Network Working Group Lars-Erik Jonsson, Ericsson INTERNET-DRAFT Ghyslain Pelletier, Ericsson Expires: January 2002 Sweden July 20, 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 an individual submission to the IETF. Comments should be directed to the authors. 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 lower 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 July 20, 2001 Table of contents 1. Introduction....................................................3 2. Terminology.....................................................5 3. Overview of the link-layer assisted profile.....................6 3.1. Providing packet type identification.....................6 3.2. Replacing the sequence number............................7 3.3. CRC replacement..........................................7 3.4. Applicability of this profile............................8 4. Additions and exceptions compared to ROHC RTP...................8 4.1. Additional packet types..................................8 4.1.1. A no-header packet (NHP)........................8 4.1.2. A context check packet (CCP)....................8 4.1.3. A context synchronization packet (CSP)..........9 4.2. Interfaces towards the assisting lower layers...........10 4.2.1. Interface between compressor and lower layer...11 4.2.2. Interface between lower layer and decompressor.12 4.3. Agreement on optimistic approach........................12 4.4. Feedback option RHP-REQUEST.............................13 4.5. Periodic context verification...........................13 4.6. Use of context identifiers..............................13 5. Implementation issues..........................................14 5.1. Implementation parameters and signals...................14 5.1.1. Implementation parameters at compressor........14 5.1.2. Implementation parameters at decompressor......15 5.2. Implementation structures...............................16 5.2.1. Compressor context.............................16 5.2.2. Decompressor context...........................16 5.3. Implementation over various link technologies...........16 6. IANA considerations............................................17 7. Security considerations........................................17 8. Acknowledgements...............................................17 9. References.....................................................17 10. Authors addresses..............................................18 Jonsson, Pelletier [Page 2] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP July 20, 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 [IP, 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 July 20, 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 the ROHC RTP profile in [ROHC] (profile number 1). 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 regular 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 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 layer and sacrificing efficiency for less frequently occurring larger header packets. 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 layer together. Therefore, additional standards-track 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 July 20, 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 LL Link Layer LLA Link Layer Assisted ROHC RTP Profile MSB Most Significant Bit 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] Link layer Link layer in this document refers to the physical link layer. Lower layer Lower layer in this document 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 between the ROHC implementation and the physical link layer used to adapt both implementations. ROHC RTP ROHC RTP in this document refers to the IP/UDP/RTP profile as defined in [ROHC]. Jonsson, Pelletier [Page 5] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP July 20, 2001 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 to be transmitted together with payloads corresponding to these optimal sizes. 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 lower layer. +---------------------------------------+ | | | | The LLA ROHC | ROHC RTP, +-----------------+ profile | Profile #1 | | | | LLA Additions | | | | +---------------------+-----------------+ By putting additional requirements on the lower layer compared to [RTP-LLG], it is possible for ROHC 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 (optional in PT0 for Reliable 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 which context the compressed packet belongs to and how it should be interpreted. This is a functionality that must be provided by some means. ROHC RTP packets with header will still be possible to distinguish between since they have this identifier, but Jonsson, Pelletier [Page 6] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP July 20, 2001 what must be provided is a mechanism to separate those packets with header from the packets without header. This functionality MUST therefore be provided by the lower 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 lower layer MUST guarantee in-order delivery and provide an indication of packet loss over the link. This is basically the same principle as VJ header compression [VJHC] relies on. 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 All RRP packets carry a CRC calculated over the uncompressed header (optional in PT0 for Reliable mode). 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 Unidirectional and Optimistic mode) - 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 a 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 lower 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 checks SHOULD be performed. Jonsson, Pelletier [Page 7] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP July 20, 2001 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 profile #1 should be implemented thus depends on the characteristics of the link itself. For most RTP packet streams, LLA will work exactly as profile #1, while it will be more efficient for packet streams with certain characteristics. LLA will never be less efficient than ROHC RTP profile #1. 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. 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. A 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) when the sequence number has incremented with one from the previous packet. 