INTERNET-DRAFT                                           Carsten Bormann
Expires: May 2001                                         TZI/Uni Bremen
                                                           November 2000

                             ROHC over PPP

Status of this memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This document is a submission to the IETF ROHC WG.  Comments should
   be directed to its mailing list,


   This document describes an option for negotiating the use of robust
   header compression (ROHC) on IP datagrams transmitted over the Point-
   to-Point Protocol [RFC1661]. It defines extensions to the PPP Control
   Protocols for IPv4 and IPv6 [RFC1332, RFC2023].

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1.  Introduction

   Robust Header Compression (ROHC) as defined in [ROHC] 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

   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.

   ROHC does not rely on any link layer's ability to indicate the types
   of datagrams carried in the link layer frames. Therefore, in this
   document a single new type for the PPP Data Link Layer Protocol Field
   is defined.

   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

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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.

        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, p 6] it follows that it can only be used to

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        select a single compression protocol at any time.

        NOTE: [RFC1332] is not explicit about whether the option
        negotiates the capabilities of the receiver or of the sender.
        In keeping with current practice, we assume that the option
        describes the capabilities of the decompressor (receiving side)
        of the peer that sends the Config-Req.

2.1.  Configuration Option Format

   Both the network control protocol for IPv4, IPCP [RFC1332] and the
   IPv6 NCP, IPV6CP [RFC2023] 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.


        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.

                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...


      >= 14

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

      00XX (hex) [to be assigned]

      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).

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

         Suggested value: 0

      The largest header size in octets that may be compressed.

         Suggested value: 168 octets

      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

      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.

          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.  LARGE_CIDS Suboption

   Without specifying further options, the LARGE_CIDS per-channel
   parameter (see [ROHC], section 5.1.1) is false.


        Set LARGE_CIDS to true.

                          LARGE_CIDS suboption
          0                   1
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
         |     Type      |    Length     |


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2.3.  PROFILES Suboption

   The set of profiles to be enabled is subject to negotiation.  Most
   initial implementations of ROHC implement profiles 0 to 3.  This
   option MUST be supplied.


        Define the set of profiles supported by the decompressor.

                           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...



            n octets in ascending order, each specifying a profile

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.  These values apply to the compression of packets where the
   outer header is an IPv4 header and an IPv6 header, respectively.

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.

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4.  Demultiplexing of Datagrams

   The ROHC specification [ROHC] defines a single header format for all
   different types of compressed headers.  One PPP Data Link Layer
   Protocol Field value is specified below.
           The frame contains a ROHC packet as defined in [ROHC].
           Value: 00XX (hex)  [to be assigned -- same XX as above]

5.  Security Considerations

   Negotiation of the option defined here imposes no additional security
   considerations beyond those that otherwise apply to PPP [RFC1661].

   The security considerations of ROHC [ROHC] apply.

   The use of header compression can, in rare cases, cause the
   misdelivery of packets. If necessary, confidentiality of packet
   contents should be assured by encryption.

   Encryption applied at the IP layer (e.g., using IPSEC mechanisms)
   precludes header compression of the encrypted headers, though
   compression of the outer IP header and authentication/security
   headers is still possible as described in [ROHC].  For RTP packets,
   full header compression is possible if the RTP payload is encrypted
   by itself without encrypting the UDP or RTP headers, as described in
   [RFC1889].  This method is appropriate when the UDP and RTP header
   information need not be kept confidential.

6.  Acknowledgments

   The present document borrows heavily from [RFC2509].

7.  References

   [ROHC]      Carsten Bormann (ed.) et al., "RObust Header Compression
               (ROHC)", work in progress, November 2000 (draft-ietf-

   [RFC2023]   Haskin, E. and E. Allan, "IP Version 6 over PPP", RFC
               2023, October 1996.

   [RFC1144]   Jacobson, V., "Compressing TCP/IP Headers for Low- Speed
               Serial Links", RFC 1144, February 1990.

   [RFC1332]   McGregor, G., "The PPP Internet Protocol Control Protocol
               (IPCP)", RFC 1332, May 1992.

   [RFC1889]   Schulzrinne, H., Casner, S., Frederick, R. and V.
               Jacobson, "RTP: A Transport Protocol for real-time
               applications", RFC 1889, January 1996.

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   [RFC1661]   Simpson, W., Ed., "The Point-To-Point Protocol (PPP)",
               STD 51, RFC 1661, July 1994.

   [RFC2686]   Bormann, C., "The Multi-Class Extension to Multi-Link
               PPP", RFC 2686, September 1999.

   [RFC2509]   M. Engan, S. Casner, C. Bormann, "IP Header Compression
               over PPP", RFC 2509, February 1999.

8.  Authors' addresses

   Carsten Bormann
   Universitaet Bremen FB3 TZI
   Postfach 330440
   D-28334 Bremen, GERMANY
   phone +49.421.218-7024
   fax +49.421.218-7000

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