RFC2393 - IP Payload Compression Protocol (IPComp)

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　　Network Working Group A. Shacham
Request for Comments: 2393 Cisco
Category: Standards Track R. Monsour
Hi/fn
R. Pereira
TimeStep
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.

Abstract

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

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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [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.

Protocol

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
options.

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-
1700].

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.

Flags

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
datagram.

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
Identifiers.

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
methods.

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, RFC791,
September 1981.

[RFC-1700] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,
RFC1700, October 1994. Or see:
http://www.iana.org/numbers.html

[RFC-2460] Deering, S., and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC2460, December 1998.

[RFC-1962] Rand, D., "The PPP Compression Control Protocol (CCP)",
RFC1962, June 1996.

[RFC-2003] Perkins, C., "IP Encapsulation within IP", RFC2003,
October 1996.

[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.

[ISAKMP] Maughan, D., Schertler, M., Schneider, M., and J. Turner,
"Internet Security Association and Key Management Protocol
(ISAKMP)", RFC2408, November 1998.

[SECDOI] Piper, D., "The Internet IP Security Domain of
Interpretation for ISAKMP", RFC2407, 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
Canada

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

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