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Rosenberg 3 Internet-Draft Cisco Systems 4 Expires: December 21, 2006 June 19, 2006 6 Rejecting Anonymous Requests in the Session Initiation Protocol (SIP) 7 draft-ietf-sip-acr-code-01 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 December 21, 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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3 52 3. Server Behavior . . . . . . . . . . . . . . . . . . . . . . . . 3 53 4. UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . . . 4 54 5. 433 (Anonymity Disallowed) Definition . . . . . . . . . . . . . 4 55 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 56 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 57 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5 58 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 59 9.1. Normative References . . . . . . . . . . . . . . . . . . . 6 60 9.2. Informative References . . . . . . . . . . . . . . . . . . 6 61 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7 62 Intellectual Property and Copyright Statements . . . . . . . . . . 8 64 1. Introduction 66 The Session Initiation Protocol (SIP) [1] allows for users to make 67 anonymous calls. In RFC 3261, this is done by including a From 68 header field whose display name has the value of "Anonymous". 69 Greater levels of anonymity were subsequently defined in RFC 3323 70 [2], which introduces the Privacy header field. The Privacy header 71 field allows a requesting UA to ask for various levels of anonymity, 72 including user level anonymity, header level anonymity, and session 73 level anonymity. RFC 3325 [3] additionally defined the P-Asserted-ID 74 header field, used to contain an asserted identity. RFC 3325 also 75 defined the 'id' value for the Privacy header field, which is used to 76 request the network to remove the P-Asserted-ID header field. 78 Though users need to be able to make anonymous calls, users that 79 receive such calls retain the right to reject the call because it is 80 anonymous. SIP does not provide a response code that allows the UAS, 81 or a proxy acting on its behalf, to explicitly to indicate that the 82 request was rejected because it was anonymous. The closest response 83 code is 403 (Forbidden), which doesn't convey a specific reason. 84 While it is possible to include a reason phrase in a 403 response 85 that indicates to the human user that the call was rejected because 86 it was anonymous, that reason phrase is not useful for automata. An 87 indication that can be understood by an automata would allow for 88 programmatic handling, including user interface prompts, automatic 89 retries, or conversion to equivalent error codes in the Public 90 Switched Telephone Network (PSTN) when the client is a gateway. 92 To remedy this, this specification defines the 433 (Anonymity 93 Disallowed) response code. 95 2. Terminology 97 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 98 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 99 document are to be interpreted as described in RFC 2119 [4]. 101 3. Server Behavior 103 A server acting on behalf of the called party, such as the UAS or a 104 proxy in their domain, MAY generate a 433 (Anonymity Disallowed) 105 response when it receives an anonymous request, and the called party 106 refuses to fulfill the request because the requestor is anonymous. A 107 request is considered anonymous when the identity of the originator 108 of the request has been explicitly withheld by the originator. This 109 occurs in any one of the following cases: 111 o The From header field contains a URI within the anonymous.invalid 112 domain. 114 o The From header field contains a display name whose value is 115 either 'Anonymous' or 'anonymous'. Note that display names make a 116 poor choice for indicating anonymity, since they are meant to be 117 consumed by humans, not automata. Thus, language variations and 118 even misspelling can cause an automata to miss a hint in the 119 display name. Despite these problems, a check on the display name 120 is included here because RFC 3261 explicitly calls out the usage 121 of the display name as a way to declare anonymity. 123 o The request contained a Privacy header field whose value was 'id' 124 [3] or 'user'. This explicitly excludes the 'header' and 125 'session' privacy services, since those do not directly convey the 126 identity of the requestor. 128 o The From or P-Asserted-ID header field contains a URI which has an 129 explicit indication that it is anonymous. One such example of a 130 mechanism that would meet this criteria is [5]. 