Network Working Group Lars-Erik Jonsson INTERNET-DRAFT Ghyslain Pelletier Expires: February 2002 Ericsson August 15, 2001 A Link-Layer Assisted ROHC Profile for IP/UDP/RTP Status of this memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/lid-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This document is a submission of the IETF ROHC WG. Comments should be directed to its mailing list, rohc@cdt.luth.se. Abstract This document defines a ROHC profile for compression of IP/UDP/RTP packets, utilizing functionality provided by the lower layers to increase compression efficiency by completely eliminating the header for most packets during normal operation. The profile is built as an extension to the ROHC RTP profile [ROHC]. It defines additional mechanisms needed in ROHC, states requirements on the assisting layer to guarantee transparency, and specifies general logic for compression and decompression making use of this header-free packet. Jonsson, Pelletier [Page 1] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP August 15, 2001 Table of contents 1. Introduction....................................................3 2. Terminology.....................................................5 3. Overview of the link-layer assisted profile.....................5 3.1. Providing packet type identification.....................6 3.2. Replacing the sequence number............................6 3.3. CRC replacement..........................................7 3.4. Applicability of this profile............................7 4. Additions and exceptions compared to ROHC RTP...................8 4.1. Additional packet types..................................8 4.1.1. No-Header Packet (NHP)..........................8 4.1.2. Context Synchronization Packet (CSP)............8 4.1.3. Context Check Packet (CCP)......................9 4.2. Interfaces towards the assisting layer..................10 4.2.1. Compressor to assisting layer interface........10 4.2.2. Assisting layer to decompressor interface......11 4.3. Optimistic approach agreement (U/O-mode)................12 4.4. NHP and the secure reference principle (R-mode).........12 4.5. Fast context initialization, IR redefinition............13 4.6. Feedback option, RHP-REQUEST............................14 4.7. Periodic context verification...........................14 4.8. Use of context identifier...............................14 5. Implementation issues..........................................15 5.1. Implementation parameters and signals...................15 5.1.1. Implementation parameters at the compressor....15 5.1.2. Implementation parameters at the decompressor..16 5.2. Implementation structures...............................17 5.2.1. Compressor context.............................17 5.2.2. Decompressor context...........................17 5.3. Implementation over various link technologies...........17 6. IANA considerations............................................18 7. Security considerations........................................18 8. Acknowledgements...............................................18 9. References.....................................................18 10. Author's addresses.............................................19 11. Full copyright statement.......................................20 Jonsson, Pelletier [Page 2] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP August 15, 2001 1. Introduction Header compression is a technique used to compress and transparently decompress the header information of a packet on a per-hop basis, utilizing redundancy within individual packets and between consecutive packets within a packet stream. Over the years, several protocols [VJHC, IPHC] have been developed to compress the network and transport protocol headers [IPv4, IPv6, UDP, TCP] and these schemes have been successful at improving efficiency over many wired bottleneck links, such as modem connections over telephone networks. In addition to IP, UDP and TCP compression, an additional compression scheme called Compressed RTP [CRTP] has been developed to further improve compression efficiency for the case of real-time traffic using the Real-time Transport Protocol [RTP]. The schemes mentioned above have all been designed taking into account normal assumptions about link characteristics, which traditionally have been based on wired links only. However, with an increasing number of wireless links in the Internet paths, these assumptions are not valid as general anymore. In wireless environments, especially wide coverage cellular environments, the error rates are relatively high to provide efficient usage of the radio resources. For real-time traffic, which is more sensitive to delays than to errors, this will be normal operating conditions over links such as the 3rd generation cellular links and header compression must therefore tolerate packet loss. However, with the previously mentioned schemes, especially for real-time traffic compressed by CRTP, high error rates have been shown to significantly reduce header compression performance [CRTPC]. This problem was the driving force for the creation of the RObust Header Compression (ROHC) WG in the IETF. 