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Network Working Group                                       L-E. JonssonRequest for Comments: 4163                                      EricssonCategory: Informational                                      August 2005RObust Header Compression (ROHC):Requirements on TCP/IP Header CompressionStatus of This Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2005).Abstract   This document contains requirements on the TCP/IP header compression   scheme (profile) to be developed by the RObust Header Compression   (ROHC) Working Group.  The document discusses the scope of TCP   compression, performance considerations, assumptions about the   surrounding environment, as well as Intellectual Property Rights   concerns.  The structure of this document is inherited fromRFC 3096,   which defines IP/UDP/RTP requirements for ROHC.Table of Contents1. Introduction ....................................................22. Header Compression Requirements .................................22.1. Impact on Internet Infrastructure ..........................22.2. Supported Headers and Kinds of TCP Streams .................32.3. Performance Issues .........................................42.4. Requirements Related to Link Layer Characteristics .........62.5. Intellectual Property Rights (IPR) .........................73. Security Consideration ..........................................74. IANA Considerations .............................................75. Acknowledgements ................................................76. Informative References ..........................................7Jonsson                      Informational                      [Page 1]

RFC 4163              Requirements on ROHC TCP/IP            August 20051.  Introduction   The goal of the ROHC WG is to develop header compression schemes that   perform well over links with high error rates and long link roundtrip   times.  The schemes must perform well for cellular links that use   technologies such as Wideband Code Division Multiple Access (W-CDMA),   Enhanced Data rates for GSM Evolution (EDGE), and CDMA2000.  However,   the schemes should also be applicable to other link technologies with   high loss and long roundtrip times.   The main objective for ROHC has been robust compression of IP/UDP/RTP   [5], but the WG is also chartered to develop new header compression   solutions for IP/TCP [1], [2].  Because TCP traffic, in contrast to   RTP, has usually been sent over reliable links, existing schemes for   TCP, [3] and [4], have not experienced the same robustness problems   as RTP compression.  However, there are still many scenarios where   TCP header compression will be implemented over less reliable links   [11], [12], making robustness an important objective for the new TCP   compression scheme.  Other, equally important, objectives for ROHC   TCP compression are: improved compression efficiency, enhanced   capabilities for compression of header fields including TCP options,   and finally incorporation of TCP compression into the ROHC framework   [6].2.  Header Compression Requirements   The following requirements have, more or less arbitrarily, been   divided into five groups.  The first group deals with requirements   concerning the impact of a header compression scheme on the rest of   the Internet infrastructure.  The second group defines what kind of   headers must be compressed efficiently.  The third and fourth groups   concern performance requirements and capability requirements that   stem from the properties of link technologies where ROHC TCP is   expected to be used.  Finally, the fifth section discusses   Intellectual Property Rights related to ROHC TCP compression.2.1.  Impact on Internet Infrastructure   1.  Transparency: When a header is compressed and then decompressed,       the resulting header must be semantically identical to the       original header.  If this cannot be achieved, the packet       containing the erroneous header must be discarded.       Justification: The header compression process must not produce       headers that might cause problems for any current or future part       of the Internet infrastructure.Jonsson                      Informational                      [Page 2]

RFC 4163              Requirements on ROHC TCP/IP            August 2005       Note: The ROHC WG has not found a case where "semantically       identical" is not the same as "bitwise identical".   2.  Ubiquity: Must not require modifications to existing IP (v4 or       v6) or TCP implementations.       Justification: Ease of deployment.       Note: The ROHC WG may recommend changes that would increase the       compression efficiency for the TCP streams emitted by       implementations.  However, ROHC cannot assume such       recommendations will be followed.       Note: Several TCP variants are currently in use on the Internet.       This requirement implies that the header compression scheme must       work efficiently and correctly for all expected TCP variants.2.2.  Supported Headers and Kinds of TCP Streams   1.  IPv4 and IPv6: Must support both IPv4 and IPv6.  This means that       all expected changes in the IP header fields must be handled by       the compression scheme, and commonly changing fields should be       compressed efficiently.  Compression must still be possible when       IPv6 Extensions are present in the header.  When designing the       compression scheme, the usage of Explicit Congestion Notification       (ECN) [10] should be considered as a common behavior.  Therefore,       the scheme must also compress efficiently in the case when the       ECN bits are used.       Justification: IPv4 and IPv6 will both be around for the       foreseeable future, and Options/Extensions are expected to be       more commonly used.  ECN is expected to have a breakthrough and       be widely deployed, especially in combination with TCP.   2.  Mobile IP: The kinds of headers used by Mobile IP{v4,v6} must be       supported and should be compressed efficiently.  For IPv4 these       include headers of tunneled packets.  For IPv6 they include       headers containing the Routing Header and the Home Address       Option.       Justification: It is very likely that Mobile IP will be used by       cellular devices.   3.  Generality: Must handle all headers from arbitrary TCP streams.       Justification: There must be a generic scheme that can compress       reasonably well for any TCP traffic pattern.  This does not       preclude optimizations for certain traffic patterns.Jonsson                      Informational                      [Page 3]

