Network Working Group Ghyslain Pelletier INTERNET-DRAFT Ericsson AB Expires: October 2003 April 11, 2003 RObust Header Compression (ROHC): Profiles for UDP-Lite 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 ROHC (Robust Header Compression) profiles for compression of RTP/UDP-Lite/IP packets (Real-Time Transport Protocol, User Datagram Protocol Lite, Internet Protocol) and UDP-Lite/IP. These profiles are defined based on their differences with the profiles specified in [RFC-3095] for UDP [RFC-768]. Although both transport protocols are very similar, ROHC profiles must be defined separately for robust compression of UDP-Lite headers because it does not share protocol identifier with UDP. Also, the UDP-Lite Checksum Coverage field does not share the semantics of the corresponding UDP Length field and as a consequence cannot always be inferred anymore. Pelletier [Page 1] INTERNET-DRAFT ROHC Profiles for UDP-Lite April 11, 2003 Table of contents 1. Introduction....................................................3 2. Terminology.....................................................3 3. Background......................................................4 3.1. Overview of the UDP-Lite protocol.........................4 3.2. Expected behaviours of UDP-Lite flows.....................5 3.3. Header field classification...............................6 4. Rationale behind the design of ROHC profiles for UDP-Lite.......6 4.1. Design motivations........................................6 4.2. ROHC considerations.......................................6 5. ROHC profiles for UDP-Lite......................................7 5.1. Context parameters........................................7 5.2. Initialization............................................8 5.2.1. Initialization of the UDP-Lite header [UDP-Lite]........8 5.2.2. Compressor and decompressor logic.......................9 5.3. Packet formats............................................9 5.3.1. General packet format...................................9 5.3.2. Packet type CE: CE(), CE(ON) and CE(OFF)...............10 5.4. Compression logic........................................11 5.5. Decompression logic......................................12 6. Security considerations........................................12 7. IANA considerations............................................12 8. Acknowledgements...............................................12 9. References.....................................................12 10. Authors address...............................................13 Appendix A. Detailed classification of header fields..............14 A.1. UDP-Lite header fields...................................14 A.2. Header compression strategies for UDP-Lite...............16 Appendix B. Detailed format of the CE packet type.................17 Pelletier [Page 2] INTERNET-DRAFT ROHC Profiles for UDP-Lite April 11, 2003 1. Introduction 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 demands of the cellular industry for an efficient way of transporting voice over IP over wireless, ROHC [RFC-3095] has mainly focused on compression of IP/UDP/RTP headers, which are generous in size, especially compared to the payloads often carried by 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. UDP-Lite [UDP-Lite] is a transport protocol similar to the UDP protocol [RFC-768]. UDP-Lite is useful for applications that are designed with the capability to tolerate errors in the payload and for which receiving damaged data is better than dealing with the loss of entire packets. This may be particularly suitable when packets are transported over link technologies where data can be partially damaged, such as wireless links. Separate ROHC profiles are needed for UDP-Lite because it does not share protocol identifier with UDP. Also, the UDP-Lite Checksum Coverage field does not share the semantics of the corresponding UDP Length field and cannot always be inferred. This document defines two ROHC profiles for efficient compression of UDP-Lite headers. The objectives of these profiles are to provide simple modifications to the corresponding ROHC profiles for UDP, as specified in [RFC-3095]. In addition, the ROHC profiles for UDP-Lite support compression of multiple IP headers using the mechanisms defined in [IP-ONLY]. 