ROHC TCP requirements: RObust Header Compression (ROHC): Requirements on TCP/IP Header Compression

Network Working Group                                       L-E. Jonsson
Request for Comments: 4163                                      Ericsson
Category: Informational                                      August 2005


                   RObust Header Compression (ROHC):
               Requirements on TCP/IP Header Compression

Status 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 from RFC 3096,
   which defines IP/UDP/RTP requirements for ROHC.

Table of Contents

   1. Introduction ....................................................2
   2. Header Compression Requirements .................................2
      2.1. Impact on Internet Infrastructure ..........................2
      2.2. Supported Headers and Kinds of TCP Streams .................3
      2.3. Performance Issues .........................................4
      2.4. Requirements Related to Link Layer Characteristics .........6
      2.5. Intellectual Property Rights (IPR) .........................7
   3. Security Consideration ..........................................7
   4. IANA Considerations .............................................7
   5. Acknowledgements ................................................7
   6. Informative References ..........................................7

1.  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.

       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.

   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 for RFC 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.

       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.

   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.

2.5.  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.

   [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.com

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