IPHC] Mikael Degermark, Bjorn Nordgren, Stephen Pink, "IP Header Compression", RFC 2507, February 1999. [CRTP] Steven Casner, Van Jacobson, "Compressing IP/UDP/RTP Headers for Low-Speed Serial Links", RFC 2508, February 1999. [PPPHC] Mathias Engan, Steven Casner, Carsten Bormann, "IP Header Compression over PPP", RFC 2509, February 1999. [CRTPC] Mikael Degermark, Hans Hannu, Lars-Erik Jonsson, Krister Svanbro, "CRTP over cellular radio links", Internet Draft (work in progress), December 1999. [REQ] Mikael Degermark, "Requirements for robust IP/UDP/RTP header compression", Internet Draft (work in progress), June 2000. [LLG] Krister Svanbro, "Lower Layer Guidelines for Robust Header Compression", Internet Draft (work in progress), May 2000. [CELL] Lars Westberg, Morgan Lindqvist, "Realtime traffic over cellular access networks", Internet Draft (work in progress), May 2000. [WCDMA] "Universal Mobile Telecommunications System (UMTS); Selection procedures for the choice of radio transmission technologies of the UMTS (UMTS 30.03 version 3.1.0)". ETSI TR 101 112 V3.0.1, November 1997. Bormann (ed.) [Page 113] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 13. Authors' addresses [[Editor's note: Multiple authors at the same address have been compressed. I hope your heads are robust enough for this...]] Carsten Bormann Tel: +49 421 218 7024 Universitaet Bremen TZI Fax: +49 421 218 7000 Postfach 330440 EMail: cabo@tzi.org D-28334 Bremen, GERMANY Carsten Burmeister Tel. +49-6103-766-263 Rolf Hakenberg Tel. +49-6103-766-162 Thomas Wiebke Tel. +49-6103-766-161 Panasonic European Laboratories GmbH Fax. +49-6103-766-166 Monzastr. 4c Email: burmeister@panasonic.de 63225 Langen, Germany Email: hakenberg@panasonic.de Email: wiebke@panasonic.de Mikael Degermark Tel: +1 520 621-3498 The University of Arizona Fax: +1 520 621-4642 Dept of Computer Science Email: micke@cs.arizona.edu P.O. Box 210077 Tucson, AZ 85721-0077, USA Hideaki Fukushima Email: fukusima@isl.mei.co.jp Akihiro Miyazaki Email: akihiro@isl.mei.co.jp Matsushita Electric Industrial Co., Ltd Tel. +81-6-6900-9192 1006, Kadoma, Kadoma City, Osaka, Japan Fax. +81-6-6900-9193 Hans Hannu Tel: +46 920 20 21 84 Lars-Erik Jonsson Tel: +46 920 20 21 07 Krister Svanbro Tel: +46 920 20 20 77 Box 920 Fax: +46 920 20 20 99 Ericsson Erisoft AB EMail: lars-erik.jonsson@ericsson.com SE-971 28 Lulea, Sweden Email: krister.svanbro@ericsson.com EMail: hans.hannu@ericsson.com Zhigang Liu Tel: +1 972 894-5935 Nokia Research Center Fax: +1 972 894-4589 6000 Connection Drive EMail: zhigang.liu@nokia.com Irving, TX 75039, USA Anton Martensson Tel: +46 8 404 3881 Ericsson Radio Systems AB Fax: +46 8 757 5550 SE-164 80 Stockholm, Sweden Email: anton.martensson@era.ericsson.se Bormann (ed.) [Page 114] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Appendix A. Detailed classification of header fields Header compression is possible due to the fact that most header fields do not vary randomly from packet to packet. Many of the fields exhibit static behavior or changes in a more or less predictable way. When designing a header compression scheme, it is of fundamental importance to understand the behavior of the fields in detail. In this appendix, all IP, UDP and RTP header fields are classified and analyzed in two steps. First, we have a general classification in A.1 where the fields are classified based on stable knowledge and assumptions. The general classification does not take into account the change characteristics of changing fields because those will vary more or less depending on the implementation and on the application used. A less stable but more detailed analysis considering the change characteristics is then done in A.2. Finally, A.3 summarizes this appendix with conclusions about how the various header fields should be handled by the header compression scheme to optimize compression and functionality. A.1. General classification On a general level, the header fields are separated into 5 classes: INFERRED These fields contain values that can be inferred from other values, for example the size of the frame carrying the packet, and thus does not have to be handled at all by the compression scheme. STATIC These fields are expected to be constant throughout the lifetime of the packet stream. Static information must in some way be communicated once. STATIC-DEF STATIC fields whose values define a packet stream. They are in general handled as STATIC. STATIC-KNOWN These STATIC fields are expected to have well-known values and therefore do not need to be communicated at all. CHANGING These fields are expected to vary in some way, either randomly, within a limited value set or range, or in some other manner. In this section, each of the IP, UDP and RTP header fields is assigned to one of these classes. For all fields