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RFC 2393 - IP Payload Compression Protocol (IPComp)

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Network Working Group                                         A. Shacham
Request for Comments: 2393                                         Cisco
Category: Standards Track                                     R. Monsour
                                                              R. Pereira
                                                               M. Thomas
                                                      AltaVista Internet
                                                           December 1998

                IP Payload Compression Protocol (IPComp)

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 (1998).  All Rights Reserved.


   This document describes a protocol intended to provide lossless
   compression for Internet Protocol datagrams in an Internet

1. Introduction

   IP payload compression is a protocol to reduce the size of IP
   datagrams.  This protocol will increase the overall communication
   performance between a pair of communicating hosts/gateways ("nodes")
   by compressing the datagrams, provided the nodes have sufficient
   computation power, through either CPU capacity or a compression
   coprocessor, and the communication is over slow or congested links.

   IP payload compression is especially useful when encryption is
   applied to IP datagrams.  Encrypting the IP datagram causes the data
   to be random in nature, rendering compression at lower protocol
   layers (e.g., PPP Compression Control Protocol [RFC-1962])
   ineffective.  If both compression and encryption are required,
   compression MUST be applied before encryption.

   This document defines the IP payload compression protocol (IPComp),
   the IPComp packet structure, the IPComp Association (IPCA), and
   several methods to negotiate the IPCA.

   Other documents shall specify how a specific compression algorithm
   can be used with the IP payload compression protocol.  Such
   algorithms are beyond the scope of this document.

1.1. Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC-2119].

2. Compression Process

   The compression processing of IP datagrams has two phases:
   compressing of outbound IP datagrams ("compression") and
   decompressing of inbound datagrams ("decompression").  The
   compression processing MUST be lossless, ensuring that the IP
   datagram, after being compressed and decompressed, is identical to
   the original IP datagram.

   Each IP datagram is compressed and decompressed by itself without any
   relation to other datagrams ("stateless compression"), as IP
   datagrams may arrive out of order or not arrive at all.  Each
   compressed IP datagram encapsulates a single IP payload.

   Processing of inbound IP datagrams MUST support both compressed and
   non-compressed IP datagrams, in order to meet the non-expansion
   policy requirements, as defined in section 2.2.

   The compression of outbound IP datagrams MUST be done before any IP
   security processing, such as encryption and authentication, and
   before any fragmentation of the IP datagram.  In addition, in IP
   version 6 [RFC-2460], the compression of outbound IP datagrams MUST
   be done before the addition of either a Hop-by-Hop Options header or
   a Routing Header, since both carry information that must be examined
   and processed by possibly every node along a packet's delivery path,
   and therefore MUST be sent in the original form.

   Similarly, the decompression of inbound IP datagrams MUST be done
   after the reassembly of the IP datagrams, and after the completion of
   all IP security processing, such as authentication and decryption.

2.1. Compressed Payload

   The compression is applied to a single array of octets, which are
   contiguous in the IP datagram.  This array of octets always ends at
   the last octet of the IP packet payload.  Note: a contiguous array of
   octets in the IP datagram may be not contiguous in physical memory.

   In IP version 4 [RFC-0791], the compression is applied to the upper
   layer protocol (ULP) payload of the IP datagram.  No portion of the
   IP header or the IP header options is compressed.

   In the IPv6 context, IPComp is viewed as an end-to-end payload, and
   MUST not apply to hop-by-hop, routing, and fragmentation extension
   headers.  The compression is applied starting at the first IP Header
   Option field that does not carry information that must be examined
   and processed by nodes along a packet's delivery path, if such IP
   Header Option field exists, and continues to the ULP payload of the
   IP datagram.

   The size of a compressed payload, generated by the compression
   algorithm, MUST be in whole octet units.

   As defined in section 3, an IPComp header is inserted immediately
   preceding the compressed payload.  The original IP header is modified
   to indicate the usage of the IPComp protocol and the reduced size of
   the IP datagram.  The original content of the Next Header (IPv6) or
   protocol (IPv4) field is stored in the IPComp header.

   The decompression is applied to a single contiguous array of octets
   in the IP datagram.  The start of the array of octets immediately
   follows the IPComp header and ends at the last octet of the IP
   payload.  If the decompression process is successfully completed, the
   IP header is modified to indicate the size of the decompressed IP
   datagram, and the original next header as stored in the IPComp
   header.  The IPComp header is removed from the IP datagram and the
   decompressed payload immediately follows the IP header.