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. ROHC RTP : RTP/UDP/IP profile 0x0001 defined in [RFC-3095]. ROHC UDP : UDP/IP profile 0x0002 defined in [RFC-3095]. ROHC UDP-Lite : UDP-Lite/IP profile defined in this document. ROHC RTP/UDP-Lite : RTP/UDP-Lite/IP profile defined in this document. Pelletier [Page 3] INTERNET-DRAFT ROHC Profiles for UDP-Lite April 11, 2003 3. Background 3.1. Overview of the UDP-Lite protocol UDP-Lite is a transport protocol defined as an independent variant of the UDP transport protocol. UDP-Lite is very similar to UDP, and allow applications that can tolerate errors in the payload to use a checksum with an optional partial coverage. This is particularly useful with IPv6 [RFC-2460], where the use of the transport-layer checksum is mandatory. UDP-Lite replaces the Length field of the UDP header with a Checksum Coverage field. This field indicates the number of octets covered by the 16-bit checksum, and it is applied on a per-packet basis. The coverage area must always include the UDP-Lite header and may cover the entire packet, in which case UDP-Lite becomes semantically identical to UDP. UDP-Lite and UDP do not share protocol identifier. The UDP-Lite header format: 0 15 16 31 +--------+--------+--------+--------+ | Source | Destination | | Port | Port | +--------+--------+--------+--------+ | Checksum | | | Coverage | Checksum | +--------+--------+--------+--------+ | | : Payload : | | +-----------------------------------+ The UDP-Lite checksum, like the UDP checksum, is an end-to-end mechanism against erroneous delivery of error sensitive data. However, as opposed to UDP, the UDP-Lite checksum may not be transmitted as all zeroes and cannot be disabled for IPv4 [RFC-791]. For UDP, in the case where the checksum is disabled (IPv4 only), the Checksum field maintains a constant value and is normally not sent by the header compression scheme. In the case where the UDP checksum is enabled (mandatory for IPv6), such an unpredictable field cannot be compressed and is sent uncompressed. The UDP Length field, however, is always redundant and can be provided by the IP module. Header compression schemes do not normally transmit any bits of information for this field, as its value can be inferred from the link layer. For UDP-Lite, the checksum also has unpredictable values and this field must always be included as-is in the compressed header, for both IPv4 and IPv6. Furthermore, as the UDP Length field is redefined Pelletier [Page 4] INTERNET-DRAFT ROHC Profiles for UDP-Lite April 11, 2003 as the Checksum Coverage field by UDP-Lite, this leads to different properties for this field from a header compression perspective. The following summarizes the relationship between UDP and UDP-Lite: - UDP-Lite and UDP have different protocol identifiers; - The UDP-Lite checksum cannot be disabled for IPv4; - UDP-Lite redefines the UDP Length field as the Checksum Coverage field, with different semantics; - UDP-Lite is semantically equivalent to UDP when the Checksum Coverage field indicates the total length of the packet. The next section provides a more detailed discussion of the behavior of the Checksum Coverage field of UPD-Lite in relation to header compression. 3.2. Expected behaviours of UDP-Lite flows Per-packet behavior As mentioned in the previous section, the checksum coverage value is applied independently of other packets that may belong to the same flow. Specifically, the value of the checksum coverage may indicate that the UDP-Lite packet is either entirely covered by the checksum, or covered up to some boundary less than the packet size but including the UDP-Lite header. Inter-packet behavior In relation to each other, UDP-Lite packets may exhibit either one of three possible change patterns, where within a sequence of packets the value of the Checksum Coverage field is: 1. changing, while covering the entire packet; 2. unchanging, covering up to a fixed boundary within the packet; 3. changing, but does not follow any specific pattern. The first pattern above corresponds to the semantics of UDP, when the UDP checksum is enabled. For this case, the checksum coverage field varies according to the packet length and may be inferred from the IP module similarly as for the UDP Length field. The second pattern corresponds to the case where the coverage is the same from one packet to another within a particular sequence. For this case, the Checksum Coverage field may be a static value defined in the context and it does not need to be sent in the compressed header. For the third case, no useful change pattern can be identified from packet to packet for the value of the checksum coverage field, and it must be included in the compressed header. Pelletier [Page 5] INTERNET-DRAFT ROHC Profiles for UDP-Lite April 11, 2003 Per-flow behavior Finally, it can be expected that any one of these change patterns for sequences of packets may be predominant at any time during the lifetime of the UDP-Lite flow. A flow that predominantly follows the first two change patterns described above may provide opportunities for compressing the Checksum Coverage field for most of the packets. 