RFC 3545 (rfc3545) - Page 2 of 22


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



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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
   light delays within the layer 2 transport network will result in
   higher round trip delays between the endpoints of the circuit.  In



Koren, et al.               Standards Track


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