Network Working Group R. Sparks Internet-Draft dynamicsoft Expires: November 20, 2002 May 22, 2002 The Referred-By Mechanism draft-ietf-sip-referredby-00 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http:// www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on November 20, 2002. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract The SIP REFER method [2] provides a mechanism where one party (the referror) provides a second party (the referree) with an arbitrary URI to reference. If that URI is a SIP URI, the referree will send a SIP request, often an INVITE, to that URI (the refer target). This document extends the REFER method allowing the referror to provide information about the reference to the refer target using the referree as an intermediary. This information includes the identity of the referror and the URI to which the referror referred. The mechanism utilizes S/MIME to help protect this information from a malicious intermediary. This protection is optional, but a recipient may refuse to accept a request unless it is present. Sparks Expires November 20, 2002 [Page 1] Internet-Draft The Referred-By Mechanism May 2002 Table of Contents 1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. The Referred-By Mechanism . . . . . . . . . . . . . . . . . 4 2.1 Referror behavior . . . . . . . . . . . . . . . . . . . . . 4 2.2 Referree behavior . . . . . . . . . . . . . . . . . . . . . 5 2.2.1 OPEN ISSUE - changing Content-ID . . . . . . . . . . . . . . 5 2.3 Refer Target behavior . . . . . . . . . . . . . . . . . . . 5 3. The Referred-By Header Field . . . . . . . . . . . . . . . . 6 3.1 OPEN ISSUE - remove the support table . . . . . . . . . . . 6 4. The Referred-By Token . . . . . . . . . . . . . . . . . . . 7 5. The 429 Provide Referror Identity error response . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . 7 7. Using an Authentication Service . . . . . . . . . . . . . . 9 8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 9 8.1 Basic REFER . . . . . . . . . . . . . . . . . . . . . . . . 9 8.2 Insecure REFER . . . . . . . . . . . . . . . . . . . . . . . 12 8.3 Requiring Referror Identity . . . . . . . . . . . . . . . . 13 8.4 Nested REFER . . . . . . . . . . . . . . . . . . . . . . . . 16 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 20 10. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 21 References . . . . . . . . . . . . . . . . . . . . . . . . . 21 Author's Address . . . . . . . . . . . . . . . . . . . . . . 22 Full Copyright Statement . . . . . . . . . . . . . . . . . . 23 Sparks Expires November 20, 2002 [Page 2] Internet-Draft The Referred-By Mechanism May 2002 1. Overview The SIP REFER method [2] provides a mechanism where one party (the referror) provides a second party (the referree) with an arbitrary URI to reference. If that URI is a SIP URI, the referree will send a SIP request, often an INVITE, to that URI (the refer target). Nothing provided in [2] distinguishes this referenced request from any other request the referree might have sent to the refer target. --------------------------------------------------------------------- Referror Referree Refer Target | | | | REFER | | | Refer-To: target | | |----------------->| INVITE target | | |------------------->| Classic REFER --------------------------------------------------------------------- There are applications of REFER, such as call transfer [4], where it is desirable to provide the refer target with certain information about the referror and the REFER request itself. This information may include, but is not limited to, the referror's identity, the referred to URI, and the time of the referral. The refer target can use this information when deciding whether to admit the referenced request. This draft defines one set of mechanisms to provide that information. All of the mechanisms in this draft involve placing information in the REFER request that the referee copies into the referenced request. This necessarily establishes the referee as an eavesdropper and places the referree in a position to launch man-in-the-middle attacks on that information. At the simplest level, this draft defines a mechanism for carrying the referror's identity, expressed as a SIP URI in a new header: Referred-By. The refer target can use that information, even if it has not been protected from the referree, at the perils and with the limitations documented here. The draft proceeds to define an S/MIME based mechanism for expressing the identity of the referror and capturing other information about the REFER request, allowing the refer target to detect tampering (and other undesirable behaviors) by the referree. Finally, the draft discusses how an authentication service as defined in [5] may be used to generate that S/MIME Sparks Expires November 20, 2002 [Page 3] Internet-Draft The Referred-By Mechanism May 2002 content. 