Movatterモバイル変換

Network Working Group                                      S. HollenbeckRequest for Comments: 3749                                VeriSign, Inc.Category: Standards Track                                       May 2004Transport Layer Security Protocol Compression MethodsStatus 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 (2004).  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.Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .22.  Compression Methods  . . . . . . . . . . . . . . . . . . . . .22.1.  DEFLATE Compression. . . . . . . . . . . . . . . . . . .33.  Compression History and Packet Processing  . . . . . . . . . .44.  Internationalization Considerations  . . . . . . . . . . . . .45.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .46.  Security Considerations  . . . . . . . . . . . . . . . . . . .57.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .68.  References . . . . . . . . . . . . . . . . . . . . . . . . . .68.1.  Normative References . . . . . . . . . . . . . . . . . .68.2.  Informative References . . . . . . . . . . . . . . . . .6       Author's Address . . . . . . . . . . . . . . . . . . . . . . .7       Full Copyright Statement . . . . . . . . . . . . . . . . . . .8Hollenbeck                  Standards Track                     [Page 1]

RFC 3749                TLS Compression Methods                 May 20041.  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.   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 inRFC 2119 [1].2.  Compression Methods   TLS [2] includes the following compression method structure in   sections6.1 and7.4.1.2 and Appendix sections A.4.1 and A.6:   enum { null(0), (255) } CompressionMethod;Hollenbeck                  Standards Track                     [Page 2]

RFC 3749                TLS Compression Methods                 May 2004   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 IETF Standards Track protocols.   2. Values from 64 decimal (0x40) through 223 decimal (0xDF) inclusive      are reserved for assignment for non-Standards Track methods.   3. Values from 224 decimal (0xE0) through 255 decimal (0xFF)      inclusive are reserved for private use.   Additional information describing the role of the IANA in the   allocation of compression method identifiers is described inSection5.   In addition, this definition is updated to include assignment of an   identifier for the DEFLATE compression method:   enum { null(0), DEFLATE(1), (255) } CompressionMethod;   As described insection 6 of RFC 2246 [2], TLS is a stateful   protocol.  Compression methods used with TLS can be either stateful   (the compressor maintains its 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 DEFLATE 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 insection 6.2.2 of RFC 2246 [2] MUST NOT be used with TLS.2.1.  DEFLATE Compression   The DEFLATE compression method and encoding format is described inRFC 1951 [5].  Examples of DEFLATE use in IETF protocols can be found   inRFC 1979 [6],RFC 2394 [7], andRFC 3274 [8].   DEFLATE 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,Hollenbeck                  Standards Track                     [Page 3]

RFC 3749                TLS Compression Methods                 May 2004   with no data retained to be included in a later output payload.   Flushing ensures that each compressed packet payload can be   decompressed completely.3.  Compression History and Packet Processing   Some compression methods have the ability to maintain state/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.   As described insection 7 of RFC 2246 [2], TLS allows multiple   connections to be instantiated using the same session through the   resumption feature of the TLS Handshake Protocol.  Session resumption   has operational implications when multiple compression methods are   available for use within the session.  For example, load balancers   will need to maintain additional state information if the compression   state is not cleared when a session is resumed.  As a result, the   following restrictions MUST be observed when resuming a session:   1.  The compression algorithm MUST be retained when resuming a       session.   2.  The compression state/history MUST be cleared when resuming a       session.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 ConsiderationsSection 2 of this document describes a registry of compression method   identifiers to be maintained by the IANA, including assignment of an   identifier for the DEFLATE compression method.  Identifier values   from the range 0-63 (decimal) inclusive are assigned viaRFC 2434   Standards Action [3].  Values from the range 64-223 (decimal)Hollenbeck                  Standards Track                     [Page 4]

RFC 3749                TLS Compression Methods                 May 2004   inclusive are assigned viaRFC 2434 Specification Required [3].   Identifier values from 224-255 (decimal) inclusive are reserved forRFC 2434 Private Use [3].6.  Security Considerations   This document does not introduce any topics that alter the threat   model addressed by TLS.  The security considerations described   throughoutRFC 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 do not 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.   Compression algorithms tend to be mathematically complex and prone to   implementation errors.  An implementation error that can produce a   buffer overrun introduces a potential security risk for programming   languages and operating systems that do not provide buffer overrun   protections.  Careful consideration should thus be given to   protections against implementation errors that introduce security   risks.   As described inSection 2, compression algorithms can occasionally   expand, rather than compress, input data.  This feature introduces   the ability to construct rogue data that expands to some enormous   size when compressed or decompressed.RFC 2246 describes several   methods to ameliorate this kind of attack.  First, compression has to   be lossless.  Second, a limit (1,024 bytes) is placed on the amount   of allowable compression content length increase.  Finally, a limit   (2^14 bytes) is placed on the total content length.  Seesection6.2.2 of RFC 2246 [2] for complete details.Hollenbeck                  Standards Track                     [Page 5]

RFC 3749                TLS Compression Methods                 May 20047.  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, Bodo Moeller, Win Treese, and the   IESG.8.  References8.1.  Normative References   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement        Levels",BCP 14,RFC 2119, March 1997.   [2]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",RFC2246, January 1999.   [3]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA        Considerations Section in RFCs",BCP 26,RFC 2434, October 1998.8.2.  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, P., "DEFLATE Compressed Data Format Specification        version 1.3",RFC 1951, May 1996.   [6]  Woods, J., "PPP Deflate Protocol",RFC 1979, August 1996.   [7]  Pereira, R., "IP Payload Compression Using DEFLATE",RFC 2394,        December 1998.   [8]  Gutmann, P., "Compressed Data Content Type for Cryptographic        Message Syntax (CMS)",RFC 3274, June 2002.Hollenbeck                  Standards Track                     [Page 6]

RFC 3749                TLS Compression Methods                 May 2004Author's Address   Scott Hollenbeck   VeriSign, Inc.   21345 Ridgetop Circle   Dulles, VA  20166-6503   US   EMail: shollenbeck@verisign.comHollenbeck                  Standards Track                     [Page 7]

RFC 3749                TLS Compression Methods                 May 2004Full Copyright Statement   Copyright (C) The Internet Society (2004).  This document is subject   to the rights, licenses and restrictions contained inBCP 78, and   except as set forth therein, the authors retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Hollenbeck                  Standards Track                     [Page 8]