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Versions: 00 01 02 03 04 05 06 07 RFC 3749

Network Working Group                                      S. Hollenbeck
Internet-Draft                                            VeriSign, Inc.
Updates: 2246 (if approved)                           September 20, 2002
Expires: March 21, 2003


         Transport Layer Security Protocol Compression Methods
                   draft-ietf-tls-compression-01.txt

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

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   The list of Internet-Draft Shadow Directories can be accessed at
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   This Internet-Draft will expire on March 21, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2002).  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,
   CompressionMethod.null, which specifies that data exchanged via the
   record protocol will not be compressed.  This document describes
   additional compression methods associated with lossless data
   compression algorithms for use with TLS.

Conventions Used In This Document



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   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
   3. Intellectual Property Considerations . . . . . . . . . . . . .   5
   4. Internationalization Considerations  . . . . . . . . . . . . .   6
   5. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .   7
   6. Security Considerations  . . . . . . . . . . . . . . . . . . .   8
   7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .   9
      Normative References . . . . . . . . . . . . . . . . . . . . .  10
      Informative References . . . . . . . . . . . . . . . . . . . .  11
      Author's Address . . . . . . . . . . . . . . . . . . . . . . .  11
      Full Copyright Statement . . . . . . . . . . . . . . . . . . .  12


































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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 additional compression methods associated
   with lossless data compression algorithms for use with TLS.
   Standardization of the compressed data formats and compression
   algorithms associated with the compression methods 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 two
   additional compression methods:

   enum { null(0), ZLIB(1), LZS(2), (255) } CompressionMethod;

   The ZLIB compression method is described in RFC 1950 [5] and RFC 1951
   [6].  The Lempel Zif Stac (LZS) compression method is described in
   ANSI publication X3.241 [7].

   As described in section 6 of RFC 2246, 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.  Compression methods SHOULD be
   stateful to take advantage of the state management features offered
   by TLS.






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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 methods 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

   This document does not have a direct impact on the IANA, but it does
   define ranges of compression method values for future assignment.
   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.

   Data compression prior to encryption typically "flattens" the
   distribution of unencrypted octets (or very slightly increases the
   unicity distance) by using fewer bits to represent common characters.
   An increase in unicity distance typically indicates an increase in
   the amount of work required of an attacker to recover the original
   plaintext.  However, compression methods often require a structured
   header at the beginning of the compressed data stream, giving an
   attacker a target for testing keys in a brute force search.
   Compression can thus decrease and not increase the security of
   encryption if an attacker has little prior knowledge of the original
   plaintext.


































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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, Nikos
   Mavroyanopoulos, 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]  American National Standards Institute, "Data Compression Method,
        Adaptive Coding with Sliding Window of Information Interchange",
        ANSI X3.241, 1994.


Author's Address

   Scott Hollenbeck
   VeriSign, Inc.
   21345 Ridgetop Circle
   Dulles, VA  20166-6503
   US

   EMail: shollenbeck@verisign.com

























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Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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