Network Working Group                        Lars-Erik Jonsson, Ericsson
INTERNET-DRAFT                                                    Sweden
Expires: August 23, 2001                               February 23, 2001








            Requirements for ROHC IP/TCP Header Compression
               <draft-ietf-rohc-tcp-requirements-00.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
   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 a submission of the IETF ROHC WG. Comments should be
   directed to the authors or the ROHC WG mailing list,
   rohc@cdt.luth.se.


Abstract

   This document contains requirements for the IP/TCP header compression
   scheme (profile) to be developed by the ROHC WG. The structure of
   this document is inherited from the document defining IP/UDP/RTP
   requirements for ROHC.







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.  Document history

   February 23, 2001 - draft-ietf-rohc-rtp-requirements-00.txt

     Initial version of this document to initiate discussion on
     requirements for TCP compression in ROHC.


.  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]. However,
   existing schemes for TCP [RFC-1144, RFC-2507] do not have the same
   robustness problems as RTP compression has had since overall
   efficiency for TCP traffic is usually achieved by using reliable
   links where losses do not occur. Therefore the objectives for a new
   TCP compression are instead ; improved compression efficiency,
   enhanced capabilities for compression of header fields including TCP
   options and finally incorporation of TCP compression into the ROHC
   framework [ROHC].


.  Header compression requirements

   The following requirements have, more or less arbitrarily, been
   divided into four 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 concerns what kind of
   headers that must be compressed efficiently. The third and forth
   groups finally concern performance requirements and capability
   requirements which stem from the properties of the anticipated link
   technologies.

   ---- TO BE REMOVED ----
   In chapter 2.5, some issues are listed that MUST be subject to
   further discussions within the WG.
   --------- END ---------









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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 rely on such
      recommendations being followed.


2.2.  Supported headers and kinds of TCP streams

   1. IPv4 and IPv6: Must support both IPv4 and IPv6.

      Justification: IPv4 and IPv6 will both be around during the
      foreseeable future.

   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 these 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 support compression of headers of 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.

      Note: This applies to the TCP stream before as well as after it
      has passed through an internet.




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   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.

      Note: It is of course 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, and in
      addition to that also the case when the SYN or FIN bits are set.

      Justification: These bits are commonly used today.

   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:
              - Window scale, [RFC-1323]
              - Selective Acknowledgement (SACK), [RFC-2018, RFC-2883]
              - Timestamp, [RFC-1323]


2.3.  Performance issues

   1. Performance/Spectral Efficiency: 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 the primary goal here since
      the requirements for robustness [see 2. below] is less stringent
      for TCP traffic than for UDP(/RTP) [RTP-REQ].

      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. Error propagation: For TCP traffic, link layer retransmissions
      should be applied to make use of the bandwidth in the most
      efficient way. Lost or damaged headers should thus not occur and
      therefore it is not a primary goal to have mechanisms for error
      propagation avoidance in case of such events.




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      Justification: To provide robustness against loss or damage
      introduced by the link, efficiency must be sacrificed. Since loss
      or damage is not expected for TCP traffic, efficiency should
      instead be prioritized. This does not mean that some robustness
      should not be provided, if efficiency can still be optimized.

      Note: In general, error propagation due to header compression
      should be kept at an absolute minimum. Error propagation is
      defined as the loss or damage of headers subsequent to headers
      lost or damaged by the link, even if those subsequent headers are
      not lost or damaged.

      Note: There are at least two kinds of error propagation; loss
      propagation, where a lost header causes subsequent headers to be
      lost or damaged, and damage propagation, where a damaged header
      causes subsequent headers to be lost or damaged.

   3. 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 means
      that the gain of header compression could be low since normally
      header compression sends full headers initially and small
      compressed headers first after the initiation phase.

   4a. Moderate Packet Reordering: The scheme should efficiently handle
      moderate reordering (2-3 packets) in the packet stream reaching
      the compressor.

      Justification: This kind of reordering is common.

   4b. Packet Reordering: The scheme should be able to compress when
      there are reordered packets in the TCP stream reaching the
      compressor.

      Justification: Reordering happens regularly in the Internet.
      However, since the Internet is engineered to run TCP reasonably
      well, excessive reordering will not be common and need not be
      handled with optimum efficiency.

   5. Processing delay: The scheme must not contribute significantly to
      system delay budget.











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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 feedback is not desirable for other reasons.

   2. Configurable frame size fluctuation: It should be possible to
      restrict the number of different frame sizes used by the scheme.

      Justification: Some radio technologies support only a limited
      number of frame sizes efficiently.

      Note: Somewhat degraded performance is to be expected when such
      restrictions are applied.

      Note: This implies that a list of "good" frame sizes must be made
      available and that ROHC may pick a suitable header format to
      utilize available space as well as possible.

   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


2.5.  Open issues - For further discussions

   1. As this draft is written today it assumes that TCP traffic is sent
      over links where link-layer retransmissions are applied. This
      approached was chosen based on previous discussions but may of
      course be questioned. One alternative could be to require TWO
      different solutions, one for links with retransmissions and one
      for links where errors and loss may occur.

   2. Should there be any exceptions (notes) to the generality section,
      2.2.->3.?

   3. What should be the approach for residual bit errors? Should we
      split the error propagation section into two sections, one for
      loss propagation and one for damage propagation?











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.  IANA Considerations

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


.  Security Considerations

   A protocol specified to meet these requirements, e.g., [ROHC], 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.


.  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.


   [RTP-REQ]   Mikael Degermark, "Requirements for IP/UDP/RTP header
               compression", Internet draft (work in progress),
               February 2001.
               <draft-ietf-rohc-rtp-requirements-05.txt>

   [ROHC]      C. Bormann, "Robust Header Compression (ROHC)", Internet
               draft (work in progress), February 2001.
               <draft-ietf-rohc-rtp-08.txt>

   [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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.  Author's address

   Lars-Erik Jonsson              Tel: +46 920 20 21 07
   Ericsson Erisoft AB            Fax: +46 920 20 20 99
   Box 920                        Mobile: +46 70 554 82 71
   SE-971 28 Lulea
   Sweden                         EMail: lars-erik.jonsson@ericsson.com





This Internet-Draft expires August 23, 2001.








































Jonsson                                                         [Page 8]