2. The Referred-By Mechanism The following figure summarizes how Referred-By information is carried to the Refer Target. The Referror provides a Referred-By header with its SIP identity, optionally associating an S/MIME protected token reflecting the identity of the referror and details of the REFER request. The Referree copies this header and the token, if provided, into the triggered request (shown here as an INVITE). --------------------------------------------------------------------- Referror Referree Refer Target | | | | REFER | | | Refer-To: target | | | Referred-By: referror;cid=X | | | | | | (one of the body parts is) | | | Content-ID: X | | | | | |----------------------------->| | | | INVITE target | | | Referred-By: referror;cid=X | | | | | | (one of the body parts is) | | | Content-ID: X | | | | | |---------------------------->| REFER with Referred-By --------------------------------------------------------------------- 2.1 Referror behavior A UA sending a REFER request (a referror) MAY provide a Referred-By header field value in the request. A REFER request MUST NOT contain more than one Referred-By header field value. A referror MAY include a Referred-By token in a REFER request. A REFER request containing a Referred-By token MUST contain a Referred- By header field value with a cid parameter value equal to the Content-ID of the body part containing the token. Sparks Expires November 20, 2002 [Page 4] Internet-Draft The Referred-By Mechanism May 2002 The referror will receive a NOTIFY with a sipfrag indicating a final response of 429 "Provide Referror Identity" to the referrenced request if the refer target requires a valid Referred-By token to accept the request. The can occur when either no token is provided or a provided token is invalid. The referror will recieve a 429 "Provide Referror Identity" response to the REFER if the referee requires a Referred-By token to be present in order to accept the REFER. 2.2 Referree behavior A UA receiving a REFER request (a referree) to a SIP URI (using either the sip: or sips: scheme) MUST copy any Referred-By header field value and token into the referrenced request without modification. [5] 2.2.1 OPEN ISSUE - changing Content-ID 2.3 Refer Target behavior A UA receiving a non-REFER SIP request MAY inspect the request for a Referred-By header field and token. If a Referred-By header field value is not present, this UA can not distinguish this request from any other the UA acting as the referree might have sent. Thus, the UA would apply exactly the admissions policies and processing described in [1] to the request. If a Referred-By header field value is present, the receiving UA can consider itself a refer target and MAY apply additional admission policies based on the contents of the Referred-By header field and token. The referror is in a position to modify the contents of the Referred- By header field value, or falsely provide one even if no REFER actually exists. If such behavior could affect admission policy (including influencing the agent's user by rendering misleading content), the refer target SHOULD require that a valid Referred-By token be present. The refer target MAY reject a request if no Referred-By token is present or if the token is stale using the 429 "Provide Referror Identity" error response defined in Section 5. The 428 error response from [5] is not appropriate for this purpose - it is needed for the refer target to request an authentication token from the referee. Sparks Expires November 20, 2002 [Page 5] Internet-Draft The Referred-By Mechanism May 2002 If no Referred-By token is present, the refer target MAY proceed with processing the request. If the agent provides any information from the Referred-By header to its user as part of processing the requst, it MUST notify the user that the information is suspect. The refer target MUST reject an otherwise well-formed request with an invalid Referred-By token (see Section 4) with a 429 error response. 3. The Referred-By Header Field Referred-By is a request header field as defined by [1]. It can appear in any request. It carries a SIP URI representing the identity of the referror and, optionally, the Content-ID of a body part (the Referred-By token) that provides a more secure statement of that identity. --------------------------------------------------------------------- Referred-By = ("Referred-By" / "b") HCOLON referrer-uri *( SEMI (referredby-id-param / generic-param) ) referrer-uri = ( name-addr / addr-spec ) referredby-id-param = "cid" EQUAL msg-id msg-id = TO BE INCORPORATED from rfc2822 (at great pain) Referred-By Syntax --------------------------------------------------------------------- The Referred-By header field MAY appear in any sip request, but is meaningless for ACK and CANCEL. Proxies do not need to be able to read Referred-By header field values