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 restrictions on when NHP should be used (see section 4.4 and 4.5). Note that since the lower layer is responsible of separating NHP packets from RHP packets, an indication from the compressor to the lower layer MUST be provided upon delivery of an NHP packet. 4.1.2. A 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 compressor in addition to the compressed packet and provided to the lower layer. Whether the Jonsson, Pelletier [Page 8] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP July 20, 2001 packet is sent over the link and delivered to the decompressor is not decided by the compressor, but by the lower 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 | +---+---+---+---+---+---+---+---+ This packet is defined by one of the unused packet type identifiers from ROHC RTP, carried in the first octet of the packet, and the first bit of the following octet being set to 0. As for any ROHC packet, except NHP, the packet MAY begin with ROHC padding and/or carry context identification. The CRC is the 7-bits CRC over the original uncompressed header as described in [ROHC section 5.9.2]. If the lower layer implementation makes use of the CCP feature, the last CCP packet received from the compressor MUST always be used, i.e. the CCP corresponding to the last data packet sent (NHP or RRP). Accordingly, if a CCP packet is received by the decompressor, it MUST be used as a context verification 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]. 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 the link layer implementation specifications for this profile. 4.1.3. A context synchronization packet (CSP) The CCP packet can be used to ensure correctness of the decompressor context, but when this verification fails a request must be sent to get an update. All packets must then be discarded until the proper context is re-established. However, it would in many cases be beneficial to send not only the header verifying CRC of the previous packet but also the header itself without its associated payload. This would allow not only to Jonsson, Pelletier [Page 9] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP July 20, 2001 perform a context verification but also to provide a context repair mechanism. Another situation when it could be useful to send a packet with only the header information is when the packet stream overruns the channel bandwidth and data must be discarded. Instead of discarding the whole packet at the compressor side, which would mean that the decompressor context might be invalidated, only the payload could be discarded and the header sent to keep the decompressor context synchronized while still saving bandwidth. Both cases above 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 : +---+---+---+---+---+---+---+---+ The CSP packet is identified using the same packet type identifier as the CCP packet with the first bit of the second octet being set to 1, and it uses the same CRC. The first two octets of the CSP packet are thus identical to the CCP packet, except for the first bit of the second octet being set to 1 instead of 0. As for any ROHC packet, except NHP, the packet MAY begin with ROHC padding and/or carry context identification. The difference of the CSP from the CCP packet is the presence of a compressed ROHC header that can be used to synchronize the decompressor context. 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. 4.2. Interfaces towards the assisting lower layers 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 lower layers is defined as interfaces between the ROHC LLA compressor/decompressor elements 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. Jonsson, Pelletier [Page 10] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP July 20, 2001 | | + + +---------------------+ +---------------------+ | ROHC RTP HC | | ROHC RTP HD | +---------------------+ +---------------------+ | LLA profile | | LLA profile | +=====================+ +=====================+ | ROHC to lower layer | | Lower layer to ROHC | | interface | | interface | +=====================+ +=====================+ | Applicable | | Applicable | | link technology | | link technology | +=====================+ +=====================+ | | +------>---- CHANNEL ---->-----+ The figure also underline the need for additional standards-track documents to specify how to fulfill these interfaces for a link technology for which this profile is relevant. 4.2.1. Interface between compressor and lower layer This section defines the interface semantics between the compressor and the lower layer, providing rules for packet delivery from the compressor. +------------------+-----+-----+-----+-----+-----+-----+-----+ | \ Interface | RRP | Seg | NHP | CCP | Seq | Seg | NHP | | Case \ Parameter | | RRP | | | Num | Flg | Flg | +------------------+-----+-----+-----+-----+-----+-----+-----+ | 1 - NHP, No Seg. | x | | x | x | x | | x | +------------------+-----+-----+-----+-----+-----+-----+-----+ | 2 - NHP, Seg. | | x | x | x | x | x | x | +------------------+-----+-----+-----+-----+-----+-----+-----+ | 3 - No Seg. | x | | | x | x | | | +------------------+-----+-----+-----+-----+-----+-----+-----+ | 4 - Seg. | | x | | x | x | x | | +------------------+-----+-----+-----+-----+-----+-----+-----+ Table 1: Data delivery from the compressor As seen in table 1, four different delivery scenarios are possible. Case 1 represent what needs to be delivered from compressor to lower layers when the compressor allows sending of an NHP packet, without any segmentation applied to the corresponding RRP packet. Case 2 differs from case 1 in that a segmented RRP is being delivered. Case 3 and case 4 are the cases where a packet without header cannot be delivered. If the compressor delivers a NHP packet to the lower layer, it MUST also provide the RRP packet, a CCP packet, the Sequence Number and Jonsson, Pelletier [Page 11] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP July 20, 2001 the indication that a NHP packet MAY be used. Otherwise the RRP packet MUST be delivered together with a CCP packet. Furthermore, for the case where the RRP packet is delivered to the lower layer as a segmented ROHC packet according to the rules in chapter 5.5.1, an indication MUST be provided by the compressor. 