except those classified as CHANGING, the motives for the classification are also stated. CHANGING fields are in A.2 further examined and classified based on their expected change behavior. Bormann (ed.) [Page 115] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 A.1.1. IPv6 header fields +---------------------+-------------+----------------+ | Field | Size (bits) | Class | +---------------------+-------------+----------------+ | Version | 4 | STATIC-KNOWN | | Traffic Class | 8 | CHANGING | | Flow Label | 20 | STATIC-DEF | | Payload Length | 16 | INFERRED | | Next Header | 8 | STATIC-KNOWN | | Hop Limit | 8 | CHANGING | | Source Address | 128 | STATIC-DEF | | Destination Address | 128 | STATIC-DEF | +---------------------+-------------+----------------+ Version The version field states which IP version the packet is based on. Packets with different values in this field must be handled by different IP stacks. For header compression, different compression profiles must also be used. When compressor and decompressor have negotiated which profile to use, the IP version is also known to both parties. The field is therefore classified as STATIC-KNOWN. Flow Label This field may be used to identify packets belonging to a specific packet stream. If not used, the value should be set to zero. Otherwise, all packets belonging to the same stream must have the same value in this field, it being one of the fields defining the stream. The field is therefore classified as STATIC-DEF. Payload Length Information about the packet length (and then also payload length) is expected to be provided by the link layer. The field is therefore classified as INFERRED. Next Header This field is expected to have the same value in all packets of a packet stream. As for the version number, a certain compression profile can only handle a specific next header which means that this value is known when profile has been negotiated. The field is therefore classified as STATIC-KNOWN. Bormann (ed.) [Page 116] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Source and Destination addresses These fields are part of the definition of a stream and must thus be constant for all packets in the stream. The fields are therefore classified as STATIC-DEF. Summarizing the bits corresponding to the classes gives: +--------------+--------------+ | Class | Size (octets)| +--------------+--------------+ | INFERRED | 2 | | STATIC-DEF | 34.5 | | STATIC-KNOWN | 1.5 | | CHANGING | 2 | +--------------+--------------+ A.1.2. IPv4 header fields +---------------------+-------------+----------------+ | Field | Size (bits) | Class | +---------------------+-------------+----------------+ | Version | 4 | STATIC-KNOWN | | Header Length | 4 | STATIC-KNOWN | | Type Of Service | 8 | CHANGING | | Packet Length | 16 | INFERRED | | Identification | 16 | CHANGING | | Reserved flag | 1 | STATIC-KNOWN | | May Fragment flag | 1 | STATIC | | Last Fragment flag | 1 | STATIC-KNOWN | | Fragment Offset | 13 | STATIC-KNOWN | | Time To Live | 8 | CHANGING | | Protocol | 8 | STATIC-KNOWN | | Header Checksum | 16 | INFERRED | | Source Address | 32 | STATIC-DEF | | Destination Address | 32 | STATIC-DEF | +---------------------+-------------+----------------+ Version The version field states which IP version the packet is based on and packets with different values in this field must be handled by different IP stacks. For header compression, different compression profiles must also be used. When compressor and decompressor has negotiated which profile to use, the IP version is also well known to both parties. The field is therefore classified as STATIC-KNOWN. Bormann (ed.) [Page 117] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Header Length As long as there are no options present in the IP header, the header length is constant and well known. If there are options, the fields would be STATIC, but we assume no options. The field is therefore classified as STATIC-KNOWN. Packet Length Information about the packet length is expected to be provided by the link layer. The field is therefore classified as INFERRED. Flags The Reserved flag must be set to zero and is therefore classified as STATIC-KNOWN. The May Fragment flag will be constant for all packets in a stream and is therefore classified as STATIC. Finally, the Last Fragment bit is expected to be zero because fragmentation is NOT expected, due to the small packet size expected. The Last Fragment bit is therefore classified as STATIC-KNOWN. Fragment Offset With the assumption that no fragmentation occurs, the fragment offset is always zero. The field is therefore classified as STATIC- KNOWN. Protocol This field is expected to have the same value in all packets of a packet stream. As for the version number, a certain compression profile can only handle a specific next header