Network Working Group Ghyslain Pelletier, Editor, Ericsson INTERNET-DRAFT Lars-Erik Jonsson, Ericsson Expires: August 2005 Mark A West, Siemens/Roke Manor Carsten Bormann, TZI/Uni Bremen Kristofer Sandlund, Effnet February 21, 2005 RObust Header Compression (ROHC): A Profile for TCP/IP (ROHC-TCP) Status of this memo By submitting this Internet-Draft, I (we) certify that any applicable patent or other IPR claims of which I am (we are) aware have been disclosed, and any of which I (we) become aware will be disclosed, in accordance with RFC 3668 (BCP 79). By submitting this Internet-Draft, I (we) accept the provisions of Section #3 of RFC 3667 (BCP 78). 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 submission of the IETF ROHC WG. Comments should be directed to the ROHC WG mailing list, rohc@ietf.org. Abstract This document specifies a ROHC (Robust Header Compression) profile for compression of TCP/IP packets. The profile, called ROHC-TCP, is a robust header compression scheme for TCP/IP that provides improved compression efficiency and enhanced capabilities for compression of various header fields including TCP options. Pelletier, et al. [Page 1] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 Existing TCP/IP 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. In addition, existing schemes (RFC 1144 [14], RFC 2507 [21]) have not addressed how to compress TCP options such as SACK (Selective Acknowledgements) (RFC 2018 [20], RFC 2883 [22]) and Timestamps (RFC 1323 [15]). Table of Contents 1. Introduction.....................................................4 2. Terminology......................................................4 3. Background.......................................................5 3.1. Existing TCP/IP Header Compression Schemes..................6 3.2. Classification of TCP/IP Header Fields......................7 3.3. Characteristics of Short-lived TCP Transfers................8 4. Overview of the TCP/IP Profile...................................9 4.1. General Concepts............................................9 4.2. Context Replication.........................................9 4.3. State Machines and Profile Operation........................9 4.4. Packet Formats and Encoding Methods........................10 4.5. Irregular Chain............................................10 4.6. TCP Options................................................10 4.6.1. Compressing Extension Headers.........................10 5. Compressor and decompressor State Machines......................10 5.1. Compressor States and Logic................................11 5.1.1. Initialization and Refresh (IR) State.................11 5.1.2. Compression (CO) State................................11 5.1.3. Feedback Logic........................................12 5.1.4. State Transition Logic................................12 5.1.4.1. Optimistic Approach, Upward Transition...........12 5.1.4.2. Optional Acknowledgements (ACKs), Upward Transition ..........................................................12 5.1.4.3. Timeouts, Downward Transition....................13 5.1.4.4. Negative ACKs (NACKs), Downward Transition.......13 5.1.4.5. Need for Updates, Downward Transition............13 5.1.5. State Machine Supporting Context Replication..........13 5.2. Decompressor States and Logic..............................14 5.2.1. No Context (NC) State.................................15 5.2.2. Static Context (SC) State.............................15 5.2.3. Full Context (FC) State...............................15 5.2.4. Allowing Decompression................................16 5.2.5. Reconstruction and Verification.......................16 5.2.6. Actions upon CRC Failure..............................16 5.2.7. Feedback Logic........................................17 6. ROHC-TCP - TCP/IP Compression (Profile 0x0006)..................18 6.1. Profile-specific Encoding Methods..........................18 6.1.1. inferred_mine_header_checksum().......................18 6.1.2. inferred_ip_v4_header_checksum()......................19 6.1.3. inferred_ip_v4_length()...............................19 Pelletier, et. al [Page 2] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 6.1.4. inferred_ip_v6_length()...............................19 6.1.5. inferred_offset().....................................20 6.1.6. Scaled TCP Sequence Number Encoding...................20 6.1.7. Scaled Acknowledgement Number Encoding................21 6.2. Considerations for the Feedback Channel....................22 6.3. Control Fields in the ROHC-TCP Context.....................22 6.3.1. Master Sequence Number (MSN)..........................23 6.3.2. IP-ID Behavior........................................23 6.3.3. Explicit Congestion Notification (ECN) in ROHC-TCP....24 6.4. CRC Calculations...........................................24 6.5. Initialization.............................................24 7. Packet Types....................................................25 7.1. Compressed Header Chains...................................25 7.2. Compressing TCP Options with List Compression..............26 7.2.1. List Compression......................................26 7.2.2. Table-based Item Compression..........................27 7.2.3. Item Tables...........................................28 7.2.4. Constraints to List Compression.......................29 7.2.5. Item Table Mappings...................................29 7.2.6. Compressed Lists in Dynamic Chain.....................30 7.2.7. Irregular Chain Items for TCP Options.................30 7.2.8. Replication of TCP Options............................31 7.3. Initialization and Refresh Packets (IR)....................31 7.4. Context Replication Packets (IR-CR)........................33 7.5. Compressed Packets (CO)....................................34 8. Packet Formats..................................................35 8.1. Design rationale for compressed base headers...............35 8.2. Global Control Fields......................................38 8.3. General Structures.........................................38 8.4. Extension Headers..........................................42 8.4.1. IPv6 DEST opt header..................................42 8.4.2. IPv6 HOP opt header...................................43 8.4.3. IPv6 Routing Header...................................43 8.4.4. GRE Header............................................44 8.4.5. MINE header...........................................47 8.4.6. Authentication Header (AH) header.....................48 8.4.7. Encapsulation Security Payload (ESP) header...........49 8.5. IP Header..................................................51 8.5.1. Structures Common for IPv4 and IPv6...................51 8.5.2. IPv6 Header...........................................51 8.5.3. IPv4 Header...........................................54 8.6. TCP Header.................................................58 8.7. TCP Options................................................64 8.8. Structures used in Compressed Base Headers.................74 8.9. Feedback Formats and Options...............................89 8.9.1. Feedback Formats......................................89 8.9.2. Feedback Options......................................90 8.9.2.1. The CRC option...................................90 8.9.2.2. The REJECT option................................90 8.9.2.3. The MSN-NOT-VALID option.........................91 8.9.2.4. The MSN option...................................91 Pelletier, et. al [Page 3] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 8.9.2.5. The LOSS option..................................91 8.9.2.6. Unknown option types.............................91 8.9.2.7. The CONTEXT_MEMORY Feedback Option...............92 9. Security Consideration..........................................92 10. IANA Considerations............................................93 11. Acknowledgments................................................93 12. Authors' Addresses.............................................93 13. References.....................................................94 13.1. Normative references......................................94 13.2. Informative References....................................95 1. Introduction There are several reasons to perform header compression on low- or medium-speed links for TCP/IP traffic, and these have already been discussed in RFC 2507 [21]. Additional considerations that make robustness an important objective for a TCP compression scheme are introduced in [10]. Finally, existing TCP/IP header compression schemes (RFC 1144 [14], RFC 2507 [21]) are limited in their handling of the TCP options field and cannot compress the headers of handshaking packets (SYNs and FINs). It is thus desirable for a header compression scheme to be able to handle loss on the link between the compression and decompression point as well as loss before the compression point. The header compression scheme also needs to consider how to efficiently compress short-lived TCP transfers and TCP options, such as SACK (RFC 2018 [20], RFC 2883 [22]) and Timestamps (RFC 1323 [15]). The ROHC WG has developed a header compression framework on top of which various profiles can be defined for different protocol sets, or for different compression strategies. This document defines a TCP/IP compression profile for the ROHC framework [2], compliant with the requirements on ROHC TCP/IP header compression [10]. Specifically, it describes a header compression scheme for TCP/IP header compression (ROHC-TCP) that is robust against packet loss and that offers enhanced capabilities, in particular for the compression of header fields including TCP options. The profile identifier for TCP/IP compression is 0x0006. 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 [1]. This document reuses some of the terminology found in RFC 3095 [2]. In addition, this document uses or defines the following terms: Pelletier, et. al [Page 4] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 Base context The base context is a context that has been validated by both the compressor and the decompressor. A base context can be used as the reference when building a new context using replication. Base CID The Base Context Identifier is the CID used to identify the Base Context, where information needed for context replication can be extracted from. Context replication Context replication is the mechanism that establishes and initializes a new context based on another existing valid context (a base context). This mechanism is introduced to reduce the overhead of the context establishment procedure, and is especially useful for compression of multiple short-lived TCP connections that may be occurring simultaneously or near-simultaneously. ROHC Context Replication (ROHC-CR) "ROHC-CR" in this document normatively refers to the context replication mechanism for ROHC profiles defined in [3]. ROHC Formal Notation (ROHC-FN) "ROHC-FN" in this document normatively refers to the formal notation for ROHC profiles defined in [4], including the library of encoding methods it specifies. Short-lived TCP Transfer Short-lived TCP transfers refer to TCP connections transmitting only small amounts of data for each single connection. Short TCP flows seldom need to operate beyond the slow-start phase of TCP to complete their transfer, which also means that the transmission ends before any significant increase of the TCP congestion window may occur. 3. Background This chapter provides some background information on TCP/IP header compression. The fundamentals of general header compression may be found in [2]. In the following sections, two existing TCP/IP header compression schemes are first described along with a discussion of their limitations, followed by the classification of TCP/IP header fields. Finally, some of the characteristics of short-lived TCP transfers are summarized. Pelletier, et. al [Page 5] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 The behavior analysis of TCP/IP header fields among multiple short- lived connections may be found in [11]. 3.1. Existing TCP/IP Header Compression Schemes Compressed TCP (CTCP) and IP Header Compression (IPHC) are two different schemes that may be used to compress TCP/IP headers. Both schemes transmit only the differences from the previous header in order to reduce the large overhead of the TCP/IP header. The CTCP (RFC 1144 [14]) compressor detects transport-level retransmissions and sends a header that updates the context completely when they occur. While CTCP works well over reliable links, it is vulnerable when used over less reliable links as even a single packet loss results in loss of synchronization between the compressor and the decompressor. This in turn leads to the TCP receiver discarding all remaining packets in the current window because of a checksum error. This effectively prevents the TCP Fast Retransmit algorithm (RFC 2001) from being triggered. In such case, the compressor must wait until the TCP timeout to resynchronize. To reduce the errors due to the inconsistent contexts between compressor and decompressor when compressing TCP, IPHC (RFC 2507 [21]) improves somewhat on CTCP by augmenting the repair mechanism of CTCP with a local repair mechanism called TWICE and with a link-level nacking mechanism to request a header that updates the context. The TWICE algorithm assumes that only the Sequence Number field of TCP segments are changing with the deltas between consecutive packets being constant in most cases. This assumption is however not always true, especially when TCP Timestamps and SACK options are used. The full header request mechanism requires a feedback channel that may be unavailable in some circumstances. This channel is used to explicitly request that the next packet be sent with an uncompressed header to allow resynchronization without waiting for a TCP timeout. In addition, this mechanism does not perform well on links with long round-trip time. Both CTCP and IPHC are also limited in their handling of the TCP options field. For IPHC, any change in the options field (caused by timestamps or SACK, for example) renders the entire field uncompressible, while for CTCP such a change in the options field effectively disables TCP/IP header compression altogether. Finally, existing TCP/IP compression schemes do not compress the headers of handshaking packets (SYNs and FINs). Compressing these packets may greatly improve the overall header compression ratio for the cases where many short-lived TCP connections share the same link. Pelletier, et. al [Page 6] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 3.2. Classification of TCP/IP Header Fields Header compression is possible due to the fact that there is much redundancy between header field values within packets, especially between consecutive packets. To utilize these properties for TCP/IP header compression, it is important to understand the change patterns of the various header fields. All fields of the TCP/IP packet header have been classified in detail in [11]. The main conclusion is that most of the header fields can easily be compressed away since they never or seldom change. The following fields do however require more sophisticated mechanisms: - IPv4 Identification (16 bits) - IP-ID - TCP Sequence Number (32 bits) - SN - TCP Acknowledgement Number (32 bits) - ACKN - TCP Reserved (4 bits) - TCP ECN flags (2 bits) - ECN - TCP Window (16 bits) - WINDOW - TCP Options - Maximum Segment Size (4 octets) - MSS - Window Scale (3 octets) - WSopt - SACK Permitted (2 octets) - TCP SACK - SACK - TCP Timestamp (32 bits) - TS The assignment of IP-ID values can be done in various ways, which are Sequential, Sequential jump, Random or constant to a value of zero. However, designers of IPv4 stacks for cellular terminals should use an assignment policy close to Sequential. Some IPv4 stacks do use a sequential assignment when generating IP-ID values but do not transmit the contents this field in network byte order; instead it is sent with 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. In RFC 3095 [2], the IP-ID is generally inferred from the RTP Sequence Number. However, with respect to TCP compression, the analysis in [11] reveals that there is no obvious candidate to this purpose among the TCP fields. The change pattern of several TCP fields (Sequence Number, Acknowledgement Number, Window, etc.) is very hard to predict and differs entirely from the behavior of RTP fields discussed in [2]. Of particular importance to a TCP/IP header compression scheme is the understanding of the sequence and acknowledgement number [11]. Specifically, the sequence number can be anywhere within a range defined by the TCP window at any point on the path (i.e. wherever a compressor might be deployed). Missing packets or retransmissions can cause the TCP sequence number to fluctuate within the limits of this Pelletier, et. al [Page 7] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 window. The TCP window also bound the jumps in acknowledgement number. Another important behavior of the TCP/IP header is the dependency between the sequence number and the acknowledgment number. It is well known that most TCP connections only have one-way traffic (web browsing and FTP downloading, for example). This means that on the forward path (from server to client), only the sequence number is changing while the acknowledgement number remains constant for most packets; on the backward path (from client to server), only the sequence number is changing and the acknowledgement number remains constant for most packets. With respect to TCP options, it is noted that most options (such as MSS, WSopt, SACK-permitted, etc.) may appear only on a SYN segment. Every implementation should (and we expect most will) ignore unknown options on SYN segments. Headers specific to Mobile IP (for IPv4 or IPv6) do not receive any special treatment in this document, for reasons similar as those described in [2]. 