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Network Working Group                                       J. RosenbergRequest for Comments: 5079                                         CiscoCategory: Standards Track                                  December 2007Rejecting 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 Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .22.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . .23.  Server Behavior . . . . . . . . . . . . . . . . . . . . . . . .34.  UAC Behavior  . . . . . . . . . . . . . . . . . . . . . . . . .45.  433 (Anonymity Disallowed) Definition . . . . . . . . . . . . .46.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . .47.  Security Considerations . . . . . . . . . . . . . . . . . . . .58.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . .59.  References  . . . . . . . . . . . . . . . . . . . . . . . . . .69.1.  Normative References  . . . . . . . . . . . . . . . . . . .69.2.  Informative References  . . . . . . . . . . . . . . . . . .6Rosenberg                   Standards Track                     [Page 1]

RFC 5079                   ACR Response Code               December 20071.  Introduction   The Session Initiation Protocol (SIP) [RFC3261] allows for users to   make anonymous calls.  InRFC 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 20073.  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 becauseRFC 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 ofRFC 3261.   RFC Number:RFC 5079   Response Code Number:  433   Default Reason Phrase:  Anonymity DisallowedRosenberg                   Standards Track                     [Page 4]

RFC 5079                   ACR Response Code               December 20077.  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 20079.  References9.1.  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 2007Author's Address   Jonathan Rosenberg   Cisco   Edison, NJ   US   EMail: jdrosen@cisco.com   URI:http://www.jdrosen.netRosenberg                   Standards Track                     [Page 7]

RFC 5079                   ACR Response Code               December 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   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, THE IETF TRUST 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.Rosenberg                   Standards Track                     [Page 8]