which means that this value is well known when profile has been negotiated. The field is therefore classified as STATIC-KNOWN. Header Checksum The header checksum protects individual hops from processing a corrupted header. When almost all IP header information is compressed away, there is no need to have this additional checksum; instead it can be regenerate at the decompressor side. The field is therefore classified as INFERRED. Source and Destination addresses Bormann (ed.) [Page 118] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 These fields are part of the definition of a stream and must thus be constant for all packets in the stream. The fields are therefore classified as STATIC-DEF. Summarizing the bits corresponding to the classes gives: +--------------+--------------+ | Class | Size (octets)| +--------------+--------------+ | INFERRED | 4 | | STATIC | 1 bit | | STATIC-DEF | 8 | | STATIC-KNOWN | 3 +7 bits | | CHANGING | 4 | +--------------+--------------+ A.1.3. UDP header fields +------------------+-------------+-------------+ | Field | Size (bits) | Class | +------------------+-------------+-------------+ | Source Port | 16 | STATIC-DEF | | Destination Port | 16 | STATIC-DEF | | Length | 16 | INFERRED | | Checksum | 16 | CHANGING | +------------------+-------------+-------------+ Source and Destination ports These fields are part of the definition of a stream and must thus be constant for all packets in the stream. The fields are therefore classified as STATIC-DEF. Length This field is redundant and is therefore classified as INFERRED. Summarizing the bits corresponding to the classes gives: +------------+--------------+ | Class | Size (octets)| +------------+--------------+ | INFERRED | 2 | | STATIC-DEF | 4 | | CHANGING | 2 | +------------+--------------+ Bormann (ed.) [Page 119] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 A.1.4. RTP header fields +-----------------+-------------+----------------+ | Field | Size (bits) | Class | +-----------------+-------------+----------------+ | Version | 2 | STATIC-KNOWN | | Padding | 1 | STATIC | | Extension | 1 | STATIC | | CSRC Counter | 4 | CHANGING | | Marker | 1 | CHANGING | | Payload Type | 7 | CHANGING | | Sequence Number | 16 | CHANGING | | Timestamp | 32 | CHANGING | | SSRC | 32 | STATIC-DEF | | CSRC | 0(-480) | CHANGING | +-----------------+-------------+----------------+ Version There exists only one working RTP version and that is version 2. The field is therefore classified as STATIC-KNOWN. Padding The use of this field depends on the application, but when payload padding is used it is likely to be present in all packets. The field is therefore classified as STATIC. Extension If RTP extensions is used by the application, it is likely to be an extension present in all packets (but use of extensions is very uncommon). However, for safety's sake this field is classified as STATIC and not STATIC-KNOWN. SSRC This field is part of the definition of a stream and must thus be constant for all packets in the stream. The field is therefore classified as STATIC-DEF. Bormann (ed.) [Page 120] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Summarizing the bits corresponding to the classes gives: +--------------+--------------+ | Class | Size (octets)| +--------------+--------------+ | STATIC | 2 bits | | STATIC-DEF | 4 | | STATIC-KNOWN | 2 bits | | CHANGING | 7.5(-67.5) | +--------------+--------------+ A.1.5. Summary for IP/UDP/RTP If we summarize this for IP/UDP/RTP we get: +----------------+--------------+--------------+ | Class \ IP ver | IPv6 (octets)| IPv4 (octets)| +----------------+--------------+--------------+ | INFERRED | 4 | 6 | | STATIC | 2 bits | 3 bits | | STATIC-DEF | 42.5 | 16 | | STATIC-KNOWN | 1 +6 bits | 4 +1 bit | | CHANGING | 11.5(-71.5) | 13.5(-73.5) | +----------------+--------------+--------------+ | Total | 60(-120) | 40(-100) | +----------------+--------------+--------------+ A.2. Analysis of change patterns of header fields To design suitable mechanisms for efficient compression of all header fields, their change patterns must be analyzed. For this reason, an extended classification is done based on the general classification in A.1, considering the fields which were labeled CHANGING in that classification. Different applications will use the fields in different ways, which may affect their behavior. When this is the case, typical behavior for conversational audio and video will be discussed. The CHANGING fields are separated into five different subclasses: STATIC These are fields that were classified as CHANGING on a general basis, but are classified as STATIC here due to certain additional assumptions. SEMISTATIC These fields are STATIC most of the time. However, occasionally the value changes but Bormann (ed.) [Page 121] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 reverts to its original value after a known number of packets. RARELY-CHANGING (RC) These are fields that change their values occasionally and then keep their new values. ALTERNATING These fields alternate between a small number of different values. IRREGULAR These, finally, are the fields for which no useful change pattern can be identified. To further expand the classification possibilities without increasing complexity, the classification can be done either according to the values of the field and/or according to the values of the deltas for the field. When the classification is done, other details are also stated regarding possible additional knowledge about the field values and/or field deltas, according to the classification. For fields classified as STATIC or SEMISTATIC, the case could be that the value of the field is not only STATIC but also well KNOWN a priori (two states for SEMISTATIC fields). For fields with non-irregular change behavior, it could be known that changes usually are within a LIMITED range compared to the maximal change for the field. For other fields, the values are completely UNKNOWN. Table A.1 classifies all the CHANGING fields based on their expected change patterns, especially for conversational audio and video. +------------------------+-------------+-------------+-------------+ | Field | Value/Delta | Class | Knowledge | +========================+=============+=============+=============+ | Sequential | Delta | STATIC | KNOWN | | -----------+-------------+-------------+-------------+ | IPv4 Id: Seq. jump | Delta | RC | LIMITED | | -----------+-------------+-------------+-------------+ | Random | Value | IRREGULAR | UNKNOWN | +------------------------+-------------+-------------+-------------+ | IP TOS / Tr. Class | Value | RC | UNKNOWN | +------------------------+-------------+-------------+-------------+ | IP TTL / Hop Limit | Value | ALTERNATING | LIMITED | +------------------------+-------------+-------------+-------------+ | Disabled | Value | STATIC | KNOWN | | UDP Checksum: ---------+-------------+-------------+-------------+ | Enabled | Value | IRREGULAR | UNKNOWN | +------------------------+-------------+-------------+-------------+ | No mix | Value | STATIC | KNOWN | | RTP CSRC Count: -------+-------------+-------------+-------------+ Bormann (ed.) [Page 122] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 | Mixed | Value | RC | LIMITED | +------------------------+-------------+-------------+-------------+ | RTP Marker | Value | SEMISTATIC | KNOWN/KNOWN | +------------------------+-------------+-------------+-------------+ | RTP Payload Type | Value | RC | UNKNOWN | +------------------------+-------------+-------------+-------------+ | RTP Sequence Number | Delta | STATIC | KNOWN | +------------------------+-------------+-------------+-------------+ | RTP Timestamp | Delta | RC | LIMITED | +------------------------+-------------+-------------+-------------+ | No mix | - | - | - | | RTP CSRC List: -------+-------------+-------------+-------------+ | Mixed | Value | RC | UNKNOWN | +------------------------+-------------+-------------+-------------+ Table A.1 : Classification of CHANGING header fields The following subsections discuss the various header fields in detail. Note that table A.1 and the discussions below do not consider changes caused by loss or reordering before the compression point. A.2.1. IPv4 Identification The Identification field (IP ID) of the IPv4 header is there to identify which fragments constitute a datagram when reassembling fragmented datagrams. The IPv4 specification does not specify exactly how this field is to be assigned values, only that each packet should get an IP ID that is unique for the source-destination pair and protocol for the time the datagram (or any of its fragments) could be alive in the network. This means that assignment of IP ID values can be done in various ways, which we have separated into three classes. Sequential This assignment policy keeps a separate counter for each outgoing packet stream and thus the IP ID value will increment by one for each packet in the stream. Therefore, the delta value of the field is constant and well known a priori. When RTP is used on top of UDP and IP, the IP ID value follows the RTP sequence number. This assignment policy is the most desirable for header compression purposes but its usage is not as common as it should be. The reason is that it can be realized only if UDP and IP are implemented together so that UDP, which separates packet streams by the port identification, can make IP use separate ID counters for each packet stream. Sequential jump This is the most common assignment policy in today's IP stacks. The difference from the sequential method is that only one Bormann (ed.) [Page 123] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 counter is used for all connections. When the sender is running more than one packet stream simultaneously, the IP ID can increase by more than one. The IP ID values will be much more predictable and require less bits to transfer than random values, and the packet-to-packet increment (determined by the number of active outgoing packet streams and sending frequencies) will usually be limited. Random Some IP stacks assign IP ID values using a pseudo-random number generator. There is thus no correlation between the ID values of subsequent datagrams. Therefore there is no way to predict the IP ID value for the next datagram. For header compression purposes, this means that the IP ID field needs to be sent uncompressed with each datagram, resulting in two extra octets of header. IP stacks in cellular terminals SHOULD NOT use this IP ID assignment policy. It should be noted that the ID is an IPv4 mechanism and is therefore not needed at all in IPv6 profiles. For IPv4 the ID could be handled in three different ways. Firstly, we have the inefficient but reliable solution where the ID field is sent as-is in all packets, increasing the compressed headers with two octets. This is the best way to handle the ID field if the sender uses random assignment of the ID field. Secondly, there can be solutions with more flexible mechanisms requiring less bits for the ID handling as long as sequential jump assignment is used. Such solutions will probably require even more bits if random assignment is used by the sender. Knowledge about the sender's assignment policy could therefore be useful when choosing between the two solutions above. Finally, even for IPv4, header compression could be designed without any additional information for the ID field included in compressed headers. To use such schemes, it must be known that the sender makes use of the pure sequential assignment policy for the ID field. That might not be possible to know, which implies that the applicability of such solutions is very uncertain. However, designers of IPv4 stacks for cellular terminals SHOULD use the sequential policy. A.2.2. IP Traffic-Class / Type-Of-Service The Traffic-Class (IPv6) or Type-Of-Service (IPv4) field is expected to be constant during the lifetime of a packet stream or to change relatively seldom. Bormann (ed.) [Page 124] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 A.2.3. IP Hop-Limit / Time-To-Live The Hop-Limit (IPv6) or Time-To-Live (IPv4) field is expected to be constant during the lifetime of a packet stream or to alternate between a limited number of values due to route changes. A.2.4. UDP Checksum The UDP checksum is optional. If disabled, its value is constantly zero and could be compressed away. If enabled, its value depends on the payload, which for compression purposes is equivalent to it changing randomly with every packet. A.2.5. RTP CSRC Counter This is a counter indicating the number of CSRC items present in the CSRC list. This number is expected to be almost constant on a packet- to-packet basis and change by small amount. As long as no RTP mixer is used, the value of this field is zero. A.2.6. RTP Marker For audio the marker bit should be set only in the first packet of a talkspurt while for video it should be set in the last packet of every picture. This means that in both cases the RTP marker is classified as SEMISTATIC with well-known values for both states. A.2.7. RTP Payload Type Changes of the RTP payload type within a packet stream are expected to be rare. Applications could adapt to congestion by changing payload type and/or frame sizes, but that is not expected to happen frequently. A.2.8. RTP Sequence Number The RTP sequence number will be incremented by one for each packet sent. A.2.9. RTP Timestamp In the audio case: As long as there are no pauses in the audio stream, the RTP timestamp will be incremented by a constant delta, corresponding to the number of samples in the speech frame. It will thus mostly Bormann (ed.) [Page 125] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 follow the RTP sequence number. When there has been a silent period and a new talkspurt begins, the timestamp will jump in proportion to the length of the silent period. However, the increment will probably be within a relatively limited range. In the video case: The timestamp change between two consecutive packets will either be zero or increase by a multiple of a fixed value corresponding to the picture clock frequency. The timestamp can also decrease by a multiple of the fixed value if B-pictures are used. The delta interval, expressed as a multiple of the picture clock frequency, is in most cases very limited. A.2.10. RTP Contributing Sources (CSRC) The participants in a session, which are identified