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SIP                                                         J. Rosenberg
Internet-Draft                                               dynamicsoft
Expires: June 26, 2003                                 December 26, 2002

  The Session Initiation Protocol (SIP) INFO Method Considered Harmful

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 (2002).  All Rights Reserved.


   The Session Initiation Protocol (SIP) INFO method defines a means for
   transporting mid-dialog application layer data between user agents.
   Its initial use was to support the transport of ISUP mid-call
   messages which could not be mapped to any other SIP request method.
   However, since its initial usage for that purpose, INFO has seen
   widespread abuse as a means for introducing non-standard and
   non-interoperable extensions to SIP.  For this reason, we now believe
   INFO should be considered harmful, and therefore, deprecated in its
   current form.

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Table of Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2. Problems with INFO . . . . . . . . . . . . . . . . . . . . . .   4
   3. Proposed Solution  . . . . . . . . . . . . . . . . . . . . . .   6
   4. Security Considerations  . . . . . . . . . . . . . . . . . . .   7
   5. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .   8
      Informative References . . . . . . . . . . . . . . . . . . . .   9
      Author's Address . . . . . . . . . . . . . . . . . . . . . . .   9
      Intellectual Property and Copyright Statements . . . . . . . .  10

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1. Introduction

   The Session Initiation Protocol (SIP), was first published as RFC
   2543 [2] in March of 1999, and later revised as RFC 3261 [1] in June
   2002.  SIP was engineered to support extensions, through new methods,
   new headers, new parameters, and new body types.  The first extension
   to SIP that was published after RFC 2543 (and indeed, the only one
   published before SIP itself was revised), was the SIP INFO method,
   documented in RFC 2976 [3].

   The original application of the INFO method was to support the
   transport of ISUP mid-call signaling messages, as part of the SIP-T
   [6] framework.  However, the group believed that transport of ISUP
   messages was an overly narrow problem space.  So, the scope of the
   INFO method was expanded to allow for the transport of any
   application layer mid-call signaling.  As long as it didn't modify
   the dialog or transaction state, it was a legal usage of INFO.

   Unfortunately, since the publication of RFC 2976, there has been
   extensive misuse of INFO to support vendor proprietary extensions,
   resulting in loss of interoperability, amongst other things.  Section
   2 overviews the problems with the current specification, and Section
   3 proposes a path forward.

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2. Problems with INFO

   The problems with INFO derive from its lack of any well defined
   semantic.  Section 2, which overviews the INFO method, has this to

      There are no specific semantics associated with INFO.  The
      semantics are derived from the body or new headers defined for
      usage in INFO.

   This means that INFO is purposefully defined as a tunneling protocol
   for other extensions.  Indeed, it has been proposed (and in fact,
   used), for a wide variety of functions, including:

   o  The transport of DTMF digits to proxies and user agents

   o  Control of media servers

   o  Control of video encoding (fast intra updates, picture freeze)[9]

   o  Managing QoS reservations [8]

   o  Floor control

   There are several problems with these usages:

      Lack of Interoperability.  None of these usages interoperate.
      Part of the problem is that many are vendor extensions which are
      not described through any IETF specifications.  Another problem is
      that INFO itself provides no way to signal the actual usage.
      Interoperation depends on configuration to properly determine the
      implied INFO usage.

      Inappropriateness.  Many of these usages are not proper usages for
      SIP.  The guidelines for authors of SIP extensions [7] clearly
      rules some of them, such as media server control, out of scope.
      Since INFO is effectively a way to tunnel information between two
      endpoints connected through SIP, it appears to attract any kind of
      usage that requires bidirectional information transfer.  Many such
      usages are inappropriate because of the scaling issues (proxy
      overload), timing issues, and sequencing issues that arise when
      SIP is used.

      Incorrectness.  Many of these extensions and usages have protocol
      errors, security weaknesses, race conditions, and so on.  These
      are typically the result of insufficient review by the working
      group.  Such reviews never take place because the INFO method does
      not define any framework by which its usages can be defined.  It

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      does not specify any means by which vendors can publish
      informational usages, and thereby obtain IETF review.

   It is our belief that the root cause of these troubles is that INFO
   is a "content-free framework".  It is a framework because it leaves
   actual protocol semantics to usages - other extensions - which ride
   ontop of INFO.  Its goal as a framework is to support mid-call
   information transfer unrelated to SIP dialog or transaction state.
   However, it is "content-free" because it doesn't describe any means
   by which those extensions can be used in an interoperable way.  This
   is in contrast to the SIP events framework [4], which defines a way
   to specify packages that build on the framework.  It defines an IANA
   registration procedure, describes guidelines for when the SIP events
   framework should be used, specifies framework-specific behaviors, and
   specifies a means for indicating support for particular packages.  We
   believe all of these features are essential for any protocol that
   defines itself as a framework.