3.3. Characteristics of Short-lived TCP Transfers Recent studies shows that the majority of TCP flows are short-lived transfers with an average and a median size no larger than 10KB. Short-lived TCP transfers will degrade the performance of header compression schemes that establish a new context by initially sending full headers. It is hard to improve the performance for a single, unpredictable, short-lived connection. However, there are common cases where there will be multiple TCP connections between the same pair of hosts. A mobile user browsing several web pages from the same web server (this is more the case with HTTP/1.0 than HTTP/1.1) is one example. In such case, multiple short-lived TCP/IP flows occur simultaneously or near simultaneously within a relatively short time interval. It may be expected that most (if not all) of the IP header of the these connections will be almost identical to each other, with only small relative jumps for the IP-ID field. Furthermore, a subset of the TCP fields may also be very similar from one connection to another. For example, one of the port numbers may be reused (the service port) while the other (the ephemeral port) may be changed only by a small amount relative to the just-closed connection. With regard to header compression, this means that parts of a compression context used for a TCP connection may be reusable for Pelletier, et. al [Page 8] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 another TCP connection. A mechanism supporting context replication, where a new context is initialized from an existing one, provide useful optimizations for a sequence of short-lived TCP connections. Context replication is possible due to the fact that there is much similarity in header field values and context values among multiple simultaneous or near simultaneous connections. All header fields and related context values have been classified in detail in [11]. The main conclusion is that most part of the IP sub-context, some TCP fields, and some context values can easily be replicated since they seldom change or change with only a small jump. 4. Overview of the TCP/IP Profile 4.1. General Concepts Many of the concepts behind the ROHC-TCP profile are similar to those described in RFC 3095 [2]. Like for other ROHC profiles, ROHC-TCP makes use of the ROHC protocol as described in [2], in sections 5.1 to 5.2.6. This includes data structures, reserved packet types, general packet formats, segmentation and initial decompressor processing. 4.2. Context Replication For ROHC-TCP, context replication may be particularly useful for short-lived TCP flows [10]. ROHC-TCP therefore supports context replication as defined in ROHC-CR [3]; the compressor MAY support context replication, while a decompressor implementation is REQUIRED to support decompression of the IR-CR packet type. 4.3. State Machines and Profile Operation 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. For ROHC-TCP compression, the compressor has two states and the decompressor has three states. The two compressor states are the Initialization and Refresh (IR) state, and the Compression (CO) state. The three states of the decompressor are No Context (NC), Static Context (SC) and Full Context (FC). The compressor may also implement a third state, the Context Replication (CR) state, to support context replication ROHC-CR [3]. Transitions need not be synchronized between the two state machines. Pelletier, et. al [Page 9] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 4.4. Packet Formats and Encoding Methods The packet formats used for ROHC-TCP and found in this document are defined using the formal notation, ROHC-FN. The formal notation is used to provide an unambiguous representation of the packet formats and a clear definition of the encoding methods. The encoding methods used in the packet formats for ROHC-TCP are defined in [4]. 4.5. Irregular Chain The ROHC-TCP profile defines an irregular chain for each header type, in addition to the static and dynamic chains as used in RFC 3095 [2]. The irregular chain handles fields for which no predictable change pattern could be identified, i.e. fields from the TCP, IP and extension headers that have an irregular behavior and therefore have to be included in each compressed packet. This chain is attached to compressed packet in order to make it possible to carry arbitrary combinations of headers. 4.6. TCP Options The list compression scheme in ROHC-TCP is a downscaled version of the list compression in [2], allowing option content to be established so that TCP options can be added or removed from the packet without having to send the entire option uncompressed. 4.6.1. Compressing Extension Headers In RFC 3095 [2], list compression is used to compress extension headers. ROHC-TCP compresses the same type of extension headers as in [2]. However, these headers are treated exactly as other headers and thus have a static chain, a dynamic chain, an irregular chain and a replicate chain. The consequence is that headers appearing in or disappearing from the flow being compressed will lead to changes to the static chain. However, the change pattern of extension headers is not deemed to impair compression efficiency with respect to this design strategy. 5. Compressor and decompressor State Machines The header compression state machines and their associated logic as specified in this section are a simplified version of the ones found in [2]. Pelletier, et. al [Page 10] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 5.1. Compressor States and Logic The two compressor states are the Initialization and Refresh (IR) state, and the Compression (CO) state. The compressor always starts in the lower compression state (IR). The compressor will normally operate in the higher compression state (CO), under the constraint that the compressor is sufficiently confident that the decompressor has the information necessary to reconstruct a header compressed according to this state. The figure below shows the state machine for the compressor. The details of each state, state transitions, and compression logic are given in sub-sections following the figure. Optimistic approach / ACK ACK +------>------>------>------+ +->-+ | | | | | v | v +----------+ +----------+ | IR State | | CO State | +----------+ +----------+ ^ | | Timeout / NACK / STATIC-NACK | +-------<-------<-------<--------+ The transition from IR state to CO state is based on the following principles: the need for update and the optimistic approach principle or, if a feedback channel is established, feedback received from the decompressor. 5.1.1. 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 static and non-static fields in uncompressed form plus some additional information. The compressor stays in the IR state until it is fairly confident that the decompressor has received the static information correctly. 5.1.2. Compression (CO) State The purpose of the CO state is to efficiently communicate irregularities in the packet stream when needed while maintaining the most optimal compression ratio. When operating in this state, the compressor normally sends most or all of the information in a compressed form. Pelletier, et. al [Page 11] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 5.1.3. Feedback Logic The compressor state machine makes use of feedback from decompressor to compressor for transitions in the backward direction, and optionally to improve the forward transition. The reception of either positive feedback (ACKs) or negative feedback (NACKs) establishes the feedback channel from the decompressor. Once there is an established feedback channel, the compressor makes use of this feedback for optionally improving the transitions among different states. This helps increasing the compression efficiency by providing the information needed for the compressor to achieve the necessary confidence level. When the feedback channel is established, it becomes superfluous for the compressor to send periodic refreshes. 5.1.4. State Transition Logic The compressor makes its decisions about when to transit between the IR and the CO states on the basis of: - variations in the packet headers - positive feedback from decompressor (Acknowledgements -- ACKs) - negative feedback from decompressor (Negative ACKS -- NACKs) - confidence level regarding error-free decompression of a packet 5.1.4.1. Optimistic Approach, Upward Transition Transition to the CO state is carried out according to the optimistic approach principle. This means that the compressor transits to the CO state when it is fairly confident that the decompressor has received enough information to correctly decompress packets sent according to the higher compression state. In general, there are many approaches where the compressor can obtain such information. A simple and general approach can be achieved by sending uncompressed or partial full headers periodically. 5.1.4.2. Optional Acknowledgements (ACKs), Upward Transition The compressor can also transit to the CO state based on feedback received by the decompressor. If a feedback channel is available, the decompressor MAY use positive feedback (ACKs) to acknowledge successful decompression of packets. Upon reception of an ACK for a context-updating packet, the compressor knows that the decompressor has received the acknowledged packet and the transition to the CO state can be carried out immediately. Pelletier, et. al [Page 12] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 This functionality is optional, so a compressor MUST NOT expect to get such ACKs initially or during normal operation, even if a feedback channel is available or established. 5.1.4.3. Timeouts, Downward Transition When the optimistic approach is used (i.e. until a feedback channel is established), there will always be a possibility of failure since the decompressor may not have received sufficient information for correct decompression. Therefore, unless the decompressor has established a feedback channel, the compressor MUST periodically transit to the IR state. 5.1.4.4. Negative ACKs (NACKs), Downward Transition Negative acknowledgments (NACKs) are also called context requests. Upon reception of a NACK, the compressor transits back to the IR state and sends updates (such as IR-DYN or IR) to the decompressor. 5.1.4.5. Need for Updates, Downward Transition When the header to be compressed does not conform to the established pattern or when the compressor is not confident whether the decompressor has the synchronized context, the compressor will transit to the IR state. 5.1.5. State Machine Supporting Context Replication For a profile supporting context replication, the additional compressor logic (including corresponding state transition and feedback logic) defined by ROHC-CR [3] must be added to the compressor state machine described above. Pelletier, et. al [Page 13] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 The following figure shows the resulting state machine: Optimistic approach / ACK +--->------>------>------>------>------>------>---+ | | | BCID Selection Optimistic approach / ACK | ACK | +------>----->------+ +----->----->----->-----+ | +->-+ | | | | | | | | | | v | v v | v +---------+ +---------+ +---------+ | IR | | CR | | CO | | State | | State | | State | +---------+ +---------+ +---------+ ^ ^ | | | | NACK / STATIC-NACK | | | +---<-----<-----<----+ | | | | Timeout / NACK / STATIC-NACK | +-----<-------<-------<-------<-------<-------<----+ 5.2. Decompressor States and Logic The three states of the decompressor are No Context (NC), Static Context (SC) and Full Context (FC). The decompressor starts in its lowest compression state, the NC state. Successful decompression will always move the decompressor to the FC state. The decompressor state machine normally never leaves the FC state once it has entered this state; only repeated decompression failures will force the decompressor to transit downwards to a lower state. Below is the state machine for the decompressor. Details of the transitions between states and decompression logic are given in the sub-sections following the figure. Success +-->------>------>------>------>------>--+ | | No Static | No Dynamic Success | Success +-->--+ | +-->--+ +--->----->---+ +-->--+ | | | | | | | | | | v | | v | v | v +-----------------+ +---------------------+ +-------------------+ | No Context (NC) | | Static Context (SC) | | Full Context (FC) | +-----------------+ +---------------------+ +-------------------+ ^ | ^ | | k_2 out of n_2 failures | | k_1 out of n_1 failures | +-----<------<------<-----+ +-----<------<------<-----+ Pelletier, et. al [Page 14] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 5.2.1. No Context (NC) State Initially, while working in the NC state, the decompressor has not yet successfully decompressed a packet. Upon receiving an IR or an IR-DYN packet, the decompressor will verify the correctness of this packet by validating its header using the CRC check. If the decompressed packet is successfully verified, the decompressor will update the context and use this packet as the reference packet. Once a packet has been decompressed correctly, the decompressor can transit to the FC state, and only upon repeated failures will it transit back to a lower state. 5.2.2. Static Context (SC) State In the SC state, the decompressor assumes static context damage when the CRC check of k_2 out of the last n_2 decompressed packets have failed. The decompressor moves to the NC state and discards all packets until a packet (e.g. IR or IR-DYN packet) that successfully passes the verification check is received. The decompressor may send feedback (see section 5.2.7) when assuming static context damage. Note that appropriate values for k and n, are related to the residual error rate of the link. When the residual error rate is close to zero, k = n = 1 may be appropriate. 5.2.3. Full Context (FC) State In the FC state, the decompressor assumes context damage when the CRC check of k_1 out of the last n_1 decompressed packets have failed, (where k and n are related to the residual error rate of the link as in section 5.2.2). The decompressor moves to the SC state and discards all packets until a packet carrying a 7- or 8-bit CRC that successfully passes the verification check is received. The decompressor may send feedback (see section 5.2.7) when assuming context damage. Upon receiving an IR or an IR-DYN packet, the decompressor MUST verify the correctness of its header using CRC validation. If the verification succeeds, the decompressor will update the context and use this packet as the reference packet. Consequently, the decompressor will convert the packet into the original packet and pass it to the network layer of the system. Upon receiving other types of packet, the decompressor will decompress it. The decompressor MUST verify the correctness of the decompressed packet by CRC check. If this verification succeeds, the decompressor passes the decompressed packet to the system's network Pelletier, et. al [Page 15] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 layer. The decompressor will then use this packet as the reference value. If the received packet is older than the current reference packet (based on sequence numbers in the compressed packet or in the uncompressed header), the decompressor MAY refrain from using this packet as the new reference value, even if the correctness of its header was successfully verified. 5.2.4. Allowing Decompression In the No Context state, only packets carrying sufficient information on the static fields (i.e. IR packets) can be decompressed. In the Static Context state, only packets carrying a 7- or 8-bit CRC may be decompressed (i.e. IR, IR-DYN and some CO packets). In the Full Context state, decompression may be attempted regardless of the type of packet received. If decompression may not be performed, the packet is discarded. As per ROHC-CR [3], IR-CR packets may be decompressed in any state. 5.2.5. Reconstruction and Verification The CRC carried within compressed headers MUST be used to verify decompression. When the decompression is verified and successful, the decompressor updates the context with the information received in the current header; otherwise if the reconstructed header fails the CRC check, these updates MUST NOT be performed. 5.2.6. Actions upon CRC Failure When a CRC check fails, the decompressor MUST discard the packet. The actions to be taken when CRC verification fails following the decompression of an IR-CR packet are specified in [3]. For other packet types carrying a CRC, if feedback is used the logic specified in section 5.2.7 must be followed when CRC verification fails. Note: Decompressor implementations may attempt corrective or repair measures prior to performing the above actions, and the result of any attempt MUST be verified using the CRC check. Pelletier, et. al [Page 16] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 5.2.7. Feedback Logic The decompressor may send positive feedback (ACKs) to initially establish the feedback channel for a particular flow. Either positive feedback (ACKs) or negative feedback (NACKs) will establish this channel. The decompressor will then use the feedback channel to send error recovery requests and (optionally) acknowledgements of significant context updates. Once the decompressor establishes a feedback channel, the compressor will operate using an optimistic logic. In particular, this means that the compressor will rely on specific decompressor feedback logic: - the decompressor will send negative acknowledgements in case when context damage is assumed or in other failure situations; - the decompressor is not strictly expected to send feedback upon successful decompression, other than for the purpose of improving the forward state transition. Once the feedback channel is established, the decompressor is REQUIRED to continue sending feedback (subject to the feedback rate limiting considerations later in this section) for the lifetime of the packet stream as follow: In NC state: The decompressor SHOULD send a STATIC-NACK if a packet of a type other than IR is received, or if an IR packet has failed the CRC check. In SC state: The decompressor SHOULD send a STATIC-NACK when decompression of an IR, an IR-DYN or a CO packet carrying a 7-bit CRC fails and if static context damage is assumed (see also section 5.2.2). If any other packet type is received, the decompressor SHOULD treat it as a CRC mismatch when deciding if feedback is to be sent. In FC state: The decompressor SHOULD send a NACK when decompression of any packet type fails and if context damage is assumed (see also section 5.2.3). When decompression fails, the feedback rate SHOULD be limited. For example, feedback could be sent only when decompression of several consecutive packets have failed. In addition, the decompressor should also limit the rate at which feedback is sent on successful Pelletier, et. al [Page 17] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 decompression, if sent at all. The decompressor may limit the feedback rate by sending feedback for one out of a number of packets providing the same type of feedback. The decompressor MAY optionally send ACKs upon successful decompression of any packet type. In particular, when an IR, an IR- DYN or any CO packet carrying a 7- or 8-bit CRC is correctly decompressed, the compressor may optionally send an ACK. Finally, when the decompressor ACKs an IR packet, it MUST use the CRC option (see [2], section 5.7.6.3) when sending this feedback. This is necessary to ensure that a context does not erroneously become a candidate for later use as a base context for replication [3]. 