Network Working Group S. Hollenbeck
Internet-Draft VeriSign, Inc.
Updates: 2246 (if approved) May 27, 2003
Expires: November 25, 2003
Transport Layer Security Protocol Compression Methods
draft-ietf-tls-compression-05.txt
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Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
The Transport Layer Security (TLS) protocol (RFC 2246) includes
features to negotiate selection of a lossless data compression method
as part of the TLS Handshake Protocol and to then apply the algorithm
associated with the selected method as part of the TLS Record
Protocol. TLS defines one standard compression method which
specifies that data exchanged via the record protocol will not be
compressed. This document describes an additional compression method
associated with a lossless data compression algorithm for use with
TLS, and it describes a method for the specification of additional
TLS compression methods.
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Conventions Used In This Document
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 [1].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Compression Methods . . . . . . . . . . . . . . . . . . . . . 4
2.1 Compression History and Packet Processing . . . . . . . . . . 5
2.2 ZLIB Compression . . . . . . . . . . . . . . . . . . . . . . . 5
3. Intellectual Property Considerations . . . . . . . . . . . . . 6
4. Internationalization Considerations . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
Normative References . . . . . . . . . . . . . . . . . . . . . 11
Informative References . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 13
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1. Introduction
The Transport Layer Security (TLS) protocol (RFC 2246, [2]) includes
features to negotiate selection of a lossless data compression method
as part of the TLS Handshake Protocol and to then apply the algorithm
associated with the selected method as part of the TLS Record
Protocol. TLS defines one standard compression method,
CompressionMethod.null, which specifies that data exchanged via the
record protocol will not be compressed. While this single
compression method helps ensure that TLS implementations are
interoperable, the lack of additional standard compression methods
has limited the ability of implementers to develop interoperable
implementations that include data compression.
TLS is used extensively to secure client-server connections on the
World Wide Web. While these connections can often be characterized
as short-lived and exchanging relatively small amounts of data, TLS
is also being used in environments where connections can be
long-lived and the amount of data exchanged can extend into thousands
or millions of octets. XML [4], for example, is increasingly being
used as a data representation method on the Internet, and XML tends
to be verbose. Compression within TLS is one way to help reduce the
bandwidth and latency requirements associated with exchanging large
amounts of data while preserving the security services provided by
TLS.
This document describes an additional compression method associated
with a lossless data compression algorithm for use with TLS.
Standardization of the compressed data formats and compression
algorithms associated with this compression method is beyond the
scope of this document.
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2. Compression Methods
TLS [2] includes the following compression method structure in
sections 6.1 and 7.4.1.2 and Appendix sections A.4.1 and A.6:
enum { null(0), (255) } CompressionMethod;
which allows for later specification of up to 256 different
compression methods. This definition is updated to segregate the
range of allowable values into three zones:
1. Values from 0 (zero) through 63 decimal (0x3F) inclusive are
reserved for future standardization efforts of the IETF TLS
working group.
2. Values from 64 decimal (0x40) through 192 decimal (0xC0) are
reserved for assignment by the IANA for specifications developed
outside the TLS working group. Assignments from this range of
values MUST be made by the IANA and MUST be associated with a
formal reference that describes the compression method.
3. Values from 193 decimal (0xC1) through 255 decimal (0xFF) are
reserved for private use.
Additional information describing the role of the IANA in the
allocation of compression method identifiers is described in Section
5.
In addition, this definition is updated to include assignment of an
identifier for the ZLIB compression method:
enum { null(0), ZLIB(1), (255) } CompressionMethod;
As described in section 6 of RFC 2246 [2], TLS is a stateful
protocol. Compression methods used with TLS can be either stateful
(the compressor maintains it's state through all compressed records)
or stateless (the compressor compresses each record independently),
but there seems to be little known benefit in using a stateless
compression method within TLS.
The ZLIB compression method described in this document is stateful.
It is recommended that other compression methods that might be
standardized in the future be stateful as well.
