RObust Header Compression (ROHC): Requirements and Assumptions for 0-byte IP/UDP/RTP Compression
RFC 3243

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Network Working Group                                       L-E. Jonsson
Request for Comments: 3243                                      Ericsson
Category: Informational                                       April 2002

                   RObust Header Compression (ROHC):
    Requirements and Assumptions for 0-byte IP/UDP/RTP 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 (2002).  All Rights Reserved.

Abstract

   This document contains requirements for the 0-byte IP/UDP/RTP
   (Internet Protocol/User Datagram Protocol/Real-Time Transport
   Protocol) header compression scheme to be developed by the Robust
   Header Compression (ROHC) Working Group.  It also includes the basic
   assumptions for the typical link layers over which 0-byte compression
   may be implemented, and assumptions about its usage in general.

1.  Introduction

   The goal of the Robust Header Compression (ROHC) Working Group 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.

   ROHC RTP has become a very efficient, robust and capable compression
   scheme, able to compress the IP/UDP/RTP headers down to a total size
   of only one octet.  This makes ROHC RTP an excellent solution for
   future cellular environments with new air interfaces, such as WCDMA,
   making even speech services possible over IP with an insignificantly
   lower spectrum efficiency compared to existing circuit switched
   solutions.

Jonsson                      Informational                      [Page 1]
RFC 3243        Reqs and Assumptions for 0-byte ROHC RTP      April 2002

   However, all-IP cellular networks will also be built with already
   existing air interfaces such as GSM and IS-95, which are less
   flexible using radio bearers optimized for specific frame sizes
   matching the speech codecs used.  This means that not a single octet
   of header can be added without switching to the next higher fixed
   packet size supported by the link, something which is obviously very
   costly.  In the long term, this drawback should of course be
   eliminated with new, more flexible air interfaces, but in the short
   term it would be desirable if an efficiency comparable to the circuit
   switched case could also be achieved for already deployed speech
   codecs when used over the existing air interfaces.  To achieve that,
   it must be possible to completely eliminate the headers for a
   majority of the packets during normal operation, and this is the
   purpose of 0-byte header compression.  All functionality normally
   provided by the 1-octet header must then be provided by some other
   means, typically by utilizing functionality from the lower layer.  It
   is important to remember that the purpose of 0-byte header
   compression is to provide optimal efficiency for applications
   matching the link layer characteristics, not efficiency in general.

   As a starting point for these requirements, the well-established
   requirements base developed in the ROHC WG has been used.  From that,
   the requirements have evolved through input from the 3GPP2 community
   and from discussions within the WG.

2.  Assumptions for the Applicability of 0-byte RTP Header Compression

   The purpose of 0-byte header compression is to provide optimal usage
   of certain links when the traffic pattern of a packet stream
   completely matches the characteristics of that link.  There are no
   assumptions that only packet streams complying with that pattern will
   occur, but optimal efficiency cannot of course be provided when this
   is not the case.

   To make 0-byte header compression feasible, it is assumed that lower
   layers can provide the necessary functionality needed to replace the
   1-octet headers and fulfill the requirements defined in section 3.
   An example is the synchronized nature of most cellular links, which
   can provide sequencing and timing information and make packet loss
   detection possible.

3.  Requirements on 0-byte RTP Header Compression

   Since 0-byte header compression for ROHC IP/UDP/RTP is a variant of
   regular ROHC RTP compression [ROHC], these requirements are described
   as deltas to those defined in the regular RTP requirements [RTP-REQ].
   For simplicity, this section is also separated into the same three
   subsections as the requirements in [RTP-REQ], where the first deals

Jonsson                      Informational                      [Page 2]
RFC 3243        Reqs and Assumptions for 0-byte ROHC RTP      April 2002