6. ROHC-TCP - TCP/IP Compression (Profile 0x0006) This section describes a ROHC profile for TCP/IP compression. The profile identifier for ROHC-TCP is 0x0006. 6.1. Profile-specific Encoding Methods This section defines encoding methods that are specific to this profile. These methods are used in the formal definition of the packet formats in section 8. 6.1.1. inferred_mine_header_checksum() This encoding method compresses the minimal encapsulation header checksum. This checksum is defined in RFC 2004 [25] as follow: Header Checksum The 16-bit one's complement of the one's complement sum of all 16-bit words in the minimal forwarding header. For purposes of computing the checksum, the value of the checksum field is 0. The IP header and IP payload (after the minimal forwarding header) are not included in this checksum computation. The "inferred_mine_header_checksum()" encoding method compresses the minimal encapsulation header checksum down to a size of zero bit, i.e. no bits are transmitted in compressed headers for this field. Using this encoding method, the decompressor infers the value of this field using the above computation. Pelletier, et. al [Page 18] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 6.1.2. inferred_ip_v4_header_checksum() This encoding method compresses the header checksum field of the IPv4 header. This checksum is defined in RFC 791 [5] as follows: Header Checksum: 16 bits A checksum on the header only. Since some header fields change (e.g., time to live), this is recomputed and verified at each point that the internet header is processed. The checksum algorithm is: The checksum field is the 16 bit one's complement of the one's complement sum of all 16 bit words in the header. For purposes of computing the checksum, the value of the checksum field is zero. The "inferred_ip_v4_header_checksum()" encoding method compresses the IPv4 header checksum down to a size of zero bit, i.e. no bits are transmitted in compressed headers for this field. Using this encoding method, the decompressor infers the value of this field using the above computation. 6.1.3. inferred_ip_v4_length() This encoding method compresses the total length field of the IPv4 header. The total length field of the IPv4 header is defined in RFC 791 [5] as follows: Total Length: 16 bits Total Length is the length of the datagram, measured in octets, including internet header and data. This field allows the length of a datagram to be up to 65,535 octets. The "inferred_ip_v4_length()" encoding method compresses the IPv4 header checksum down to a size of zero bit, i.e. no bits are transmitted in compressed headers for this field. Using this encoding method, the decompressor infers the value of this field by counting in octets the length of the entire packet after decompression. 6.1.4. inferred_ip_v6_length() This encoding method compresses the payload length field in the IPv6 header. This length field is defined in RFC 2460 [9] as follow: Payload Length: 16-bit unsigned integer Pelletier, et. al [Page 19] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 Length of the IPv6 payload, i.e., the rest of the packet following this IPv6 header, in octets. (Note that any extension headers present are considered part of the payload, i.e., included in the length count.) The "inferred_ip_v6_length()" encoding method compresses the payload length field of the IPv6 header down to a size of zero bit, i.e. no bits are transmitted in compressed headers for this field. Using this encoding method, the decompressor infers the value of this field by counting in octets the length of the entire packet after decompression. 6.1.5. inferred_offset() This encoding method compresses The inferred_offset encoding method is used on the data offset field of the TCP header. This field is defined in RFC 793 as: Data Offset: 4 bits The number of 32 bit words in the TCP Header. This indicates where the data begins. The TCP header (even one including options) is an integral number of 32 bits long. The "inferred_offset()" encoding method compresses the data offset field of the TCP header down to a size of zero bit, i.e. no bits are transmitted in compressed headers for this field. Using this encoding method, the decompressor infers the value of this field by first decompressing the TCP options list, and by then setting data offset = (options length / 4) + 5. 6.1.6. Scaled TCP Sequence Number Encoding On some TCP streams, such as data transfers, the payload size will be constants over periods of time. For such streams, the TCP sequence number is bound to increase by multiples of the payload size between packets. ROHC-TCP provides a method to use scaled compression of the TCP sequence number to improve compression efficiency in such case. When scaling the TCP sequence number, the residue is the sequence number offset from a multiple of the payload size. The precondition for the compressor to start using this type of encoding is that the compressor must be confident that the decompressor has received a number of packets sufficient to establish the value of the residue of the scaling function. This confidence can be established by sending a number of packets that are compressed using an unscaled representation of the sequence Pelletier, et. al [Page 20] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 numbers, when the payload size is constant. The compressor can then start using the scaled sequence number encoding, where the sequence number is first downscaled by the value of the payload size and then LSB encoded. Packets incoming to the compressor for which the value of the residue is different than the one that has previously been established MUST be sent in a compressed packet that carry the sequence number compressed using its unscaled representation, until a stable residue value can once again be established at the decompressor. Note that when the sequence number wraps around, the value of the residue of the scaling function is likely to change, even when the payload size remains constant. When this occurs, the compressor MUST reestablish the new residue value using the unscaled representation of the sequence number as described above. Note also that the scaling function applied to the TCP sequence number does not use an explicit scaling factor, such as the TS_STRIDE used in RFC 3095 [2]. Instead, the payload size is used as the scaling factor; as this value can be inferred from the length of the packet, there is no need to transmit this field explicitly. The expressions for compressing and decompressing the scaled sequence number are specified in the definitions of the packet format. 6.1.7. Scaled Acknowledgement Number Encoding Similar to the pattern exhibited by sequence numbers, the expected increase in the TCP Acknowledgment number will often be a multiple of the packet size. For the Sequence Number, the compression scheme can use the payload size of the packets as a scaling factor (see section 6.1.6 above). For the Acknowledgement Number, the scaling factor depends on the size of packets flowing in the opposite direction; this information might not be available to the compressor/decompressor pair. For this reason, ROHC-TCP uses an explicit scaling factor to compress the TCP Acknowledgement Number. For the compressor to use the scaled acknowledgement number encoding, it MUST first explicitly transmit the value of the scaling factor (ack_stride) to the decompressor, using one of the packet types that can carry this information. Once the value of the scaling factor is established, before using this scaled encoding the compressor must have enough confidence that the decompressor has successfully calculated the residue of the scaling function for the acknowledgement number. This is done the same way as for the scaled sequence number encoding (see section 6.1.6 above). Pelletier, et. al [Page 21] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 Once the compressor has gained enough confidence that both the value of the scaling factor and the value of the residue have been established in the decompressor, the compressor can start compress packets using the scaled representation of the Acknowledgement Number. The compressor MUST NOT use the scaled acknowledgement number encoding with the value of the scaling factor (ack_stride) set to zero. The compressor MAY use the scaled acknowledgement number encoding; what value it will use as the scaling factor is up to the compressor implementation. In the case where there is a co-located decompressor processing packets of the same TCP flow in the opposite direction, the scaling factor for the acknowledgement numbers can be set to the same value as the scaling factor of the sequence numbers used for that flow. 6.2. Considerations for the Feedback Channel The ROHC-TCP profile may be used in environments with or without feedback capabilities from decompressor to compressor. ROHC-TCP however assumes that if a ROHC feedback channel is available and is used at least once by the decompressor, this channel will be present during the entire compression operation. Otherwise, if the connection is broken and the channel disappears, header compression should be restarted. To parallel RFC 3095 [2], this is similar to allowing only one mode transition per compressor: from the initial unidirectional mode to the bi-directional mode of operation, with the transition being triggered by the reception of the first packet containing feedback from the decompressor. This effectively means that ROHC-TCP does not explicitly define any operational modes. 6.3. Control Fields in the ROHC-TCP Context A control field is a field that is transmitted from the compressor to the decompressor, but is not part of the uncompressed header. Values for control fields can be set up in the context of both the compressor and the decompressor. In ROHC-TCP, a number of control fields are used by the decompressor in its interpretation of the packet formats for packets received from the compressor. These control fields are not a part of the uncompressed header, but are explicitly transmitted inside ROHC-TCP packets. Once established at the decompressor, the values of these fields should be kept until updated by another packet. Pelletier, et. al [Page 22] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 6.3.1. Master Sequence Number (MSN) Feedback packets of types ACK and NACK carry information about sequence number or acknowledgement number from decompressor to compressor. Unfortunately, there is no guarantee that sequence number and acknowledgement number fields will be used by every IP protocol stack. In addition, the combined size of the sequence number field and the acknowledgement number field is rather large, and they can therefore not be carried efficiently within the feedback packet. To overcome this problem, ROHC-TCP introduces a control field called the Master Sequence Number (MSN) field. The MSN field is created at the compressor, rather than using one of the fields already present in the uncompressed header. The compressor increments the value of the MSN by one for each packet that it sends. The MSN field has the following two functions: 1. Differentiating between packets when sending feedback data. 2. Inferring the value of incrementing fields such as the IP-ID. The MSN field is present in every packets sent by the compressor. The MSN is LSB encoded within the CO packets, and the 16-bit MSN is sent in full in IR/IR-DYN packets. The decompressor always sends the MSN as part of the feedback information. The compressor can later use the MSN to infer which packet the decompressor is acknowledging. When the MSN is initialized, it is initialized to a random value. The compressor should only initialize a new MSN for the initial IR or IR- CR packet sent for a CID that corresponds to a context that is not already associated with this profile. In other words, if the compressor reuses the same CID to compress many TCP flows one after the other, the MSN is not reinitialized but rather continues to increment monotonously. For context replication, the compressor does not use the MSN of the base context when sending the IR-CR packet, unless the replication process overwrites the base context (i.e. BCID == CID). Instead, the compressor uses the value of the MSN if it already exists in the context being associated with the new flow (CID); otherwise, the MSN is initialized to a new value. 6.3.2. IP-ID Behavior The IP-ID field of the Ipv4 header can have different change patterns. RFC 3095 [2] describes three behaviors: sequential (NBO), sequential byteswapped, and random (RND). In addition, this profile uses a fourth behavior, the constant zero IP-ID behavior as defined in RFC 3843 [12] (SID). Pelletier, et. al [Page 23] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 The compressor monitors changes in the value of the IP-ID field for a number of packets, to identify which one of the above listed behavior is the closest match to the observed change pattern. The compressor can then select packet formats based the identified field behavior. If more than one level of IP headers is present, ROHC-TCP can assign a sequential behavior (NBO or byteswapped) only to the IP-ID of innermost IP header. This is because only this IP-ID is likely to have a close correlation with the MSN (see also section 6.3.1). Therefore, a compressor MUST assign either the constant zero IP-ID or the random behavior to tunneling headers. The IP-ID behavior control fields are transmitted in certain packet formats, as a two-bit field and for each IP header. When the compressor sends compressed packets, this control field is used to determine which set of packet formats will be used. Note these control fields are also used to determine the contents of the irregular chain item for each IP header. 6.3.3. Explicit Congestion Notification (ECN) in ROHC-TCP When ECN is used once on a stream, it can be expected that ECN bits will be change quite often. ROHC-TCP maintains a control field in the context to indicate if ECN is used or not. This control field is transmitted in the dynamic chain of the TCP header, and its value can be updated using specific compressed headers carrying a 7-bit CRC. When this control field indicates that ECN is being used, items of IP and TCP headers in the irregular chain will include bits used for ECN. To preserve octet-alignment, all of the TCP reserved bits are transmitted and, for outer IP headers, the entire TOS/TC field is included in the irregular chain. The design rationale behind this is the possible use of the "full- functionality option" of section 9.1 of RFC 3168 [23]. 6.4. CRC Calculations The 3-bit and 7-bit CRCs both cover the entire uncompressed header chain. Note that there is no division between CRC-STATIC or CRC- DYNAMIC fields in ROHC-TCP, as opposed to profiles defined in [2]. 6.5. Initialization The static context of ROHC TCP streams can be initialized in either two ways: Pelletier, et. al [Page 24] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 1) By using an IR packet as in section 7.3, where the profile is six (6) and the static chain ends with the static part of a TCP header. 2) By replicating an existing context using the mechanism defined by ROHC-CR. This is done with the IR-CR packet defined in section 7.4, where the profile number is six (6). 7. Packet Types ROHC-TCP defines two different packet types: the Initialization and Refresh (IR) packet type, and the Compressed packet type (CO). Each type corresponds to one of the possible states of the compressor. Each packet type also defines a number of packet formats: [TBD] packet formats are defined for compressed headers (CO), and two for initialization and refresh (IR). Finally, the profile-specific part of the IR-CR packet [3] is also defined in this section. 