Internet DRAFT - draft-ietf-rohc-rtp-requirements


Network Working Group   Mikael Degermark (editor) /University of Arizona
INTERNET-DRAFT                                                       USA
Expires: Aug 5, 2001                                         Feb 5, 2001

         Requirements for robust IP/UDP/RTP header compression

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-

   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 to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This document is a submission of the IETF ROHC WG. Comments should be
   directed to its mailing list,


   This document contains requirements for robust IP/UDP/RTP header
   compression to be developed by the ROHC WG. It is based on the ROHC
   charter, discussions in the WG, the 3GPP document "3GPP TR 23.922",
   version 1.0.0 of October 1999 [TR], as well as contributions from

Degermark (Ed)                                                  [Page 1]

INTERNET-DRAFT      Requirements for IP/UDP/RTP ROHC         Feb 5, 2001

0.  History and Change Log
   Feb 5, 2001 - 05

      Changes requested by the IESG:

      Removal of note to Transparency requirement.

      Addition of the sections Security Considerations and IANA

   Dec 5, 2000 - 04

      Changed the definitions of loss propagation and damage propagation
      to coincide with how they are used in draft-ietf-rohc-rtp-XX.txt

   Nov 1, 2000 - 03

      Changed "packet" to "header" on several places.

   June 7, 2000 - 02

      Transparency: Added note saying that the ROHC WG has not found an
      instance where semantically identical is not the same as bitwise

      Ubiquity: Added note saying that ROHC may recommend changes that
      will improve compression efficiency.

      2.2.4 IPSEC: new requirement

      Performance/Spectral Efficiency: changed "error conditions" to
      "operating conditions".

      Split Cellular Handover requirement in two: one dealing with loss
      events caused by handover, and another dealing with context
      recreation after switching compressor or decompressor to a new

      Added note to Genericity requirement stating that it applies to an
      RTP stream before as well as after it has passed through IP-

      Added note to Error Propagation requirement that explains the
      terms "loss propagation" and "damage propagation".

      2.3.11: Residual bit-error rate: New requirement

   May 20, 2000 - 01

Degermark (Ed)                                                  [Page 2]

INTERNET-DRAFT      Requirements for IP/UDP/RTP ROHC         Feb 5, 2001

      added "robust" to the title of the document.

      2.1.2 Mobile-IP: Added requirement for compression of Home Address

      2.3.3 Cellular handover: Added note to requirement stating that
      handover procedure will not be prescribed as the procedure will be
      highly system-dependent. However, it must be possible to run ROHC
      such that handover is an efficient operation.

      2.3.7 Packet misordering: Added note to explain why only moderate
      misordering needs to be handled efficiently.

      2.3.10 Delay. New requirement.

   March 29, 2000 - 00

      Initial version of this document. Distributed over the ROHC WG
      mailing list prior to 47th IETF in Adelaide.

Degermark (Ed)                                                  [Page 3]

INTERNET-DRAFT      Requirements for IP/UDP/RTP ROHC         Feb 5, 2001

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 round
   trip times. The schemes must perform well for cellular links build
   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 round trip times.

   The following requirements have, more or less arbitrarily, been
   divided into three groups. The first group deals with requirements
   concerning the impact of an header compression scheme on the rest of
   the Internet infrastructure. The second group concerns what kind of
   headers that must be compressed efficiently. The final group concerns
   efficiency requirements and requirements which stem from the
   properties of the anticipated link technologies.

2. Header compression requirements

   Several current standardization efforts in the cellular arena aim at
   supporting voice over IP and other real-time services over IP, e.g.,
   GERAN (specified by the ETSI SMG2 standards group), and UTRAN
   (specified by the 3GPP standards organization). It is critical for
   these standardization efforts that a suitable header compression
   scheme is developed before completion of the Release 2000 standards.
   Therefore, it is imperative that the ROHC WG keeps its schedule.

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), UDP, or RTP implementations.

   Justification: Ease of deployment.

Degermark (Ed)                                                  [Page 4]

INTERNET-DRAFT      Requirements for IP/UDP/RTP ROHC         Feb 5, 2001

   Note: The ROHC WG may recommend changes that would increase the
   compression efficiency for the RTP streams emitted by
   implementations.  However, ROHC cannot rely on such recommendations
   being followed.

2.2 Supported headers and kinds of RTP 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} 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

   Justification: It is very likely that Mobile IP will be used by
   cellular devices.

   3. Genericity: Must support compression of headers of arbitrary RTP

   Justification: There must be a generic scheme which can compress
   reasonably well for any payload type and traffic pattern. This does
   not preclude optimizations for certain media types where the traffic
   pattern is known, e.g., for low-bandwidth voice and low-bandwidth

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

   4. IPSEC: ROHC should be able to compress headers containing IPSEC

   Note: It is of course not possible to compress the encrypted part of
   an ESP header, nor the cryptographic data in an AH header.

2.3 Efficiency

   1. Performance/Spectral Efficiency: Must provide low relative
   overhead under expected operating conditions; compression efficiency
   should be better than for RFC2508 under equivalent operating
   conditions. The error rate should only marginally increase the
   overhead under expected operating conditions.

