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Rosenberg 3 Internet-Draft Cisco Systems 4 Expires: July 20, 2006 January 16, 2006 6 Rejecting Anonymous Requests in the Session Initiation Protocol (SIP) 7 draft-ietf-sip-acr-code-00 9 Status of this Memo 11 By submitting this Internet-Draft, each author represents that any 12 applicable patent or other IPR claims of which he or she is aware 13 have been or will be disclosed, and any of which he or she becomes 14 aware will be disclosed, in accordance with Section 6 of BCP 79. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as Internet- 19 Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt. 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 This Internet-Draft will expire on July 20, 2006. 34 Copyright Notice 36 Copyright (C) The Internet Society (2006). 38 Abstract 40 The Session Initiation Protocol (SIP) allows for users to make 41 anonymous calls. However, users receiving such calls have the right 42 to reject them because they are anonymous. SIP has no way to 43 indicate to the caller that the reason for call rejection was that 44 the call was anonymous. Such an indication is useful to allow the 45 call to be retried without anonymity. This specification defines a 46 new SIP response code for this purpose. 48 Table of Contents 50 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 51 2. Server Behavior . . . . . . . . . . . . . . . . . . . . . . . 3 52 3. UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . . . 4 53 4. 433 (Anonymity Disallowed) Definition . . . . . . . . . . . . 4 54 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 55 6. Security Considerations . . . . . . . . . . . . . . . . . . . 5 56 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5 57 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5 58 8.1 Normative References . . . . . . . . . . . . . . . . . . . 5 59 8.2 Informative References . . . . . . . . . . . . . . . . . . 6 60 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 6 61 Intellectual Property and Copyright Statements . . . . . . . . 7 63 1. Introduction 65 The Session Initiation Protocol (SIP) [1] allows for users to make 66 anonymous calls. In RFC 3261, this is done by including a From 67 header field whose display name has the value of "Anonymous". 68 Greater levels of anonymity were subsequently defined in RFC 3323 69 [2], which introduces the Privacy header field. The Privacy header 70 field allows a requesting UA to ask for various levels of anonymity, 71 including user level anonymity, header level anonymity, and session 72 level anonymity. RFC 3325 [3] additionally defined the P-Asserted-ID 73 header field, used to contain an asserted identity. RFC 3325 also 74 defined the 'id' value for the Privacy header field, which is used to 75 request the network to remove the P-Asserted-ID header field. 77 Though users need to be able to make anonymous calls, users that 78 receive such calls retain the right to reject the call because it is 79 anonymous. SIP does not provide a response code that allows the UAS, 80 or a proxy acting on its behalf, to explicitly to indicate that the 81 request was rejected because it was anonymous. The closest response 82 code is 403 (Forbidden), which doesn't convey a specific reason. 83 While it is possible to include a reason phrase in a 403 response 84 that indicates to the human user that the call was rejected because 85 it was anonymous, that reason phrase is not useful for automata. An 86 indication that can be understood by an automata would allow for 87 programmatic handling, including user interface prompts, automatic 88 retries, or conversion to equivalent error codes in the Public 89 Switched Telephone Network (PSTN) when the client is a gateway. 91 To remedy this, this specification defines the 433 (Anonymity 92 Disallowed) response code. 94 2. Server Behavior 96 A server acting on behalf of the called party, such as the UAS or a 97 proxy in their domain, MAY generate a 433 (Anonymity Disallowed) 98 response when it receives an anonymous request, and the called party 99 refuses to fulfill the request because the requestor is anonymous. A 100 request is considered anonymous when the identity of the originator 101 of the request has been explicitly witheld by the originator. This 102 occurs in any one of the following cases: 104 o The From header field contains a URI within the anonymous.invalid 105 domain. 107 o The From header field contains a display name whose value is 108 either 'Anonymous' or 'anonymous'. Note that display names make a 109 poor choice for indicating anonymity, since they are meant to be 110 consumed by humans, not automata. Thus, language variations and 111 even misspelling can cause an automata to miss a hint in the 112 display name. Despite these problems, a check on the display name 113 is included here because RFC 3261 explicitly calls out the usage 114 of the display name as a way to declare anonymity. 116 o The request contained a Privacy header field whose value was 'id' 117 [3] or 'user'. This explicitly excludes the 'header' and 118 'session' privacy services, since those do not directly convey the 119 identity of the requestor. 