Network Working Group                                      S. Hollenbeck
Internet-Draft                                            VeriSign, Inc.
Updates: 2246 (if approved)                                 May 27, 2003
Expires:                            November 25, 20, 2003
Expires: May 20, 2004

         Transport Layer Security Protocol Compression Methods

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.


   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.

Conventions Used In This Document

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

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

   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.

2. Compression Methods

   TLS [2] includes the following compression method structure in
   sections 6.1 and 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

   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,
   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

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.

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.

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.

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

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.

   However, combining compression with encryption can sometimes reveal
   information that would not have been revealed without compression:
   data that is the same length before compression might be a different
   length after compression, so adversaries that observe the length of
   the compressed data might be able to derive information about the
   corresponding uncompressed data.  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.  For example, ciphersuites that use stream cipher encryption
   without padding do not hide length at all; ciphersuites that use
   Cipher Block Chaining (CBC) encryption with padding provide some
   length hiding, depending on how the amount of padding is chosen.  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.

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. Moeller, and Win Treese.

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

   [3]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
        Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

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, <>.

   [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


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