RFC 3241 (rfc3241) - Page 2 of 12

Robust Header Compression (ROHC) over PPP

RFC 3241                     ROHC over PPP                    April 2002


   ROHC does not require that the link layer be able to indicate the
   types of datagrams carried in the link layer frames.  However, there
   are two basic types of ROHC headers defined in the ROHC framework:
   small-CID headers (zero or one bytes are used to identify the
   compression context) and large-CID headers (one or two bytes are used
   for this purpose).  To keep the PPP packets self-describing, in this
   document two new types for the PPP Data Link Layer Protocol Field are
   defined, one for small-CID ROHC packets and one for large-CID ROHC
   packets.  (This also avoids a problem that would occur if PPP were to
   negotiate which of the formats to use in each of IPCP and IPV6CP and
   the two negotiation processes were to arrive at different results.)
   A PPP ROHC sender may send packets in either small-CID or large-CID
   format at any time, i.e., the LARGE_CIDS parameter from [RFC 3095] is
   not used.  Any PPP ROHC receiver MUST be able to process both small-
   CID and large-CID ROHC packets, therefore no negotiation of this
   function is required.

   ROHC assumes that the link layer delivers packets in sequence.  PPP
   normally does not reorder packets.  When using reordering mechanisms
   such as multiclass multilink PPP [RFC 2686], care must be taken so
   that packets that share the same compression context are not
   reordered.  (Note that in certain cases, reordering may be acceptable
   to ROHC, such as within a sequence of packets that all do not change
   the decompression context.)

   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 RFC 2119.

2.  Configuration Option

   This document specifies a new compression protocol value for the IPCP
   IP-Compression-Protocol option as specified in [RFC 1332].  The new
   value and the associated option format are described in section 2.1.

   The option format is structured to allow future extensions to the
   ROHC scheme.

   It may be worth repeating [RFC 1332], section 4: "The IP-Compression-
   Protocol Configuration Option is used to indicate the ability to
   receive compressed packets.  Each end of the link must separately
   request this option if bi-directional compression is desired."  I.e.,
   the option describes the capabilities of the decompressor (receiving
   side) of the peer that sends the Configure-Request.







Bormann                     Standards Track