The headers of IP, UDP and RTP contribute 40 bytes of overhead to each packet, even without IP options and RTP CSRC lists or header extensions. Thus, at a packetization interval 20 ms, headers alone would generate 16 kb/s, too much for slow serial links.
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.
Existing header compression schemes do not work well when used over links with significant error rates, especially when the round-trip time of the link is long. For many bandwidth limited links where header compression is essential, such characteristics are common. A header compression framework and a highly robust and efficient header compression scheme is introduced in this document, adaptable to the characteristics of the link over which it is used and also to the properties of the packet streams it compresses.
The IESG has approved [Dec. 30, 1998] the Internet-Draft "Compressing IP/UDP/RTP Headers for Low-Speed Serial Links"as a Proposed Standard. This document is the product of the Audio/Video Transport Working Group. The IESG contact persons are Scott Bradner and Vern Paxson. Technical Summary
This documents defines a protocol for reducing the size of IP, UDP and RTP packets sent over a serial link by compressing out header information that is constant, changes rarely, or only changes by small increments. For small packets and slow links, the savings in transmission time can be quite significant.Working Group Summary
The proposed protocol received solid working group support for its technical merits. There was however controversy within the WG that the protocol could not be deployed soon enough to meet the quickly emerging needs of Internet Telephony and similar applications. The alternative was to specify an end-to-end compression of just the RTP header that could be implemented in the applications themselves as an interim solution until the link-level compression specified in this draft could be deployed. Considering that the gain of compressing RTP alone would be relatively small and that it could not be standardized in the necessary timeframe, the prevalent position was that AVT should not define such an interim solution.Protocol Quality
Vern Paxson reviewed the specification for the IESG. The protocol is sound, thoroughly analyzed, and builds appropriately on the earlier work of RFC 1144.
Last updated by Henning Schulzrinne