and MUST NOT remove or modify them. 3.1 OPEN ISSUE - remove the support table Is this form of table still useful? It is inherently obsolete. I propose removing it leaving only the text above paragraph. The following row should be interpreted as if it appeared in Table 3 of RFC 3261. Sparks Expires November 20, 2002 [Page 6] Internet-Draft The Referred-By Mechanism May 2002 Header field where proxy ACK BYE CAN INV OPT REG ___________________________________________________________________ Referred-By R - o - o o o 4. The Referred-By Token The Referred-By token is an Authenticated Identity Body as defined by [5]. This body part MUST be identified with a MIME [6] Content-ID: field. In addition to the From, Date, and Call-ID header fields required by [5], the sipfrag inside a Referred-By token MUST contain copies of the Refer-To and Referred-By header fields from the REFER request. As in [5] additional header fields and body parts MAY be included. As described in [5], a Referred-By token MAY be encrypted as well as signed. A Referred-By token is judged suspect or valid using the criteria in [5] with the addition that the mandatory headers enumerated in this document MUST be included in the validity test. 5. The 429 Provide Referror Identity error response 6. Security Considerations This mechanism defined in this specification relies on an intermediary (the referree) to forward information from the referror to the refer target. This necessarily establishes the referree as an eavesdropper of that information and positions him perfectly to launch man-in-the-middle attacks using the mechanism. A SIP proxy is similarly positioned. Protecting SIP messaging from malicious proxy implementations is discussed in [1]. In contrast to a proxy, the referree's agent is an endpoint. Proxies will typically be managed and monitored by service providers. Malicious behavior by a proxy is more likely to be noticed and result in negative repercussions for the provider than malicious behavior by an endpoint would be. The behavior of an endpoint can be entirely under the control of a single user. Thus, it is more feasible for an endpoint acting as referree to behave maliciously than it is for a proxy being operated by a service provider. This specification uses an S/MIME based mechanism to enable the refer target to detect manipulation of the Referred-By information by the referree. Use of this protection is optional! The community has asserted that there are systems where trust in the validity of this Sparks Expires November 20, 2002 [Page 7] Internet-Draft The Referred-By Mechanism May 2002 information is either not important or can be established through other means. Any implementation choosing not to use this optional mechanism needs to provide its own defense to the following risks: o The Referred-By information is highly likely to influence request admission policy. For instance, it may be displayed to the user of the agent with a "This call was transferred to you by X. Accept?" prompt. A malicious referree can unduly influence that policy decision by providing falsified referred-by information. This includes falsely claiming to have been referred in the first place. (The S/MIME mechanism protects the information with a signature, hampering the referree's ability to inject or modify information without knowing the key used for that signature). o A referree is by definition an eavesdropper of the referred-by information. Parts of that information may be sensitive. (The S/ MIME mechanism allows encryption). o The referree may store any referred-by information it sees and paste it into future unrelated requests. (The S/MIME mechanism allows detection of stale assertions by covering a timestamp with the signature and allows detection of use in unrelated requests by covering the Refer-To header field with the signature). The mechanisms in this specification do NOT prevent the referree from deleting ALL referred-by information from the referrenced request. A refer target can not detect such deletion. This is not a problem since removing all referred-by information from a referrenced request transforms it into an ordinary SIP request as described in [1]. Thus the referree gains no new influence over processing logic at the refer target by removing the referred-by information. Applications using the mechanims in this draft may be able to take advantage of pre-existing relationships between the participants to mitigate the risks of its use. In some transfer scenarios, A has the choice of referring B to C or referring C to B. If A and B have a pre-existing trust relationship leading A to have greater confidence that B will not behave maliciously (B is A's administrative assistant for example), referring B to C may make more sense. This mechanism involves two SIP messages between three endpoints, the REFER and the referrenced request. The