7.1. Compressed Header Chains Some packet types use one or more chains containing sub-header information. The function of a chain is to group items based on similar characteristics, i.e. grouping fields that either are static, dynamic or irregular in behavior. Chaining is done by appending each item to the chain in their order of appearance in the original header, starting from the fields in the outermost header. Static chain: The static chain is consists of one item for each header of the chain of headers to be compressed, starting from the outermost IP header and ending with a TCP header. In the formal description of the packet formats, this static chain item for each header type is named format__static. Dynamic chain: The dynamic chain consists of one item for each header of the chain of headers to be compressed, starting from the outermost IP header and ending with a TCP header. It should be noted that the dynamic chain item for the TCP header also contains a compressed list of TCP options (see section 7.2). In the formal description of the packet formats, this dynamic chain item for each header type is named format__dynamic. Pelletier, et. al [Page 25] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 Replicate chain: The replicate chain consists of one item for each header in the header chain to be compressed, starting from the outermost IP header and ending with a TCP header. It should be noted that the replicate chain item for the TCP header also contains a compressed list of TCP options (see section 7.2). In the formal description of the packet formats, this replicate chain item for each header type is named format__replicate. Header fields that are not present in the replicate chain are replicated from the base context. Irregular chain: The structure of the irregular chain is analogous to the structure of the static chain. For each compressed packet, the irregular chain is appended at the specified location in the general format of the compressed packets as defined in section 7.5. This chain also includes the irregular chain items for TCP options as defined in section 7.2.7. Note that the format of the irregular chain for the innermost IP header differs from the format of outer IP headers, since this header is a part of the compressed base header. The name of the chain item for the innermost header is postfixed with_innermost_irregular, while the irregular chain item for outer IP headers is postfixed by_outer_irregular. The format of the irregular chain item for the outer IP headers also determined using a flag for TTL/Hoplimit; this flag can be present in some compressed base headers. 7.2. Compressing TCP Options with List Compression This section describes in details how list compression is applied to the TCP options. In the definition of the packet formats for ROHC-TCP, the most frequent type of TCP options are described. Each of these options has an uncompressed format, a format_[option_type]_list_item format and a format_[option_type]_irregular format, where [option_type] is the name of the actual field item in the option list. 7.2.1. List Compression The TCP options in the uncompressed packet can be structured as an ordered list, whose order and presence are most of the time constant between packets. The generic structure of such a list is as follows: Pelletier, et. al [Page 26] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 +--------+--------+--...--+--------+ list: | item 1 | item 2 | | item n | +--------+--------+--...--+--------+ The basic principles of list-based compression are the following: 1) When a context is being initialized, a complete representation of the compressed list of options is transmitted. All options that have any content are present in the compressed list of items sent to the decompressor. Then, once the context has been initialized: 2) When the structure AND the content of the list are not changing, no information about the list is sent in compressed headers. 3) When the structure of the list is constant, and when only the content of one or more options that are defined within the irregular format is changing, no information about the list needs to be sent in compressed headers; the irregular content is sent as part of the irregular chain (as described in section 7.2.7) in the generalcompressed packet format (section 7.5). 4) When the structure of the list changes, a compressed list is sent in the compressed header, including a representation of its structure and order. 7.2.2. Table-based Item Compression The Table-based item compression compresses individual items sent in compressed lists. The compressor assigns a unique identifier, "Index", to each item "Item" of a list. Compressor Logic The compressor conceptually maintains an Item Table containing all items, indexed using "Index". The (Index, Item) pair is sent together in compressed lists until the compressor gains enough confidence that the decompressor has observed the mapping between items and their respective index. Confidence is obtained from the reception of an acknowledgment from the decompressor, or by sending L (Index, Item) pairs (not necessarily consecutively). The value for L is maintained by the compressor. Once confidence is obtained, the index alone is sent in compressed lists to indicate the presence of the item corresponding to this index. The compressor may reassign an existing index to a new item, by re-establishing the mapping using the procedure described above. Pelletier, et. al [Page 27] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 Decompressor Logic The decompressor conceptually maintains an Item Table that contains all (Index, Item) pairs received. The Item Table is updated whenever an (Index, Item) pair is received and decompression is successfully verified using the CRC. The decompressor retrieves the item from the table whenever an Index without an accompanying Item is received. 7.2.3. Item Tables Compressor Logic The compressor uses the following structure to represent an entry in the Item Table: +-------+------+---------+--------------------------+ Index i | Known | Item | Counter | MSN_1, MSN_2, ..., MSN_L | +-------+------+---------+--------------------------+ The flag "Known" indicates whether the mapping between Index i and Item has been established, i.e., if Index i can be sent in compressed lists without its corresponding Item. The "Counter" field is useful to obtain confidence that the context at the decompressor contains the (Index, Item) pair. The list of sequence numbers, [MSN 1, ..., MSN L], is useful in relating an acknowledgment received from the decompressor with the (Index, Item) pair, meaning that it is now part of the decompressor context. The flag "Known" is initially set to a value of zero. It is also set to zero whenever Index i is assigned to a new Item. "Known" is set to a non-zero value when either of the following occur: a) The corresponding (Index, Item) pair is acknowledged; b) Counter >= L (confidence based of the optimistic approach). When the compressor sets the flag "Known", the list of sequence numbers can be discarded. Decompressor Logic The decompressor uses the following structure to represent an entry in the Item Table: +-------+------+ Index i | Known | item | +-------+------+ Pelletier, et. al [Page 28] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 The flag "Known" is initially set to a value of zero. "Known" is set to a non-zero value when the decompressor receives an (Index, Item) pair and inserts the item "Item" into the table at position "Index". If an index without an accompanying item is received for which the value of the "Known" flag is zero, the header MUST be discarded and a NACK SHOULD be sent. 7.2.4. Constraints to List Compression List compression, as defined in the ROHC-FN [4], allows 7-bit indexes to be used in the Item table. For ROHC-TCP, the compressor MUST use the low-order 4 bits of the item count (i.e. large_xi of [4], section 5.5.5) to describe an index. In other words, the compressor MUST NOT map items with indexes larger than a value of 15. This is because no more than 16 different options are expected to be used in a TCP flow. 7.2.5. Item Table Mappings The mapping between TCP option type and table indexes are listed in the table below: +-----------------+---------------+ | Option name | Table index | +-----------------+---------------+ | NOP | 0 | | EOL | 1 | | MSS | 2 | | WINDOW SCALE | 3 | | TIMESTAMP | 4 | | SACK-PERMITTED | 5 | | SACK | 6 | | Generic options | 7-15 | +-----------------+---------------+ Some TCP options are used more frequently than others. To simplify their compression, a part of the item table is reserved for these option types, as shown on the table above. The decompressor MUST use these mappings between item and indexes to decompress TCP options compressed using list compression. The compressor can thus omit from the compressed packet format an option type that corresponds to a reserved item in the item table. This is because the type of the option can be known based on the index number. It is expected that the option types for which an index is reserved in the item table will only appear once in a list. However, if an Pelletier, et. al [Page 29] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 option type is detected twice in the same options list and if both options have a different content, the compressor should compress the second occurrence of the option type by mapping it to a generic compressed option. Otherwise, if the options have the exact same content, the compressor can still use the same table index for both. The NOP option The NOP option can appear more than once in the list. However, since its value is always the same, no context information needs to be transmitted. Multiple NOP options can thus be mapped to the same index. Since the NOP option does not have any content when compressed as a list_item, it will never be present in the item list. For consistency, the compressor should still establish an entry in the list by setting the presence bit, as done for the other type of options. The EOL option The size of the compressed format for the EOL option can be larger than one octet, and it is defined so that it includes the option padding. This is because the EOL should terminate the parsing of the options, but it can also be followed by padding octets that all have the value zero. The Generic option The generic option can be used to compress any type of TCP option that do not have a reserved index in the item table. 7.2.6. Compressed Lists in Dynamic Chain When a compressed list for TCP options is part of the dynamic chain (i.e. IR or IR-DYN packets), the compressed list must have all its list items present, i.e. all x-bits in the XI list must be set. 7.2.7. Irregular Chain Items for TCP Options The list_item represents the option inside the compressed item list, and the irregular format is used for the option fields that are expected to change with each packet. When an item of the specified type is present in the current context, these irregular fields are present in each compressed packet, as part of the irregular chain. Since many of the TCP option types are expected to stay static for the duration of a flow, many of the irregular_formats are empty. The irregular chain for TCP options is structured analogously to the structure of the current TCP options in the uncompressed packet. If a compressed type 0 list is present in the compressed packet, then the Pelletier, et. al [Page 30] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 irregular chain for TCP options MUST NOT contain irregular items for the list items that are transmitted inside the compressed list (i.e. items in the list that have the x-bit set in its xi). The items that are not present in the compressed list, but are present in the current list, MUST have their respective irregular items present in the irregular chain. 7.2.8. Replication of TCP Options The entire table of TCP options items is always replicated when using the IR-CR packet. In the IR-CR packet, the current list of options for the new flow is also transmitted as a generic compressed list in the IR-CR packet. 7.3. Initialization and Refresh Packets (IR) ROHC-TCP uses the basic structure of the ROHC IR and IR-DYN packets as defined in [2] (section 5.2.3. and 5.2.4. respectively). The 8-bit CRC is computed according to section 5.9.1 of [2]. o Packet type: IR This packet type communicates the static part and the dynamic part of the context. For the ROHC-TCP IR packet, the value of the x bit must be set to one. It has the following format: 0 1 2 3 4 5 6 7 --- --- --- --- --- --- --- --- : Add-CID octet : if for small CIDs and (CID != 0) +---+---+---+---+---+---+---+---+ | 1 1 1 1 1 1 0 1 | IR type octet +---+---+---+---+---+---+---+---+ : : / 0-2 octets of CID / 1-2 octets if for large CIDs : : +---+---+---+---+---+---+---+---+ | Profile | 1 octet +---+---+---+---+---+---+---+---+ | CRC | 1 octet +---+---+---+---+---+---+---+---+ | | / Static chain / variable length | | - - - - - - - - - - - - - - - - | | / Dynamic chain / variable length | | Pelletier, et. al [Page 31] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 - - - - - - - - - - - - - - - - | | / Payload / variable length | | - - - - - - - - - - - - - - - - CRC: 8-bit CRC, computed according to section 5.9.1 of [2]. Static chain: See section 7.1. Dynamic chain: See section 7.1. Payload: The payload of the corresponding original packet, if any. The presence of a payload is inferred from the packet length. o Packet type: IR-DYN This packet type communicates the dynamic part of the context. The ROHC-TCP IR-DYN packet has the following format: 0 1 2 3 4 5 6 7 --- --- --- --- --- --- --- --- : Add-CID octet : if for small CIDs and (CID != 0) +---+---+---+---+---+---+---+---+ | 1 1 1 1 1 0 0 0 | IR-DYN type octet +---+---+---+---+---+---+---+---+ : : / 0-2 octets of CID / 1-2 octets if for large CIDs : : +---+---+---+---+---+---+---+---+ | Profile | 1 octet +---+---+---+---+---+---+---+---+ | CRC | 1 octet +---+---+---+---+---+---+---+---+ | | / Profile_Specific_Part / variable length | | - - - - - - - - - - - - - - - - | | / Payload / variable length | | - - - - - - - - - - - - - - - - CRC: 8-bit CRC, computed according to section 5.9.1 of [2]. Dynamic chain: See section 7.1. Payload: The payload of the corresponding original packet, if any. The presence of a payload is inferred from the packet length. Pelletier, et. al [Page 32] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 7.4. Context Replication Packets (IR-CR) Context replication requires a dedicated IR packet format that uniquely identifies the IR-CR packet for the ROHC-TCP profile. o Packet type: IR-CR This packet type communicates a reference to a base context along with the static and dynamic parts of the replicated context that differs from the base context. The ROHC-TCP IR-CR packet follows the general format of the ROHC CR packet, as defined in ROHC-CR [3], section 3.4.2. With consideration to the extensibility of the IR packet type defined in RFC 3095 [2], the ROHC-TCP profile supports context replication through the profile specific part of the IR packet. This is achieved using the bit (x) left in the IR packet header for "Profile specific information". For ROHC-TCP, this bit is defined as a flag indicating whether this packet is an IR packet or an IR-CR packet. For the ROHC-TCP IR-CR packet, the value of the x bit must be set to zero. The ROHC-TCP IR-CR has the following format: 0 1 2 3 4 5 6 7 --- --- --- --- --- --- --- --- : Add-CID octet : if for small CIDs and (CID != 0) +---+---+---+---+---+---+---+---+ | 1 1 1 1 1 1 0 0 | IR-CR type octet +---+---+---+---+---+---+---+---+ : : / 0-2 octets of CID / 1-2 octets if for large CIDs : : +---+---+---+---+---+---+---+---+ | Profile | 1 octet +---+---+---+---+---+---+---+---+ | CRC | 1 octet +---+---+---+---+---+---+---+---+ | B | CRC7 | 1 octet +---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+ : Reserved | Base CID : 1 octet, for small CID, if B=1 +---+---+---+---+---+---+---+---+ : : / Base CID / 1-2 octets, for large CIDs, if B=1 : : +---+---+---+---+---+---+---+---+ | | | Replicate chain / variable length | | - - - - - - - - - - - - - - - - Pelletier, et. al [Page 33] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 | | / Payload / variable length | | - - - - - - - - - - - - - - - - B: B = 1 indicates that the Base CID field is present. CRC7: The CRC over the original, uncompressed, header. This 7-bit CRC is computed according to section 3.4.1.1 of [3]. Replicate chain: See section 7.1. Payload: The payload of the corresponding original packet, if any. The presence of a payload is inferred from the packet length. 