3.3. Header field classification In relation to the header field classification from [RFC-3095], the first two patterns represent the case where the value of the Checksum Coverage field behavior is fixed and may be either INFERRED (pattern 1) or STATIC (pattern 2); pattern 3 is for the case where the value varies unpredictably, the field is CHANGING and the value must be sent along with every packet. Additional information regarding the analysis of the behavior of the UDP-Lite fields may be found in the Appendix A. 4. Rationale behind the design of ROHC profiles for UDP-Lite 4.1. Design motivations Simplicity is a strong motivation for the design of the UDP-Lite header compression profiles. The profiles defined for UDP-Lite should entail only a few simple modifications to the corresponding profiles defined for UDP in [RFC-3095]. In addition, whenever UDP-Lite is used in a manner that is semantically identical to UDP, the compression efficiency should be similar. 4.2. ROHC considerations The simplest approach to the definition of ROHC profiles for UDP-Lite is to treat the Checksum Coverage field as an irregular value, and to send it uncompressed for every packet. This may be achieved simply by adding the field to the definition of the general packet format [RFC- 3095]. However, the compression efficiency would then always be less than for UDP. Some care should be given to achieve similar compression efficiency for UDP-Lite as for UDP when the Checksum Coverage field behaves like the UDP Length field. This requires the possibility to infer the Checksum Coverage field when it is equal to the length of the packet. This would otherwise put the UDP-Lite protocol at a disadvantage over links where header compression is used, when its behavior is made similar to the semantics of UDP. Pelletier [Page 6] INTERNET-DRAFT ROHC Profiles for UDP-Lite April 11, 2003 A mechanism to detect the presence of the Checksum Coverage field in compressed headers is thus needed. This is achieved by defining a new packet type, using the unused identifiers from [RFC-3095]. 5. ROHC profiles for UDP-Lite This section describes two ROHC profiles: - RTP/UDP-Lite/IP compression (profile 0x0007) - UDP-Lite/IP compression (profile 0x0008) These profiles build on the specifications found in [RFC-3095] with as little modifications as possible. Unless explicitly stated otherwise, the profiles defined herein follow the specifications of ROHC UDP and ROHC RTP, respectively. Note that this document also reuses the notation found in [RFC-3095]. 5.1. Context parameters As described in [RFC-3095], information relevant to previous packets is maintained in a context. This includes information describing the packet stream, or parameters. While the UDP and UDP-Lite protocols share many commonalities, the differences in semantics as described earlier renders the following parameter inapplicable: The parameter context(UDP Checksum) The UDP-Lite checksum cannot be disabled, as opposed to UDP. The parameter context(UDP Checksum) of [RFC-3095, section 5.7] is therefore not used for compression of UDP-Lite. In addition, the UDP-Lite checksum is always sent as-is in every compressed packet. However, the Checksum Coverage field may not always be sent in each compressed packet, and the following context parameter is used to indicate whether or not the field is sent: The parameter context(UDP-Lite Coverage Field Present) Whether the UDP-Lite Checksum Coverage field is present or not in the general packet format (see 5.3.1.) is controlled by the value of the Coverage Field Present (CFP) flag in the context. If context(CFP) is nonzero, the Checksum Coverage field is not compressed and it is present within compressed packets. If context(CFP) is zero, the Checksum Coverage field is compressed and it is not sent. This is the case when the value of the Checksum Coverage field follows a stable