2.2. Non-Expansion Policy

   If the total size of a compressed ULP payload and the IPComp header,
   as defined in section 3, is not smaller than the size of the original
   ULP payload, the IP datagram MUST be sent in the original non-
   compressed form.  To clarify:  If an IP datagram is sent non-
   compressed, no IPComp header is added to the datagram.  This policy
   ensures saving the decompression processing cycles and avoiding
   incurring IP datagram fragmentation when the expanded datagram is
   larger than MTU.

   Small IP datagrams are likely to expand as a result of compression.
   Therefore, a numeric threshold should be applied before compression,
   where IP datagrams of size smaller than the threshold are sent in the
   original form without attempting compression.  The numeric threshold
   is implementation dependent.

   An IP datagram with payload that has been previously compressed tends
   not to compress any further.  The previously compressed payload may
   be the result of external processes, such as compression applied by
   an upper layer in the communication stack, or by an off-line
   compression utility.  An adaptive algorithm should be implemented to
   avoid the performance hit.  For example, if the compression of i
   consecutive IP datagrams of an IPCA fails, the next k IP datagrams
   are sent without attempting compression.  If the next j datagrams are
   also failing to compress, the next k+n datagrams are sent without
   attempting compression.  Once a datagram is compressed successfully,
   the normal process of IPComp restarts.  Such an adaptive algorithm,
   including all the related thresholds, is implementation dependent.

   During the processing of the payload, the compression algorithm MAY
   periodically apply a test to determine the compressibility of the
   processed data, similar to the requirements of [V42BIS].  The nature
   of the test is algorithm dependent.  Once the compression algorithm
   detects that the data is non-compressible, the algorithm SHOULD stop
   processing the data, and the payload is sent in the original non-
   compressed form.

3. Compressed IP Datagram Header Structure

   A compressed IP datagram is encapsulated by modifying the IP header
   and inserting an IPComp header immediately preceding the compressed
   payload.  This section defines the IP header modifications both in
   IPv4 and IPv6, and the structure of the IPComp header.

3.1. IPv4 Header Modifications

   The following IPv4 header fields are set before transmitting the
   compressed IP datagram:

      Total Length

         The length of the entire encapsulated IP datagram, including
         the IP header, the IPComp header and the compressed payload.


         The Protocol field is set to 108, IPComp Datagram, [RFC-1700].

      Header Checksum

         The Internet Header checksum [RFC-0791] of the IP header.

   All other IPv4 header fields are kept unchanged, including any header

3.2. IPv6 Header Modifications

   The following IPv6 header fields are set before transmitting the
   compressed IP datagram:

      Payload Length

         The length of the compressed IP payload.

      Next Header

         The Next Header field is set to 108, IPComp Datagram, [RFC-

   All other IPv6 header fields are kept unchanged, including any non-
   compressed header options.

   The IPComp header is placed in an IPv6 packet using the same rules as
   the IPv6 Fragment Header.  However if an IPv6 packet contains both an
   IPv6 Fragment Header and an IPComp header, the IPv6 Fragment Header
   MUST precede the IPComp header in the packet.

3.3.  IPComp Header Structure

   The four-octet header has the following structure:

   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
   |  Next Header  |     Flags     | Compression Parameter Index |

   Next Header

        8-bit selector.  Stores the IPv4 Protocol field or the IPv6 Next
        Header field of the original IP header.


        8-bit field.  Reserved for future use.  MUST be set to zero.
        MUST be ignored by the receiving node.

   Compression Parameter Index (CPI)

        16-bit index.  The CPI is stored in network order.  The values
        0-63 define well-known compression algorithms, which require no
        additional information, and are used for manual setup.  The
        values themselves are identical to IPCOMP Transform identifiers
        as defined in [SECDOI].  Consult [SECDOI] for an initial set of
        defined values and for instructions on how to assign new values.
        The values 64-255 are reserved for future use.  The values
        256-61439 are negotiated between the two nodes in definition of
        an IPComp Association, as defined in section 4.  Note: When
        negotiating one of the well-known algorithms, the nodes MAY
        select a CPI in the pre-defined range 0-63.  The values
        61440-65535 are for private use among mutually consenting
        parties.  Both nodes participating can select a CPI value
        independently of each other and there is no relationships
        between the two separately chosen CPIs.  The outbound IPComp
        header MUST use the CPI value chosen by the decompressing node.
        The CPI in combination with the destination IP address uniquely
        identifies the compression algorithm characteristics for the

4. IPComp Association (IPCA) Negotiation

   To utilize the IPComp protocol, two nodes MUST first establish an
   IPComp Association (IPCA) between them.  The IPCA includes all
   required information for the operation of IPComp, including the
   Compression Parameter Index (CPI), the mode of operation, the
   compression algorithm to be used, and any required parameter for the
   selected compression algorithm.  The IPComp mode of operation is
   either a node-to-node policy where IPComp is applied to every IP
   packet between the nodes, or an ULP session based policy where only
   selected ULP sessions between the nodes are using IPComp.  For each
   IPCA, a different compression algorithm may be negotiated in each
   direction, or only one direction may be compressed.  The default is
   "no IPComp compression".

