rfc3241.txt

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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.





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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.







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RFC 3241                     ROHC over PPP                    April 2002


      NOTE: The specification of link and network layer parameter
      negotiation for PPP [RFC1661], [RFC1331], [RFC1332] does not
      prohibit multiple instances of one configuration option but states
      that the specification of a configuration option must explicitly
      allow multiple instances.  From the current specification of the
      IPCP IP-Compression-Protocol configuration option [RFC1332] one
      can infer that it can only be used to select a single compression
      protocol at any time.

      This was appropriate at a time when only one header compression
      scheme existed.  With the advent of IP header compression
      [RFC2507, RFC2509], this did not really change, as RFC 2507
      essentially superseded RFC 1144.  However, with ROHC, it may now
      very well be desirable to use RFC 2507 TCP compression in
      conjunction with RFC 3095 RTP/UDP compression.

   The present document now updates RFC 1332 by explicitly allowing the
   sending of multiple instances of the IP-Compression-Protocol
   configuration option, each with a different value for IP-
   Compression-Protocol.  Each type of compression protocol may
   independently establish its own parameters.

   This change is believed to not cause significant harm in existing PPP
   implementations, as they would most likely Configure-Nak or
   Configure-Reject the duplicate option, or simply happen to accept the
   one option they understand.  To aid interoperability, the peer
   implementing the present specification SHOULD react to a Configure-
   Nak or Configure-Reject by reducing the number of options offered to
   one.

2.1.  Configuration Option Format

   Both the network control protocol for IPv4, IPCP [RFC1332] and the
   IPv6 NCP, IPV6CP [RFC2472] may be used to negotiate IP Header
   Compression parameters for their respective protocols.  The format of
   the configuration option is the same for both IPCP and IPV6CP.

   Description

      This NCP configuration option is used to negotiate parameters for
      Robust Header Compression.  The option format is summarized below.
      The fields are transmitted from left to right.









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           Figure 1: Robust Header Compression (ROHC) Option

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |    IP-Compression-Protocol    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            MAX_CID            |             MRRU              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           MAX_HEADER          |          suboptions...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      2

   Length
      >= 10

      The length may be increased if the presence of additional
      parameters is indicated by additional suboptions.

   IP-Compression-Protocol
      0003 (hex)

   MAX_CID
      The MAX_CID field is two octets and indicates the maximum value of
      a context identifier.

         Suggested value: 15

      MAX_CID must be at least 0 and at most 16383 (The value 0 implies
      having one context).

   MRRU
      The MRRU field is two octets and indicates the maximum
      reconstructed reception unit (see [RFC3095], section 5.1.1).

         Suggested value: 0

   MAX_HEADER
      The largest header size in octets that may be compressed.

         Suggested value: 168 octets








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      The value of MAX_HEADER should be large enough so that at least
      the outer network layer header can be compressed.  To increase
      compression efficiency MAX_HEADER should be set to a value large
      enough to cover common combinations of network and transport layer
      headers.

      NOTE: The four ROHC profiles defined in RFC 3095 do not provide
      for a MAX_HEADER parameter.  The parameter MAX_HEADER defined by
      this document is therefore without consequence in these profiles.
      Other profiles (e.g., ones based on RFC 2507) can make use of the
      parameter by explicitly referencing it.

   suboptions
      The suboptions field consists of zero or more suboptions.  Each
      suboption consists of a type field, a length field and zero or
      more parameter octets, as defined by the suboption type.  The
      value of the length field indicates the length of the suboption in
      its entirety, including the lengths of the type and length fields.

                          Figure 2: Suboption

             0                   1                   2
             0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |     Type      |    Length     |  Parameters...
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.2.  PROFILES Suboption

   The set of profiles to be enabled is subject to negotiation.  Most
   initial implementations of ROHC implement profiles 0x0000 to 0x0003.
   This option MUST be supplied.

   Description

      Define the set of profiles supported by the decompressor.

                       Figure 3: PROFILES suboption

             0                   1                   2
             0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |     Type      |    Length     |  Profiles...
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type
         1




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RFC 3241                     ROHC over PPP                    April 2002


      Length
         2n+2

      Value
         n octet-pairs in ascending order, each octet-pair specifying a
         ROHC profile supported.

3.  Multiple Network Control Protocols

   The ROHC protocol is able to compress both IPv6 and IPv4 datagrams.
   Both IPCP and IPV6CP are able to negotiate option parameter values
   for ROHC.  The ROHC capability negotiated as a whole applies to the
   compression of packets where the outer header is an IPv4 header and
   an IPv6 header, respectively; e.g., an outer IPv6 header MUST NOT be
   sent if the ROHC IP-Compression-Protocol option was not negotiated
   for IPV6CP.

   Offering a specific ROHC capability in a Configure-Request in either
   IPCP or IPV6CP indicates that the capability is provided for the
   entire ROHC channel formed by the PPP link.  When the option has been
   negotiated with different values in IPCP and IPV6CP, the result is
   that the set of parameter values for the entire ROHC channel is the
   logical union of the two values, i.e., the maximum for MAX_CID, MRRU
   or MAX_HEADER, and the logical union of the suboptions.  For the
   PROFILES suboption, the logical union is the union of the two sets of
   profiles.  The unified values are kept as valid parameter values for
   the ROHC channel even when either of the NCPs is taken down.

   Note that each new suboption for this option must define the meaning
   of "logical union", if the concept applies.

3.1.  Sharing Context Identifier Space

   For the compression and decompression of IPv4 and IPv6 datagram
   headers, the context identifier space is shared.  While the parameter
   values are independently negotiated, sharing the context identifier
   spaces becomes more complex when the parameter values differ.  Since
   the compressed packets share context identifier space, the
   compression engine must allocate context identifiers out of a common
   pool; for compressed packets, the decompressor has to examine the
   context state to determine what parameters to use for decompression.

   In particular, the context identifier space is shared between ROHC
   small-CID packets and ROHC large-CID packets.  From the point of view
   of the ROHC framework, the PPP NCP instances for IPCP and IPV6CP
   together constitute exactly one ROHC channel; its feedback is
   destined for the ROHC channel defined by the NCP instances for IPCP
   and IPV6CP in the reverse direction on the same PPP link.



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