RFC 3243 (rfc3243) - Page 2 of 6

RObust Header Compression (ROHC): Requirements and Assumptions for 0-byte IP/UDP/RTP Compression

RFC 3243        Reqs and Assumptions for 0-byte ROHC RTP      April 2002


   However, all-IP cellular networks will also be built with already
   existing air interfaces such as GSM and IS-95, which are less
   flexible using radio bearers optimized for specific frame sizes
   matching the speech codecs used.  This means that not a single octet
   of header can be added without switching to the next higher fixed
   packet size supported by the link, something which is obviously very
   costly.  In the long term, this drawback should of course be
   eliminated with new, more flexible air interfaces, but in the short
   term it would be desirable if an efficiency comparable to the circuit
   switched case could also be achieved for already deployed speech
   codecs when used over the existing air interfaces.  To achieve that,
   it must be possible to completely eliminate the headers for a
   majority of the packets during normal operation, and this is the
   purpose of 0-byte header compression.  All functionality normally
   provided by the 1-octet header must then be provided by some other
   means, typically by utilizing functionality from the lower layer.  It
   is important to remember that the purpose of 0-byte header
   compression is to provide optimal efficiency for applications
   matching the link layer characteristics, not efficiency in general.

   As a starting point for these requirements, the well-established
   requirements base developed in the ROHC WG has been used.  From that,
   the requirements have evolved through input from the 3GPP2 community
   and from discussions within the WG.

2.  Assumptions for the Applicability of 0-byte RTP Header Compression

   The purpose of 0-byte header compression is to provide optimal usage
   of certain links when the traffic pattern of a packet stream
   completely matches the characteristics of that link.  There are no
   assumptions that only packet streams complying with that pattern will
   occur, but optimal efficiency cannot of course be provided when this
   is not the case.

   To make 0-byte header compression feasible, it is assumed that lower
   layers can provide the necessary functionality needed to replace the
   1-octet headers and fulfill the requirements defined in section 3.
   An example is the synchronized nature of most cellular links, which
   can provide sequencing and timing information and make packet loss
   detection possible.

3.  Requirements on 0-byte RTP Header Compression

   Since 0-byte header compression for ROHC IP/UDP/RTP is a variant of
   regular ROHC RTP compression [ROHC], these requirements are described
   as deltas to those defined in the regular RTP requirements [RTP-REQ].
   For simplicity, this section is also separated into the same three
   subsections as the requirements in [RTP-REQ], where the first deals



Jonsson                      Informational