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rfc:rfc5079

Network Working Group J. Rosenberg Request for Comments: 5079 Cisco Category: Standards Track December 2007

Rejecting Anonymous Requests in the Session Initiation Protocol (SIP)

Status 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.

Abstract

 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.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
 2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 2
 3.  Server Behavior . . . . . . . . . . . . . . . . . . . . . . . . 3
 4.  UAC Behavior  . . . . . . . . . . . . . . . . . . . . . . . . . 4
 5.  433 (Anonymity Disallowed) Definition . . . . . . . . . . . . . 4
 6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
 7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
 8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
 9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
   9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
   9.2.  Informative References  . . . . . . . . . . . . . . . . . . 6

Rosenberg Standards Track [Page 1] RFC 5079 ACR Response Code December 2007

1. Introduction

 The Session Initiation Protocol (SIP) [RFC3261] 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 [RFC3323],
 which introduces the Privacy header field.  The Privacy header field
 allows a requesting User Agent (UA) to ask for various levels of
 anonymity, including user level anonymity, header level anonymity,
 and session level anonymity.  [RFC3325] additionally defined the
 P-Asserted-Identity 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-Identity 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 User
 Agent Server (UAS), or a proxy acting on its behalf, to explicitly
 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 and cannot be interpreted by callers that speak a
 different language.  An indication that can be understood by an
 automaton would allow for programmatic handling, including user
 interface prompts, 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. Terminology

 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 [RFC2119].

Rosenberg Standards Track [Page 2] RFC 5079 ACR Response Code December 2007

3. Server Behavior

 A server (generally acting on behalf of the called party, though this
 need not be the case) MAY generate a 433 (Anonymity Disallowed)
 response when it receives an anonymous request, and the server
 refuses to fulfill the request because the requestor is anonymous.  A
 request SHOULD be considered anonymous when the identity of the
 originator of the request has been explicitly withheld by the
 originator.  This occurs in any one of the following cases:
 o  The From header field contains a URI within the anonymous.invalid
    domain.
 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
    even misspelling can cause an automaton 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 indicates
    that the user wishes its identity withheld.  Values meeting this
    criteria are 'id' [RFC3325] or 'user'.
 o  The From header field contains a URI that has an explicit
    indication that it is anonymous.  One such example of a mechanism
    that would meet this criteria is [coexistence].  This criteria is
    true even if the request has a validated Identity header field
    [RFC4474], which can be used in concert with anonymized From
    header fields.
 Lack of a network-asserted identity (such as the P-Asserted-Identity
 header field), in and of itself, SHOULD NOT be considered an
 indication of anonymity.  Even though a Privacy header field value of
 'id' will cause the removal of a network-asserted identity, there is
 no way to differentiate this case from one in which a network-
 asserted identity was not supported by the originating domain.  As a
 consequence, a request without a network-asserted identity is
 considered anonymous only when there is some other indication of
 this, such as a From header field with a display name of 'Anonymous'.
 In addition, requests where the identity of the requestor cannot be
 determined or validated, but it is not a consequence of an explicit
 action on the part of the requestor, are not considered anonymous.
 For example, if a request contains a non-anonymous From header field,
 along with the Identity and Identity-Info header fields [RFC4474],

Rosenberg Standards Track [Page 3] RFC 5079 ACR Response Code December 2007

 but the certificate could not be obtained from the reference in the
 Identity-Info header field, it is not considered an anonymous
 request, and the 433 response code SHOULD NOT be used.

4. UAC Behavior

 A User Agent Client (UAC) receiving a 433 (Anonymity Disallowed) MUST
 NOT retry the request without anonymity unless 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.  If the user has indicated that this is desirable,
 the UAC MAY retry the request without requesting anonymity.  Note
 that if the UAC were to automatically retry the request without
 anonymity in the absence of an indication from the user that this
 treatment is desirable, then the user's expectations would not be
 met.  Consequently, a user might think it had completed a call
 anonymously when it is not actually anonymous.
 Receipt of a 433 response to a mid-dialog request SHOULD NOT cause
 the dialog to terminate, and SHOULD NOT cause the specific usage of
 that dialog to terminate [RFC5057].
 A UAC that does not understand or care about the specific semantics
 of the 433 response will treat it as a 400 response.

5. 433 (Anonymity Disallowed) Definition

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

6. IANA Considerations

 This section registers a new SIP response code according to the
 procedures of RFC 3261.
 RFC Number:  RFC 5079
 Response Code Number:  433
 Default Reason Phrase:  Anonymity Disallowed

Rosenberg Standards Track [Page 4] RFC 5079 ACR Response Code December 2007

7. Security Considerations

 The fact that a request was rejected because it was anonymous does
 reveal information about the called party -- that the called party
 does 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, anonymous call rejection 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 [sipping-spam] for a
 discussion of the problem.
 When anonymity services are being provided as a consequence of an
 anonymizer function acting as a back-to-back user agent (B2BUA)
 [RFC3323], and the anonymizer receives a 433 response, the anonymizer
 MUST NOT retry the request without anonymization unless it has been
 explicitly configured by the user to do so.  In essence, the same
 rules that apply to a UA in processing of a 433 response apply to a
 network-based anonymization function, and for the same reasons.

8. Acknowledgements

 This document was motivated based on the requirements in
 [tispan-req], and has benefited from the concepts in [hautakorpi].
 Thanks to Keith Drage, Paul Kyzivat, and John Elwell for their
 reviews of this document.

Rosenberg Standards Track [Page 5] RFC 5079 ACR Response Code December 2007

9. References

9.1. Normative References

 [RFC3261]       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.
 [RFC3323]       Peterson, J., "A Privacy Mechanism for the Session
                 Initiation Protocol (SIP)", RFC 3323, November 2002.
 [RFC2119]       Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC4474]       Peterson, J. and C. Jennings, "Enhancements for
                 Authenticated Identity Management in the Session
                 Initiation Protocol (SIP)", RFC 4474, August 2006.

9.2. Informative References

 [RFC3325]       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.
 [coexistence]   Rosenberg, J., "Coexistence of P-Asserted-ID and SIP
                 Identity", Work in Progress, June 2006.
 [tispan-req]    Jesske, R., "Input Requirements for the Session
                 Initiation Protocol (SIP) in support for  the
                 European Telecommunications Standards Institute",
                 Work in Progress, July 2007.
 [hautakorpi]    Hautakorpi, J. and G. Camarillo, "Extending the
                 Session Initiation Protocol Reason Header with
                 Warning Codes", Work in Progress, October 2005.
 [RFC5057]       Sparks, R., "Multiple Dialog Usages in the Session
                 Initiation Protocol", RFC in 5057, November 2007.
 [sipping-spam]  Jennings, C. and J. Rosenberg, "The Session
                 Initiation Protocol (SIP) and Spam", Work
                 in Progress, August 2007.

Rosenberg Standards Track [Page 6] RFC 5079 ACR Response Code December 2007

Author's Address

 Jonathan Rosenberg
 Cisco
 Edison, NJ
 US
 EMail: jdrosen@cisco.com
 URI:   http://www.jdrosen.net

Rosenberg Standards Track [Page 7] RFC 5079 ACR Response Code December 2007

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Rosenberg Standards Track [Page 8]

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