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. Due to the packet loss robustness problems of CRTP and the demands of the cellular industry for an efficient way of transporting voice over IP over wireless, the main focus of ROHC has so far been on compression of IP/UDP/RTP headers, which are generous in size, especially compared to the payloads often carried by the packets with these headers. ROHC RTP has become a very efficient, robust and capable compression scheme, able to compress the headers down to a total size of one octet only. Also, transparency is guaranteed to an extremely high extent even when residual bit errors are present in compressed headers delivered to the decompressor. The requirements for RTP compression [RTP-REQ], defined by the WG before and during the development process, has thus been fulfilled. As mentioned above, the 3rd generation cellular systems, where IP will be used end-to-end, has been one of the driving forces for ROHC Jonsson, Pelletier [Page 3] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP August 15, 2001 RTP and the scheme has been designed to also suit new cellular air interfaces, such as WCDMA, making even speech services possible with an insignificantly lower spectrum efficiency than with existing one- service circuit switched solutions [VTC2000]. However, other existing air interfaces such as GSM and IS-95 will be used in all-IP networks, adding new implications to the header compression issue. These air interfaces are less flexible with radio bearers optimized for specific payload sizes. This means that not even a single octet of header can be added without using the next higher fixed packet size of the link and that is obviously very costly. For the already deployed speech vocoders, the spectrum efficiency over these links will thus be low compared to the existing circuit switched solutions. To achieve high spectrum efficiency overall with any application, more flexible air interfaces must be deployed and then the ROHC RTP scheme will be excellent, as shown for WCDMA [MOMUC01]. For deployment reasons, it is however important to also achieve efficiency with already existing vocoders and air interfaces, such as GSM and IS-95, with minimal effects on spectral efficiency. This document defines a new link-layer assisted ROHC RTP profile extending ROHC RTP (profile #1) [ROHC], compliant to the ROHC 0-byte requirements [0B-REQ]. The purpose of this new profile is to provide a header free packet format that, for a certain application behavior, can replace a majority of the 1-octet header ROHC RTP packets during normal operation, while still being fully transparent and comply with all the requirements of ROHC RTP [RTP-REQ]. For other applications, compression will be carried out as with normal ROHC RTP. To completely eliminate the compressed header, all functionality normally provided by the 1-octet header has to be provided by other means, typically by utilizing functionality provided by the lower layers and sacrificing efficiency for less frequently occurring larger compressed headers. The latter is not a contradiction since the argument for eliminating the last octet for most packets is not overall efficiency in general. It is important to remember that the purpose of this profile is to provide efficient matching of existing applications to existing link technologies, not efficiency in general. The additional complexity introduced by this profile, although minimized by a tight integration with already existing ROHC functionality, implies that it should therefore only be used to optimize performance of specific applications over specific links. When implementing this profile over various link technologies, care must be taken to guarantee that all the functionality needed is provided by ROHC and the lower layers together. Therefore, additional documents should specify how to incorporate this profile on top of various link technologies. Jonsson, Pelletier [Page 4] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP August 15, 2001 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. CCP Context Check Packet CRC Cyclic Redundancy Check CSP Context Synchronization Packet LLA Link Layer Assisted ROHC RTP Profile NHP No Header Packet ROHC RObust Header Compression RHP ROHC Header Packet (a non-NHP packet, i.e. RRP, CSP or CCP) RRP ROHC RTP Packet as defined in [ROHC, profile 1] Assisting layer Assisting layer refers to any entity implementing the interface to ROHC as defined in section 4.2. It may, as an example, refer to a sub-layer used to adapt the ROHC implementation and the physical link layer. Lower layers Lower layers in this document refers to entities located below ROHC in the protocol stack, including the assisting layer. ROHC RTP ROHC RTP in this document refers to the IP/UDP/RTP profile (profile #1) as defined in [ROHC]. 