SIPPING                                                     J. Rosenberg
Internet-Draft                                             Cisco Systems
Expires: May 12, 2006                                   November 8, 2005

 Rejecting Anonymous Requests in the Session Initiation Protocol (SIP)

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   The Session Initiation Protocol (SIP) allows for users to make
   anonymous calls.  However, users receiving such calls have the right
   to reject them because they are anonymous.  SIP has no way to
   indicate to the caller that the reason for call rejection was that
   the call was anonymous.  Such an indication is useful to allow the
   call to be retried without anonymity.  This specification defines a
   new SIP response code for this purpose.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  UAS Behavior . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  433 (Anonymity Disallowed) Definition  . . . . . . . . . . . .  4
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  4
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  4
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  5
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     8.1   Normative References . . . . . . . . . . . . . . . . . . .  5
     8.2   Informative References . . . . . . . . . . . . . . . . . .  5
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  6
       Intellectual Property and Copyright Statements . . . . . . . .  7

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1.  Introduction

   The Session Initiation Protocol (SIP) [1] allows for users to make
   anonymous calls.  In RFC 3261, this is done by including a From
   header field whose display name has the value of "Anonymous".
   Greater levels of anonymity were subsequently defined in RFC 3323
   [2], which introduces the Privacy header field.  The Privacy header
   field allows a requesting UA to ask for various levels of anonymity,
   including user level anonymity, header level anonymity, and session
   level anonymity.  RFC 3325 [3] additionally defined the P-Asserted-ID
   header field, used to contain an asserted identity.  RFC 3325 also
   defined the 'id' value for the Privacy header field, which is used to
   request the network to remove the P-Asserted-ID header field.

   Though users need to be able to make anonymous calls, users that
   receive such calls retain the right to reject the call because it is
   anonymous.  SIP does not provide a response code that allows the UAS
   to explicitly to indicate that the request was rejected because it
   was anonymous.  The closest response code is 403 (Forbidden), which
   doesn't convey a specific reason.  While it is possible to include a
   reason phrase in a 403 response that indicates to the human user that
   the call was rejected because it was anonymous, that reason phrase is
   not useful for automata.  An indication that can be understood by an
   automata would allow for programmatic handling, including user
   interface prompts, automatic retries, or conversion to equivalent
   error codes in the Public Switched Telephone Network (PSTN) when the
   client is a gateway.

   To remedy this, this specification defines the 433 (Anonymity
   Disallowed) response code.

2.  UAS Behavior

   A UAS MAY generate a 433 (Anonymity Disallowed) response when it
   receives an anonymous request, and the UAS refuses to fulfill the
   request because the requestor is anonymous.  A request is considered
   anonymous when the identity of the originator of the request has been
   explicitly witheld by the originator.  This occurs in several cases:

   o  The From header field contains a display name of anonymous or a
      URI within the anonymous.invalid domain.

   o  The request contained a Privacy header field whose value was 'id'
      [3] or 'user'.  This explicitly excludes the 'header' and
      'session' privacy services, since those do not directly convey the
      identity of the requestor.

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   o  The From or  P-Asserted-ID header field contains a URI which has
      an explicit indication that it is anonymous.  One such example of
      a mechanism that would meet this criteria is [4].

   It is important to note that lack of a P-Asserted-ID header field, in
   and of itself, is not an indication of anonymity.  Even though a
   Privacy header field value of 'id' will cause the removal of the
   P-Asserted-ID header field, there is no way to differentiate this
   case from one in which P-Asserted-ID was not supported by the
   originating domain.  As a consequence, a request without a
   P-Asserted-ID is considered anonymous only when there is some other
   indication of this, such as a From header field with a display name
   of 'Anonymous'.

3.  UAC Behavior

   A UAC receiving a 433 (Anonymity Disallowed) response MAY retry the
   request without requesting anonymity.  It SHOULD only do so if it
   obtains confirmation that the user that this is desirable.  Such
   confirmation could be obtained through the user interface, or by
   accessing user defined policy.  The UAC SHOULD NOT retry the request
   if it continues to request anonymity.

   A UAC the does not understand or care about the specific semantics of
   the 433 response will treat it as a 400 response.

4.  433 (Anonymity Disallowed) Definition

   This response indicates that the UAS refused the fulfill the request
   because the requestor was anonymous.  Its default reason phrase is
   "Anonymity Disallowed".

5.  IANA Considerations

   This section registers a new SIP response code according to the
   procedures of RFC 3261.

   RFC Number: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the RFC
      number of this specification]]

   Response Code Number: 433

   Default Reason Phrase: Anonymity Disallowed

6.  Security Considerations

   The fact that an request was rejected because it was anonymous does

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   reveal information about the called party - that they do not accept
   anonymous calls.  This information may or may not be sensitive.  If
   it is, a UAS SHOULD reject the request with a 403 instead.

7.  Acknowledgements

   This draft was motivated based on the requirements in [6], and has
   benefitted from the concepts in [5].

8.  References

8.1  Normative 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]  Peterson, J., "A Privacy Mechanism for the Session Initiation
        Protocol (SIP)", RFC 3323, November 2002.

   [3]  Jennings, C., Peterson, J., and M. Watson, "Private Extensions
        to the Session Initiation Protocol (SIP) for Asserted Identity
        within Trusted Networks", RFC 3325, November 2002.

8.2  Informative References

   [4]  Rosenberg, J., "Identity Privacy in the Session Initiation
        Protocol (SIP)", draft-rosenberg-sip-identity-privacy-00 (work
        in progress), July 2005.

   [5]  Hautakorpi, J. and G. Camarillo, "Extending the Session
        Initiation Protocol Reason Header with Warning Codes",
        draft-hautakorpi-reason-header-for-warnings-00 (work in
        progress), October 2005.

   [6]  Jesske, R., "Input Requirements for the Session Initiation
        Protocol (SIP) in support for  the European Telecommunications
        Standards Institute",
        draft-jesske-sipping-tispan-requirements-02 (work in progress),
        October 2005.

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Author's Address

   Jonathan Rosenberg
   Cisco Systems
   600 Lanidex Plaza
   Parsippany, NJ  07054

   Phone: +1 973 952-5000

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