Enhanced Compressed RTP (CRTP) for Links with High Delay, Packet Loss and Reordering
RFC 3545

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Network Working Group                                           T. Koren
Request for Comments: 3545                                 Cisco Systems
Category: Standards Track                                      S. Casner
                                                           Packet Design
                                                          J. Geevarghese
                                         Motorola India Electronics Ltd.
                                                             B. Thompson
                                                                P. Ruddy
                                                           Cisco Systems
                                                               July 2003

       Enhanced Compressed RTP (CRTP) for Links with High Delay,
                      Packet Loss and Reordering

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This document describes a header compression scheme for point to
   point links with packet loss and long delays.  It is based on
   Compressed Real-time Transport Protocol (CRTP), the IP/UDP/RTP header
   compression described in RFC 2508.  CRTP does not perform well on
   such links: packet loss results in context corruption and due to the
   long delay, many more packets are discarded before the context is
   repaired.  To correct the behavior of CRTP over such links, a few
   extensions to the protocol are specified here.  The extensions aim to
   reduce context corruption by changing the way the compressor updates
   the context at the decompressor: updates are repeated and include
   updates to full and differential context parameters.  With these
   extensions, CRTP performs well over links with packet loss, packet
   reordering and long delays.

Koren, et al.               Standards Track                     [Page 1]
RFC 3545             Enhanced Compressed RTP (CRTP)            July 2003

Table of Contents

   1.  Introduction .................................................  2
       1.1.  CRTP Operation .........................................  4
       1.2.  How do contexts get corrupted? .........................  4
       1.3.  Preventing context corruption ..........................  5
       1.4.  Specification of Requirements ..........................  5
   2.  Enhanced CRTP ................................................  5
       2.1.  Extended COMPRESSED_UDP packet .........................  6
       2.2.  CRTP Headers Checksum .................................. 11
       2.3.  Achieving robust operation ............................. 13
             2.3.1.  Examples ....................................... 15
   3.  Negotiating usage of enhanced-CRTP ........................... 18
   4.  Security Considerations ...................................... 18
   5.  Acknowledgements ............................................. 19
   6.  References ................................................... 19
       6.1.  Normative References ................................... 19
       6.2.  Informative References ................................. 20
   7.  Intellectual Property Rights Notice .......................... 20
   8.  Authors' Addresses ........................................... 21
   9.  Full Copyright Statement ..................................... 22

1.  Introduction

   RTP header compression (CRTP) as described in RFC 2508 was designed
   to reduce the header overhead of IP/UDP/RTP datagrams by compressing
   the three headers.  The IP/UDP/RTP headers are compressed to 2-4
   bytes most of the time.

   CRTP was designed for reliable point to point links with short
   delays.  It does not perform well over links with high rate of packet
   loss, packet reordering and long delays.

   An example of such a link is a PPP session that is tunneled using an
   IP level tunneling protocol such as L2TP.  Packets within the tunnel
   are carried by an IP network and hence may get lost and reordered.
   The longer the tunnel, the longer the round trip time.

   Another example is an IP network that uses layer 2 technologies such
   as ATM and Frame Relay for the access portion of the network.  Layer
   2 transport networks such as ATM and Frame Relay behave like point to
   point serial links in that they do not reorder packets.  In addition,
   Frame Relay and ATM virtual circuits used as IP access technologies
   often have a low bit rate associated with them.  These virtual
   circuits differ from low speed serial links in that they may span a
   larger physical distance than a point to point serial link. Speed of