3. Overview of the link-layer assisted profile This ROHC IP/UDP/RTP profile is designed to be used over channels that have been optimized for specific payload sizes and therefore cannot efficiently accommodate header information when transmitted together with payloads corresponding to these optimal sizes. +---------------------------------------+ | | The LLA ROHC | ROHC RTP, | profile | Profile #1 +-----------------+ | | LLA Additions | +---------------------+-----------------+ The LLA profile extends, thus also inherits all functionality from, the ROCH RTP profile by defining some additional functionality and an interface between the ROHC component towards the assisting layer. Jonsson, Pelletier [Page 5] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP August 15, 2001 By putting additional requirements on the lower layers compared to [ROHC], it is possible to infer the information needed to maintain robust and transparent header compression even though the headers are completely eliminated during most of the operation time. Basically, what this profile does is to replace the smallest ROHC header of one octet with a no-header format by providing the header functionality by other means. Smallest header in Smallest header in ROHC RTP (profile #1) LLA ROHC RTP profile +--+--+--+--+--+--+--+--+ ++ | 1 octet | -----> || No Header +--+--+--+--+--+--+--+--+ ++ | | Header field functionality +-------------------> provided by other means The fields present in the one octet ROHC RTP header for PT0 are the packet type identifier, the sequence number and the CRC (U/O-mode). The subsequent sections elaborate more on the replacement of the functionality of these fields. 3.1. Providing packet type identification All ROHC headers carry a packet type identifier, indicating to the decompressor how it should be interpreted. This is a functionality that must be provided by some means. ROHC RTP packets with compressed header will still be possible to distinguish between since they have this identifier, but a mechanism to separate those packets with header from the packets without header is needed. This functionality MUST therefore be provided by the assisting layer in one way or another. 3.2. Replacing the sequence number From the sending application, the RTP sequence number is increased by one for each packet sent. The purpose of the sequence number is thus to cope with packet reordering and packet loss. If reordering or loss has occurred before the compression point, the compressor can easily avoid problems by not allowing usage of a header-free packet. However, the compressor can not in beforehand anticipate loss or reordering that may occur between compressor and decompressor. Therefore, the assisting layer MUST guarantee in-order delivery (already assumed by [ROHC]) and it MUST provide an indication for each packet loss over the link. This is basically the same principle as VJ header compression [VJHC] relies on. Jonsson, Pelletier [Page 6] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP August 15, 2001 Note that guarantees for in-order delivery and packet loss indication not only makes it possible to infer the sequence number information, it also supersedes the main functionality of the CRC, which normally takes care of errors due to long losses and bit errors in the compressed sequence number. 3.3. CRC replacement Most RRP packets carry a CRC calculated over the uncompressed header. This CRC is used by the decompressor to verify that the decompressed header is correct. This verification serves three purposes: - Detection of longer losses than can be covered by the sequence number LSBs (this applies to U/O-mode only) - Protection against failures caused by residual bit errors in the compressed header - Protection against faulty implementations or other causes of error Since this profile defines an NHP packet without this CRC, care must be taken to fulfill these purposes by other means. Detection of long losses is already covered since the assisting layer MUST provide indication of all packet losses. Furthermore, the NHP packet has one important advantage compared to RHP packets because residual bit errors can not damage a header that is not even sent. It is thus reasonable to assume that compression and decompression transparency can be guaranteed even without a CRC in header-free packets. However, to detect also unexpected errors and thereby provide transparency in the ROHC class, periodical context verifications SHOULD be performed (see section 4.7). 