7.5. Compressed Packets (CO) The ROHC-TCP CO packets communicate irregularities in the packet header. All CO packets carry a CRC and can update the context. The general format for a compressed TCP header is as follows: 0 1 2 3 4 5 6 7 --- --- --- --- --- --- --- --- : Add-CID octet : if for small CIDs and CID 1-15 +---+---+---+---+---+---+---+---+ | 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 octets +---+---+---+---+---+---+---+---+ : : / Irregular Chain / variable : : --- --- --- --- --- --- --- --- : : / TCP Options Irregular Part / variable : : --- --- --- --- --- --- --- --- The base header in the figure above is the compressed representation of the innermost IP header and the TCP header in the uncompressed packet. The full set of base headers are described in section 8.8. Irregular chain: See section 7.1. TCP options irregular part: See section 7.2.7. Pelletier, et. al [Page 34] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 8. Packet Formats This section describes the set of compressed TCP/IP packet formats. The normative description of the packet formats is given using a formal notation, the ROHC-FN [4]. The formal description of the packet formats specifies all of the information needed to compress and decompress a header relative to the context. In particular, the notation provides a list of all the fields present in the uncompressed and compressed TCP/IP headers, and defines how to map from each uncompressed packet to its compressed equivalent and vice versa. See the ROHC-FN [4] for an explanation of the formal notation itself, and the encoding methods used to compress each of the fields in the TCP/IP header. Note that the formal definition of the packet formats for ROHC-TCP includes comments that follow a specific syntax. These comments, called annotations, make use of square brackets as delimiters; numbers in between the "[" and the "]" are used to provide additional information about the expected number of bits for the field(s) that appears as a right-hand operand. These are not normative in any way. 8.1. Design rationale for compressed base headers The compressed packet formats are defined as two separate sets: one set for the packets where the innermost IP header contains a sequential IP-ID (either network byte order or byte swapped), and one set for the packets without sequential IP-ID (either random, zero, or no IP-ID). These two sets of packet formats are referred to as the "sequential" and the "random" set of packet format. In addition, there is a common compressed packet that can be used regardless of the type of IP-ID behaviour. This common packet can transmit rarely changing fields and also send the frequently changing field coded in variable lengths. The common packet format can also change the value of control fields such as IP-ID and ECN behaviour. All compressed base headers contain a 3-bit CRC, unless they update control fields such as "ip_id_behavior" or "ecn_used" that affect the interpretation of subsequent packets. Packets that can modify these control fields will carry a 7-bit CRC instead. The encoding methods used in the compressed base headers are based on the following design criteria: o MSN Since the MSN is a number generated by the compressor, it only Pelletier, et. al [Page 35] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 needs to be large enough to ensure robust operation and to accommodate a small amount of reordering. Therefore, each compressed base header contains 4 bits of MSN. To handle reordering, the LSB offset value is set to p=4. o Sequence number For ROHC-TCP compression to have the capability to handle bulk data transfers efficiently, and for such connections, the sequence number is expected to increase by about 1460 bytes (which can be represented by 11 bits). For the compressed base headers to handle retransmissions (i.e. negative delta to the sequence number), the LSB interpretation interval must handle negative offsets about as large as positive offset, which means that one more bit is needed. Also, for ROHC-TCP to be robust to losses, two additional bits are added to the LSB encoding of the sequence number. This means that the base headers should contain at least 14 bits of LSB-encoded sequence number when present. According to the logic above, the LSB offset value p, is set to be as large as the positive offset, i.e. p=2^(k-1)-1. o Acknowledgement number The design criterion for the acknowledgement number is similar to that of the sequence number. However, often only every other data packet is acknowledged, which means that the expected delta value is twice as large as for sequence numbers. Therefore, at least 15 bits of acknowledgement number should be used in compressed base headers. Since the acknowledgement number is expected to constantly increase, and the only exception to this is packet reordering (either on link or pre-link), the negative offset for LSB encoding is set to be 25% of the total interval, i.e. p=2^(k-2)-1. The offset value p has been set the same way as for the sequence number (p=2^(k-1)-1). o Window The TCP window field is expected to increase in increments of similar size as the sequence number, and therefore the design criterion for the TCP window has been to send at least 14 bits when used. o IP-ID For the "sequential" set of packet formats, all the compressed base headers contains LSB encoded IP-ID offset bits. The requirement is that at least 3 bits of IP-ID should always be present, but it is preferable to use 4 to 7 bits. When k=3, p=1 and if k>3, then p=3 since the offset is expected to increase most of the time. Pelletier, et. al [Page 36] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 Each set of packet formats contains 10 different compressed base headers. The reason for having this large number of packets is that the TCP sequence number, TCP acknowledgement number, TCP window and MSN are frequently changing in a non-linear pattern. All of the compressed base headers transmit a LSB-encoded MSN, the push flag and a CRC, and in addition to this, all the base headers in the sequential packet format set contains LSB encoded IP-ID. The following packet formats exist in both the sequential and random packet format sets: - Format 1: This packet format transmits changes to the TCP sequence number and its principal use should be on the downstream of a data transfer. - Format 2: This packet format transmits the TCP sequence number in scaled form, and will normally be used on the downstream of a data transfer where the payload size is constant for multiple packets. - Format 3: This packet format transmits changes in the TCP acknowledgement number, and will be used in the acknowledgement direction of data transfer. - Format 4: This packet format is similar to format 3, but sends scaled Acknowledgement number. - Format 5: This packet format transmits both the TCP sequence number and the acknowledgement number, and should be particularly useful for streams that send data in both directions. - Format 6: This packet format is similar to format 5, but sends the sequence number in scaled form, when the payload size is static for certain intervals in a data stream. - Format 7: This packet format transmits changes to both the TCP sequence number and the TCP window, and is expected to be useful for any type of data transfer. - Format 8: This packet format transmits changes to both the TCP acknowledgement number and the TCP window, and is expected to be useful for the acknowledgement streams of data connections. - Format 9: This packet format is similar to format 7, but sends the sequence number in scaled form to allow higher compression rates on streams with a constant payload size, - Format 10: This packet format is used to transmit changes to some of the more seldom changing fields in the streams, such as ECN behaviour, Reset/SYN/FIN flags, the TTL/Hop Limit and the TCP options list. This format carries a 7-bit CRC, since it can change the contents of the irregular chain in later packets. Note that Pelletier, et. al [Page 37] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 this can be seen as a reduced form of the common packet format. - Common packet format: The common packet format can be used for all kinds of IP-ID behaviour, and should be used when some of the more rarely changing fields in the IP or TCP header changes. Since this packet format can be used to change what set of packet formats is to be used for future packets, it carries a 7-bit CRC to reduce the probability of context corruption. This packet can basically change all the dynamic fields in the IP and TCP header, and it uses a large set of flags to control which fields that are present in the packet. 8.2. Global Control Fields control_fields = ecn_used, %[ 1 ] msn, %[ 16 ] ip_inner_ecn, %[ 2 ] seq_number_scaled, %[ 32 ] seq_number_residue, %[ 32 ] ack_stride, %[ 16 ] ack_number_scaled, %[ 16 ] ack_number_residue; %[ 16 ] 8.3. General Structures static_or_irreg32(flag) === { uc_format = field; %[ 32 ] co_format_irreg_enc = field, %[ 32 ] { let (flag == 1); field ::= irregular(32); }; co_format_static_enc = field, %[ 0 ] { let (flag == 0); field ::= static; }; }; static_or_irreg16(flag) === { uc_format = field; %[ 16 ] co_format_irreg_enc = field, %[ 16 ] { let (flag == 1); Pelletier, et. al [Page 38] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 field ::= irregular(16); }; co_format_static_enc = field, %[ 0 ] { let (flag == 0); field ::= static; }; }; static_or_irreg8(flag) === { uc_format = field; %[ 8 ] co_format_irreg_enc = field, %[ 8 ] { let (flag == 1); field ::= irregular(8); }; co_format_static_enc = field, %[ 0 ] { let (flag == 0); field ::= static; }; }; variable_length_32_enc(flag) === { uc_format = field; %[ 32 ] co_format_not_present = field, %[ 0 ] { let(flag == 0); field ::= static; }; co_format_8_bit = field, %[ 8 ] { let(flag == 1); field ::= lsb(8, 63); }; co_format_16_bit = field, %[ 16 ] { let(flag == 2); field ::= lsb(16, 16383); }; co_format_32_bit = field, %[ 32 ] { let(flag == 3); field ::= irregular(32); }; }; Pelletier, et. al [Page 39] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 variable_length_16_enc(flag) === { uc_format = field; %[ 16 ] co_format_not_present = field, %[ 0 ] { let(flag == 0); field ::= static; }; co_format_8_bit = field, %[ 8 ] { let(flag == 1); field ::= lsb(8, 63); }; co_format_16_bit = field, %[ 16 ] { let(flag == 2); field ::= irregular(16); }; }; optional32 (flag) === { uc_format = item; % 0 or 32 bits co_format_present = item, %[ 32 ] { let (flag == 1); item ::= irregular (32); }; co_format_not_present = item, %[ 0 ] { let (flag == 0); item ::= compressed_value (0, 0); }; }; lsb_7_or_31 === { uc_format = item; % 7 or 31 bits co_format_lsb_7 = discriminator, %[ 1 ] item, %[ 7 ] { discriminator ::= '0'; item ::= lsb (7, 8); }; Pelletier, et. al [Page 40] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 co_format_lsb_31 = discriminator, %[ 1 ] item, %[ 31 ] { discriminator ::= '1'; item ::= lsb (31, 256); }; }; opt_lsb_7_or_31 (flag) === { uc_format = item; % 32 bits co_format_present = item, % 8 or 32 bits { let (flag == 1); item ::= lsb_7_or_31; }; co_format_not_present = item, %[ 0 ] { let (flag == 0); item ::= compressed_value (0, 0); }; }; crc3 (data_value, data_length) === { uc_format = ; co_format = crc_value, %[ 3 ] { crc_value ::= crc(3, 0x06, 0x07, data_value, data_length); }; }; crc7 (data_value, data_length) === { uc_format = ; co_format = crc_value, %[ 7 ] { crc_value ::= crc(7, 0x79, 0x7f, data_value, data_length); }; }; Pelletier, et. al [Page 41] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 8.4. Extension Headers 8.4.1. IPv6 DEST opt header ip_dest_opt === { uc_format = next_header, %[ 8 ] length, %[ 8 ] value; % n bits default_methods = { next_header ::= static; length ::= static; value ::= static; }; co_format_dest_opt_static = next_header, %[ 8 ] length, %[ 8 ] { next_header ::= irregular(8); length ::= irregular(8); }; co_format_dest_opt_dynamic = value, % n bits { value ::= irregular(length:uncomp_value * 64 + 48); }; co_format_dest_opt_replicate_0 = discriminator, %[ 8 ] { discriminator ::= '00000000'; }; co_format_dest_opt_replicate_1 = discriminator, %[ 8 ] length, %[ 8 ] value, % n bits { discriminator ::= '10000000'; length ::= irregular(8); value ::= irregular(length:uncomp_value * 64 + 48); }; }; Pelletier, et. al [Page 42] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 8.4.2. IPv6 HOP opt header ip_hop_opt === { uc_format = next_header, %[ 8 ] length, %[ 8 ] value; % n bits default_methods = { next_header ::= static; length ::= static; value ::= static; }; co_format_hop_opt_static = next_header, %[ 8 ] length, %[ 8 ] { next_header ::= irregular(8); length ::= irregular(8); }; co_format_hop_opt_dynamic = value, % n bits { value ::= irregular(length:uncomp_value * 64 + 48); }; co_format_hop_opt_replicate_0 = discriminator, %[ 8 ] { discriminator ::= '00000000'; }; co_format_hop_opt_replicate_1 = discriminator, %[ 8 ] length, %[ 8 ] value, % n bits { discriminator ::= '10000000'; length ::= irregular(8); value ::= irregular(length:uncomp_value * 64 + 48); }; }; 8.4.3. IPv6 Routing Header ip_rout_opt === { uc_format = next_header, %[ 8 ] length, %[ 8 ] value; % n bits Pelletier, et. al [Page 43] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 default_methods = { next_header ::= static; length ::= static; value ::= static; }; co_format_rout_opt_static = next_header, %[ 8 ] length, %[ 8 ] value, % n bits { next_header ::= irregular(8); length ::= irregular(8); value ::= irregular(length:uncomp_value * 64 + 48); }; co_format_rout_opt_dynamic = { }; co_format_rout_opt_replicate_0 = discriminator, %[ 8 ] { discriminator ::= '00000000'; }; co_format_rout_opt_replicate_1 = discriminator, %[ 8 ] length, %[ 8 ] value, % n bits { discriminator ::= '10000000'; length ::= irregular(8); value ::= irregular(length:uncomp_value * 64 + 48); }; }; 8.4.4. GRE Header optional_checksum (flag_value) === { uc_format = value, % 0 or 16 bits reserved1; % 0 or 16 bits co_format_cs_present = value, %[ 16 ] reserved1, %[ 0 ] { let (flag_value == 1); value ::= irregular (16); reserved1 ::= uncompressed_value (16, 0); }; Pelletier, et. al [Page 44] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 co_format_not_present = value, %[ 0 ] reserved1, %[ 0 ] { let (flag_value == 0); value ::= compressed_value (0, 0); reserved1 ::= compressed_value (0, 0); }; }; gre_proto === { uc_format = protocol; %[ 16 ] default_methods = { }; co_format_ether_v4 = discriminator, %[ 1 ] { discriminator ::= compressed_value (1, 0); protocol ::= uncompressed_value (16, 0x0800); }; co_format_ether_v6 = discriminator, %[ 1 ] { discriminator ::= compressed_value (1, 1); protocol ::= uncompressed_value (16, 0x86DD); }; }; gre === { uc_format = c_flag, %[ 1 ] r_flag, %[ 1 ] k_flag, %[ 1 ] s_flag, %[ 1 ] reserved0, %[ 9 ] version, %[ 3 ] protocol, %[ 16 ] checksum_and_res, % 0 or 32 bits key, % 0 or 32 bits sequence_number; % 0 or 32 bits default_methods = { c_flag ::= static; r_flag ::= static; k_flag ::= static; s_flag ::= static; reserved0 ::= uncompressed_value (9, 0); version ::= static; Pelletier, et. al [Page 45] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 protocol ::= static; key ::= static; checksum_and_res ::= optional_checksum (c_flag:uncomp_value); }; co_format_gre_static = protocol, %[ 1 ] c_flag, %[ 1 ] r_flag, %[ 1 ] k_flag, %[ 1 ] s_flag, %[ 1 ] version, %[ 3 ] key, % 0 or 32 bits { protocol ::= gre_proto; c_flag ::= irregular (1); r_flag ::= irregular (1); k_flag ::= irregular (1); s_flag ::= irregular (1); version ::= irregular (3); key ::= optional32 (k_flag:uncomp_value); sequence_number ::= static; }; co_format_gre_dynamic = checksum_and_res, % 0 or 16 bits sequence_number, % 0 or 32 bits { sequence_number ::= optional32 (s_flag:uncomp_value); }; co_format_gre_replicate_0 = discriminator, %[ 8 ] checksum_and_res, % 0 or 16 bits sequence_number, % 0, 8 or 32 bits { discriminator ::= '00000000'; sequence_number ::= opt_lsb_7_or_31 (s_flag:uncomp_value); }; co_format_gre_replicate_1 = discriminator, %[ 8 ] c_flag, %[ 1 ] r_flag, %[ 1 ] k_flag, %[ 1 ] s_flag, %[ 1 ] reserved, %[ 1 ] version, %[ 3 ] checksum_and_res, % 0 or 16 bits key, % 0 or 32 bits sequence_number, % 0 or 32 bits { discriminator ::= '10000000'; c_flag ::= irregular (1); Pelletier, et. al [Page 46] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 r_flag ::= irregular (1); k_flag ::= irregular (1); s_flag ::= irregular (1); reserved ::= '0'; version ::= irregular (3); key ::= optional32 (k_flag:uncomp_value); sequence_number ::= optional32 (s_flag:uncomp_value); }; co_format_gre_irregular = checksum_and_res, % 0 or 16 bits sequence_number, % 0, 8 or 32 bits { sequence_number ::= opt_lsb_7_or_31 (s_flag:uncomp_value); }; }; 8.4.5. MINE header mine === { uc_format = next_header, %[ 8 ] s_bit, %[ 1 ] res_bits, %[ 7 ] checksum, %[ 16 ] orig_dest, %[ 32 ] orig_src; % 0 or 32 bits default_methods = { next_header ::= static; s_bit ::= static; res_bits ::= static; checksum ::= inferred_mine_header_checksum; orig_dest ::= static; orig_src ::= static; }; co_format_mine_static = next_header, %[ 8 ] s_bit, %[ 1 ] res_bits, %[ 7 ] orig_dest, %[ 32 ] orig_src, % 0 or 32 bits { next_header ::= irregular (8); s_bit ::= irregular (1); res_bits ::= irregular (7); % include reserved - no benefit in removing them orig_dest ::= irregular (32); orig_src ::= optional32 (s_bit:uncomp_value); }; Pelletier, et. al [Page 47] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 co_format_mine_dynamic = { }; co_format_mine_replicate_0 = discriminator, %[ 8 ] checksum, %[ 0 ] { discriminator ::= '00000000'; }; co_format_mine_replicate_1 = discriminator, %[ 8 ] s_bit, %[ 1 ] res_bits, %[ 7 ] orig_dest, %[ 32 ] orig_src, % 0 or 32 bits { discriminator ::= '10000000'; s_bit ::= irregular (1); res_bits ::= irregular (7); orig_dest ::= irregular (32); orig_src ::= optional32 (s_bit:uncomp_value); }; }; 8.4.6. Authentication Header (AH) header ah === { uc_format = next_header, %[ 8 ] length, %[ 8 ] res_bits, %[ 16 ] spi, %[ 32 ] sequence_number, %[ 32 ] auth_data; % n bits default_methods = { next_header ::= static; length ::= static; res_bits ::= static; spi ::= static; sequence_number ::= static; auth_data ::= irregular (length:uncomp_value * 32 - 32); }; co_format_ah_static = next_header, %[ 8 ] length, %[ 8 ] spi, %[ 32 ] { next_header ::= irregular(8); Pelletier, et. al [Page 48] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 length ::= irregular (8); spi ::= irregular (32); }; co_format_ah_dynamic = res_bits, %[ 16 ] sequence_number, %[ 32 ] auth_data, % n bits { res_bits ::= irregular (16); sequence_number ::= irregular (32); }; co_format_ah_replicate_0 = discriminator, %[ 8 ] sequence_number, % 8 or 32 bits auth_data, % n bits { discriminator ::= '00000000'; sequence_number ::= lsb_7_or_31; }; co_format_ah_replicate_1 = discriminator, %[ 8 ] length, %[ 8 ] res_bits, %[ 16 ] spi, %[ 32 ] sequence_number, %[ 32 ] auth_data, % n bits { discriminator ::= '10000000'; length ::= irregular (8); res_bits ::= irregular (16); spi ::= irregular (32); sequence_number ::= irregular (32); }; co_format_ah_irregular = sequence_number, % 8 or 32 bits auth_data, % n bits { sequence_number ::= lsb_7_or_31; }; }; 8.4.7. Encapsulation Security Payload (ESP) header esp_null === { uc_format = spi, %[ 32 ] sequence_number, %[ 32 ] next_header; %[ 8 ] default_methods = Pelletier, et. al [Page 49] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 { spi ::= static; % Next header will always be present in the trailer part, % but sometimes it will ALSO be present in the header % (static chain only). nh_field ::= static; % Control field next_header ::= static; sequence_number ::= static; }; co_format_esp_static = nh_field, %[ 8 ] spi, %[ 32 ] { % identify next header assume next 96 bits skipped % to get to end of packet (i.e. this is