inter-packet change pattern; the field has either a constant value or it has a value equal to the packet length for most packets in a sequence (see 3.2.). Pelletier [Page 7] INTERNET-DRAFT ROHC Profiles for UDP-Lite April 11, 2003 Finally, the following context parameter is needed to indicate whether the field should be inferred or taken from a value previously saved in the context: The parameter context(UDP-Lite Coverage Field Inferred) When the UDP-Lite Checksum Coverage field is not present in the compressed header (CFP=0), whether it is inferred or not is controlled by the value of the Coverage Field Inferred (CFI) flag in the context. If context(CFI) is nonzero, the Checksum Coverage field is inferred from the packet length, similarly as for the UDP Length field in ROHC RTP. If context(CFI) is zero, the Checksum Coverage field is decompressed using context(UDP-Lite Checksum Coverage). Therefore, when context(CFI) is updated to a nonzero value, the value of the Checksum Coverage field stored in the context must also be updated. 5.2. Initialization Unless stated otherwise, the mechanisms of ROHC RTP and ROHC UDP found in [RFC-3095] are used also for the ROHC RTP/UDP-Lite and the ROHC UDP-Lite profiles, respectively. In particular, the considerations of ROHC UDP regarding the UDP SN taking the role of the RTP Sequence Number applies to ROHC UDP-Lite. Also, the static context for ROHC UDP-Lite may be initialized by reusing an existing context belonging to a stream compressed using ROHC RTP/UDP-Lite (profile 0x0007), similarly as for ROHC UDP. 5.2.1. Initialization of the UDP-Lite header [UDP-Lite] The structure of the IR and IR-DYN packets and the initialization procedures are the same as for the ROHC profiles for UDP [RFC-3095], with the exception of the dynamic part as specified for UDP. A 2- octet field containing the checksum coverage is added before the Checksum field. This affects the format of dynamic chains in both IR and IR-DYN packets. Dynamic part: +---+---+---+---+---+---+---+---+ / Checksum Coverage / 2 octets +---+---+---+---+---+---+---+---+ / Checksum / 2 octets +---+---+---+---+---+---+---+---+ CRC-DYNAMIC: Checksum Coverage field, Checksum field (octets 5-8). CRC-STATIC: All other fields (octets 1-4). Pelletier [Page 8] INTERNET-DRAFT ROHC Profiles for UDP-Lite April 11, 2003 5.2.2. Compressor and decompressor logic The following logic must be used by both the compressor and the decompressor for assigning values to the parameters context(CFP) and context(CFI) during initialization: Context(CFP) During context initialization, the value of context(CFP) MUST be set to a nonzero value if the Checksum Coverage field differs from the length of the UDP-Lite packet, for any one IR or IR-DYN packet sent (compressor) or received (decompressor); otherwise the value MUST be set to zero. Context(CFI) During context initialization, the value of context(CFI) MUST be set to a nonzero value if the Checksum Coverage field is equal to the length of the UDP-Lite packet within an IR or an IR-DYN packet sent (compressor) or received (decompressor); otherwise the value MUST be set to zero. 5.3. Packet formats The general packet format as defined in [RFC-3095] is modified to include an additional field for the UDP-Lite checksum coverage. A packet type is also defined to handle the specific semantics and characteristics of this field. 5.3.1. General packet format The general packet format of a compressed ROHC UDP-Lite header is similar to the compressed ROHC RTP header [RFC-3095, section 5.7], with modifications to the Checksum field, as well as additional fields for handling multiple IP headers [IP-ONLY, section 3.3.] and for the UDP-Lite checksum coverage: --- --- --- --- --- --- --- --- : List of : / dynamic chains / variable, given by static chain : for additional IP headers : see [IP-ONLY, section 3.3]. --- --- --- --- --- --- --- --- : : 2 octets, + UDP-Lite Checksum Coverage + if context(CFP) = 1 or : : if packet type = CE (see 5.3.2.) --- --- --- --- --- --- --- --- : : + UDP-Lite Checksum + 2 octets : : --- --- --- --- --- --- --- --- Pelletier [Page 9] INTERNET-DRAFT ROHC Profiles for UDP-Lite April 11, 2003 Note that the order of the fields following the optional extension of the general ROHC packet format is the same as the order between the fields in the uncompressed header. Note also that when calculating the CRC for this profile, the Checksum Coverage field is CRC-DYNAMIC. 