3.4. Applicability of this profile The LLA profile can be used on any link technology capable of providing the necessary required functionality described in previous sections. Whether LLA ROHC RTP or ROHC RTP should be implemented thus depends on the characteristics of the link itself. For most RTP packet streams, LLA will work exactly as ROHC RTP, while it will be more efficient for packet streams with certain characteristics. LLA will never be less efficient than ROHC RTP. Note as well that LLA, like all other ROHC profiles, is fully transparent to any packet stream reaching the compressor. LLA does not make any assumptions about the packet stream but will produce optimal performance for packet streams with certain characteristics, e.g. synchronized streams exactly matching the timing of the assisting link over which the LLA profile is implemented. Jonsson, Pelletier [Page 7] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP August 15, 2001 4. Additions and exceptions compared to ROHC RTP 4.1. Additional packet types The LLA profile defines three new packet types to be used in addition to the RRP packet types defined by [ROHC]. The following sections describe these packet types and their purpose in detail. 4.1.1. No-Header Packet (NHP) A no-header packet (NHP), thus a packet consisting only of a payload, is defined and MAY be used instead of ROHC RTP packet type 0 (PT0). Note that the requirement for using PT0 in the first place is basically that all header fields must be unchanged or follow the currently established change pattern. In addition, there are some considerations for the use of the NHP (see 4.3, 4.4, 4.6 and 4.7). The NHP packet, if delivered by the LLA compressor, can only be sent by the assisting layer if the corresponding RTP Sequence Number for this packet has incremented by one from the value for the packet previously sent. Otherwise, the RHP or CSP MUST be sent. 4.1.2. Context Synchronization Packet (CSP) The case where the packet stream overruns the channel bandwidth may lead to data being discarded, which may result in decompressor context invalidation. It might therefore be beneficial to send a packet with only the header information and discard only the payload. This would allow for both context verification and context synchronization at the decompressor, while still saving bandwidth. This case can be handled with the Context Synchronization Packet (CSP), which has the following format: 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 1 1 1 1 0 0 1 | Packet type identifier +---+---+---+---+---+---+---+---+ | 1 | CRC | +---+---+---+---+---+---+---+---+ : ROHC header without padding : : or context identification : +---+---+---+---+---+---+---+---+ This packet is defined by one of the unused packet type identifiers from ROHC RTP, carried in the first octet of the base header, and the first bit of the following octet being set to 1. As for any ROHC packet, except NHP, the packet MAY begin with ROHC padding and/or Jonsson, Pelletier [Page 8] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP August 15, 2001 carry context identification. ROHC segmentation might also be applied to CSP packets. The CRC used in the CSP packet is the 7-bits CRC over the original uncompressed header defined in [ROHC section 5.9.2]. Note that when the decompressor has received a CSP packet and updated the context accordingly, the packet including any possible data following the CSP encapsulated compressed header MUST be discarded. 4.1.3. Context Check Packet (CCP) A Context Check Packet (CCP), which does not carry any payload but only a CRC value in addition to the packet type identifier, is defined. The CCP packet MAY be created by the assisting layer from the CSP packet provided by the compressor. Whether the packet is sent over the link and delivered to the decompressor is therefore decided by the assisting layer. The purpose of the CCP is to provide a useful packet that MAY be sent by a synchronized physical link layer in the case where data must be sent at fixed intervals, even if no compressed packet is available. The CCP has the following format: 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 1 1 1 1 0 0 1 | Packet type identifier +---+---+---+---+---+---+---+---+ | 0 | CRC | +---+---+---+---+---+---+---+---+ The CPP packet is identified using the same packet type identifier as the CSP packet with the first bit of the second octet being set to 0, and it uses the same CRC. The two octets of the CPP packet are thus identical to the first two octets of the CSP packet, except for the first bit of the second octet being set to 0 instead of 1. The difference of the CPP from the CSP is the absence of a compressed ROHC header. This packet can therefore only be used by the decompressor to perform context verification. An assisting layer MAY use the CCP packet