draft-ietf-avt-crtp-enhance-05.txt   draft-ietf-avt-crtp-enhance-06.txt 
Audio/Video Transport Working Group Tmima Koren Audio/Video Transport Working Group Tmima Koren
Internet Draft Cisco Systems Internet Draft Cisco Systems
November 2, 2002 Stephen Casner January 28, 2003 Stephen Casner
Expires June 2003 Packet Design Expires August 2003 Packet Design
draft-ietf-avt-crtp-enhance-05.txt John Geevarghese draft-ietf-avt-crtp-enhance-06.txt John Geevarghese
Telseon Telseon
Bruce Thompson Bruce Thompson
Patrick Ruddy Patrick Ruddy
Cisco Systems Cisco Systems
Compressing IP/UDP/RTP headers on links with high delay, Compressing IP/UDP/RTP headers on links with high delay,
packet loss and reordering packet loss and reordering
Status of this memo Status of this memo
skipping to change at line 89 skipping to change at line 89
technologies often have a low bit rate associated with them. These technologies often have a low bit rate associated with them. These
virtual circuits differ from low speed serial links in that they may virtual circuits differ from low speed serial links in that they may
span a larger physical distance than a point to point serial link. span a larger physical distance than a point to point serial link.
Speed of light delays within the layer 2 transport network will Speed of light delays within the layer 2 transport network will
result in higher round trip delays between the endpoints of the result in higher round trip delays between the endpoints of the
circuit. In addition, congestion within the layer 2 transport circuit. In addition, congestion within the layer 2 transport
network may result in an effective drop rate for the virtual circuit network may result in an effective drop rate for the virtual circuit
which is significantly higher than error rates typically experienced which is significantly higher than error rates typically experienced
on point to point serial links. on point to point serial links.
CRTP is widely deployed and has relatively low computational It may be desirable to extend existing CRTP implementations for use
complexity. It is desirable to extend its usage over such links. also over IP tunnels and other virtual circuits, where packet
This can be achieved with a few simple extensions to the protocol. losses, reordering, and long delays are common characteristics. To
address these scenarios, this document defines modifications and
extensions to CRTP to increase robustness to both packet loss and
misordering between the compressor and the decompressor. This is
achieved by repeating updates and allowing the sending of absolute
(uncompressed) values in addition to delta values for selected
context parameters. Although these new mechanisms impose some
additional overhead, the overall compression is still substantial.
The enhanced CRTP, as defined in this document, is thus suitable for
many applications in the scenarios discussed above, e.g. tunneling
and other virtual circuits.
RFC 3095 defines another RTP header compression scheme called Robust RFC 3095 defines another RTP header compression scheme called Robust
Header Compression (ROHC) which is very efficient and robust against Header Compression [ROHC]. ROHC was developed with wireless links
packet loss. However, ROHC is designed for point-to-point links and as the main target, and introduced new compression mechanisms with
does not tolerate the misordering of compressed packets between the the primary objective to achieve the combination of robustness
compressor and decompressor that may occur when these packets are against packet loss and maximal compression efficiency. ROHC is
carried in an IP tunnel across multiple hops. The enhanced CRTP expected to be the preferred compression mechanism over links where
scheme defined in this document includes optimizations to CRTP to compression efficiency is important. However, ROHC was designed
deal with both packet loss and misordering between the compressor with the same link assumptions as CRTP, e.g. that the compression
and decompressor. scheme should not have to tolerate misordering of compressed packets
between the compressor and decompressor, which may occur when
packets are carried in an IP tunnel across multiple hops.
At some time in the future, enhancements may be defined for ROHC to
allow it to perform well in the presence of misordering of
compressed packets. The result might be more efficient than the
compression protocol specified in this document. However, there are
many environments for which the enhanced CRTP defined here may be
the preferred choice. In particular, for those environments where
CRTP is already implemented, the additional effort required to
implement the extensions defined here is expected to be small.
There are also cases where the implementation simplicity of this
enhanced CRTP relative to ROHC is more important than the
performance advantages of ROHC.
1.1 CRTP Operation 1.1 CRTP Operation
During compression of an RTP stream, a session context is defined. During compression of an RTP stream, a session context is defined.
For each context, the session state is established and shared For each context, the session state is established and shared
between the compressor and the decompressor. Once the context state between the compressor and the decompressor. Once the context state
is established, compressed packets may be sent. is established, compressed packets may be sent.
The context state consists of the full IP/UDP/RTP headers, a few The context state consists of the full IP/UDP/RTP headers, a few
first order differential values, a link sequence number, a first order differential values, a link sequence number, a
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Cisco Systems, Inc. Cisco Systems, Inc.
3rd Floor 3rd Floor
96 Commercial Street 96 Commercial Street
Leith, Edinburgh EH6 6LX Leith, Edinburgh EH6 6LX
Scotland Scotland
Email: pruddy@cisco.com Email: pruddy@cisco.com
8. Copyright 8. Copyright
Copyright (C) The Internet Society 1999-2001. All Rights Reserved. Copyright (C) The Internet Society 1999-2003. All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However, this are included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for developing Internet standards in which case the procedures for
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