Robust Header Compression (ROHC) over PPP
RFC 3241

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Network Working Group                                         C. Bormann
Request for Comments: 3241                                TZI/Uni Bremen
Updates: 1332                                                 April 2002
Category: Standards Track

               Robust Header Compression (ROHC) over PPP

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document describes an option for negotiating the use of robust
   header compression (ROHC) on IP datagrams transmitted over the
   Point-to-Point Protocol (PPP).  It defines extensions to the PPP
   Control Protocols for IPv4 and IPv6.

1.  Introduction

   Robust Header Compression (ROHC) as defined in [RFC3095] may be used
   for compression of both IPv4 and IPv6 datagrams or packets
   encapsulated with multiple IP headers.  The initial version of ROHC
   focuses on compression of the packet headers in RTP streams, while
   supporting compression of other UDP flows; however, it also defines a
   framework into which further header compression mechanisms can be
   plugged as new profiles.  Planned additions to the set of profiles
   supported by ROHC will be capable of compressing TCP transport
   protocol headers as well.

   In order to establish compression of IP datagrams sent over a PPP
   link each end of the link must agree on a set of configuration
   parameters for the compression.  The process of negotiating link
   parameters for network layer protocols is handled in PPP by a family
   of network control protocols (NCPs).  Since there are separate NCPs
   for IPv4 and IPv6, this document defines configuration options to be
   used in both NCPs to negotiate parameters for the compression scheme.

Bormann                     Standards Track                     [Page 1]
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 [RFC3095] 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 [RFC2686], 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 [RFC1332].  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 [RFC1332], 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                     [Page 2]
RFC 3241                     ROHC over PPP                    April 2002

      NOTE: The specification of link and network layer parameter
      negotiation for PPP [RFC1661], [RFC1331], [RFC1332] does not