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Network Working Group                                  Lars-Erik Jonsson
INTERNET-DRAFT                                                  Ericsson
Expires: April 2003                                     October 23, 2002








              Requirements on ROHC TCP/IP Header Compression
                <draft-ietf-rohc-tcp-requirements-05.txt>


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
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   The list of current Internet-Drafts can be accessed at
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   This document is a submission of the IETF ROHC WG. Comments should be
   directed to the ROHC WG mailing list, rohc@ietf.org.

Abstract

   This document contains requirements on the TCP/IP header compression
   scheme (profile) to be developed by the ROHC WG. The document
   discusses the scope of TCP compression, performance considerations,
   assumptions on the surrounding environment, as well as IPR concerns.
   The structure of this document is inherited from the document
   defining RTP/UDP/IP requirements for ROHC.







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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, using
   technologies such as WCDMA, EDGE, and CDMA-2000. However, the schemes
   should also be applicable to other future link technologies with high
   loss and long roundtrip times.

   The main objective for ROHC has been robust compression of
   IP/UDP/RTP, but the WG is also chartered to develop new header
   compression solutions for IP/TCP [RFC-791, RFC-793]. Since TCP
   traffic, in contrast to RTP, has usually been sent over reliable
   links, existing schemes for TCP [RFC-1144, RFC-2507] 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 [RFC-3150, PILC-ARQ], making
   robustness an important objective also 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 [RFC-3095].

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, while the third and fourth
   groups concern performance requirements and capability requirements
   which stem from the properties of the anticipated link technologies.
   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".




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    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 rely on such
       recommendations being 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 possible changes in the IP header fields must be handled by
       the compression scheme, and commonly changing fields should be
       compressed efficiently. Compression must not be disabled if IPv4
       Options or IPv6 Extensions are present. The compression scheme
       must further consider as normal operation the scenario where
       Explicit Congestion Notification (ECN) [RFC-3168] is applied and
       support efficient compression also 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, the Binding Update
       Destination Option, 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 which 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 sub-headers.

       Justification: IPSEC is expected to be used to provide necessary
       end-to-end security.



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       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 [RFC-2507] should be handled with
       efficient compression, and so also the cases when the SYN, FIN or
       TCP ECN [RFC-3168] 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 provide compression mechanisms also for the options.

       Justification: Since 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), [RFC-2018, RFC-2883]
               - Timestamp, [RFC-1323]

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 RFC2507 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 that losses between compressor and decompressor do not
       result in losses of subsequent packets, or cause damage to the
       context that result 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 [RFC-
       3150, PILC-ARQ]. In such cases, loss propagation due to header
       compression could affect certain TCP mechanisms that are capable
       of handling some losses, and have a negative impact on the
       performance of TCP loss recovery.



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    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, since 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, and 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 a rather weak protection.
       Therefore, although it is not a requirement on the compression
       scheme, the decompressor should discard packets which are known
       to contain residual errors.

    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 all new
       packet streams require full headers to be sent initially and
       allow small compressed headers only after the initiation phase.

       Note: This requirement implies that mechanisms for "context
       sharing" (concurrent packet streams share context information) or
       "context re-use" (new contexts can be built on information from
       previous contexts) 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
       and preferably compress also when there are misordered packets in
       the TCP stream reaching the compressor.

       Justification: Misordering happens regularly in the Internet.
       However, since the Internet is engineered to run TCP reasonably



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       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.  Capability 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. Misordering between compressor and decompressor: The header
       compression scheme must be able to handle misordered packets
       between compressor and decompressor, but can assume that packet
       misordering is indicated to the decompressor by the lower layers.

       Justification: When compression is applied over tunnels,
       misordering often cannot be completely avoided. A header
       compression scheme that prohibits misordering between compressor
       and decompressor 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.

    3. Link delay: Must operate under all expected link delay conditions.

    4. Header compression coexistence: The scheme must fit into the ROHC
       framework together with other ROHC profiles (e.g. [RFC-3095]).

2.5.  Intellectual Property Rights (IPR)

       The ROHC WG must spend effort to achieve a high degree of
       confidence that there is no IPR covering a 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 will be acceptable to the Internet at
       large.





Jonsson                                                         [Page 6]

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3.  IANA Considerations

   A protocol which meets these requirements will require the IANA to
   assign various numbers. This document by itself, however, does not
   require any IANA involvement.

4.  Security Considerations

   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.

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.

6.  References

    [RFC-791]   Jon Postel, Internet Protocol, RFC 791, September 1981.

    [RFC-793]   Jon Postel, Transport Control Protocol, RFC 793,
                September 1981.

    [RFC-1144]  Van Jacobson, "Compressing TCP/IP Headers for Low-Speed
                Serial Links", RFC 1144, February 1990.

    [RFC-2507]  Mikael Degermark, Bjorn Nordgren, Stephen Pink, "IP
                Header Compression", RFC 2507, February 1999.

    [RFC-3096]  Mikael Degermark, "Requirements for IP/UDP/RTP header
                compression", RFC 3096, July 2001.

    [RFC-3095]  Carsten Bormann, et. al., "Robust Header Compression
                (ROHC)", RFC 3095, July 2001.

    [RFC-1323]  Van Jacobson, Bob Braden, Dave Borman, "TCP Extensions
                for High Performance", RFC 1323, May 1992.

    [RFC-2018]  Matt Mathis, Jamshid Mahdavi, Sally Floyd, Allyn
                Romanow, "TCP Selective Acknowledgement Option", RFC
                2018, October 1996.

    [RFC-2883]  Sally Floyd, Jamshid Mahdavi, Matt Mathis, Matthew
                Podolsky, "An Extension to the Selective Acknowledgement
                (SACK) Option for TCP", RFC 2883, July 2000.



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    [RFC-3168]  K. K. Ramakrishnan, Sally Floyd, David L. Black, "The
                Addition of Explicit Congestion Notification (ECN) to
                IP", RFC 3168, September 2001.

    [RFC-3150]  Spencer Dawkins, Gabriel Montenegro, Markku Kojo,
                Vincent Magret, "End-to-end Performance Implications of
                Slow Links", RFC 3150, July 2001.

    [PILC-ARQ]  Gorry Fairhurst, Lloyd Wood, "Advice to link designers
                on link Automatic Repeat reQuest (ARQ)", Internet Draft
                (work in progress), March 2002.
                <draft-ietf-pilc-link-arq-issues-04.txt>

7.  Author's Address

   Lars-Erik Jonsson              Tel: +46 920 20 21 07
   Ericsson AB                    Fax: +46 920 20 20 99
   Box 920
   SE-971 28 Lulea
   Sweden                         EMail: lars-erik.jonsson@ericsson.com

































Jonsson                                                         [Page 8]

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This Internet-Draft expires April 23, 2003.



















Jonsson                                                         [Page 9]


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