by the CSRC fields, are expected to be almost the same on a packet-to-packet basis with relatively few additions or removals. As long as RTP mixers are not used, no CSRC fields are present at all. A.3. Header compression strategies This section elaborates on what has been done in previous sections. Based in the classifications, recommendations are given on how to handle the various fields in the header compression process. Seven different actions are possible and these are listed together with the fields to which each action applies. A.3.1. Do not send at all The fields that have well known values a priori do not have to be sent at all. These are: - IP Version - IPv6 Payload Length - IPv6 Next Header - IPv4 Header Length - IPv4 Reserved Flag - IPv4 Last Fragment Flag - IPv4 Fragment Offset - IPv4 Protocol - UDP Checksum (if disabled) - RTP Version Bormann (ed.) [Page 126] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 A.3.2. Transmit only initially The fields that are constant throughout the lifetime of the packet stream have to be transmitted and correctly delivered to the decompressor only once. These are: - IP Source Address - IP Destination Address - IPv6 Flow Label - IPv4 May Fragment Flag - UDP Source Port - UDP Destination Port - RTP Padding Flag - RTP Extension Flag - RTP SSRC A.3.3. Transmit initially, but be prepared to update The fields that are changing only occasionally must be transmitted initially but there must also be a way to update these fields with new values if they change. These fields are: - IPv6 Traffic Class - IPv6 Hop Limit - IPv4 Type Of Service (TOS) - IPv4 Time To Live (TTL) - RTP CSRC Counter - RTP Payload Type - RTP CSRC List A.3.4. Be prepared to update or send as-is frequently For fields that normally are either constant or whose values can be deduced from some other field but frequently diverge from that behavior, there must be an efficient way to update the field value or send it as-is in some packets. Those fields are: - IPv4 Identification (if not sequentially assigned) - RTP Marker - RTP Timestamp A.3.5. Guarantee continuous robustness Fields that behave like a counter with a fixed delta for ALL packets, the only requirement on the transmission encoding is that packet losses between compressor and decompressor must be tolerable. If more than one such field exists, all these can be communicated together. Such fields can also be used to interpret the values for fields Bormann (ed.) [Page 127] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 listed in the previous section. Fields that have this counter behavior are: - IPv4 Identification (if sequentially assigned) - RTP Sequence Number A.3.6. Transmit as-is in all packets Fields that have completely random values for each packet must be included as-is in all compressed headers. Those fields are: - IPv4 Identification (if randomly assigned) - UDP Checksum (if enabled) A.3.7. Establish and be prepared to update delta Finally, there is a field that is usually increasing by a fixed delta and is correlated to another field. For this field it would make sense to make that delta part of the context state. The delta must then be possible to initiate and update in the same way as the fields listed in A.3.3. The field to which this applies is: - RTP Timestamp Bormann (ed.) [Page 128] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Appendix E - Encoding Examples [[Editor's note: Need to update this with current terminology.]] E.1. Basic VLE The examples below illustrate the operation of VLE under various scenarios. The field values used in the examples could correspond to any fields that we wish to compress. The examples illustrate the scenario where the compressed field has resolution of one bit. Example 1: Normal operation (no packet loss prior to compressor, no reodering prior to compressor). Suppose packets with header fields 279, 280, 281, 282, and 283 have been sent, and 279 and 283 are fields of potential reference packets. The current VLE window is {279, 283}. and a packet with field value = 284 is received next, VLE computes the following values New Value VMax VMin r # LSBs 284 283 279 max[|284-279|,|284-283|]=5 4 The window is unmodified if we assuming the new packet {284} is not a potential reference. The field is encoded using 4 bits in this case, and the actual encoded value is the 4 least significant bits of 284 (10011100) which = 1100. Example 2: Packet Loss prior to compressor. Suppose packets with header fields 279, 280, 281, 282, and 283 have been sent, and 279 and 283 are fields of potential reference packets such that the VSW is again {279, 283}. If a packet with field value = 290 is received next, VLE computes the following values New Value VMax VMin r # LSBs 290 283 279 max[|290-283|,|290-279|]=11 5 So the field is encoded using 5 