121 o The From or P-Asserted-ID header field contains a URI which has 122 an explicit indication that it is anonymous. One such example of 123 a mechanism that would meet this criteria is [4]. 125 It is important to note that lack of a P-Asserted-ID header field, in 126 and of itself, is not an indication of anonymity. Even though a 127 Privacy header field value of 'id' will cause the removal of the 128 P-Asserted-ID header field, there is no way to differentiate this 129 case from one in which P-Asserted-ID was not supported by the 130 originating domain. As a consequence, a request without a 131 P-Asserted-ID is considered anonymous only when there is some other 132 indication of this, such as a From header field with a display name 133 of 'Anonymous'. 135 3. UAC Behavior 137 A UAC receiving a 433 (Anonymity Disallowed) response MAY retry the 138 request without requesting anonymity. It SHOULD only do so if it 139 obtains confirmation from the user that this is desirable. Such 140 confirmation could be obtained through the user interface, or by 141 accessing user defined policy. The UAC SHOULD NOT retry the request 142 if user continues to request anonymity. 144 A UAC the does not understand or care about the specific semantics of 145 the 433 response will treat it as a 400 response. 147 4. 433 (Anonymity Disallowed) Definition 149 This response indicates that the server refused the fulfill the 150 request because the requestor was anonymous. Its default reason 151 phrase is "Anonymity Disallowed". 153 5. IANA Considerations 155 This section registers a new SIP response code according to the 156 procedures of RFC 3261. 158 RFC Number: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the RFC 159 number of this specification]] 161 Response Code Number: 433 163 Default Reason Phrase: Anonymity Disallowed 165 6. Security Considerations 167 The fact that an request was rejected because it was anonymous does 168 reveal information about the called party - that they do not accept 169 anonymous calls. This information may or may not be sensitive. If 170 it is, a UAS SHOULD reject the request with a 403 instead. 172 In the Public Switched Telephone Network (PSTN), the Anonymous Call 173 Rejection (ACR) feature is commonly used to prevent unwanted calls 174 from telemarketers (also known as spammers). Since telemarketers 175 frequently withhold their identity, this has the desired effect in 176 many (but not all) cases. It is important to note that the response 177 code described here is likely to be ineffective in blocking SIP-based 178 spam. The reason is that a malicious caller can include a From 179 header field and display name that is not anonymous, but is 180 meaningless and invalid. Without a Privacy header field, such a 181 request will not appear anonymous and thus not be blocked by an 182 anonymity screening service. Dealing with SIP-based spam is not a 183 simple problem. The reader is referred to [7] for a discussion of 184 the problem. 186 7. Acknowledgements 188 This draft was motivated based on the requirements in [6], and has 189 benefitted from the concepts in [5]. 191 8. References 193 8.1 Normative References 195 [1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., 196 Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: 197 Session Initiation Protocol", RFC 3261, June 2002. 199 [2] Peterson, J., "A Privacy Mechanism for the Session Initiation 200 Protocol (SIP)", RFC 3323, November 2002. 202 [3] Jennings, C., Peterson, J., and M. Watson, "Private Extensions 203 to the Session Initiation Protocol (SIP) for Asserted Identity 204 within Trusted Networks", RFC 3325, November 2002. 206 8.2 Informative References 208 [4] Rosenberg, J., "Identity Privacy in the Session Initiation 209 Protocol (SIP)", draft-rosenberg-sip-identity-privacy-00 (work 210 in progress), July 2005. 212 [5] Hautakorpi, J. and G. Camarillo, "Extending the Session 213 Initiation Protocol Reason Header with Warning Codes", 214 draft-hautakorpi-reason-header-for-warnings-00 (work in 215 progress), October 2005. 217 [6] Jesske, R., "Input Requirements for the Session Initiation 218 Protocol (SIP) in support for the European Telecommunications 219 Standards Institute", 220 draft-jesske-sipping-tispan-requirements-02 (work in progress), 221 October 2005. 223 [7] Rosenberg, J., "The Session Initiation Protocol (SIP) and Spam", 224 draft-ietf-sipping-spam-01 (work in progress), July 2005. 226 [8] Peterson, J. and C. Jennings, "Enhancements for Authenticated 227 Identity Management in the Session Initiation Protocol (SIP)", 228 draft-ietf-sip-identity-05 (work in progress), May 2005. 230 Author's Address 232 Jonathan Rosenberg 233 Cisco Systems 234 600 Lanidex Plaza 235 Parsippany, NJ 07054 236 US 238 Phone: +1 973 952-5000 239 Email: jdrosen@cisco.com 240 URI: http://www.jdrosen.net 242 Intellectual Property Statement 244 The IETF takes no position regarding the validity or scope of any 245 Intellectual Property Rights or other rights that might be claimed to 246 pertain to the implementation or use of the technology described in 247 this document or the extent to which any license under such rights 248 might or might not be available; nor does it represent that it has 249 made any independent effort to identify any such rights. 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