RFC 3408 (rfc3408) - Page 2 of 7

Zero-byte Support for Bidirectional Reliable Mode (R-mode) in Extended Link-Layer Assisted RObust Header Compression (ROHC) Profile

RFC 3408               0-byte Support for R-mode           December 2002


   For simplification, this profile is defined in the form of the
   additions and exceptions to [RFC 3242] that are required to extend the
   RFC 3242 profile with zero-byte support for R-mode.  All terminology
   used in this document is the same as in [RFC 3242].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119
   [RFC 2119].

2.  Extensions to the assisting layer (AL) interface

   This section describes additions (some are optional) to the assisting
   layer interface as defined in [RFC 3242, section 4.2].

2.1.  Additional parameters to the compressor to AL interface

   -  Mode, indicating the mode in which the compressor is operating.
      The AL has slightly different logic depending on the mode value.

   -  SN_ACKed, indicating the latest RTP SN that has been acknowledged.
      It is used only when Mode value = R-mode.

   Note that these two parameters MUST always be attached to every
   packet delivered to the AL.

2.2.  Additional interface, assisting layer to compressor

   To improve the compression efficiency of this profile in some
   specific cases, e.g., when the AL operates in such a way that it
   often becomes unsafe to send NHPs, it is RECOMMENDED to implement
   this additional interface.  Here, the word "unsafe" means that the
   compressor allows the AL to send NHP but the AL cannot guarantee that
   the RTP SN of the NHP will be correctly decompressed at the receiving
   side.  The interface is used to carry update_request as described in
   section 3.  Note that this interface is not required in the sense
   that the impossibility of implementing such an interface should not
   be an obstacle to implement this profile over a specific link.

3.  R-mode operation

   For the R-mode, this profile extends ROHC RTP by performing a mapping
   of the R-0 packet to the NHP packet.  Note that R-0 is the only type
   of packets in R-mode that can be replaced with NHP.

   On the receiving side, the RTP SN of an NHP is determined by the
   decompressor as = SN_Ref_D + Offset_D, where SN_Ref_D is the RTP SN
   of the last update packet received by the decompressor, and Offset_D



Liu & Le                    Standards Track