Compression algorithms can occasionally expand, rather than compress,
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input data. A compression method that exceeds the expansion limits
described in section 6.2.2 of RFC 2246 [2] MUST NOT be used with TLS.
2.1 Compression History and Packet Processing
Some compression methods have the ability to maintain history
information when compressing and decompressing packet payloads. The
compression history allows a higher compression ratio to be achieved
on a stream as compared to per-packet compression, but maintaining a
history across packets implies that a packet might contain data
needed to completely decompress data contained in a different packet.
History maintenance thus requires both a reliable link and sequenced
packet delivery. Since TLS and lower-layer protocols provide
reliable, sequenced packet delivery, compression history information
MAY be maintained and exploited if supported by the compression
method.
2.2 ZLIB Compression
The ZLIB compression method and encoding format is described in RFC
1950 [5] and RFC 1951 [6]. Examples of ZLIB use in IETF protocols can
be found in RFC 1979 [7], RFC 2394 [8], and RFC 3274 [9].
ZLIB allows the sending compressor to select from among several
options to provide varying compression ratios, processing speeds, and
memory requirements. The receiving decompressor MUST automatically
adjust to the parameters selected by the sender. All data that was
submitted for compression MUST be included in the compressed output,
with no data retained to be included in a later output payload.
Flushing ensures that each compressed packet payload can be
decompressed completely.
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3. Intellectual Property Considerations
Many compression algorithms are subject to patent or other
intellectual property rights claims. Implementers are encouraged to
seek legal guidance to better understand the implications of
developing implementations of the compression method described in
this document or other documents that describe compression methods
for use with TLS.
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4. Internationalization Considerations
The compression method identifiers specified in this document are
machine-readable numbers. As such, issues of human
internationalization and localization are not introduced.
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5. IANA Considerations
Section 2 of this document describes a registry of compression method
identifiers to be maintained by the IANA, including assignment of an
identifier for the ZLIB compression method. Identifier values from
the range reserved for future standardization efforts of the TLS
working group MUST be assigned according to the "Standards Action"
policy described in RFC 2434 [3]. Values from the range reserved for
private use MUST be used according to the "Private Use" policy
described in RFC 2434. Values from the general IANA pool MUST be
assigned according to the "IETF Consensus" policy described in RFC
2434.
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6. Security Considerations
This document does not introduce any topics that alter the threat
model addressed by TLS. The security considerations described
throughout RFC 2246 [2] apply here as well.
Some symmetric encryption ciphersuites do not hide the length of
symmetrically encrypted data at all. Others hide it to some extent,
but still don't hide it fully. Use of TLS compression SHOULD take
into account that the length of compressed data may leak more
information than the length of the original uncompressed data.
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7. Acknowledgements
The concepts described in this document were originally discussed on
the IETF TLS working group mailing list in December, 2000. The
author acknowledges the contributions to that discussion provided by
Jeffrey Altman, Eric Rescorla, and Marc Van Heyningen. Later
suggestions that have been incorporated into this document were
provided by Tim Dierks, Pasi Eronen, Peter Gutmann, Elgin Lee, Nikos
Mavroyanopoulos, Alexey Melnikov, and Bodo Moeller.
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Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and
P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, January
1999.
[3] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
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Informative References
[4] Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler,
"Extensible Markup Language (XML) 1.0 (2nd ed)", W3C REC-xml,
October 2000, <http://www.w3.org/TR/REC-xml>.
[5] Deutsch, L. and J-L. Gailly, "ZLIB Compressed Data Format
Specification version 3.3", RFC 1950, May 1996.
[6] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, May 1996.
[7] Woods, J., "PPP Deflate Protocol", RFC 1979, August 1996.
[8] Pereira, R., "IP Payload Compression Using DEFLATE", RFC 2394,
December 1998.
[9] Gutmann, P., "Compressed Data Content Type for Cryptographic
Message Syntax (CMS)", RFC 3274, June 2002.
Author's Address
Scott Hollenbeck
VeriSign, Inc.
21345 Ridgetop Circle
Dulles, VA 20166-6503
US
EMail: shollenbeck@verisign.com
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