   The IPCA is established by dynamic negotiations or by manual
   configuration.  The dynamic negotiations SHOULD use the Internet
   Security Association and Key Management Protocol [ISAKMP], where
   IPSec is present.  The dynamic negotiations MAY be implemented
   through a different protocol.

4.1. Use of ISAKMP

   For IPComp in the context of IP Security, ISAKMP provides the
   necessary mechanisms to establish IPCA.  IPComp Association is
   negotiated by the initiator using a Proposal Payload, which would

   include one or more Transform Payloads.  The Proposal Payload would
   specify a compression protocol in the protocol id field and each
   Transform Payload would contain the specific compression method(s)
   being offered to the responder.

   In the Internet IP Security Domain of Interpretation (DOI), IPComp is
   negotiated as the Protocol ID PROTO_IPCOMP.  The compression
   algorithm is negotiated as one of the defined IPCOMP Transform

4.2. Use of Non-ISAKMP Protocol

   The dynamic negotiations MAY be implemented through a protocol other
   than ISAKMP.  Such protocol is beyond the scope of this document.

4.3. Manual Configuration

   Nodes may establish IPComp Associations using manual configuration.
   For this method, a limited number of Compression Parameters Indexes
   (CPIs) is designated to represent a list of specific compression

5. Security Considerations

   When IPComp is used in the context of IPSec, it is believed not to
   have an effect on the underlying security functionality provided by
   the IPSec protocol; i.e., the use of compression is not known to
   degrade or alter the nature of the underlying security architecture
   or the encryption technologies used to implement it.

   When IPComp is used without IPSec, IP payload compression potentially
   reduces the security of the Internet, similar to the effects of IP
   encapsulation [RFC-2003].  For example, IPComp may make it difficult
   for border routers to filter datagrams based on header fields.  In
   particular, the original value of the Protocol field in the IP header
   is not located in its normal positions within the datagram, and any
   transport layer header fields within the datagram, such as port
   numbers, are neither located in their normal positions within the
   datagram nor presented in their original values after compression.  A
   filtering border router can filter the datagram only if it shares the
   IPComp Association used for the compression.  To allow this sort of
   compression in environments in which all packets need to be filtered
   (or at least accounted for), a mechanism must be in place for the
   receiving node to securely communicate the IPComp Association to the
   border router.  This might, more rarely, also apply to the IPComp
   Association used for outgoing datagrams.

6. References

   [RFC-0791] Postel, J., Editor, "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [RFC-1700] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,
              RFC 1700, October 1994.  Or see:

   [RFC-2460] Deering, S., and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC-1962] Rand, D., "The PPP Compression Control Protocol (CCP)",
              RFC 1962, June 1996.

   [RFC-2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
              October 1996.

   [RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [ISAKMP]   Maughan, D., Schertler, M., Schneider, M., and J. Turner,
              "Internet Security Association and Key Management Protocol
              (ISAKMP)", RFC 2408, November 1998.

   [SECDOI]   Piper, D., "The Internet IP Security Domain of
              Interpretation for ISAKMP", RFC 2407, November 1998.

   [V42BIS]   CCITT, "Data Compression Procedures for Data Circuit
              Terminating Equipment (DCE) Using Error Correction
              Procedures", Recommendation V.42 bis, January 1990.

Authors' Addresses

   Abraham Shacham
   Cisco Systems
   170 West Tasman Drive
   San Jose, California 95134
   United States of America

   EMail: shacham@cisco.com

   Robert Monsour
   Hi/fn Inc.
   2105 Hamilton Avenue, Suite 230
   San Jose, California 95125
   United States of America

   EMail: rmonsour@hifn.com

   Roy Pereira
   TimeStep Corporation
   362 Terry Fox Drive
   Kanata, Ontario K2K 2P5

   EMail: rpereira@timestep.com

   Matt Thomas
   AltaVista Internet Software
   30 Porter Road
   Littleton, Massachusetts 01460
   United States of America

   EMail: matt.thomas@altavista-software.com

Working Group

   The IP Payload Compression Protocol (IPPCP) working group can be
   contacted through its chair:

   Naganand Dorswamy
   Bay Networks

   EMail: naganand@baynetworks.com

Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

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   This document and the information contained herein is provided on an


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