Degermark (Ed)                                                  [Page 5]

INTERNET-DRAFT      Requirements for IP/UDP/RTP ROHC         Feb 5, 2001

   Justification: Spectrum efficiency is a primary goal. RFC2508 does
   not perform well enough.

   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: 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

   Justification: Error propagation reduces spectral efficiency and
   reduces quality. CRTP suffers severely from error propagation.

   Note: There are at least two kinds of error propagation; loss
   propagation, where an error causes subsequent headers to be lost, and
   damage propagation, where an error causes subsequent headers to be

   3a. Handover loss events: There must be a way to run ROHC where loss
   events of length 150 milliseconds are handled efficiently and where
   loss or damage propagation is not introduced by ROHC during such

   Justification: Such loss events can be introduced by handover
   operations in a (radio) network between compressor and decompressor.
   Handover operations can be frequent in cellular systems.  Failure to
   handle handover well can adversely impact spectrum efficiency and

   3b. Handover context recreation: There must be a way to run ROHC that
   deals well with events where the route of an RTP conversation changes
   such that either the compressor or the decompressor of the
   conversation is relocated to another node, where the context needs to
   be recreated. ROHC must not introduce erroneous headers during such
   events, and should not introduce packet loss during such events.

   Justification: Such events can be frequent in cellular systems where
   the compressor/decompressor on the network side is close to the radio
   base stations.

   Note: In order to aid developers of context recreation schemes where
   context is transfered to the new node, the specification shall
   outline what parts of the context is to be transfered, as well as
   conditions for its use. Procedures for context recreation (and

Degermark (Ed)                                                  [Page 6]

INTERNET-DRAFT      Requirements for IP/UDP/RTP ROHC         Feb 5, 2001

   discard) without such context transfer will also be provided.

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

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

   6. Multiple links: The scheme must perform well when there are two or
   more cellular links in the end-to-end path.

   Justification: Such paths will occur.

   Note: loss on previous links will cause irregularities in the RTP
   stream reaching the compressor. Such irregularities should only
   marginally affect performance.

   7a. Packet Misordering: The scheme should be able to compress when
   there are misordered packets in the RTP stream reaching the
   compressor.  No misordering is expected on the link between
   compressor and decompressor.

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

   7b. Moderate Packet Misordering: The scheme should efficiently handle
   moderate misordering (2-3 packets) in the packet stream reaching the

   Note: This kind of reordering is common.

   8. Unidirectional links/multicast: Must operate (possibly with less
   efficiency) over links where there is no feedback channel or where
   there are several receivers.

   9. 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.

Degermark (Ed)                                                  [Page 7]

INTERNET-DRAFT      Requirements for IP/UDP/RTP ROHC         Feb 5, 2001

   10. Delay: ROHC should not noticeably add to the end-to-end delay.

   Justification: RTP packets carrying data for interactive voice or
   video have a limited end-to-end delay budget.

   Note: This requirement is intended to prevent schemes that achieve
   robustness at the expense of delay, for example schemes that require
   that header i+x, x>0, is received before header i can be

   Note: Together with 2.3.5, this requirement implies that ROHC will
   not noticeably add to the jitter of the RTP stream, other than what
   is caused by variations in header size.

   Note: This requirement does not prevent a queue from forming upstream
   a bottleneck link. Nor does it prevent a compressor from utilizing
   information from more than one header in such a queue.

   11. Residual errors: For a residual bit-error rate of at most 1e-5,
   the ROHC scheme must not increase the error rate.

   Justification: Some target links have a residual error rate, i.e,
   rate of undetected errors, of this magnitude.

   Note: In the presence of residual bit-errors, ROHC will need error
   detection mechanisms to prevent damage propagation.  These mechanisms
   will catch some residual errors, but those not caught might cause
   damage propagation.  This requirement states that the reduction of
   the damage rate due to the error detection mechanisms must not be
   less than the increase in error rate due to damage propagation.

3.  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 involvment.

4.  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.

Degermark (Ed)                                                  [Page 8]

INTERNET-DRAFT      Requirements for IP/UDP/RTP ROHC         Feb 5, 2001

5.  Editor's address

     Mikael Degermark                    Tel (May-July): +46 70 833-8933
     Dept. of Computer Science                      Tel: +1 520 621-3489
     University of Arizona                          Fax: +1 520 621-4246
     P.O. Box 210077
     Tucson, AZ 85721-0077
     USA                                     EMail:

6.  References

   [TR]        3GPP TR 23.922 version 1.0.0, 3rd Generation partnership
               Project; Technical Specification Group Services and
               Systems Aspects; Architecture for an All IP network.

   [TS]        TS 22.101 version 3.6.0: Service Principles

   [RFC-768]   J. Postel, User Datagram Protocol, RFC 761, August 1980.

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

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

   [CRTP]      S. Casner, V. Jacobson, Compressing IP/UDP/RTP Headers
               for Low-Speed Serial Links, RFC 2508.

   [ROHC]      C. Bormann (ed), Robust Header Compression (ROHC), draft-

This draft expires Aug 5, 2001.

Degermark (Ed)                                                  [Page 9]