RFC 4163              Requirements on ROHC TCP/IP            August 2005   4.  IPSEC: The scheme should be able to compress headers containing       IPSEC subheaders where the NULL encryption algorithm is used.       Justification: IPSEC is expected to be used to provide necessary       end-to-end security.       Note: It is not possible to compress the encrypted part of an ESP       header, nor the cryptographic data in an AH header.   5.  TCP: All fields supported by [4] should be handled with efficient       compression, as should be the cases when the SYN, FIN or TCP ECN       [10] bits are set.       Justification: These bits are expected to be commonly used.   6.  TCP options: The scheme must support compression of packets with       any TCP option present, even if the option itself is not       compressed.  Further, for some commonly used options the scheme       should also provide compression mechanisms for the options.       Justification: Because various TCP options are commonly used,       applicability of the compression scheme would be significantly       reduced if packets with options could not be compressed.       Note: Options that should be compressed are:                     - Selective Acknowledgement (SACK), [8], [9]                     - Timestamp, [7]2.3.  Performance Issues   1.  Performance/Spectral Efficiency: The scheme must provide low       relative overhead under expected operating conditions;       compression efficiency should be better than forRFC 2507 [4]       under equivalent operating conditions.       Justification: Spectrum efficiency is a primary goal.       Note: The relative overhead is the average header overhead       relative to the payload.  Any auxiliary (e.g., control or       feedback) channels used by the scheme should be taken into       account when calculating the header overhead.   2.  Losses between compressor and decompressor: The scheme should       make sure losses between compressor and decompressor do not       result in losses of subsequent packets, or cause damage to the       context that results in incorrect decompression of subsequent       packet headers.Jonsson                      Informational                      [Page 4]

RFC 4163              Requirements on ROHC TCP/IP            August 2005       Justification: Even though link layer retransmission in most       cases is expected to almost eliminate losses between compressor       and decompressor, there are still many scenarios where TCP header       compression will be implemented over less reliable links [11],       [12].  In such cases, loss propagation due to header compression       could affect certain TCP mechanisms that are capable of handling       some losses; loss propagation could also have a negative impact       on the performance of TCP loss recovery.   3.  Residual errors in compressed headers: Residual errors in       compressed headers may result in delivery of incorrectly       decompressed headers not only for the damaged packet itself, but       also for subsequent packets, because errors may be saved in the       context state.  For TCP, the compression scheme is not required       to implement explicit mechanisms for residual error detection,       but the compression scheme must not affect TCP's end-to-end       mechanisms for error detection.       Justification: For links carrying TCP traffic, the residual error       rate is expected to be insignificant.  However, residual errors       may still occur, especially in the end-to-end path.  Therefore,       it is crucial that TCP is not prevented from handling these.       Note: This requirement implies that the TCP checksum must be       carried unmodified in all compressed headers.       Note: The error detection mechanism in TCP may be able to detect       residual bit errors, but the mechanism is not designed for this       purpose, and might actually provide rather weak protection.       Therefore, although it is not a requirement of the compression       scheme, it should be possible for the decompressor to detect       residual errors and discard such packets.   4.  Short-lived TCP transfers: The scheme should provide mechanisms       for efficient compression of short-lived TCP transfers,       minimizing the size of context initiation headers.       Justification: Many TCP transfers are short-lived.  This may lead       to a low gain for header compression schemes that, for each new       packet stream, requires full headers to be sent initially and       allows small compressed headers only after the initialization       phase.       Note: This requirement implies that mechanisms for building new       contexts that are based on information from previous contexts or       for concurrent packet streams to share context information should       be considered.Jonsson                      Informational                      [Page 5]