to signal a packet loss before the link. If the assisting layer implementation makes use of the CCP, the last CCP packet received from the compressor MUST always be used, i.e. the CCP corresponding to the last non-CCP packet sent (NHP, RRP or CSP). Accordingly, if a CCP packet is received by the decompressor and used for context verification, it MUST be for the last packet decompressed unless a packet loss indication was previously received. The 7-bit CRC MUST always be calculated for all decompressed packets and saved in the decompressor context in order to use the CCP. Upon CRC failure, actions MUST be taken as specified in [ROHC, section 5.3.2.2.3]. Jonsson, Pelletier [Page 9] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP August 15, 2001 Note that the usage of CCP is optional and depends on the characteristics of the actual link. Whether it is used or not MUST therefore be specified in link layer implementation specifications for this profile. 4.2. Interfaces towards the assisting layer This profile relies on the lower layers to provide the necessary functionality to allow NHP packets to be sent. This interaction between LLA and the assisting layer is defined as interfaces between the ROHC LLA compressor/decompressor and the LLA applicable link technology. The figure below shows the various levels, as defined in [ROHC] and this document, making up for a complete implementation of the LLA profile. | | + + +-------------------------+ +-------------------------+ | ROHC RTP HC | | ROHC RTP HD | +-------------------------+ +-------------------------+ | LLA profile | | LLA profile | +=========================+ +=========================+ | ROHC to assisting layer | | Assisting layer to ROHC | | interface | | interface | +=========================+ +=========================+ | Applicable | | Applicable | | link technology | | link technology | +=========================+ +=========================+ | | +------>---- CHANNEL ---->-----+ The figure also underline the need for additional documents to specify how to fulfill these interfaces for a link technology for which this profile is relevant. 4.2.1. Compressor to assisting layer interface This section defines the interface semantics between the compressor and the assisting layer, providing rules for packet delivery from the compressor. The interface defines the following parameters: RRP, RRP segmentation flag, CSP, CSP segmentation flag, NHP and RTP Sequence Number. All parameters, except the NHP, MUST always be delivered to the assisting layer. This leads to two possible delivery scenarios: a. RRP, CSP, NHP and RTP Sequence Number are delivered, along with the corresponding segmentation flags accordingly set. Jonsson, Pelletier [Page 10] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP August 15, 2001 This corresponds to the case when the compressor allows sending of an NHP packet, with or without segmentation being applied to the corresponding RRP/CSP packets. Recall that delivery of an NHP packet occurs when the ROHC RTP compressor would have used a ROHC PT0. b. RRP, CSP and RTP Sequence Number are delivered, along with the corresponding segmentation flags accordingly set. This corresponds to the case when the compressor does not allow sending of an NHP packet. Segmentation might be applied to the corresponding RRP and CSP packets. Segmentation may be applied independently to an RRP or a CSP packet if its size exceeds the largest value provided in the PREFERRED PACKET_SIZES list and if the LARGE_PACKET_ALLOWED parameter is set to false. Note that the CCP can always be inferred from the CSP, which in turn must always be delivered by the compressor. The CCP is not required to be provided by the compressor and therefore does not appear in the list of required interface parameters. 4.2.2. Assisting layer to decompressor interface The interface semantics between the assisting layer and the decompressor are defined here, and provide simple rules for the delivery of received packets to the decompressor. The decompressor needs a way to identify NHP packets from RHP packets. Also, when receiving packets without header, the decompressor needs a way to infer the sequencing information to keep synchronization between received payload and the sequence information of the decompressed headers. To achieve this, the assisting layer MUST provide the following to the decompressor: - an indication for each packet loss for CID=0 - the received packet together with a indication whether the packet received is an NHP or not Jonsson, Pelletier [Page 11] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP August 15, 2001 4.3. Optimistic approach agreement (U/O-mode) ROHC defines an optimistic approach for updates to reduce the header overhead. This approach is fully exploited in the Optimistic and Unidirectional modes of operation. Due