SIP                                                         J. Rosenberg
Internet-Draft                                             Cisco Systems
Expires: July 20, 2006                                  January 16, 2006

 Rejecting Anonymous Requests in the Session Initiation Protocol (SIP)

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   Copyright (C) The Internet Society (2006).


   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.  Server Behavior  . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  433 (Anonymity Disallowed) Definition  . . . . . . . . . . . .  4
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  4
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  5
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  5
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     8.1   Normative References . . . . . . . . . . . . . . . . . . .  5
     8.2   Informative References . . . . . . . . . . . . . . . . . .  6
       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,
   or a proxy acting on its behalf, 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.  Server Behavior

   A server acting on behalf of the called party, such as the UAS or a
   proxy in their domain, MAY generate a 433 (Anonymity Disallowed)
   response when it receives an anonymous request, and the called party
   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 any one of the following cases:

   o  The From header field contains a URI within the anonymous.invalid

   o  The From header field contains a display name whose value is
      either 'Anonymous' or 'anonymous'.  Note that display names make a
      poor choice for indicating anonymity, since they are meant to be
      consumed by humans, not automata.  Thus, language variations and

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      even misspelling can cause an automata to miss a hint in the
      display name.  Despite these problems, a check on the display name
      is included here because RFC 3261 explicitly calls out the usage
      of the display name as a way to declare anonymity.

   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.

   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 from 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 user 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 server 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.

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   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
   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.

   In the Public Switched Telephone Network (PSTN), the Anonymous Call
   Rejection (ACR) feature is commonly used to prevent unwanted calls
   from telemarketers (also known as spammers).  Since telemarketers
   frequently withhold their identity, this has the desired effect in
   many (but not all) cases.  It is important to note that the response
   code described here is likely to be ineffective in blocking SIP-based
   spam.  The reason is that a malicious caller can include a From
   header field and display name that is not anonymous, but is
   meaningless and invalid.  Without a Privacy header field, such a
   request will not appear anonymous and thus not be blocked by an
   anonymity screening service.  Dealing with SIP-based spam is not a
   simple problem.  The reader is referred to [7] for a discussion of
   the problem.

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.

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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.

   [7]  Rosenberg, J., "The Session Initiation Protocol (SIP) and Spam",
        draft-ietf-sipping-spam-01 (work in progress), July 2005.

   [8]  Peterson, J. and C. Jennings, "Enhancements for Authenticated
        Identity Management in the Session Initiation  Protocol (SIP)",
        draft-ietf-sip-identity-05 (work in progress), May 2005.

Author's Address

   Jonathan Rosenberg
   Cisco Systems
   600 Lanidex Plaza
   Parsippany, NJ  07054

   Phone: +1 973 952-5000

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