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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)                             October 23, 2002
Expires: April 23, 2003


         Transport Layer Security Protocol Compression Methods
                   draft-ietf-tls-compression-03.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
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   This Internet-Draft will expire on April 23, 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.





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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
   2.3 LZS Compression  . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Intellectual Property Considerations . . . . . . . . . . . . .  7
   4.  Internationalization Considerations  . . . . . . . . . . . . .  8
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
       Normative References . . . . . . . . . . . . . . . . . . . . . 12
       Informative References . . . . . . . . . . . . . . . . . . . . 13
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 13
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 14





























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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;

   These two compression methods are defined to provide implementers
   with alternatives based on compression performance, ease of
   implementation, and licensing requirements (see Section 3 for a
   description of intellectual property considerations).  ZLIB is
   generally known as a freely-available, widely-deployed compression
   method, whereas LZS is generally known to provide memory footprint
   and performance advantages in stateful networking applications.

   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.




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   All of the compression methods described in this document are
   stateful.  It is recommended that other compression methods that
   might be standardized in the future be stateful as well.

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.  History information MAY be maintained and exploited
   when using the compression methods described in this document if TLS
   is being used with a protocol that provides reliable, sequenced
   packet delivery.

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 will automatically
   adjust to the parameters selected by the sender.

   ZLIB has the ability to maintain history information when compressing
   and decompressing packet payloads.  If TLS is not being used with a
   protocol that provides reliable, sequenced packet delivery, the
   sender MUST flush the compressor completely each time a compressed
   payload is produced.  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.

2.3 LZS Compression

   The Lempel Zif Stac (LZS) compression method and encoding format is
   described in ANSI publication X3.241 [10].  Examples of LZS use in
   IETF protocols can be found in RFC 1967 [11], RFC 1974 [12], and RFC
   2395 [13].

   LZS has the ability to maintain history information when compressing
   and decompressing packet payloads.  If TLS is not being used with a
   protocol that provides reliable, sequenced packet delivery, the



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   compression history MUST be reset by the sender before compressing
   data and the decompression history MUST be reset by the receiver
   before decompressing data to ensure that compressed packet payloads
   can be decompressed completely.  The sender MUST flush the compressor
   completely each time a compressed payload is produced.  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 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.

   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, 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.

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

   [11]  Schneider, K., Friend, R. and K. Fox, "PPP LZS-DCP Compression
         Protocol (LZS-DCP)", RFC 1967, August 1996.

   [12]  Friend, R., Simpson, W. and K. Fox, "PPP Stac LZS Compression
         Protocol", RFC 1974, August 1996.

   [13]  Friend, R. and R. Monsour, "IP Payload Compression Using LZS",
         RFC 2395, December 1998.


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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Acknowledgement

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



















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