to the presence of a CRC in all compressed headers, the optimistic approach is defined as a compressor issue only because the decompressor will always be able to detect an invalid context through the CRC check. However, no CRC is present in the NHP packet defined by the LLA profile. Therefore the loss of an RHP packet updating the context may not always be detected. To avoid this problem, the compressor and decompressor must agree on the principles for the optimistic approach. If, for example, the compressor sends three consecutive updates to convey a header field change, the decompressor must know this and invalidate the context in case of three or more consecutive packet losses. An LLA decompressor MUST use the optimistic approach knowledge to detect possible context loss events. If context loss is suspected it MUST invalidate the context and not forward any packets before a context synchronization event has happened. It is REQUIRED that all documents describing how the LLA profile is implemented over a certain link technology MUST define how the optimistic approach is agreed between compressor and decompressor. It could be with a fixed principle, negotiation at startup or by other means but it must be unambiguously defined. 4.4. NHP and the secure reference principle (R-mode) Similar to the optimistic approach used for U/O-mode, R-mode relies on the secure reference principle. In parallel, the NHP packet introduces some additional considerations also in relation to the secure reference principle. At the decompression side, the secure reference principle [ROHC, section 5.5] states that only packets carrying a 7- or 8-bit CRC can update the context and be used for decompression of subsequent packets. This rule is not in any way modified by LLA. However, to be able to decompress subsequent NHP packets, the decompressor must keep a counter for the number of received non- updating packets following the established linear pattern (NHP or R- 0). The application of this counter to the context defines a pseudo- context. This pseudo-context is used to decompress NHPs, while subsequent updating packets are decompressed based on the context, as usual. Jonsson, Pelletier [Page 12] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP August 15, 2001 Since the NHP packets do not update the decompressor context, the compressor sliding windows can obviously not be updated with values corresponding to these packets. The compressor must therefore also keep a counter for the number of non-updating packets sent following the established linear pattern. NHP transmission is thus allowed if PT0 would normally be allowed, not only based on the context but also based on the pseudo-context. 4.5. Fast context initialization, IR redefinition As initial IR packets might overrun the channel bandwidth and significantly delay decompressor context establishment, it might be beneficial to initially discard the payload. This allows state transitions and higher compression efficiency to be achieved with minimal delay. To serve this purpose, the D-bit from the basic structure of the ROHC RTP IR packet [ROHC section 5.7.7.1] is redefined for the LLA profile. The meaning of the D bit for D=0 (no dynamic chain) is extended to indicate that the payload has been discarded when assembling the IR packet. All other fields keep their meaning as defined for ROHC RTP. The resulting structure, using small CIDs and CID=0, becomes: 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 | 1 | 1 | 1 | 1 | 1 | 0 | D | +---+---+---+---+---+---+---+---+ | Profile | 1 octet +---+---+---+---+---+---+---+---+ | CRC | 1 octet +---+---+---+---+---+---+---+---+ | Static | variable length | chain | - - - - - - - - - - - - - - - - | Dynamic | not present if D = 0 | chain | present if D = 1, variable length - - - - - - - - - - - - - - - - | Payload | not present if D = 0 | | present if D = 1, variable length - - - - - - - - - - - - - - - - D: D = 0 indicates that the dynamic chain is not present and the payload has been discarded. After an IR packet with D=0 has been processed by the decompressor, the packet MUST be discarded. Jonsson, Pelletier [Page 13] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP August 15, 2001 4.6. Feedback option, RHP-REQUEST The RHP-REQUEST option MAY be used by the decompressor to request an RHP for context verification. This option should be used if only NHP have been received for a long time and the context therefore has not been verified recently. If the compressor receives a feedback packet with this option, the sending of an RRP with CRC SHOULD be triggered immediately. +---+---+---+---+---+---+---+---+ | Opt Type = 8 | Opt Len = 0 | +---+---+---+---+---+---+---+---+ 4.7. Periodic context verification As described in section 3.3, transparency is expected to be guaranteed by the functionality