5.3.2. Packet type CE: CE(), CE(ON) and CE(OFF) The ROHC profiles for UDP-Lite defines a packet type to handle the various possible change patterns of the checksum coverage. This packet type may be used to manipulate the context values that control the presence of the Checksum Coverage field within the general packet format, i.e. context(CFP), and how the field is decompressed, i.e. context(CFI). The 2-octet Checksum Coverage field is always present within the format of this packet (see 5.3.1.). This packet is named Coverage Extension, or CE, and its updating properties depend on the final two bits of the packet type octet (see format below). A naming scheme of the form CE() is used to uniquely identify the properties of a particular CE packet. Although this packet type defines its own format, it may be considered as an extension mechanism for packets of type 2, 1 or 0 [RFC-3095]. This is achieved by substitution of the packet type identifier of the first octet of the base header (the "outer" identifier) with one of the unused packet types from [RFC-3095]. The substituted identifier is then moved to the first octet of the remainder of the base header (the "inner" identifier). The format of the ROHC UDP-Lite CE packet type: 0 1 2 3 4 5 6 7 +---+---+---+---+---+---+---+---+ | 1 1 1 1 1 0 F | K | Outer packet type identifier +===+===+===+===+===+===+===+===+ : : (with inner type identifier) / Inner Base header / variable number of bits, given by : : the inner packet type identifier +---+---+---+---+---+---+---+---+ F,K: F,K = 00 is reserved at framework level (IR-DYN); F,K = 01 indicates CE(); F,K = 10 indicates CE(ON); F,K = 11 indicates CE(OFF). Updating properties: The updating properties of the inner packet type carried within any of the CE packets are always maintained. In addition, CE(ON) always update context(CFP); CE(OFF) always update context(CFP), context(CFI) and context(UDP-Lite Checksum Coverage). Pelletier [Page 10] INTERNET-DRAFT ROHC Profiles for UDP-Lite April 11, 2003 Appendix B provides an expanded view of the resulting format of the CE packet type. Properties of CE(): Aside from the updating properties of the inner packet type carried within CE(), this packet does not update any other context values. CE() thus is mode-agnostic, e.g. it can extend any of packet types 2, 1 and 0, regardless of the current mode of operation [RFC-3095]. CE() may be used when the checksum coverage deviates from the change pattern assumed by the compressor, while the field could previously be compressed. This packet is useful if the occurrence of such deviations are seldom. Properties of CE(ON): In addition to the updating properties of the inner packet type, CE(ON) updates context(CFP) to a nonzero value, i.e. it effectively turns on the presence of the Checksum Coverage field within the general packet format. This is useful when the predominant change pattern of the checksum coverage preclude its compression. CE(ON) can extend any of the context updating packets of type 2, 1 and 0, that is packets with a compressed header containing a CRC [RFC-3095]. Specifically, R-0 and R-1* headers MUST NOT be extended using CE(ON). Properties of CE(OFF): In addition to the updating properties of the inner packet type, CE(OFF) updates context(CFP) to a value of zero, i.e. it effectively turns off the presence of the Checksum Coverage field within the general packet format. This is useful when the change pattern of the checksum coverage seldom deviates from the pattern assumed by the compressor. CE(OFF) also updates context(CFI) to a nonzero value, if field(UDP- Lite Checksum Coverage) is equal to the packet length; otherwise it must be set to zero. Finally, context(UDP-Lite Checksum Coverage) is also updated by CE(OFF). Similarly to CE(ON), CE(OFF) can extend any of the context updating packets of type 2, 1 and 0 [RFC-3095]. 5.4. Compressor logic Should hdr(UDP-Lite Checksum Coverage) be different from context(UDP- Lite Checksum Coverage) and different from the packet length when context(CFP) is zero, the Checksum Coverage field cannot be compressed. In addition, should hdr(UDP-Lite Checksum Coverage) be Pelletier [Page 11] INTERNET-DRAFT ROHC Profiles for UDP-Lite April 11, 2003 different from the packet length when context(CFP) is zero and context(CFI) is nonzero, the Checksum Coverage field cannot be compressed either. For both cases, the field must be sent uncompressed using a CE packet or the context must be reinitialized using an IR packet. 