anchored %from the end of the packet, not the start) nh_field ::= compressed_value(8, next_header:uncomp_value); next_header ::= irregular (8); % At packet end! spi ::= irregular (32); }; co_format_esp_dynamic = sequence_number, %[ 32 ] { sequence_number ::= irregular (32); }; co_format_esp_replicate_0 = discriminator, %[ 8 ] sequence_number, % 8 or 32 bits { discriminator ::= '00000000'; sequence_number ::= lsb_7_or_31; }; co_format_esp_replicate_1 = discriminator, %[ 8 ] spi, %[ 32 ] sequence_number, %[ 32 ] { discriminator ::= '10000000'; spi ::= irregular (32); sequence_number ::= irregular (32); }; co_format_esp_irregular = sequence_number, % 8 or 32 bits { sequence_number ::= lsb_7_or_31; }; }; Pelletier, et. al [Page 50] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 8.5. IP Header 8.5.1. Structures Common for IPv4 and IPv6 irreg_tos_tc === { uc_format = tos_tc; %[ 6 ] co_format_tos_tc_present = tos_tc, %[ 6 ] { let(ecn_used:uncomp_value == 1); tos_tc ::= irregular (6); }; co_format_tos_tc_not_present = tos_tc, %[ 0 ] { let(ecn_used:uncomp_value == 0); tos_tc ::= static; }; }; ip_irreg_ecn === { uc_format = ip_ecn_flags; %[ 2 ] co_format_tc_present = ip_ecn_flags, %[ 2 ] { let(ecn_used:uncomp_value == 1); ip_ecn_flags ::= irregular (2); }; co_format_tc_not_present = ip_ecn_flags, %[ 0 ] { let(ecn_used:uncomp_value == 0); ip_ecn_flags ::= static; }; }; 8.5.2. IPv6 Header fl_enc === { uc_format = flow_label; co_format_fl_zero = discriminator, %[ 1 ] flow_label, %[ 0 ] reserved, %[ 4 ] { discriminator ::= '0'; flow_label ::= uncompressed_value (20, 0); Pelletier, et. al [Page 51] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 reserved ::= '0000'; }; co_format_fl_non_zero = discriminator, %[ 1 ] flow_label, %[ 20 ] { discriminator ::= '1'; flow_label ::= irregular (20); }; }; % The argument flag should only be used if this % flag was set when processing a compressed base % header, if not, the flag should be zero. ipv6 (ttl_irregular_chain_flag) === { uc_format = version, %[ 4 ] tos_tc, %[ 6 ] ip_ecn_flags, %[ 2 ] flow_label, %[ 20 ] payload_length, %[ 16 ] next_header, %[ 8 ] ttl_hopl, %[ 8 ] src_addr, %[ 128 ] dst_addr; %[ 128 ] default_methods = { version ::= uncompressed_value (4, 6); tos_tc ::= static; ip_ecn_flags ::= static; flow_label ::= static; payload_length ::= inferred_ip_v6_length; next_header ::= static; ttl_hopl ::= static; src_addr ::= static; dst_addr ::= static; }; co_format_ipv6_static = version_flag, %[ 1 ] reserved, %[ 2 ] flow_label, % 5 or 21 bits next_header, %[ 8 ] src_addr, %[ 128 ] dst_addr, %[ 128 ] { version_flag ::= '1'; reserved ::= '00'; flow_label ::= fl_enc; next_header ::= irregular (8); src_addr ::= irregular(128); Pelletier, et. al [Page 52] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 dst_addr ::= irregular(128); }; co_format_ipv6_dynamic = tos_tc, %[ 6 ] ip_ecn_flags, %[ 2 ] ttl_hopl, %[ 8 ] { tos_tc ::= irregular (6); ip_ecn_flags ::= irregular (2); ttl_hopl ::= irregular (8); }; co_format_ipv6_replicate = tos_tc, %[ 6 ] ip_ecn_flags, %[ 2 ] { tos_tc ::= irregular (6); ip_ecn_flags ::= irregular (2); }; co_format_ipv6_outer_irregular_without_ttl = tos_tc, % 0 or 6 bits ip_ecn_flags, % 0 or 2 bits { % for 'outer' headers only, irregular chain is required tos_tc ::= irreg_tos_tc; ip_ecn_flags ::= ip_irreg_ecn; let(ttl_irregular_chain_flag == 0); }; co_format_ipv6_outer_irregular_with_ttl = tos_tc, % 0 or 6 bits ip_ecn_flags, % 0 or 2 bits ttl_hopl, %[ 8 ] { % for 'outer' headers only, irregular chain is required tos_tc ::= irreg_tos_tc; ip_ecn_flags ::= ip_irreg_ecn; let(ttl_irregular_chain_flag == 1); ttl_hopl ::= irregular(8); }; % Note that the ECN bits are stored in the global control field % so that they can be output in TCP irregular chain. co_format_ipv6_innermost_irregular = { let(ip_inner_ecn:uncomp_value == ip_ecn_flags:uncomp_value); }; }; Pelletier, et. al [Page 53] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 8.5.3. IPv4 Header ip_id_enc_dyn (behavior) === { uc_format = ip_id; %[ 16 ] co_format_ip_id_seq = ip_id, %[ 16 ] { let ((behavior == 0) || (behavior == 1) || (behavior == 2)); % In dynamic chain, but random, seq, and seq-swapped are 16 bits ip_id ::= irregular(16); }; co_format_ip_id_zero = ip_id, %[ 0 ] { let (behavior == 3); % Zero IPID ip_id ::= uncompressed_value (16, 0); }; }; ip_id_enc_irreg (behavior) === { uc_format = ip_id; %[ 16 ] co_format_ip_id_seq = ip_id, %[ 0 ] { let (behavior == 0); % sequential ip_id ::= static; % Nothing to send in irregular chain }; co_format_ip_id_seq_swapped = ip_id, %[ 0 ] { let (behavior == 1); % sequential-swapped ip_id ::= static; % Nothing to send in irregular chain }; co_format_ip_id_rand = ip_id, %[ 16 ] { let (behavior == 2); % random ip_id ::= irregular (16); }; co_format_ip_id_zero = ip_id, %[ 0 ] { let (behavior == 3); % zero ip_id ::= uncompressed_value (16, 0); }; }; Pelletier, et. al [Page 54] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 ip_id_behavior_enc === { uc_format = ip_id_behavior; %[ 2 ] default_methods = { ip_id_behavior ::= irregular(2); } co_format_sequential = ip_id_behavior, %[ 2 ] { let (ip_id_behavior:uncomp_value = 0b00); }; co_format_sequential_swapped = ip_id_behavior, %[ 2 ] { let (ip_id_behavior:uncomp_value = 0b01); }; co_format_random = ip_id_behavior, %[ 2 ] { let (ip_id_behavior:uncomp_value = 0b10); }; co_format_zero = ip_id_behavior, %[ 2 ] { let (ip_id_behavior:uncomp_value = 0b11); }; }; % The argument flag should only be used if this flag was % set when processing a compressed base header, if not, % the flag should be zero. ipv4 (ttl_irregular_chain_flag) === { uc_format = version, %[ 4 ] hdr_length, %[ 4 ] tos_tc, %[ 6 ] ip_ecn_flags, %[ 2 ] length, %[ 16 ] ip_id, %[ 16 ] rf, %[ 1 ] df, %[ 1 ] mf, %[ 1 ] frag_offset, %[ 13 ] ttl_hopl, %[ 8 ] protocol, %[ 8 ] checksum, %[ 16 ] src_addr, %[ 32 ] dst_addr; %[ 32 ] Pelletier, et. al [Page 55] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 control_fields = ip_id_behavior; %[ 2 ] default_methods = { version ::= static; hdr_length ::= uncompressed_value (4, 5); protocol ::= static; tos_tc ::= static; ip_ecn_flags ::= static; ttl_hopl ::= static; df ::= static; mf ::= uncompressed_value (1, 0); rf ::= static; frag_offset ::= uncompressed_value (13, 0); ip_id ::= uncompressed_value (16, 0); ip_id_behavior ::= static; src_addr ::= static; dst_addr ::= static; checksum ::= inferred_ip_v4_header_checksum; length ::= inferred_ip_v4_length; }; co_format_ipv4_static = version_flag, %[ 1 ] reserved, %[ 7 ] protocol, %[ 8 ] src_addr, %[ 32 ] dst_addr, %[ 32 ] { version_flag ::= '0'; reserved ::= '0000000'; protocol ::= irregular (8); src_addr ::= irregular(32); dst_addr ::= irregular(32); }; co_format_ipv4_dynamic = reserved, %[ 5 ] df, %[ 1 ] ip_id_behavior, %[ 2 ] tos_tc, %[ 6 ] ip_ecn_flags, %[ 2 ] ttl_hopl, %[ 8 ] ip_id, % 0/16 bits { reserved ::= '00000'; % The compressor chooses % behavior of IP-ID % 00 = sequential % 01 = sequential byteswapped % 10 = random % 11 = zero ip_id_behavior ::= ip_id_behavior_enc; Pelletier, et. al [Page 56] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 df ::= irregular (1); tos_tc ::= irregular (6); ip_ecn_flags ::= irregular (2); ttl_hopl ::= irregular (8); ip_id ::= ip_id_enc_dyn (ip_id_behavior:uncomp_value); }; co_format_ipv4_replicate_0 = discriminator, %[ 8 ] ip_id, % 0 or 16 bits tos_tc, %[ 6 ] ip_ecn_flags, %[ 2 ] { discriminator ::= '00000000'; ip_id_behavior ::= static; ip_id ::= ip_id_enc_irreg (ip_id_behavior:uncomp_value); tos_tc ::= irregular (6); ip_ecn_flags ::= irregular (2); }; co_format_ipv4_replicate_1 = discriminator, %[ 5 ] df, %[ 1 ] ip_id_behavior, %[ 2 ] tos_tc, %[ 6 ] ip_ecn_flags, %[ 2 ] ttl_hopl, %[ 8 ] ip_id, % 0/16 bits { discriminator ::= '10000'; df ::= irregular (1); tos_tc ::= irregular (6); ip_ecn_flags ::= irregular (2); ttl_hopl ::= irregular (8); % The compressor chooses % behavior of IP-ID % 00 = sequential % 01 = sequential byteswapped % 10 = random % 11 = zero ip_id_behavior ::= ip_id_behavior_enc; ip_id ::= ip_id_enc_dyn (ip_id_behavior:uncomp_value); }; co_format_ipv4_outer_irregular_without_ttl = ip_id, % 0 or 16 bits tos_tc, % 0 or 6 bits ip_ecn_flags, % 0 or 2 bits { ip_id_behavior ::= static; ip_id ::= ip_id_enc_irreg (ip_id_behavior:uncomp_value); tos_tc ::= irreg_tos_tc; ip_ecn_flags ::= ip_irreg_ecn; Pelletier, et. al [Page 57] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 let(ttl_irregular_chain_flag == 0); }; co_format_ipv4_outer_irregular_with_ttl = ip_id, % 0 or 16 bits tos_tc, % 0 or 6 bits ip_ecn_flags, % 0 or 2 bits ttl_hopl, %[ 8 ] { ip_id_behavior ::= static; ip_id ::= ip_id_enc_irreg (ip_id_behavior:uncomp_value); tos_tc ::= irreg_tos_tc; ip_ecn_flags ::= ip_irreg_ecn; let(ttl_irregular_chain_flag == 1); ttl_hopl ::= irregular(8); }; % Note that the ECN bits are stored in the global control field % so that they can be output in TCP irregular chain. co_format_ipv4_innermost_irregular = ip_id, % 0 or 16 bits { ip_id_behavior ::= static; ip_id ::= ip_id_enc_irreg (ip_id_behavior:uncomp_value); let(ip_inner_ecn:uncomp_value == ip_ecn_flags:uncomp_value); }; }; 8.6. TCP Header port_replicate(flags) === { uc_format = port; %[ 16 ] co_format_port_static_enc = port, %[ 0 ] { let(flags == 0b00); port ::= static; }; co_format_port_lsb8 = port, %[ 8 ] { let(flags == 0b01); port ::= lsb (8, 64); }; co_format_port_irr_enc = port, %[ 16 ] { let(flags == 0b10); port ::= irregular (16); Pelletier, et. al [Page 58] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 }; }; zero_or_irr16_enc(flag) === { uc_format = field; %[ 16 ] co_format_non_zero = field, %[ 16 ] { let(flag == 0); field ::= irregular (16); }; co_format_zero = field, %[ 0 ] { let(flag == 1); field ::= uncompressed_value (16, 0); }; }; ack_enc_dyn(flag) === { uc_format = ack_number; %[ 32 ] co_format_ack_non_zero = ack_number, %[ 32 ] { let(flag == 0); ack_number ::= irregular (32); }; co_format_ack_zero = ack_number, %[ 0 ] { let(flag == 1); ack_number ::= uncompressed_value (32, 0); }; }; tcp_ecn_flags_enc === { uc_format = tcp_ecn_flags; %[ 2 ] co_format_irreg = tcp_ecn_flags, %[ 2 ] { let(ecn_used:uncomp_value == 1); tcp_ecn_flags ::= irregular(2); }; co_format_unused = { let(ecn_used:uncomp_value == 0); tcp_ecn_flags ::= static; Pelletier, et. al [Page 59] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 }; }; tcp_res_flags_enc === { uc_format = tcp_res_flags; %[ 4 ] co_format_irreg = tcp_res_flags, %[ 4 ] { let(ecn_used:uncomp_value == 1); tcp_res_flags ::= irregular(4); }; co_format_unused = { let(ecn_used:uncomp_value == 0); tcp_res_flags ::= uncompressed_value(4, 0); }; }; tcp_irreg_ip_ecn === { uc_format = ip_ecn_flags; %[ 2 ] co_format_tc_present = ip_ecn_flags, %[ 2 ] { let(ecn_used:uncomp_value == 1); ip_ecn_flags ::= compressed_value(2, ip_inner_ecn:uncomp_value); }; co_format_tc_not_present = ip_ecn_flags, %[ 0 ] { let(ecn_used:uncomp_value == 0); ip_inner_ecn ::= static; % Global control field ip_ecn_flags ::= compressed_value(0,0); % Nothing transmit }; }; rsf_index_enc === { uc_format = rsf_flag; %[ 3 ] co_format_none = rsf_idx, %[ 2 ] { rsf_idx ::= '00'; rsf_flag ::= uncompressed_value (3, 0x00); }; co_format_rst_only = rsf_idx, %[ 2 ] { rsf_idx ::= '01'; Pelletier, et. al [Page 60] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 rsf_flag ::= uncompressed_value (3, 0x04); }; co_format_syn_only = rsf_idx, %[ 2 ] { rsf_idx ::= '10'; rsf_flag ::= uncompressed_value (3, 0x02); }; co_format_fin_only = rsf_idx, %[ 2 ] { rsf_idx ::= '11'; rsf_flag ::= uncompressed_value (3, 0x01); }; }; optional_2bit_padding(used_flag) === { uc_format = ; co_format_used = padding, %[ 2 ] { let(used_flag == 1); padding ::= compressed_value (2, 0x0); }; co_format_unused = padding, { let(used_flag == 0); padding ::= compressed_value (0, 0x0); }; }; tcp === { uc_format = src_port, %[ 16 ] dst_port, %[ 16 ] seq_number, %[ 32 ] ack_number, %[ 32 ] data_offset, %[ 4 ] tcp_res_flags, %[ 4 ] tcp_ecn_flags, %[ 2 ] urg_flag, %[ 1 ] ack_flag, %[ 1 ] psh_flag, %[ 1 ] rsf_flags, %[ 3 ] window, %[ 16 ] checksum, %[ 16 ] urg_ptr, %[ 16 ] options; % n bits Pelletier, et. al [Page 61] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 default_methods = { src_port ::= static; dst_port ::= static; seq_number ::= static; ack_number ::= static; rsf_flags ::= static; psh_flag ::= irregular (1); urg_flag ::= static; ack_flag ::= uncompressed_value (1, 1); urg_ptr ::= static; window ::= static; checksum ::= irregular (16); tcp_ecn_flags ::= static; tcp_res_flags ::= static; }; co_format_tcp_static = src_port, %[ 16 ] dst_port, %[ 16 ] { src_port ::= irregular(16); dst_port ::= irregular(16); }; co_format_tcp_dynamic = ecn_used, %[ 1 ] ack_stride_zero, %[ 1 ] ack_zero, %[ 1 ] urp_zero, %[ 1 ] tcp_res_flags, %[ 4 ] tcp_ecn_flags, %[ 2 ] urg_flag, %[ 1 ] ack_flag, %[ 1 ] psh_flag, %[ 1 ] rsf_flags, %[ 3 ] msn, %[ 16 ] seq_number, %[ 32 ] ack_number, % 0 or 32 bits window, %[ 16 ] checksum, %[ 16 ] urg_ptr, % 0 or 16 bits ack_stride, % 0 or 16 bits options, % n bits { ecn_used ::= irregular (1); ack_stride_zero ::= irregular (1); ack_zero ::= irregular (1); urp_zero ::= irregular (1); ack_flag ::= irregular (1); urg_flag ::= irregular (1); psh_flag ::= irregular (1); tcp_ecn_flags ::= irregular (2); Pelletier, et. al [Page 62] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 rsf_flags ::= irregular (3); tcp_res_flags ::= irregular (4); msn ::= irregular (16); seq_number ::= irregular (32); window ::= irregular (16); checksum ::= irregular (16); urg_ptr ::= zero_or_irr16_enc(urp_zero:comp_value); ack_number ::= ack_enc_dyn(ack_zero:comp_value); ack_stride ::= zero_or_irr16_enc(ack_stride_zero:comp_value); data_offset ::= uncompressed_value(4, data_offset_value); options ::= list_tcp_options((data_offset_value - 5) * 32); }; co_format_tcp_replicate = reserved, %[ 2 ] window_presence, %[ 1 ] list_present, %[ 1 ] src_port_presence, %[ 2 ] dst_port_presence, %[ 2 ] ack_presence, %[ 1 ] urp_presence, %[ 1 ] urg_flag, %[ 1 ] ack_flag, %[ 1 ] psh_flag, %[ 1 ] rsf_flags, %[ 2 ] ecn_used, %[ 1 ] msn, %[ 16 ] seq_number, %[ 32 ] src_port, % 0, 8 or 16 bits dst_port, % 0, 8 or 16 bits window, % 0 or 16 bits urg_point, % 0 or 16 bits ack_number, % 0 or 32 bits ecn_padding, % 0 or 2 bits tcp_res_flags, % 0 or 4 bits tcp_ecn_flags, % 0 or 2 bits options, % n bits { reserved ::= '000'; list_present ::= irregular (1); msn ::= irregular (16); urg_flag ::= irregular (1); ack_flag ::= irregular (1); psh_flag ::= irregular (1); rsf_flags ::= rsf_index_enc; ecn_used ::= irregular (1); src_port_presence ::= compressed_value(2, src_port_presence_value); dst_port_presence ::= compressed_value(2, dst_port_presence_value); src_port ::= port_replicate(src_port_presence_value); Pelletier, et. al [Page 63] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 dst_port ::= port_replicate( dst_port_presence_value); seq_number ::= irregular(32); ack_presence ::= compressed_value(1, ack_presence_value); window_presence ::= compressed_value(1, window_presence_value); urp_presence ::= compressed_value(1, urg_presence_value); ack_number ::= static_or_irreg32(ack_presence_value); window ::= static_or_irreg16(window_presence_value); urg_point ::= static_or_irreg16(urp_presence_value); ecn_padding ::= optional_2bit_padding(ecn_used:comp_value); tcp_res_flags ::= tcp_res_flags_enc; tcp_ecn_flags ::= tcp_ecn_flags_enc; data_offset ::= uncompressed_value(4, data_offset_value); options ::= tcp_list_presence_enc ((data_offset_value - 5) * 32, list_present:comp_value, ack_number:uncomp_value); }; % ECN from innermost IP header is taken from global control field. co_format_tcp_irregular = ip_ecn_flags, % 0 or 2 bits tcp_res_flags, % 0 or 4 bits tcp_ecn_flags, % 0 or 2 bits checksum, %[ 16 ] { ip_ecn_flags ::= tcp_irreg_ip_ecn; tcp_ecn_flags ::= tcp_ecn_flags_enc; tcp_res_flags ::= tcp_res_flags_enc; checksum ::= irregular (16); }; }; 8.7. TCP Options tcp_opt_eol(nbits) === { uc_format = type, %[ 8 ] padding; % (nbits - 8) bits default_methods = { type ::= uncompressed_value (8, 0); pad_len ::= static; padding ::= uncompressed_value (nbits - 8, 0); }; co_format_eol_list_item = pad_len, % 8 bits padding, %[ 0 ] { pad_len ::= compressed_value(8, nbits - 8); }; Pelletier, et. al [Page 64] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 co_format_eol_irregular = { let(nbits - 8 == pad_len:uncomp_value); }; }; tcp_opt_nop === { uc_format = type; %[ 8 ] default_methods = { type ::= uncompressed_value (8, 1); }; co_format_nop_list_item = { }; co_format_nop_irregular = { }; }; tcp_opt_mss === { uc_format = type, %[ 8 ] length, %[ 8 ] mss; %[ 16 ] default_methods = { type ::= uncompressed_value (8, 2); length ::= uncompressed_value (8, 4); mss ::= static; }; co_format_mss_list_item = mss, %[ 16 ] { mss ::= irregular (16); }; co_format_mss_irregular = { }; }; tcp_opt_wscale === { uc_format = type, %[ 8 ] Pelletier, et. al [Page 65] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 length, %[ 8 ] wscale; %[ 8 ] default_methods = { type ::= uncompressed_value (8, 3); length ::= uncompressed_value (8, 3); wscale ::= static; }; co_format_wscale_list_item = wscale, %[ 8 ] { wscale ::= irregular (8); }; co_format_wscale_irregular = { }; }; ts_lsb === { uc_format = tsval; % Few bits (7 and 14) bits % can only increase, while % the larger formats allow % decreasing timestamp to % allow prelink reordering. co_format_tsval_7 = discriminator, %[ 1 ] tsval, %[ 7 ] { discriminator ::= '0'; tsval ::= lsb (7, -1); }; co_format_tsval_14 = discriminator, %[ 2 ] tsval, %[ 14 ] { discriminator ::= '10'; tsval ::= lsb (14, -1); }; co_format_tsval_21 = discriminator, %[ 3 ] tsval, %[ 21 ] { discriminator ::= '110'; tsval ::= lsb (21, 0x00040000); }; Pelletier, et. al [Page 66] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 co_format_tsval_29 = discriminator, %[ 3 ] tsval, %[ 29 ] { discriminator ::= '111'; tsval ::= lsb (29, 0x04000000); }; }; tcp_opt_tsopt === { uc_format = type, %[ 8 ] length, %[ 8 ] tsval, %[ 32 ] tsecho; %[ 32 ] default_methods = { type ::= uncompressed_value (8, 