provided by the lower layers. This ROHC profile would therefore be at least as reliable as the older header compression schemes [VJHC, IPHC, CRTP], which do not make use of a header compression CRC. However, since ROHC RTP normally is extremely safe to use from a transparency point of view, it would be desirable if that also could be achieved with this profile. To provide an additional guarantee for transparency and also catch non expected errors, such as errors due to faulty implementations, it is RECOMMENDED that RRP packets (with the CRC present also for R- mode PT0 packets) be sent periodically (possibly with an increasing period), even when the compressor logic allows for NHP packets to be used. 4.8. Use of context identifier Since a NHP can not carry a context identifier (CID), there is a restriction on how this profile may be used, related to context identification. Independent of which CID size has been negotiated, NHP packets can only be used for CID=0. If the decompressor receives a NHP packet, it can only belong to CID=0. Note that if multiple packet streams are handled by a compressor running LLA, the assisting layer MUST in case of packet loss be able to tell for which CID the loss occurred, at least it must be able to tell if packets with CID=0 (packet stream with NHPs) have been lost. Jonsson, Pelletier [Page 14] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP August 15, 2001 5. Implementation Issues This document specifies mechanisms for the protocol and leaves details on the use of these mechanisms to the implementers. This chapter aims to provide guidelines, ideas and suggestions for implementation of this profile. 5.1. Implementation parameters and signals As described in [ROHC, section 6.3], implementations uses parameters to set up configuration information and to stipulate how a ROHC implementation is to operate. The following are additions to the ones used by ROHC RTP implementations, needed by this profile. Note that if the PREFERRED_PACKET_SIZES parameters defined here are used, they obsolete all PACKET_SIZE and PAYLOAD_SIZE parameters of ROHC RTP. 5.1.1. Implementation parameters at the compressor ALWAYS_PAD -- value: boolean This parameter may be set by an external entity to specify to the compressor that every RHP packet MUST be padded using the ROHC padding. The assisting layer MUST provide a packet type identification. If no field is available for this purpose from the protocol at the link layer, then a leading sequence may be used to identify RHP packets from NHP packets. Although the use of a leading sequence is obviously not efficient since it sacrifices efficiency for RHP packets, this leading sequence applies only to packets with headers in order to favor the use of packets without headers. If a leading sequence is desired for RHP identification, the lower layer MAY use ROHC padding for this by setting the ALWAYS_PAD parameter. By default, this parameter is set to FALSE. PREFERRED PACKET SIZES -- list of: SIZE -- value: integer (octets) ONLY_NHP -- value: boolean This parameter set governs which packet sizes that are preferred by the assisting layer. If this parameter set is used, all RHP packets MUST be padded to fit the smallest possible preferred size. If the size of the unpadded packet, or in the case of ALWAYS_PAD being set the packet with minimal one octet padding, is larger than the maximal preferred packet size, the compressor has two options. It may either deliver this larger packet with an arbitrary size or it may split the packet into several segments using ROHC segmentation and pad each segment to one of the Jonsson, Pelletier [Page 15] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP August 15, 2001 preferred sizes. Which method to use depends on the value of the LARGE_PACKETS_ALLOWED parameter below. NHP packets can only be delivered to the lower layer if the payload size is part of the preferred packet size set. Furthermore, if ONLY_NHP is set to TRUE for any of the preferred packet sizes, that size is only allowed to be used for NHP packets. By default, no preferred packet sizes are specified and when used the default value of ONLY_NHP is FALSE for the specified sizes. LARGE_PACKETS_ALLOWED -- value: boolean This parameter may be set by an external entity to specify how to handle packets that can not fit in any of the preferred packet sizes specified. If set to TRUE, the compressor MUST deliver the larger packet as it is and not use segmentation. If set to FALSE, the ROHC segmentation scheme MUST be used to split the packet into two or more segments and each segment MUST further be padded to fit into any of the preferred packet sizes. By default, this parameter is set to TRUE, which means that segmentation is disabled. VERIFICATION_PERIOD -- value: integer (octets) This parameter may be set by an external entity to