RFC 4163              Requirements on ROHC TCP/IP            August 2005   5a. Moderate Packet Misordering: The scheme should efficiently handle       moderate misordering (2-3 packets) in the packet stream reaching       the compressor.       Justification: This kind of misordering is common.   5b. Packet Misordering: The scheme must be able to correctly handle       packet misordering and preferably compress when misordered       packets are in the TCP stream reaching the compressor.       Justification: Misordering happens regularly in the Internet.       However, because the Internet is engineered to run TCP reasonably       well, excessive misordering will not be common and need not be       handled with optimum efficiency.   6.  Processing delay: The scheme should not contribute significantly       to the system delay budget.2.4.  Requirements Related to Link Layer Characteristics   1.  Unidirectional links: Must be possible to implement (possibly       with less efficiency) without explicit feedback messages from       decompressor to compressor.       Justification: There are links that do not provide a feedback       channel or where feedback is not desirable for other reasons.   2.  Link delay: Must operate under all expected link delay       conditions.   3.  Header compression coexistence: The scheme must fit into the ROHC       framework together with other ROHC profiles (e.g., [6]).   4.  Note on misordering between compressor and decompressor:       When compression is applied over tunnels, misordering often       cannot be completely avoided.  The header compression scheme       should not prohibit misordering between compressor and       decompressor, as it would therefore not be applicable in many       tunneling scenarios.  However, in the case of tunneling, it is       usually possible to get misordering indications.  Therefore, the       compression scheme does not have to support detection of       misordering, but can assume that such information is available       from lower layers when misordering occurs.Jonsson                      Informational                      [Page 6]

RFC 4163              Requirements on ROHC TCP/IP            August 20052.5.  Intellectual Property Rights (IPR)   The ROHC WG must spend effort to achieve a high degree of confidence   that there are no known IPR claims that cover the final compression   solution for TCP.   Justification: Currently there is no TCP header compression scheme   available that can efficiently compress the packet headers of modern   TCP, e.g., with SACK, ECN, etc.  ROHC is expected to fill this gap by   providing a ROHC TCP scheme that is applicable in the wide area   Internet, not only over error-prone radio links.  It must thus   attempt to be as future-proof as possible, and only unencumbered   solutions, or solutions where the terms of any IPR are such that   there is no hindrance on implementation and deployment, will be   acceptable to the Internet at large.3.  Security Consideration   A protocol specified to meet these requirements must be able to   compress packets containing IPSEC headers according to the IPSEC   requirement, 2.2.4.  There may be other security aspects to consider   in such protocols.  This document by itself, however, does not add   any security risks.4.  IANA Considerations   A protocol that meets these requirements will require the IANA to   assign various numbers.  This document by itself, however, does not   require any IANA involvement.5.  Acknowledgements   This document has evolved through fruitful discussions with and input   from Gorry Fairhurst, Carsten Bormann, Mikael Degermark, Mark West,   Jan Kullander, Qian Zhang, Richard Price, and Aaron Falk.  The   document quality was significantly improved thanks to Peter Eriksson,   who made a thorough linguistic review.   Last, but not least, Ghyslain Pelletier and Kristofer Sandlund served   as committed working group document reviewers.6.  Informative References   [1]  Postel, J., "Internet Protocol", STD 5,RFC 791, September 1981.   [2]  Postel, J., "Transmission Control Protocol", STD 7,RFC 793,        September 1981.Jonsson                      Informational                      [Page 7]

RFC 4163              Requirements on ROHC TCP/IP            August 2005   [3]  Jacobson, V., "Compressing TCP/IP headers for low-speed serial        links",RFC 1144, February 1990.   [4]  Degermark, M., Nordgren, B., and S. Pink, "IP Header        Compression",RFC 2507, February 1999.   [5]  Degermark, M., "Requirements for robust IP/UDP/RTP header        compression",RFC 3096, July 2001.   [6]  Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,        Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, K., Liu,        Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke, T.,        Yoshimura, T., and H. Zheng, "RObust Header Compression (ROHC):        Framework and four profiles: RTP, UDP, ESP, and uncompressed",RFC 3095, July 2001.   [7]  Jacobson, V., Braden, R., and D. Borman, "TCP Extensions for        High Performance",RFC 1323, May 1992.   [8]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP        Selective Acknowledgement Options",RFC 2018, October 1996.   [9]  Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An        Extension to the Selective Acknowledgement (SACK) Option for        TCP",RFC 2883, July 2000.   [10] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of        Explicit Congestion Notification (ECN) to IP",RFC 3168,        September 2001.   [11] Dawkins, S., Montenegro, G., Kojo, M., and V. Magret, "End-to-        end Performance Implications of Slow Links",BCP 48,RFC 3150,        July 2001.   [12] Fairhurst, G. and L. Wood, "Advice to link designers on link        Automatic Repeat reQuest (ARQ)",BCP 62,RFC 3366, August 2002.Author's Address   Lars-Erik Jonsson   Ericsson AB   Box 920   SE-971 28 Lulea   Sweden   Phone: +46 8 404 29 61   Fax:   +46 920 996 21   EMail: lars-erik.jonsson@ericsson.comJonsson                      Informational                      [Page 8]

RFC 4163              Requirements on ROHC TCP/IP            August 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Jonsson                      Informational                      [Page 9]