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Protocol Action: RObust Header Compression (ROHC) to Proposed Standard





The IESG has approved the Internet-Draft 'RObust Header Compression
(ROHC)' <draft-ietf-rohc-rtp-08.txt> as a Proposed Standard.

The IESG also approved publication of Requirements for Robust Header
Compression for RTP/UDP/IP <draft-ietf-rohc-rtp-requirements-05.txt>
These documents are the product of the Robust Header Compression
Working Group.  The IESG contact persons are Allison Mankin and Scott
Bradner.  


Technical Summary

   A problem with IP over cellular links when used for interactive voice
   conversations is the large header overhead. Speech data for IP
   telephony will most likely be carried by RTP. An uncompressed packet
   has, in addition to link layer framing, an IP [IPv4] header (20
   octets), a UDP [UDP] header (8 octets), and an RTP header (12 octets)
   for a total of 40 octets. With IPv6 [IPv6], the IP header is 40
   octets for a total of 60 octets. The size of the payload depends on
   the speech coding and frame sizes being used and may be as low as 15-
   20 octets.

   From these numbers, the need for reducing header sizes for efficiency
   reasons is obvious. However, cellular links have characteristics that
   make header compression as defined in existing IETSF standards
   (RFC 2507, 2508) perform less well than desirable.  The most
   important characteristic is the lossy behavior of
   cellular links, where a bit error rate (BER) as high as 1e-3 must be
   accepted to keep the radio resources efficiently utilized. In severe
   operating situations, the BER can be as high as 1e-2. The other
   problematic characteristic is the long round-trip time (RTT) of the
   cellular link, which can be as high as 100-200 milliseconds. An
   additional problem is that the residual BER is nontrivial, i.e.,
   lower layers can sometimes deliver frames containing undetected
   errors. A viable header compression scheme for cellular links must be
   able to handle loss on the link between the compression and
   decompression point as well as loss before the compression point.

   A highly robust and efficient header compression scheme for
   RTP/UDP/IP, UDP/IP and ESP/IP headers is specified in this document,
   particularly addressing the conditions expected for cellular IP.

   This is done in a framework designed to be extensible.  For example,
   a scheme for compressing TCP/IP headers will be simple to add, and is
   in development.  Headers specific to Mobile IPv4 are not subject to
   special treatment, but are expected to be compressed sufficiently
   well by the provided methods for compression of sequences of
   extension headers and tunneling headers.  For the most part, the same
   will apply to work in progress on Mobile IPv6, but future work might
   be required to handle some extension headers, when a standards track
   Mobile IPv6 has been completed.

Working Group Summary

  The Working group strongly supported the advancement of these
  two documents.

Protocol Quality

   The document was reviewed for the IESG by Allison Mankin.
   The ROHC RTP compression protocols have been implemented
   by vendors as the specifications were developed, and critical
   aspects of the three modes were tested by at least one
   implementation, though interoperability testing has just
   begun.