specify to the compressor the minimum frequency for which a packet that validates the context must be sent. This tells the compressor that a packet containing a CRC field MUST be sent at least every number of packets equals to this value (see section 4.7). A value of 0 indicates that no periodical verification are needed. By default, this parameter is set to the value 1, which indicates that packets with the CRC field MUST always be sent. 5.1.2. Implementation parameters at the decompressor NHP_PACKET -- value: boolean This parameter informs the decompressor that the packet being delivered is an NHP packet. The decompressor MUST accept this packet type indicator from the lower layer. An assisting layer MUST set this indicator to true for every NHP packet delivered, and to false for any other packet. Jonsson, Pelletier [Page 16] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP August 15, 2001 PACKET_LOST -- signal This parameter indicates to the decompressor that a packet has been lost on the link between the compressor and the decompressor, for each packet that was lost. 5.2. Implementation structures This section provides some explanatory material on data structures specific to this profile that a ROHC implementation will have to maintain in one form or another. What is listed here are additions to ROHC RTP [ROHC section 6.5], imposed by this profile. 5.2.1. Compressor context For the compressor context, this profile does not require any additions to the data structures needed for ROHC RTP. 5.2.2. Decompressor context For all modes, an additional field MUST be kept in memory to store a CRC value. The decompressor MUST calculate the CRC for every header decompressed and always keep in one form or another the value calculated for the last packet received. CRC_PREVIOUS: CRC calculated from the decompressed header of the last packet received 5.3. Implementation over various link technologies This document provides the interface semantics and requirements needed from the ROHC compressor and decompressor towards the assisting layer to perform link-layer assisted header compression. However, the document does not provide any link layer specific operational information, except for some implementation suggestions. Further details about how this profile is to be implemented over various link technologies must be described in other documents, where specific characteristics of each link layer can be taken into account to provide optimal usage of this profile. These specifications MAY use a packet type bit pattern unused by this profile to implement signaling on the lower layer. The pattern available to lower layers implementations is [1111101]. Jonsson, Pelletier [Page 17] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP August 15, 2001 6. IANA considerations A ROHC profile identifier must be reserved by the IANA for the IP/UDP/RTP profile defined in this document. Since this additional profile will be used concurrent to the ROHC IP/UDP/RTP profile in [ROHC] and is part of the IETF standards track, an ordinary identifier in the range from 4 to 127 should be reserved. 7. Security considerations The security considerations of ROHC RTP [ROHC section 7] apply also to this document with one addition: in the case of a denial-of- service attack scenario where an intruder inject bogus CCP packets onto the link using random CRC values, the CRC check will fail for incorrect reasons at the decompressor side. This would obviously greatly reduce the advantages of ROHC and any extra efficiency provided by this profile due to unnecessary context invalidation, feedback messages and refresh packets. However, the same remarks related to the presence of such an intruder applies. 8. Acknowledgements The authors would like to thank Ulises Olvera-Hernandez and Francis Lupien for inputs about the typical links that LLA can be applied to. Thanks also to Mikael Degermark for fruitful discussions that led to improvements of this profile, and to Zhigang Liu for valuable inputs especially regarding R-mode operation. 9. References [ROHC] Bormann, C., et. al., "Robust Header Compression (ROHC)", RFC 3095, July 2001. [IPv4] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [IPv6] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [UDP] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. [RTP] Schulzrinne, H., Casner S., Frederick R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 1889, January 1996. [TCP] Postel, P., "Transmission Control Protocol", RFC 793, September 1981. Jonsson, Pelletier [Page 18] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP August 15, 2001 [RTP-REQ] Degermark, M., "Requirements for IP/UDP/RTP Header Compression", RFC 3096, July 2001. [0B-REQ] Jonsson, L-E., "Requirements and Assumptions for ROHC 0- byte Header Compression", Internet Draft, work in progress, August 2001.