5.5. Decompressor logic For packet types other than IR, IR-DYN and CE that are received when the value of context(CFP) is zero, the Checksum Coverage field must be decompressed using the value stored in the context if the value of context(CFI) is zero; otherwise the field is inferred from the length of the UDP-Lite packet derived from the IP module. 6. Security considerations The security considerations of [RFC-3095] apply integrally to this document without modifications. 7. IANA considerations A ROHC profile identifier must be reserved by the IANA for each of the profiles defined in this document, preferably as listed below: Profile Document Usage Identifier 0x0007 RFCthis ROHC RTP/UDP-Lite 0x0008 RFCthis ROHC UDP-Lite 8. Acknowledgements The author would like to thank Lars-Erik Jonsson, Mats Nordberg for reviews and discussions around this document. 9. References [RFC-3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H., Hannu, H., Jonsson, L., 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. [IP-ONLY] Jonsson, L., "RObust Header Compression (ROHC): A compression profile for IP", Internet draft (work in Pelletier [Page 12] INTERNET-DRAFT ROHC Profiles for UDP-Lite April 11, 2003 progress), January 2003, [RFC-791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC-2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC-768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. [UDP-Lite] Larzon, L., Degermark, M., Pink, S., Jonsson, L., Fairhurst, G., "The UDP-Lite Protocol", Internet draft (work in progress), December 2002, [RFC-1889] Schulzrinne, H., Casner S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 1889, January 1996. 10. Authors address Ghyslain Pelletier Tel: +46 920 20 24 32 Ericsson AB Fax: +46 920 20 20 99 Box 920 SE-971 28 Lulea Sweden EMail: ghyslain.pelletier@epl.ericsson.se Pelletier [Page 13] INTERNET-DRAFT ROHC Profiles for UDP-Lite April 11, 2003 Appendix A. Detailed classification of header fields This section summarizes the difference from the classification found in the corresponding appendix in [RFC-3095], and similarly provides conclusions about how the various header fields should be handled by the header compression scheme to optimize compression and functionality. These conclusions are separated based on the behavior of the UDP-Lite Checksum Coverage field and uses the expected change patterns described in section 3.2 of this document. A.1. UDP-Lite header fields The following table summarizes a possible classification for the UDP- Lite header fields in comparison with the classification for UDP, using the same classes as in [RFC-3095]. Header fields of UDP-Lite and UDP: +-------------------+-------------+ | UDP-Lite | UDP | +-------------------+--------+-------------------+-------------+ | Header | Size | Class | Class | | Field | (bits) | | | +-------------------+--------+-------------------+-------------+ | Source Port | 16 | STATIC-DEF | STATIC-DEF | | Destination Port | 16 | STATIC-DEF | STATIC-DEF | | Checksum Coverage | 16 | INFERRED | | | | | STATIC | | | | | CHANGING | | | Length | 16 | | INFERRED | | Checksum | 16 | CHANGING | CHANGING | +-------------------+--------+-------------------+-------------+ Source and Destination Port Same as for UDP. Specifically, these fields are part of the definition of a stream and must thus be constant for all packets in the stream. The fields are therefore classified as STATIC-DEF. Checksum Coverage This field specifies which part of the UDP-Lite datagram is covered by the checksum. It may have a value of zero or equal to the datagram length if the checksum covers the entire datagram, or it may have any value between eight octets and the length of the datagram to specify the number of octets protected by the checksum, calculated from the first octet of the UDP-Lite header. The value of this field may vary for each packet, and this makes the value unpredictable from a header compression perspective. Pelletier [Page 14] INTERNET-DRAFT ROHC Profiles for UDP-Lite April 11, 2003 Checksum The information used for the calculation of the UDP-Lite checksum is governed by the value of the checksum coverage, and minimally includes the UDP-Lite header. The checksum is a changing field that must always be sent as-is. The total size of the fields in each class, for each expected change patterns (see section 3.2), is summarized in the tables below: Pattern 1: +------------+---------------+ | Class | Size (octets) | +------------+---------------+ | INFERRED | 2 | Checksum Coverage | STATIC-DEF | 4 | Source Port / Destination Port | CHANGING | 2 | Checksum +------------+---------------+ Pattern 2: +------------+---------------+ | Class | Size (octets) | +------------+---------------+ | STATIC-DEF | 4 | Source Port / Destination Port | STATIC | 2 | Checksum Coverage | CHANGING | 2 | Checksum +------------+---------------+ Pattern 3: +------------+---------------+ | Class | Size (octets) | +------------+---------------+ | STATIC-DEF | 4 | Source Port / Destination Port | CHANGING | 4 | Checksum Coverage / Checksum +------------+---------------+ Pelletier [Page 15] INTERNET-DRAFT ROHC Profiles for UDP-Lite April 11, 2003 A.2. Header compression strategies for UDP-Lite The following table revisits the corresponding table (table A.1) for UDP from [RFC-3095, section A.2] and classifies the changing fields, based on the change patterns previously identified in section 3.2. Header compression strategies for UDP-Lite: +----------+---------+-------------+-----------+-----------+ | Field | Pattern | Value/Delta | Class | Knowledge | +==========+=========+=============+===========+===========+ | | #1 | Value | CHANGING | INFERRED | | Checksum |---------+-------------+-----------+-----------+ | Coverage | #2 | Value | RC | UNKNOWN | | |---------+-------------+-----------+-----------+ | | #3 | Value | IRREGULAR | UNKNOWN | +----------+---------+-------------+-----------+-----------+ | Checksum | All | Value | IRREGULAR | UNKNOWN | +----------+---------+-------------+-----------+-----------+ A.2.1. Transmit initially, but be prepared to update UDP-Lite Checksum Coverage (Patterns #1 and #2) A.2.2. Transmit as-is in all packets UDP-Lite Checksum UDP-Lite Checksum Coverage (Pattern #3) Pelletier [Page 16] INTERNET-DRAFT ROHC Profiles for UDP-Lite April 11, 2003 Appendix B. Detailed format of the CE packet type This section provides an expanded view of the format of the CE packet, based on the general ROHC RTP compressed header [RFC-3095] and the general format of a compressed header [IP-ONLY]. The modifications necessary to carry the base header of a packet of type 2, 1 or 0 [RFC-3095] within the CE packet format along with the additional fields to properly handle compression of multiple IP headers results in the following structure for the CE packet type: 0 1 2 3 4 5 6 7 --- --- --- --- --- --- --- --- : Add-CID octet : if for small CIDs and CID 1-15 +---+---+---+---+---+---+---+---+ | 1 1 1 1 1 0 F | K | Outer packet type identifier +---+---+---+---+---+---+---+---+ : : / 0, 1, or 2 octets of CID / 1-2 octets if large CIDs : : +---+---+---+---+---+---+---+---+ | First octet of base header | (with "inner" type indication) +---+---+---+---+---+---+---+---+ / Remainder of base header / variable number of bits +---+---+---+---+---+---+---+---+ : : / Extension / See [RFC-3095], section 5.7. : : --- --- --- --- --- --- --- --- : : + IP-ID of outer IPv4 header + See [RFC-3095], section 5.7. : : --- --- --- --- --- --- --- --- / AH data for outer list / See [RFC-3095], section 5.7. --- --- --- --- --- --- --- --- : : + GRE checksum + See [RFC-3095], section 5.7. : : --- --- --- --- --- --- --- --- : : + IP-ID of inner IPv4 header + See [RFC-3095], section 5.7. : : --- --- --- --- --- --- --- --- / AH data for inner list / See [RFC-3095], section 5.7. --- --- --- --- --- --- --- --- : : + GRE checksum + See [RFC-3095], section 5.7. : : --- --- --- --- --- --- --- --- : List of : / dynamic chains / See [IP-ONLY], section 3.3. : for additional IP headers : Pelletier [Page 17] INTERNET-DRAFT ROHC Profiles for UDP-Lite April 11, 2003 --- --- --- --- --- --- --- --- : : + UDP-Lite Checksum Coverage + 2 octets : : +---+---+---+---+---+---+---+---+ : : + UDP-Lite Checksum + 2 octets : : +---+---+---+---+---+---+---+---+ F,K: F,K = 00 is reserved at framework level (IR-DYN); F,K = 01 indicates CE(); F,K = 10 indicates CE(ON); F,K = 11 indicates CE(OFF). Note that this document does not define (F,K) = 00, as this would collide with the IR-DYN packet type already reserved at the ROHC framework level. Pelletier [Page 18] INTERNET-DRAFT ROHC Profiles for UDP-Lite April 11, 2003 Full Copyright Statement Copyright (C) The Internet Society (2003). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS 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 October 11, 2003. Pelletier [Page 19]