132 It is important to note that lack of a P-Asserted-ID header field, in 133 and of itself, is not an indication of anonymity. Even though a 134 Privacy header field value of 'id' will cause the removal of the 135 P-Asserted-ID header field, there is no way to differentiate this 136 case from one in which P-Asserted-ID was not supported by the 137 originating domain. As a consequence, a request without a 138 P-Asserted-ID is considered anonymous only when there is some other 139 indication of this, such as a From header field with a display name 140 of 'Anonymous'. 142 4. UAC Behavior 144 A UAC receiving a 433 (Anonymity Disallowed) response MAY retry the 145 request without requesting anonymity. It SHOULD only do so if it 146 obtains confirmation from the user that this is desirable. Such 147 confirmation could be obtained through the user interface, or by 148 accessing user defined policy. The UAC SHOULD NOT retry the request 149 if user continues to request anonymity. 151 A UAC the does not understand or care about the specific semantics of 152 the 433 response will treat it as a 400 response. 154 5. 433 (Anonymity Disallowed) Definition 156 This response indicates that the server refused the fulfill the 157 request because the requestor was anonymous. Its default reason 158 phrase is "Anonymity Disallowed". 160 6. IANA Considerations 162 This section registers a new SIP response code according to the 163 procedures of RFC 3261. 165 RFC Number: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the RFC 166 number of this specification]] 168 Response Code Number: 433 170 Default Reason Phrase: Anonymity Disallowed 172 7. Security Considerations 174 The fact that an request was rejected because it was anonymous does 175 reveal information about the called party - that they do not accept 176 anonymous calls. This information may or may not be sensitive. If 177 it is, a UAS SHOULD reject the request with a 403 instead. 179 In the Public Switched Telephone Network (PSTN), the Anonymous Call 180 Rejection (ACR) feature is commonly used to prevent unwanted calls 181 from telemarketers (also known as spammers). Since telemarketers 182 frequently withhold their identity, this has the desired effect in 183 many (but not all) cases. It is important to note that the response 184 code described here is likely to be ineffective in blocking SIP-based 185 spam. The reason is that a malicious caller can include a From 186 header field and display name that is not anonymous, but is 187 meaningless and invalid. Without a Privacy header field, such a 188 request will not appear anonymous and thus not be blocked by an 189 anonymity screening service. Dealing with SIP-based spam is not a 190 simple problem. The reader is referred to [8] for a discussion of 191 the problem. 193 8. Acknowledgements 195 This draft was motivated based on the requirements in [7], and has 196 benefited from the concepts in [6]. 198 9. References 199 9.1. Normative References 201 [1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., 202 Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: 203 Session Initiation Protocol", RFC 3261, June 2002. 205 [2] Peterson, J., "A Privacy Mechanism for the Session Initiation 206 Protocol (SIP)", RFC 3323, November 2002. 208 [3] Jennings, C., Peterson, J., and M. Watson, "Private Extensions 209 to the Session Initiation Protocol (SIP) for Asserted Identity 210 within Trusted Networks", RFC 3325, November 2002. 212 [4] Bradner, S., "Key words for use in RFCs to Indicate Requirement 213 Levels", BCP 14, RFC 2119, March 1997. 215 9.2. Informative References 217 [5] Rosenberg, J., "Identity Privacy in the Session Initiation 218 Protocol (SIP)", draft-rosenberg-sip-identity-privacy-00 (work 219 in progress), July 2005. 221 [6] Hautakorpi, J. and G. Camarillo, "Extending the Session 222 Initiation Protocol Reason Header with Warning Codes", 223 draft-hautakorpi-reason-header-for-warnings-00 (work in 224 progress), October 2005. 226 [7] Jesske, R., "Input Requirements for the Session Initiation 227 Protocol (SIP) in support for the European Telecommunications 228 Standards Institute", 229 draft-jesske-sipping-tispan-requirements-02 (work in progress), 230 October 2005. 232 [8] Rosenberg, J., "The Session Initiation Protocol (SIP) and Spam", 233 draft-ietf-sipping-spam-02 (work in progress), March 2006. 235 Author's Address 237 Jonathan Rosenberg 238 Cisco Systems 239 600 Lanidex Plaza 240 Parsippany, NJ 07054 241 US 243 Phone: +1 973 952-5000 244 Email: jdrosen@cisco.com 245 URI: http://www.jdrosen.net 247 Intellectual Property Statement 249 The IETF takes no position regarding the validity or scope of any 250 Intellectual Property Rights or other rights that might be claimed to 251 pertain to the implementation or use of the technology described in 252 this document or the extent to which any license under such rights 253 might or might not be available; nor does it represent that it has 254 made any independent effort to identify any such rights. 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