bits. Actual encoded value is the 5 LSBs of 290 (100100010) which = 00010. If we assume the new value is a potential reference, the new VSW is {279, 283, 290}. Example 3: Packet Misordering prior to compressor. Bormann (ed.) [Page 129] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 Suppose packets with header fields 279, 280, 281, 282, and 283 have been sent, and 279 and 283 are fields of potential reference packets such that the VSW is again {279, 283}. If a packet with field value = 278 is received next, VLE computes the following values New Value VMax VMin r # LSBs 278 283 279 max[|278-283|,|278-279|]=5 4 So the field is encoded using 4 bits. Actual encoded value is the 4 LSBs of 278 (10010110) which = 0110. If we assume the new value is a potential reference, the new VSW is {283, 290, 278}. In any case, the VLE encoded fields must be accompanied by some bits in order to identify the different possible encoded field sizes. Sizes of this bit field can vary depending on the number of different sizes one wishes to allow. The approach in ACE is to allow only a few different sizes for byte-aligned header formats. Huffman coding of the length is used to achieve some additional efficiency, based on the expected frequency of needing the different sizes. 1 or 2 additional bits are actually sent in the ACE compressed header. The decompressor behavior in all the example cases is the same- it uses as a reference a specific decompressed header field value. The header to use might be indicated by the presence of a checksum in the compressed header packet, or by other means. They must by definition be one of the values in the compressor's window. For example let's assume that the last correctly decompressed packet which qualifies as a reference was the packet with header field = 291. Now suppose the encoded field value of 303 (10001111) is received and = 01111. The two values closest values to 291 which have LSBs = 01111 are 271 and 303. 303 is closest, therefore it is correctly selected as the uncompressed field value. E.2. Timer-Based VLE As a an example of operation, consider the case of a voice codec (20 ms), such that TS_stride = 160. Assume T_current and p_TS_current are 357 and 351, respectively, and that we have sliding window TSW which contains the following values 4 entries: j T_j p_TS_j 1 9 7 Bormann (ed.) [Page 130] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 2 8 6 3 7 4 4 3 1 j above is the packet number. In this case we have Network_jitter(1)=|(357-9)-(351-7)|=4 (80 ms Network Jitter) Network_jitter(2)=|(357-8)-(351-6)|=4 (80 ms Network Jitter) Network_jitter(3)=|(357-7)-(351-4)|=3 (60 ms Network Jitter) Network_jitter(4)=|(357-3)-(351-1)|=4 (80 ms Network Jitter) So Max_Network_Jitter = 4. We assume a maximum CD-CC jitter of 2 (40 ms); the total jitter to be handled in this case is then J = 4 + 2 + 2 = 8 packets (160 ms) and k = 5 bits (since 2 * 5 + 1 < 2^5). The compressor sends the 5 LSBs of p_TS_current to the decompressor (351 = 101011111, so the encoded TS value = 11111). When the decompressor receives this value, it first attempts to estimate the timestamp by computing the time difference between the last reference established and the current packet T_current - T_ref, where T_ref is the value of the wall clock time at which the reference headers was received by the decompressor That value is added to p_TS_ref, the packed RTP TS of the reference header, to get the estimate. Assume that at the decompressor packet #3 is used as the reference: - T_current = 359 - T_ref = 7 - p_TS_ref = 4 Note: T_current is picked here as any value; the difference between it and T_ref represents the length of the silence interval as observed at the decompressor. Then: T_current - T_ref = 359 - 7 = 352 Bormann (ed.) [Page 131] INTERNET-DRAFT Robust Header Compression Oct 03, 2000 p_TS_current(estimate) = 352 + 4 = 356 The decompressor searches for the closest value to 356 which has, in this case, LSBs = 11111. The value in this case is 351, the original p_TS. If instead the compressor were to send the timestamp jump as simply the difference in consecutive packed RTP Timestamps, that value would be p_TS_current - p_TS_ref = 351-4 = 347 = 101011011 So over twice as many bits would be sent for a silence interval of 347 (20 ms) = 6.94 seconds Due to basic conversational real-time requirements, the cumulative jitter in normal operation is expected to be at most only a few times T stride for voice. For this reason, the FO payload formats in section 4.3 are optimized (in terms of representing different k- length encoded TS values) for the case of k=4 (handles up to 16 discrepencies in the timestamp). The remaining formats allow a wide range of jitter conditions (outside of just voice) to be handled as well. This Internet-Draft expires April 02, 2001.