4.2.2. Interface between lower layer and decompressor The interface semantics between the lower 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 headers, 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 lower layer MUST provide the following to the decompressor: - an indication of packet loss - the received packets together with a indication whether the packet received is an NHP or not 4.3. Agreement on optimistic approach 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, but it may also be partially used in the Reliable mode. 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, with the LLA profile the CRC is not present in the NHP packet and 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. 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. An LLA decompressor MUST use the optimistic approach knowledge to detect possible context loss events. If context loss is suspected it Jonsson, Pelletier [Page 12] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP July 20, 2001 MUST invalidate the context and not forward any packets before a context synchronization event has happened. 4.4. Feedback option, RHP-REQUEST The RHP-REQUEST option could 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, at least one RRP with CRC SHOULD be sent immediately. +---+---+---+---+---+---+---+---+ | Opt Type = 8 | Opt Len = 0 | +---+---+---+---+---+---+---+---+ 4.5. Periodic context verification As described in chapter 3.3, transparency is expected to be guaranteed by the functionality provided by the lower layer. 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 Reliable mode PT0 packets) be sent periodically (possibly with an increasing period), even when the normal logic allows for NHP packets to be used. Since a CCP packet serves the same purpose as a regular periodic verification with RRP, indication of CCP transmission is beneficial to the compressor, which can ignore some periodic RRP verifications. 4.6. 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 lower layers 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 (NHP packet stream) have been lost. Jonsson, Pelletier [Page 13] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP July 20, 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 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 lower layer MUST provide a packet type identification. If no field is available for this purpose from the protocol at the link layer, then it is suggested to use a leading sequence 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 lower layer. If this parameter set is used, all RHP packets MUST be padded to fit the smallest possible of the preferred sizes. If the unpadded packet size, 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 14] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP July 20, 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 packet 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. 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 decompressor NHP_PACKET -- value: boolean This parameter informs the decompressor that the packet being delivered is a NHP packet. The decompressor MUST accept this packet type indicator from the lower layer. A lower layer MUST set this indicator for every NHP packet delivered to true, and to false for any other packet. Jonsson, Pelletier [Page 15] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP July 20, 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. 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 compared 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 An additional field MUST for all modes be kept in memory to store a CRC value. The decompressor MUST calculate the CRC for every packet 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 link 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 should be implemented over various link technologies must be described in additional standards track 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 a lower layer implementations is [1111101]. Jonsson, Pelletier [Page 16] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP July 20, 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 valuable inputs about the typical links that LLA can be applied to. Thanks also to Mikael Degermark and Zhigang Liu for fruitful discussions that led to improvements of this profile. 9. References [ROHC] C. Bormann, "Robust Header Compression (ROHC)", RFC 3095, July 2001. [VJHC] V. Jacobson, "Compressing TCP/IP Headers for Low-Speed Serial Links", RFC 1144, February 1990. [IPHC] M. Degermark, B. Nordgren, S. Pink, "IP Header Compression", RFC 2507, February 1999. [CRTP] S. Casner, V. Jacobson, "Compressing IP/UDP/RTP Headers for Low-Speed Serial Links", RFC 2508, February 1999. [CRTPC] M. Degermark, H. Hannu, L.-E. Jonsson, K. Svanbro, "Evaluation of CRTP Performance over Cellular Radio Networks", IEEE Personal Communications Magazine, Volume 7, number 4, pp. 20-25, August 2000. Jonsson, Pelletier [Page 17] INTERNET-DRAFT A Link-Layer Assisted ROHC RTP July 20, 2001 [RTP-REQ] M. Degermark, "Requirements for IP/UDP/RTP Header Compression", RFC 3096, July 2001. [RTP-LLG] K. Svanbro, "Lower Layer Guidelines for Robust RTP/UDP/IP Header Compression", Internet draft (work in progress), February 2001. [IP] J. Postel, "Internet Protocol", RFC 791, September 1981. [IPv6] S. Deering, R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [UDP] J. Postel, "User Datagram Protocol", RFC 768, August 1980. [TCP] J. Postel, "Transmission Control Protocol", RFC 793, September 1981. [RTP] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 1889, January 1996. [VTC2000] K. Svanbro, H. Hannu, L.-E. Jonsson, M. Degermark, "Wireless real time IP-services enabled by header compression", proceedings of IEEE VTC2000, May 2000. [MOMUC01] G. Liu, et al., "Experimental field trials results of Voice-over IP over WCDMA links", MoMuC'01 - The International Workshop on Mobile Multimedia Communications, Conference proceedings, February 2001. 10. Authors addresses Lars-Erik Jonsson Tel: +46 920 20 21 07 Ericsson Erisoft AB Fax: +46 920 20 20 99 Box 920 SE-971 28 Lulea Sweden EMail: lars-erik.jonsson@ericsson.com Ghyslain Pelletier Tel: +46 920 20 24 32 Ericsson Erisoft AB Fax: +46 920 20 20 99 Box 920 SE-971 28 Lulea Sweden EMail: ghyslain.pelletier@epl.ericsson.se This Internet-Draft expires January 20, 2002. Jonsson, Pelletier [Page 18]