   However, none of these features are defined in RFC 2976.  There are
   no IANA procedures for "INFO extensions", no guidelines for what
   makes a good INFO usage, and no means of indicating support for a
   particular INFO usage.  Perhaps most interestingly, it does not
   specify any semantics for INFO itself.  Nothing about INFO processing
   differs from any other method.  Any new feature could be defined
   using a new method, and would suffer no duplication of functionality
   compared to defining it as a new usage of INFO.  It is this latter
   characteristic which truly makes it content-free.  What is the value
   of a framework that provides no semantics whatsoever? There is no

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3. Proposed Solution

   There are a few directions that can be taken to remedy this problem.
   We enumerate them all for completeness:

   1.  Do nothing.

   2.  Revise RFC 2976, turning it into a proper framework.  That would
       mean the specification of an IANA registration procedure, usage
       guidelines, and so on, patterned after RFC 3265.

   3.  Revise RFC 2976, restricting its usage to the only approved one -
       support for SIP-T.

   We believe that the proper course of action is the third.  While the
   second of these seems attractive, there is really little value.
   Thats because INFO does not actually define any semantics.  The SIP
   events framework, by contrast, defines a significant amount of
   behavior as part of the framework.  Indeed, one might argue that the
   vast majority of the semantics of any package exist in the framework.
   It is merely the definition of some defaults, and the establishment
   of some guidelines, which rest purely with the package.  The exact
   opposite is true of INFO.  In fact, it is not clear that there are
   any general, usage independent semantics that could be defined.  In
   the absence of such semantics, the usage of a framework provides no

   Option 3 would allow us to retain backwards compatibility with the
   only approved usage (and one of the most common ones).  Any other
   usage would be non-standard.  The functions that were previously
   being provided by INFO usages would now need to be specified as
   proper SIP extensions, and would therefore be subject to the SIP
   change process [5].  That will be valuable in addressing many of the
   problems outlined in Section 2.  The SIP change process will ensure
   that the extension is a proper SIP usage, is correct in its behavior,
   and that it interoperates.

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4. Security Considerations

   Well documented and reviewed protocols always provide better security
   than hacks piled ontop of a vague framework.  Therefore, we believe
   this proposal will generally improve the security of the Internet.

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5. Acknowledgments

   Credit goes to Dean Willis for suggesting, at some point, that INFO
   could be reformulated as a framework along the lines of RFC 3265.  It
   is also worth mentioning that the faults of RFC 2976 do not lie with
   its author, who is a respected contributor in the SIP community.  At
   the time of publication, it represented our best understanding of the
   way to approach the problem.  Significant experience has been gained
   since its publication, and it is that experience which has led us to
   conclude that the direction was ultimately not the right one.

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Informative References

   [1]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
        Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
        Session Initiation Protocol", RFC 3261, June 2002.

   [2]  Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
        "SIP: Session Initiation Protocol", RFC 2543, March 1999.

   [3]  Donovan, S., "The SIP INFO Method", RFC 2976, October 2000.

   [4]  Roach, A., "Session Initiation Protocol (SIP)-Specific Event
        Notification", RFC 3265, June 2002.

   [5]  Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J. and B.
        Rosen, "Change Process for the Session Initiation Protocol
        (SIP)", BCP 67, RFC 3427, December 2002.

   [6]  Vemuri, A. and J. Peterson, "Session Initiation Protocol for
        Telephones (SIP-T): (SIP-T): Context and Architectures", BCP 63,
        RFC 3372, September 2002.

   [7]  Rosenberg, J., "Guidelines for Authors of Extensions to the
        Session Initiation Protocol  (SIP)",
        draft-ietf-sip-guidelines-06 (work in progress), November 2002.

   [8]  Salsano, S., Veltri, L. and D. Papalilo, "SIP Extensions for QoS
        support", draft-veltri-sip-qsip-01 (work in progress), October

   [9]  Levin, O., Olson, S. and R. Even, "XML Schema for Media
        Control", draft-levin-mmusic-xml-media-control-00 (work in
        progress), October 2002.

Author's Address

   Jonathan Rosenberg
   72 Eagle Rock Avenue
   East Hanover, NJ  07936

   Phone: +1 973 952-5000
   EMail: jdrosen@dynamicsoft.com
   URI:   http://www.jdrosen.net

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