8); length ::= uncompressed_value (8, 10); }; co_format_tsopt_list_item = tsval, %[ 32 ] tsecho, %[ 32 ] { tsval ::= irregular (32); tsecho ::= irregular (32); }; co_format_tsopt_irregular = tsval, % 16, 24 or 32 bits tsecho, % 16, 24 or 32 bits { tsval ::= ts_lsb; tsecho ::= ts_lsb; }; }; sack_var_length_enc (base) === { uc_format = sack_field; %[ 32 ] default_methods = { let (sack_offset:uncomp_value == sack_field:uncomp_value - base); let (sack_offset:uncomp_length == 32); let (sack_field:uncomp_length == 32); }; co_format_lsb_15 = discriminator, %[ 1 ] sack_offset, %[ 15 ] { discriminator ::= '0'; Pelletier, et. al [Page 67] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 sack_offset ::= lsb (15, -1); }; co_format_lsb_22 = discriminator, %[ 2 ] sack_offset, %[ 22 ] { discriminator ::= '10'; sack_offset ::= lsb (22, -1); }; co_format_lsb_30 = discriminator, %[ 2 ] sack_offset, %[ 30 ] { discriminator ::= '11'; sack_offset ::= lsb (30, -1); }; }; tcp_opt_sack_block (prev_block_end) === { uc_format = block_start, %[ 32 ] block_end; %[ 32 ] co_format_0 = block_start, % 16, 24 or 32 bits block_end, % 16, 24 or 32 bits { block_start ::= sack_var_length_enc (prev_block_end); block_end ::= sack_var_length_enc (block_start); }; }; tcp_opt_sack(ack_value) === { % The ACK value from the TCP header is needed as input parameter. uc_format = type, %[ 8 ] length, %[ 8 ] block_1, %[ 64 ] block_2, % 0 or 64 bits block_3, % 0 or 64 bits block_4; % 0 or 64 bits default_methods = { length ::= static; type ::= uncompressed_value (8, 5); block_2 ::= uncompressed_value (0, 0); block_3 ::= uncompressed_value (0, 0); block_4 ::= uncompressed_value (0, 0); }; Pelletier, et. al [Page 68] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 co_format_sack1_list_item = discriminator, block_1, { let(length:uncomp_value == 10); discriminator ::= '00000001'; block_1 ::= tcp_opt_sack_block (ack_value); }; co_format_sack2_list_item = discriminator, block_1, block_2, { let(length:uncomp_value == 18); discriminator ::= '00000010'; block_1 ::= tcp_opt_sack_block (ack_value); block_2 ::= tcp_opt_sack_block (block_1_end:uncomp_value); }; co_format_sack3_list_item = discriminator, block_1, block_2, block_3, { let(length:uncomp_value == 26); discriminator ::= '00000011'; block_1 ::= tcp_opt_sack_block (ack_value); block_2 ::= tcp_opt_sack_block (block_1_end:uncomp_value); block_3 ::= tcp_opt_sack_block (block_2_end:uncomp_value); }; co_format_sack4_list_item = discriminator, block_1, block_2, block_3, block_4, { let(length:uncomp_value == 34); discriminator ::= '00000100'; block_1 ::= tcp_opt_sack_block (ack_value); block_2 ::= tcp_opt_sack_block (block_1_end:uncomp_value); block_3 ::= tcp_opt_sack_block (block_2_end:uncomp_value); block_4 ::= tcp_opt_sack_block (block_3_end:uncomp_value); }; co_format_sack_unchanged_irregular = discriminator, block_1, block_2, block_3, block_4, { discriminator ::= '00000000'; Pelletier, et. al [Page 69] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 block_1 ::= static; block_2 ::= static; block_3 ::= static; block_4 ::= static; }; co_format_sack1_irregular = discriminator, block_1, { let(length:uncomp_value == 10); discriminator ::= '00000001'; block_1 ::= tcp_opt_sack_block (ack_value); }; co_format_sack2_irregular = discriminator, block_1, block_2, { let(length:uncomp_value == 18); discriminator ::= '00000010'; block_1 ::= tcp_opt_sack_block (ack_value); block_2 ::= tcp_opt_sack_block (block_1_end:uncomp_value); }; co_format_sack3_irregular = discriminator, block_1, block_2, block_3, { let(length:uncomp_value == 26); discriminator ::= '00000011'; block_1 ::= tcp_opt_sack_block (ack_value); block_2 ::= tcp_opt_sack_block (block_1_end:uncomp_value); block_3 ::= tcp_opt_sack_block (block_2_end:uncomp_value); }; co_format_sack4_irregular = discriminator, block_1, block_2, block_3, block_4, { let(length:uncomp_value == 34); discriminator ::= '00000100'; block_1 ::= tcp_opt_sack_block (ack_value); block_2 ::= tcp_opt_sack_block (block_1_end:uncomp_value); block_3 ::= tcp_opt_sack_block (block_2_end:uncomp_value); block_4 ::= tcp_opt_sack_block (block_3_end:uncomp_value); }; }; Pelletier, et. al [Page 70] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 tcp_opt_sack_permitted === { uc_format = type, %[ 8 ] length; %[ 8 ] default_methods = { type ::= uncompressed_value (8, 4); length ::= uncompressed_value (8, 2); }; co_format_sack_permitted_list_item = { }; co_format_sack_permitted_irregular = { }; }; tcp_opt_generic === { uc_format = type, %[ 8 ] length_msb, %[ 1 ] length_lsb, %[ 7 ] contents; % n bits control_fields = option_static; %[ 1 ] default_methods = { type ::= static; % lengths are always < 128 % (i.e. the msb is always 0) length_msb ::= uncompressed_value (1, 0); length_lsb ::= static; contents ::= static; let (option_static:uncomp_length == 1); }; co_format_generic_list_item = type, %[ 8 ] option_static, %[ 1 ] length_lsb, %[ 7 ] contents, % n bits { type ::= irregular (8); option_static ::= irregular (1); length_lsb ::= irregular (7); contents ::= irregular (length_len:uncomp_value * 8 - 16); }; Pelletier, et. al [Page 71] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 % Used when context of option has option_static set to one co_format_generic_irregular_static = { let(option_static:uncomp_value == 1); }; % An item that can change, but currently is unchanged co_format_generic_irregular_stable = discriminator, %[ 8 ] { let(option_static:uncomp_value == 0); discriminator ::= '11111111'; }; % An item that can change, and has changed compared to context. % Length is not allowed to change here, since a length change is % most likely to cause new NOPs or an EOL length change. co_format_generic_irregular_full = discriminator, %[ 8 ] contents, % n bits { let(option_static:uncomp_value == 0); discriminator ::= '00000000'; contents ::= irregular (length_lsb:uncomp_value * 8 - 16); }; }; list_tcp_options(nbits, ack_value) === { uc_format = item, tail; default_methods = { let(nbits >= item:uncomp_length); tail ::= list_tcp_options(nbits - item:uncomp_length, ack_value); }; co_format_list_end = { let(nbits == 0); item ::= irregular(0); tail ::= irregular(0); }; co_format_eol = item, tail { let(nbits == item:uncomp_length); % redundant item ::= tcp_opt_eol(nbits); tail ::= irregular(0); }; Pelletier, et. al [Page 72] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 co_format_nop = item, tail { item ::= tcp_opt_nop; }; co_format_mss = item, tail { item ::= tcp_opt_mss; }; co_format_wscale = item, tail { item ::= tcp_opt_wscale; }; co_format_tsopt = item, tail { item ::= tcp_opt_tsopt; }; co_format_sack = item, tail { item ::= tcp_opt_sack(ack_value); }; co_format_permitted = item, tail { item ::= tcp_opt_sack_permitted; }; co_format_generic = item, tail { item ::= tcp_opt_generic; }; }; tcp_list_presence_enc(list_length, presence, ack_value) === { uc_format = tcp_options; co_format_list_not_present = tcp_options, %[ 0 ] { let (presence == 0); tcp_options ::= static; Pelletier, et. al [Page 73] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 }; co_format_list_present = tcp_options, % 8 + n*8 bits { let (presence == 1); tcp_options ::= list_tcp_options(list_length, ack_value); }; }; 8.8. Structures used in Compressed Base Headers tos_tc_enc(flag) === { uc_format = tos_tc; %[ 6 ] co_format_static = tos_tc, %[ 0 ] { let (flag == 0); tos_tc ::= static; }; co_format_irreg = tos_tc, %[ 6 ] padding, %[ 2 ] { let (flag == 1); tos_tc ::= irregular(6); padding ::= compressed_value (2, 0); }; }; ip_id_lsb (behavior, k, p) === { uc_format = ip_id; %[ 16 ] default_methods = { let (ip_id:uncomp_length == 16); }; co_format_nbo = ip_id_offset, % k bits { let (behavior == 0); let (ip_id_offset:uncomp_value == ip_id:uncomp_value - msn:uncomp_value); let (ip_id_offset:uncomp_length == 16); ip_id_offset ::= lsb (k, p); }; Pelletier, et. al [Page 74] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 co_format_non_nbo = ip_id_offset, % k bits { let (behavior == 1); let (ip_id_nbo:uncomp_value == (ip_id:uncomp_value / 256) + (ip_id:uncomp_value & 255) * 256); let (ip_id_nbo:uncomp_length == 16); let (ip_id_offset:uncomp_value == ip_id_nbo:uncomp_value - msn:uncomp_value); let (ip_id_offset:uncomp_length == 16); ip_id_offset ::= lsb (k, p); }; }; dont_fragment(version) === { uc_format = df; %[ 1 ] co_format_v4 = df, %[ 1 ] { let (version == 4); df ::= irregular(1); }; co_format_v6 = df, { let (version == 6); df ::= compressed_value(1,0); }; }; %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % Actual start of compressed packet formats % Important note: The base header is the % compressed representation of the innermost % IP header AND the TCP header. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % ttl_irregular_chain_flag is an % "output argument" that should be passed % to the processing of the irregular chain % for outer IP headers. co_baseheader(payload_size, ttl_irregular_chain_flag) === { uc_format_v4 = version, %[ 4 ] header_length, %[ 4 ] tos_tc, %[ 6 ] ip_ecn_flags, %[ 2 ] length, %[ 16 ] ip_id, %[ 16 ] rf, %[ 1 ] Pelletier, et. al [Page 75] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 df, %[ 1 ] mf, %[ 1 ] frag_offset, %[ 13 ] ttl_hopl, %[ 8 ] next_header, %[ 8 ] checksum, %[ 16 ] src_addr, %[ 32 ] dest_addr, %[ 32 ] src_port, %[ 16 ] dest_port, %[ 16 ] seq_number, %[ 32 ] ack_number, %[ 32 ] data_offset, %[ 4 ] tcp_res_flags, %[ 4 ] tcp_ecn_flags, %[ 2 ] urg_flag, %[ 1 ] ack_flag, %[ 1 ] psh_flag, %[ 1 ] rsf_flags, %[ 3 ] window, %[ 16 ] tcp_checksum, %[ 16 ] urg_ptr, %[ 16 ] options_list, % n bits { let (version:uncomp_value == 4); }; uc_format_v6 = version, %[ 4 ] tos_tc, %[ 6 ] ip_ecn_flags, %[ 2 ] flow_label, %[ 20 ] payload_length, %[ 16 ] next_header, %[ 8 ] ttl_hopl, %[ 8 ] src_addr, %[ 128 ] dest_addr, %[ 128 ] src_port, %[ 16 ] dest_port, %[ 16 ] seq_number, %[ 32 ] ack_number, %[ 32 ] data_offset, %[ 4 ] tcp_res_flags, %[ 4 ] tcp_ecn_flags, %[ 2 ] urg_flag, %[ 1 ] ack_flag, %[ 1 ] psh_flag, %[ 1 ] rsf_flags, %[ 3 ] window, %[ 16 ] tcp_checksum, %[ 16 ] urg_ptr, %[ 16 ] options_list, % n bits Pelletier, et. al [Page 76] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 { let (version:uncomp_value == 6); }; control_fields = ip_id_behavior; % 2 bits default_methods = { version ::= static; tos_tc ::= static; ip_ecn_flags ::= static; ttl_hopl ::= static; next_header ::= static; src_addr ::= static; dest_addr ::= static; flow_label ::= static; payload_length ::= inferred_ip_v6_length; header_length ::= uncompressed_value (4,5); length ::= inferred_ip_v4_length; ip_id ::= irregular(16); ip_id_behavior ::= static; rf ::= static; df ::= static; mf ::= static; frag_offset ::= static; checksum ::= inferred_ip_v4_header_checksum; src_port ::= static; dest_port ::= static; seq_number ::= static; ack_number ::= static; data_offset ::= inferred_offset; tcp_ecn_flags ::= static; psh_flag ::= irregular (1); urg_flag ::= uncompressed_value (1, 0); ack_flag ::= uncompressed_value (1, 1); window ::= static; tcp_checksum ::= irregular(16); urg_ptr ::= static; rsf_flags ::= uncompressed_value (3, 0); tcp_res_flags ::= static; options_list ::= static; let (version:uncomp_length == 4); let (seq_number_scaled:uncomp_value == seq_number:uncomp_value / payload_size); let (seq_number_residue:uncomp_value == mod(seq_number:uncomp_value, payload_size)); let (ack_number:uncomp_value == (ack_stride:uncomp_value * ack_number_scaled:uncomp_value) + ack_number_residue:uncomp_value); Pelletier, et. al [Page 77] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 let (ack_number_residue:uncomp_value == mod(ack_number:uncomp_value, ack_stride:uncomp_value)); }; %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % Common compressed packet format %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% co_format_co_common = discriminator, %[ 7 ] ttl_hopl_outer_flag, %[ 1 ] ack_flag, %[ 1 ] psh_flag, %[ 1 ] rsf_flags, %[ 2 ] msn, %[ 4 ] seq_indicator, %[ 2 ] ack_indicator, %[ 2 ] ack_stride_indicator, %[ 1 ] window_indicator, %[ 1 ] ip_id_indicator, %[ 2 ] urg_ptr_present, %[ 1 ] ecn_used, %[ 1 ] tos_tc_present, %[ 1 ] ttl_hopl_present, %[ 1 ] list_present, %[ 1 ] ip_id_behavior, %[ 2 ] urg_flag, %[ 1 ] df, %[ 1 ] header_crc, %[ 7 ] seq_number, % 0, 8, 16, 32 bits ack_number, % 0, 8, 16, 32 bits ack_stride, % 0 or 16 bits window, % 0 or 16 bits ip_id, % 0, 8, 16 bits urg_ptr, % 0 or 16 bits tos_tc, % 0 or 8 bits ttl_hopl, % 0 or 8 bits options_list, % n bits { discriminator ::= '1111101'; ttl_hopl_outer_flag::= irregular(1); % Need to bind argument so that it can be passed to the % structure for IPv4/IPv6 irregular chain. let(ttl_irregular_chain_flag == ttl_hopl_outer_flag:uncomp_value); tos_tc_present ::= irregular(1); ttl_hopl_present ::= irregular(1); ack_flag ::= irregular(1); psh_flag ::= irregular(1); msn ::= lsb (4, 3); df ::= dont_fragment(version:uncomp_value); header_crc ::= crc7(this:uncomp_value, this:uncomp_length); Pelletier, et. al [Page 78] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 urg_flag ::= irregular(1); urg_ptr_present ::= irregular(1); ecn_used ::= irregular(1); list_present ::= irregular(1); ip_id_behavior ::= ip_id_behavior_enc; rsf_flags ::= rsf_index_enc; window_indicator ::= irregular(1); ip_id_indicator ::= irregular(2); seq_indicator ::= irregular(2); ack_indicator ::= irregular(2); ack_stride_indicator ::= irregular(1); seq_number ::= variable_length_32_enc(seq_indicator:comp_value); ack_number ::= variable_length_32_enc(ack_indicator:comp_value); ack_stride ::= static_or_irreg16(ack_stride_indicator:comp_value); window ::= static_or_irreg16(window_indicator:comp_value); ip_id ::= variable_length_16_enc(ip_id_indicator:comp_value); urg_ptr ::= static_or_irreg16(urg_ptr_present:comp_value); ttl_hopl ::= static_or_irreg8(ttl_hopl_present:comp_value); tos_tc ::= tos_tc_enc(tos_tc_present:comp_value); options_list ::= tcp_list_presence_enc ((data_offset:uncomp_value - 5) * 32, list_present:comp_value, ack_number:uncomp_value); }; % NON-SEQUENTIAL PACKET FORMATS % +------+--------+-----+-----+-----+---------+---------+ % |Name |Disc |MSN |SN |ACK |Window |Comment | % +------+--------+-----+-----+-----+---------+---------+ % |_1 |10111110|4 |16 |0 | | | % +------+--------+-----+-----+-----+---------+---------+ % |_2 |1100 |4 |*4 |0 | | | % +------+--------+-----+-----+-----+---------+---------+ % |_3 |0 |4 |0 |15 | | | % +------+--------+-----+-----+-----+---------+---------+ % |_4 |1101 |4 |0 |4* | | | % +------+--------+-----+-----+-----+---------+---------+ % |_5 |100 |4 |14 |15 | | | % +------+--------+-----+-----+-----+---------+---------+ % |_6 |10110 |4 |*4 |15 | | | % +------+--------+-----+-----+-----+---------+---------+ % |_7 |1010 |4 |14 |0 |14 | | % +------+--------+-----+-----+-----+---------+---------+ % |_8 |10111111|4 |0 |16 |16 | | % +------+--------+-----+-----+-----+---------+---------+ Pelletier, et. al [Page 79] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 % |_9 |101110 |4 |*4 |0 |14 | | % +------+--------+-----+-----+-----+---------+---------+ % |_10 |1011110 |4 |14 |16 | |and more | % +------+--------+-----+-----+-----+---------+---------+ % Send LSBs of sequence number co_format_rnd_1 = discriminator, %[ 8 ] seq_number, %[ 16 ] msn, %[ 4 ] psh_flag, %[ 1 ] header_crc, %[ 3 ] { let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '10111110'; msn ::= lsb(4, 4); header_crc ::= crc3 (this:uncomp_value, this:uncomp_length); psh_flag ::= irregular (1); seq_number ::= lsb(16, 32767); }; % Send scaled sequence number LSBs co_format_rnd_2 = discriminator, %[ 4 ] seq_number_scaled, %[ 4 ] msn, %[ 4 ] psh_flag, %[ 1 ] header_crc, %[ 3 ] { let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '1100'; msn ::= lsb(4, 4); header_crc ::= crc3 (this:uncomp_value, this:uncomp_length); psh_flag ::= irregular (1); seq_number_scaled ::= lsb(4, 7); seq_number_residue ::= static; }; % Send acknowledgement number LSBs co_format_rnd_3 = discriminator, %[ 1 ] ack_number, %[ 15 ] msn, %[ 4 ] psh_flag, %[ 1 ] header_crc, %[ 3 ] { let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '0'; Pelletier, et. al [Page 80] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 msn ::= lsb(4, 4); header_crc ::= crc3 (this:uncomp_value, this:uncomp_length); psh_flag ::= irregular (1); ack_number ::= lsb(15, 8191); }; % Send acknowledgement number scaled co_format_rnd_4 = discriminator, %[ 4 ] ack_number_scaled, %[ 4 ] msn, %[ 4 ] psh_flag, %[ 1 ] header_crc, %[ 3 ] { let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '1101'; msn ::= lsb(4, 4); header_crc ::= crc3 (this:uncomp_value, this:uncomp_length); psh_flag ::= irregular (1); ack_number_scaled ::= lsb(4, 3); ack_number_residue ::= static; }; % Send ACK and sequence number co_format_rnd_5 = discriminator, %[ 3 ] psh_flag, %[ 1 ] msn, %[ 4 ] header_crc, %[ 3 ] seq_number, %[ 14 ] ack_number, %[ 15 ] { let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '100'; msn ::= lsb(4, 4); header_crc ::= crc3 (this:uncomp_value, this:uncomp_length); psh_flag ::= irregular (1); ack_number ::= lsb(15, 8191); seq_number ::= lsb(14, 8191); }; % Send both ACK and scaled sequence number LSBs co_format_rnd_6 = discriminator, %[ 5 ] header_crc, %[ 3 ] psh_flag, %[ 1 ] ack_number, %[ 15 ] msn, %[ 4 ] seq_number_scaled, %[ 4 ], Pelletier, et. al [Page 81] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 { let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '10110'; msn ::= lsb(4, 4); header_crc ::= crc3 (this:uncomp_value, this:uncomp_length); psh_flag ::= irregular (1); ack_number ::= lsb(15, 8191); seq_number_scaled ::= lsb(4, 7); seq_number_residue ::= static; }; % Send sequence number and window co_format_rnd_7 = discriminator, %[ 4 ] seq_number, %[ 14 ] window, %[ 14 ] msn, %[ 4 ] psh_flag, %[ 1 ] header_crc, %[ 3 ] { let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '1010'; msn ::= lsb(4, 4); header_crc ::= crc3 (this:uncomp_value, this:uncomp_length); psh_flag ::= irregular (1); seq_number ::= lsb(14, 8191); window ::= lsb(14, 8191); }; % Send ACK and window co_format_rnd_8 = discriminator, %[ 8 ] ack_number, %[ 16 ] window, %[ 16 ] msn, %[ 4 ] psh_flag, %[ 1 ] header_crc, %[ 3 ] { let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '10111111'; msn ::= lsb(4, 4); header_crc ::= crc3 (this:uncomp_value, this:uncomp_length); psh_flag ::= irregular (1); ack_number ::= lsb(16, 16383); window ::= irregular(16); }; Pelletier, et. al [Page 82] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 % Send scaled sequence number and window. co_format_rnd_9 = discriminator, %[ 6 ] seq_number_scaled, %[ 4 ] window, %[ 14 ] msn, %[ 4 ] psh_flag, %[ 1 ] header_crc, %[ 3 ] { let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '101110'; msn ::= lsb(4, 4); header_crc ::= crc3 (this:uncomp_value, this:uncomp_length); psh_flag ::= irregular (1); window ::= lsb(14, 8191); seq_number_scaled ::= lsb(4, 3); seq_number_residue ::= static; }; % A packet halfway between co_common and compressed packets % Can send LSBs of TTL, RSF flags, % change ECN behavior and options list co_format_rnd_10 = discriminator, %[ 7 ] ecn_used, %[ 1 ] list_present, %[ 1 ] header_crc, %[ 7 ] msn, %[ 4 ] psh_flag, %[ 1 ] ttl_hopl, %[ 3 ] rsf_flags, %[ 2 ] seq_number, %[ 14 ] ack_number, %[ 16 ] options_list, % 0 or X bits { let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '1011110'; msn ::= lsb(4, 4); header_crc ::= crc7 (this:uncomp_value, this:uncomp_length); psh_flag ::= irregular (1); list_present ::= irregular(1); options_list ::= tcp_list_presence_enc ((data_offset:uncomp_value - 5) * 32, list_present:comp_value, ack_number:uncomp_value); rsf_flags ::= rsf_index_enc; ecn_used ::= irregular(1); ttl_hopl ::= lsb(3, 3); seq_number ::= lsb(14, 8191); Pelletier, et. al [Page 83] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 ack_number ::= lsb(16, 16383); }; % SEQUENTIAL PACKET FORMATS % +------+------+-----+-----+-----+-----+-----+---------+ % |Name |Disc |MSN |ID |SN |ACK |Win |Comment | % +------+------+-----+-----+-----+-----+-----+---------+ % |_1 |1010 |4 |4 |16 |0 | | | % +------+------+-----+-----+-----+-----+-----+---------+ % |_2 |11001 |4 |7 |*4 |0 | | | % +------+------+-----+-----+-----+-----+-----+---------+ % |_3 |1001 |4 |4 |0 |16 | | | % +------+------+-----+-----+-----+-----+-----+---------+ % |_4 |0 |4 |3 |0 |*4 | | | % +------+------+-----+-----+-----+-----+-----+---------+ % |_5 |1000 |4 |4 |16 |16 | | | % +------+------+-----+-----+-----+-----+-----+---------+ % |_6 |110110|4 |6 |*4 |16 | | | % +------+------+-----+-----+-----+-----+-----+---------+ % |_7 |11010 |4 |5 |14 |0 |16 | | % +------+------+-----+-----+-----+-----+-----+---------+ % |_8 |11000 |4 |5 |0 |16 |14 | | % +------+------+-----+-----+-----+-----+-----+---------+ % |_9 |110111|4 |6 |*4 |0 |16 | | % +------+------+-----+-----+-----+-----+-----+---------+ % |_10 |1011 |4 |4 |14 |15 | |and more | % +------+------+-----+-----+-----+-----+-----+---------+ % Send LSBs of sequence number co_format_seq_1 = discriminator, %[ 4 ] ip_id, %[ 4 ] seq_number, %[ 16 ] msn, %[ 4 ] psh_flag, %[ 1 ] header_crc, %[ 3 ] { let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '1010'; msn ::= lsb(4, 4); ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 4, 3); header_crc ::= crc3 (this:uncomp_value, this:uncomp_length); psh_flag ::= irregular (1); seq_number ::= lsb(16, 32767); }; % Send scaled sequence number LSBs co_format_seq_2 = discriminator, %[ 5 ] ip_id, %[ 7 ] Pelletier, et. al [Page 84] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 seq_number_scaled, %[ 4 ] msn, %[ 4 ] psh_flag, %[ 1 ] header_crc, %[ 3 ] { let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '11001'; msn ::= lsb(4, 4); ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 7, 3); header_crc ::= crc3 (this:uncomp_value, this:uncomp_length); psh_flag ::= irregular (1); seq_number_scaled ::= lsb(4, 7); seq_number_residue ::= static; }; % Send acknowledgement number LSBs co_format_seq_3 = discriminator, %[ 4 ] ip_id, %[ 4 ] ack_number, %[ 16 ] msn, %[ 4 ] psh_flag, %[ 1 ] header_crc, %[ 3 ] { let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '1001'; msn ::= lsb(4, 4); ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 4, 3); header_crc ::= crc3 (this:uncomp_value, this:uncomp_length); psh_flag ::= irregular (1); ack_number ::= lsb(16, 16383); }; % Send scaled acknowledgement number scaled co_format_seq_4 = discriminator, %[ 1 ] ack_number_scaled, %[ 4 ] ip_id, %[ 3 ] msn, %[ 4 ] psh_flag, %[ 1 ] header_crc, %[ 3 ] { let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '0'; msn ::= lsb(4, 4); % Note that due to having very few ip_id bits, % no reordering offset ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 3, 1); Pelletier, et. al [Page 85] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 header_crc ::= crc3 (this:uncomp_value, this:uncomp_length); psh_flag ::= irregular (1); ack_number_scaled ::= lsb(4, 3); ack_number_residue ::= static; }; % Send ACK and sequence number co_format_seq_5 = discriminator, %[ 4 ] ip_id, %[ 4 ] ack_number, %[ 16 ] seq_number, %[ 16 ] msn, %[ 4 ] psh_flag, %[ 1 ] header_crc, %[ 3 ] { let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '1000'; msn ::= lsb(4, 4); ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 4, 3); header_crc ::= crc3 (this:uncomp_value, this:uncomp_length); psh_flag ::= irregular (1); ack_number ::= lsb(16, 16383); seq_number ::= lsb(16, 32767); }; % Send both ACK and scaled sequence number LSBs co_format_seq_6 = discriminator, %[ 6 ] seq_number_scaled, %[ 4 ] ip_id, %[ 6 ] ack_number, %[ 16 ] msn, %[ 4 ] psh_flag, %[ 1 ] header_crc, %[ 3 ] { let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '110110'; seq_number_scaled ::= lsb(4, 7); seq_number_residue ::= static; ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 6, 3); ack_number ::= lsb(16, 16383); msn ::= lsb(4, 4); psh_flag ::= irregular (1); header_crc ::= crc3 (this:uncomp_value, this:uncomp_length); }; Pelletier, et. al [Page 86] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 % Send sequence number and window co_format_seq_7 = discriminator, %[ 5 ] seq_number, %[ 14 ] ip_id, %[ 5 ] window, %[ 16 ] msn, %[ 4 ] psh_flag, %[ 1 ] header_crc, %[ 3 ] { let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '11010'; msn ::= lsb(4, 4); ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 5, 3); header_crc ::= crc3 (this:uncomp_value, this:uncomp_length); psh_flag ::= irregular (1); seq_number ::= lsb(14, 8191); window ::= irregular(16); }; % Send ACK and window co_format_seq_8 = discriminator, %[ 5 ] window, %[ 14 ] ip_id, %[ 5 ] ack_number, %[ 16 ] msn, %[ 4 ] psh_flag, %[ 1 ] header_crc, %[ 3 ] { let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '11000'; msn ::= lsb(4, 4); ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 5, 3); header_crc ::= crc3 (this:uncomp_value, this:uncomp_length); psh_flag ::= irregular (1); ack_number ::= lsb(16, 32767); window ::= lsb(14, 8191); }; % Send scaled sequence number and window. co_format_seq_9 = discriminator, %[ 6 ] ip_id, %[ 6 ] seq_number_scaled, %[ 4 ] window, %[ 16 ] msn, %[ 4 ] psh_flag, %[ 1 ] header_crc, %[ 3 ] { Pelletier, et. al [Page 87] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '110111'; msn ::= lsb(4, 4); ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 6, 3); header_crc ::= crc3 (this:uncomp_value, this:uncomp_length); psh_flag ::= irregular (1); window ::= irregular(16); seq_number_scaled ::= lsb(4, 7); seq_number_residue ::= static; }; % A packet halfway between co_common and compressed packets % Can send LSBs of TTL, RSF flags, % change ECN behavior and options list co_format_seq_10 = discriminator, %[ 4 ] ip_id, %[ 4 ] list_present, %[ 1 ] header_crc, %[ 7 ] msn, %[ 4 ] psh_flag, %[ 1 ] ttl_hopl, %[ 3 ] ecn_used, %[ 1 ] ack_number, %[ 15 ] rsf_flags, %[ 2 ] seq_number, %[ 14 ] options_list, % Nx8 bits { let ((ip_id_behavior:uncomp_value == 2) || (ip_id_behavior:uncomp_value == 3)); discriminator ::= '1011'; msn ::= lsb(4, 4); ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 4, 3); header_crc ::= crc7 (this:uncomp_value, this:uncomp_length); psh_flag ::= irregular (1); list_present ::= irregular(1); options_list ::= tcp_list_presence_enc ((data_offset:uncomp_value - 5) * 32, list_present:comp_value); rsf_flags ::= rsf_index_enc; ecn_used ::= irregular(1); ttl_hopl ::= lsb(3, 3); seq_number ::= lsb(14, 8191); ack_number ::= lsb(15, 8191); }; }; Pelletier, et. al [Page 88] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 8.9. Feedback Formats and Options 8.9.1. Feedback Formats This section describes the feedback format for ROHC-TCP. ROHC-TCP uses the ROHC feedback format described in section 5.2.2 of [2]. All feedback formats carry a field labeled SN. The SN field contains LSBs of the Master Sequence Number (MSN) described in section 6.3.1. The sequence number to use is the MSN corresponding to the header that caused the feedback information to be sent. If that MSN cannot be determined, for example when decompression fails, the MSN to use is that corresponding to the latest successfully decompressed header. FEEDBACK-1 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | MSN | +---+---+---+---+---+---+---+---+ MSN: The lsb-encoded master sequence number. A FEEDBACK-1 is an ACK. In order to send a NACK or a STATIC-NACK, FEEDBACK-2 must be used. FEEDBACK-2 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ |Acktype| MSN | +---+---+---+---+---+---+---+---+ | MSN | +---+---+---+---+---+---+---+---+ / Feedback options / +---+---+---+---+---+---+---+---+ Acktype: 0 = ACK 1 = NACK 2 = STATIC-NACK 3 is reserved (MUST NOT be used for parseability) MSN: The lsb-encoded master sequence number. Feedback options: A variable number of feedback options, see section 8.9.2. Options may appear in any order. Pelletier, et. al [Page 89] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 8.9.2. Feedback Options A ROHC-TCP 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 +---+---+---+---+---+---+---+---+ 8.9.2.1. The CRC option The CRC option contains an 8-bit CRC computed over the entire feedback payload, without the packet type and code octet, but including any CID fields, using the polynomial of section 5.9.1 of [2]. If the CID is given with an Add-CID octet, the Add-CID octet immediately precedes the FEEDBACK-1 or FEEDBACK-2 format. 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 feedback information with a CRC option, the compressor MUST verify the information by computing the CRC and comparing the result with the CRC carried in the CRC option. If the two are not identical, the feedback information MUST be ignored. 8.9.2.2. The REJECT option The REJECT option informs the compressor that the decompressor does not have sufficient resources to handle the flow. +---+---+---+---+---+---+---+---+ | 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. Pelletier, et. al [Page 90] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 8.9.2.3. The MSN-NOT-VALID option The MSN-NOT-VALID option indicates that the MSN of the feedback is not valid. A compressor MUST NOT use the MSN of the feedback to find the corresponding sent header when this option is present. +---+---+---+---+---+---+---+---+ | Opt Type = 3 | Opt Len = 0 | +---+---+---+---+---+---+---+---+ 8.9.2.4. The MSN option The MSN option provides 8 additional bits of MSN. +---+---+---+---+---+---+---+---+ | Opt Type = 4 | Opt Len = 1 | +---+---+---+---+---+---+---+---+ | MSN | +---+---+---+---+---+---+---+---+ 8.9.2.5. The LOSS option The LOSS option allows the decompressor to report the largest observed number of packets lost in sequence. +---+---+---+---+---+---+---+---+ | Opt Type = 7 | 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. 8.9.2.6. 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. Pelletier, et. al [Page 91] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 8.9.2.7. The CONTEXT_MEMORY Feedback Option The CONTEXT_MEMORY option informs the compressor that the decompressor does not have sufficient memory resources to handle the context of the packet stream, as the stream is currently compressed. 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | Opt Type = 9 | Opt Len = 0 | +---+---+---+---+---+---+---+---+ When receiving a CONTEXT_MEMORY option, the compressor SHOULD take actions to compress the packet stream in a way that requires less decompressor memory resources, or stop compressing the packet stream. 9. Security Consideration 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, TCP does specify an alternative encryption method in which only the TCP payload is encrypted and the headers are left in the clear. That would still allow header compression to be applied. 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, and TCP headers and possibly also valid TCP checksums. Such corruption may be detected with end-to-end authentication and integrity mechanisms that will not be affected by the compression. Moreover, this header compression scheme uses an internal checksum for verification of reconstructed 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 IR, CO 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. Pelletier, et. al [Page 92] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 10. IANA Considerations The ROHC profile identifier 0x00XX <# Editor's Note: To be replaced before publication #> has been reserved by the IANA for the profile defined in this document. <# Editor's Note: To be removed before publication #> A ROHC profile identifier must be reserved by the IANA for the profile defined in this document. Profiles 0x0000-0x0005 have previously been reserved, which means this profile could be 0x0006. As for previous ROHC profiles, profile numbers 0xnnXX must also be reserved for future updates of this profile. A suggested registration in the "RObust Header Compression (ROHC) Profile Identifiers" name space would then be: Profile Usage Document identifier 0x0006 ROHC TCP [RFCXXXX (this)] 0xnn06 Reserved 11. Acknowledgments The authors would like to thank Qian Zhang, Hong Bin Liao and Richard Price for their work with early versions of this specification. Thanks also to Fredrik Lindstroem for reviewing the packet formats, as well as to Robert Finking for valuable input. 12. Authors' Addresses Ghyslain Pelletier Ericsson AB Box 920 SE-971 28 Lulea, Sweden Phone: +46 8 404 29 43 Fax: +46 920 996 21 EMail: ghyslain.pelletier@ericsson.com Lars-Erik Jonsson Ericsson AB Box 920 SE-971 28 Lulea, Sweden Phone: +46 8 404 29 61 Fax: +46 920 996 21 EMail: lars-erik.jonsson@ericsson.com Pelletier, et. al [Page 93] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 Mark A West Roke Manor Research Ltd Romsey, Hants, SO51 0ZN United Kingdom Phone: +44 1794 833311 Email: mark.a.west@roke.co.uk Carsten Bormann Universitaet Bremen TZI Postfach 330440 Bremen D-28334 Germany Phone: +49 421 218 7024 Fax: +49 421 218 7000 EMail: cabo@tzi.org Kristofer Sandlund Effnet AB Stationsgatan 69 S-972 34 Lulea Sweden Phone: +46 920 609 17 Fax: +46 920 609 27 EMail: kristofer.sandlund@effnet.com 13. References 13.1. Normative references [1] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [2] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H., Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke, T., Yoshimura, T. and H. Zheng, "RObust Header Compression (ROHC): Framework and four profiles: RTP, UDP, ESP, and uncompressed", RFC 3095, July 2001. [3] Pelletier, G., "Robust Header Compression (ROHC): Context Replication for ROHC profiles", Internet Draft (work in progress), , October 2004. Pelletier, et. al [Page 94] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 [4] R. Finking, C. Bormann and G. Pelletier, "Formal Notation for Robust Header Compression (ROHC-FN)", Internet Draft (work in progress), , February 2005. [5] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [6] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [7] S. Bradner, "The Internet Standards Process - Revision 3", RFC 2026, October 1996. [8] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [9] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. 13.2. Informative References [10] Jonsson, L-E., "Requirements on ROHC IP/TCP header compression", Internet Draft (work in progress), , September 2004. [11] West, M. and S. McCann, "TCP/IP Field Behavior", Internet Draft (work in progress), , October 2004. [12] Jonsson, L-E. and G. Pelletier, "RObust Header Compression (ROHC): A compression profile for IP", RFC 3843, June 2003. [13] Jacobson, V., and R. Braden, "TCP Extensions for Long-Delay Paths", LBL, ISI, October 1988. [14] Jacobson, V.,"Compressing TCP/IP Headers for Low-Speed Serial Links", RFC 1144, February 1990. [15] Jacobson, V., Braden, R. and D. Borman, "TCP Extensions for High Performance", RFC 1323, May 1992. [16] Braden, R. "T/TCP -- TCP Extensions for Transactions Functional Specification", ISI, July 1994. [17] Connolly, T., et al, "An Extension to TCP: Partial Order Service", University of Delaware, November 1994. Pelletier, et. al [Page 95] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 [18] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 1889, January 1996. [19] Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery Algorithms", NOAO, January 1997. [20] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP Selective Acknowledgment Options", RFC 2018, October 1996. [21] Degermark, M., Nordgren, B. and S. Pink, "IP Header Compression", RFC 2507, February 1999. [22] Floyd, S., Mahdavi, J., Mathis, M. and M. Podolsky, "An Extension to the Selective Acknowledgement (SACK) Option for TCP", RFC 2883, July 2000. [23] Ramakrishnan, K., Floyd and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, September 2001. [24] Jacobson, V., "Fast Retransmit", Message to the end2end-interest mailing list, April 1990. [25] Perkins, C., "Minimal Encapsulation within IP", RFC 2004, October 1996. Pelletier, et. al [Page 96] INTERNET-DRAFT ROHC Profile for TCP/IP February 21, 2005 Copyright Statement Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. This Internet-Draft expires August 21, 2005. Pelletier, et. al [Page 97]