rfc2509.txt

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Network Working Group
Request for Comments: 2509                                      M. Engan
Category: Standards Track                                         Effnet
                                                               S. Casner
                                                           Cisco Systems
                                                              C. Bormann
                                                 Universitaet Bremen TZI
                                                           February 1999


                     IP Header Compression 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 (1999).  All Rights Reserved.

Abstract

   This document describes an option for negotiating the use of header
   compression 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]. Header compression
   may be applied to IPv4 and IPv6 datagrams in combination with TCP,
   UDP and RTP transport protocols as specified in [IPHC] and [CRTP].

1. Introduction

   The IP Header Compression (IPHC) defined in [IPHC] may be used for
   compression of both IPv4 and IPv6 datagrams or packets encapsulated
   with multiple IP headers. IPHC is also capable of compressing both
   TCP and UDP transport protocol headers.  The IP/UDP/RTP header
   compression defined in [CRTP] fits within the framework defined by
   IPHC so that it may also be applied to both IPv4 and IPv6 packets.

   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



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   used in both NCPs to negotiate parameters for the compression scheme.

   IPHC relies on the link layer's ability to indicate the types of
   datagrams carried in the link layer frames. In this document nine new
   types for the PPP Data Link Layer Protocol Field are defined along
   with their meaning.

   In general, header compression schemes that use delta encoding of
   compressed packets require that the lower layer does not reorder
   packets between compressor and decompressor. IPHC uses delta encoding
   of compressed packets for TCP and RTP.  The IPHC specification [IPHC]
   includes methods that allow link layers that may reorder packets to
   be used with IPHC.  Since PPP does not reorder packets these
   mechanisms are disabled by default.  When using reordering mechanisms
   such as multiclass multilink PPP [MCML], care must be taken so that
   packets that share the same compression context are not reordered.

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
   IPHC 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 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.











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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.

   Description

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

       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    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           TCP_SPACE           |         NON_TCP_SPACE         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         F_MAX_PERIOD          |          F_MAX_TIME           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           MAX_HEADER          |          suboptions...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      2

   Length
      >= 14

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

   IP-Compression-Protocol
      0061 (hex)

   TCP_SPACE
      The TCP_SPACE field is two octets and indicates the maximum value
      of a context identifier in the space of context identifiers
      allocated for TCP.

         Suggested value: 15

      TCP_SPACE must be at least 0 and at most 255 (The value 0 implies
      having one context).





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   NON_TCP_SPACE
      The NON_TCP_SPACE field is two octets and indicates the maximum
      value of a context identifier in the space of context identifiers
      allocated for non-TCP. These context identifiers are carried in
      COMPRESSED_NON_TCP, COMPRESSED_UDP and COMPRESSED_RTP packet
      headers.

         Suggested value: 15

      NON_TCP_SPACE must be at least 0 and at most 65535 (The value 0
      implies having one context).

   F_MAX_PERIOD
      Maximum interval between full headers.  No more than F_MAX_PERIOD
      COMPRESSED_NON_TCP headers may be sent between FULL_HEADER
      headers.

         Suggested value: 256

      A value of zero implies infinity, i.e. there is no limit to the
      number of consecutive COMPRESSED_NON_TCP headers.

   F_MAX_TIME
      Maximum time interval between full headers.  COMPRESSED_NON_TCP
      headers may not be sent more than F_MAX_TIME seconds after sending
      the last FULL_HEADER header.

         Suggested value: 5 seconds

      A value of zero implies infinity.

   MAX_HEADER
      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
      headers.

   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.



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             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 RTP-Compression Suboption

   The RTP-Compression suboption is included in the NCP
   IP-Compression-Protocol option for IPHC if IP/UDP/RTP compression is
   to be enabled.

   After successful negotiation of parameters for IP Header Compression
   the use of Protocol Identifiers FULL_HEADER, COMPRESSED_TCP,
   COMPRESSED_TCP_NODELTA and COMPRESSED_NON_TCP is enabled, regardless
   of the prescence of an RTP-Compression suboption.

   Description

      Enable use of Protocol Identifiers COMPRESSED_RTP, COMPRESSED_UDP and
      CONTEXT_STATE as specified in [CRTP].


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

         Type
            1

         Length
            2

3. Multiple Network Control Protocols

   The IPHC protocol is able to compress both IPv6 and IPv4 datagrams.
   Both IPCP and IPV6CP are able to negotiate option parameter values
   for IPHC.  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



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