draft-ietf-avt-crtp-enhance-04.txt   draft-ietf-avt-crtp-enhance-05.txt 
Audio/Video Transport Working Group Tmima Koren Audio/Video Transport Working Group Tmima Koren
Internet Draft Cisco Systems Internet Draft Cisco Systems
February 24, 2002 Stephen Casner November 2, 2002 Stephen Casner
Expires September 2002 Packet Design Expires June 2003 Packet Design
draft-ietf-avt-crtp-enhance-04.txt John Geevarghese draft-ietf-avt-crtp-enhance-05.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
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group. The working group mailing list is avt@ietf.org. Subscribe via group. The working group mailing list is avt@ietf.org. Subscribe via
the web at http://www.ietf.org/mailman/listinfo/avt. the web at http://www.ietf.org/mailman/listinfo/avt.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (1999-2001). All Rights Reserved. Copyright (C) The Internet Society (1999-2001). All Rights Reserved.
Abstract Abstract
This document describes a header compression scheme for point to This document describes a header compression scheme for point to
point links with packet loss and long delays. It is based on CRTP, point links with packet loss and long delays. It is based on
the IP/UDP/RTP header compression described in [RFC2508]. CRTP does Compressed Real-time Transport Protocol (CRTP), the IP/UDP/RTP
not perform well on such links: packet loss results in context header compression described in RFC 2508. CRTP does not perform well
corruption and due to the long delay, many more packets are on such links: packet loss results in context corruption and due to
discarded before the context is repaired. To correct the behavior of the long delay, many more packets are discarded before the context
CRTP over such links, a few extensions to the protocol are specified is repaired. To correct the behavior of CRTP over such links, a few
here. The extensions aim to reduce context corruption by changing extensions to the protocol are specified here. The extensions aim to
the way the compressor updates the context at the decompressor: reduce context corruption by changing the way the compressor updates
updates are repeated and include updates to full and differential the context at the decompressor: updates are repeated and include
context parameters. With these extensions, CRTP performs well over updates to full and differential context parameters. With these
links with packet loss, packet reordering and long delays. extensions, CRTP performs well over links with packet loss, packet
reordering and long delays.
1.0 Introduction 1.0 Introduction
RTP header compression (CRTP) as described in RFC 2508 was designed RTP header compression (CRTP) as described in RFC 2508 was designed
to reduce the header overhead of IP/UDP/RTP datagrams by compressing 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 the three headers. The IP/UDP/RTP headers are compressed to 2-4
bytes most of the time. bytes most of the time.
CRTP was designed for reliable point to point links with short CRTP was designed for reliable point to point links with short
delays. It does not perform well over links with high rate of packet delays. It does not perform well over links with high rate of packet
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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 CRTP is widely deployed and has relatively low computational
complexity. It is desirable to extend its usage over such links. complexity. It is desirable to extend its usage over such links.
This can be achieved with a few simple extensions to the protocol. This can be achieved with a few simple extensions to the protocol.
RFC 3095 defines another RTP header compression scheme called Robust
Header Compression (ROHC) which is very efficient and robust against
packet loss. However, ROHC is designed for point-to-point links and
does not tolerate the misordering of compressed packets between the
compressor and decompressor that may occur when these packets are
carried in an IP tunnel across multiple hops. The enhanced CRTP
scheme defined in this document includes optimizations to CRTP to
deal with both packet loss and misordering between the compressor
and decompressor.
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
generation number and a delta encoding table. generation number and a delta encoding table.
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The authors would like to thank Van Jacobson, co-author of RFC 2508, The authors would like to thank Van Jacobson, co-author of RFC 2508,
and the authors of RFC 2507, Mikael Degermark, Bjorn Nordgren, and and the authors of RFC 2507, Mikael Degermark, Bjorn Nordgren, and
Stephen Pink. The authors would also like to thank Dana Blair, Stephen Pink. The authors would also like to thank Dana Blair,
Francois Le Faucheur, Tim Gleeson, Matt Madison, Hussein Salama, Francois Le Faucheur, Tim Gleeson, Matt Madison, Hussein Salama,
Mallik Tatipamula, Mike Thomas, Alex Tweedly, Herb Wildfeuer, and Mallik Tatipamula, Mike Thomas, Alex Tweedly, Herb Wildfeuer, and
Dan Wing. Dan Wing.
6. References 6. References
Normative References
[CRTP] S. Casner, V. Jacobson, "Compressing IP/UDP/RTP Headers for [CRTP] S. Casner, V. Jacobson, "Compressing IP/UDP/RTP Headers for
Low-Speed Serial Links", RFC2508, February 1999. Low-Speed Serial Links", RFC2508, February 1999.
[IPHCOMP] M. Degermark, B. Nordgren, S. Pink, [IPHCOMP] M. Degermark, B. Nordgren, S. Pink,
"IP Header Compression", RFC2507, February 1999. "IP Header Compression", RFC2507, February 1999.
[IPCPHC] M. Engan, S. Casner, C. Bormann, T. Koren, [IPCPHC] M. Engan, S. Casner, C. Bormann, T. Koren,
"IP Header Compression over PPP", "IP Header Compression over PPP",
draft-koren-pppext-rfc2509bis-01.txt, February 2002. draft-koren-pppext-rfc2509bis-01.txt, February 2002.
[KEYW] S. Bradner, "Key words for use in RFCs to Indicate [KEYW] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC2119, BCP 14, March 1997. Requirement Levels", RFC2119, BCP 14, March 1997.
[RTP] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, [RTP] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC1889, "RTP: A Transport Protocol for Real-Time Applications", RFC1889,
January 1996. January 1996.
Informative References
[ROHC] Bormann, C., Burmeister, C., Degermark, M., Fukushima,
H., Hannu, H., Jonsson, L., Hakenberg, R., Koren, T., Le,
K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
Wiebke, T., Yoshimura, T. and H. Zheng, "RObust Header
Compression (ROHC): Framework and four profiles: RTP,
UDP, ESP, and uncompressed", RFC 3095, July 2001.
7. Authors' Addresses 7. Authors' Addresses
Tmima Koren Tmima Koren
Cisco Systems, Inc. Cisco Systems, Inc.
170 West Tasman Drive 170 West Tasman Drive
San Jose, CA 95134-1706 San Jose, CA 95134-1706
United States of America United States of America
Email: tmima@cisco.com Email: tmima@cisco.com
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