content of those messages (including the referred-by information) is subject to the security considerations and protection mechanisms documented in [1]. This mechanism uses an Authenticated Identity Body as defined in [5]. The security considerations and protection mechanisms described in that document apply. Sparks Expires November 20, 2002 [Page 8] Internet-Draft The Referred-By Mechanism May 2002 Proxies between the participants may collect referred-by information and reinsert it in future request or make them available to hostile endpoints. The end-to-end confidentiality capabilities discussed in [1] can help reduce risk of exposing sensitive referred-by information to these proxies. The abuse possibilities in subsequent requests by proxies (or endpoints that they may leak information to) between the referree and the refer target are identical to abuse by the referree and the considerations discussed for malicious referree applies. The abuse possibilities in subsequent requests by proxies (or endpoints that they may leak information to) between the referror and the referree are identical to those discussed for the presentation of Authenticated Identity Bodies in [5]. 7. Using an Authentication Service This section will discuss considerations for using an Authentication Service to create a Referred-By token beyond those documented in [5] 8. Examples 8.1 Basic REFER This example shows the secured Referred-By mechanism applied to a REFER to an SIP INVITE URI. Details are shown only for those messages involved in exercising the mechanism defined in this document. Referror Referree Refer Target | F1 REFER | | |-------------------------->| | | 202 Accepted | | |<--------------------------| | | | F2 INVITE | | |--------------------------->| | | 200 OK | | |<---------------------------| | | ACK | | NOTIFY |--------------------------->| |<--------------------------| | | 200 OK | | |-------------------------->| | | | | F1 REFER sip:referree@referree.example SIP/2.0 Via: SIP/2.0/UDP referror.example;branch=z9hG4bK392039842 To: sip:referree@referree.example Sparks Expires November 20, 2002 [Page 9] Internet-Draft The Referred-By Mechanism May 2002 From: sip:referror@referror.example;tag=39092342 Call-ID: 2203900ef0299349d9209f023a CSeq: 1239930 REFER Max-Forwards: 70 Contact: Refer-To: sip:refertarget@target.example Referred-By: sip:referror@referror.example ;cid=%3C20398823.2UWQFN309shb3@referror.example%3E Content-Type: multipart/mixed; boundary=unique-boundary-1 Content-Length: (appropriate value) --unique-boundary-1 Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha1; boundary=dragons39 Content-ID: <20398823.2UWQFN309shb3@referror.example> Content-Length: (appropriate value) --dragons39 Content-Type: message/sipfrag Content-Disposition: auth-id; handling=optional From: sip:referror@referror.example Date: Thu, 21 Feb 2002 13:02:03 GMT Call-ID: 2203900ef0299349d9209f023a Refer-To: sip:refertarget@target.example Referred-By: sip:referror@referror.example ;cid=%3C20398823.2UWQFN309shb3@referror.example%3E --dragons39 Content-Type: application/pkcs7-signature; name=smime.p7s Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename=smime.p7s; handling=required (appropriate signature goes here) --dragons39-- --unique-boundary-1-- F2 INVITE sip:refertarget@target.example SIP/2.0 Via: SIP/2.0/UDP referree.example;branch=z9hG4bKffe209934aac To: sip:refertarget@target.example From: sip:referree@referree.example;tag=2909034023 Call-ID: fe9023940-a3465@referree.example CSeq: 889823409 INVITE Sparks Expires November 20, 2002 [Page 10] Internet-Draft The Referred-By Mechanism May 2002 Max-Forwards: 70 Contact: Referred-By: sip:referror@referror.example ;cid=%3C20398823.2UWQFN309shb3@referror.example%3E Content-Type: multipart/mixed; boundary=my-boundary-9 Content-Length: (appropriate value) --my-boundary-9 Content-Type: application/sdp Content-Length: (appropriate value) v=0 o=referree 2890844526 2890844526 IN IP4 referree.example s=Session SDP c=IN IP4 referree.example t=0 0 m=audio 49172 RTP/AVP 0 a=rtpmap:0 PCMU/8000 --my-boundary-9 Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha1; boundary=dragons39 Content-ID: <20398823.2UWQFN309shb3@referror.example> Content-Length: (appropriate value) --dragons39 Content-Type: message/sipfrag Content-Disposition: auth-id; handling=optional From: sip:referror@referror.example Date: Thu, 21 Feb 2002 13:02:03 GMT Call-ID: 2203900ef0299349d9209f023a Refer-To: sip:refertarget@target.example Referred-By: sip:referror@referror.example ;cid=%3C20398823.2UWQFN309shb3@referror.example%3E --dragons39 Content-Type: application/pkcs7-signature; name=smime.p7s Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename=smime.p7s; handling=required (appropriate signature goes here) --dragons39-- Sparks Expires November 20, 2002 [Page 11] Internet-Draft The Referred-By Mechanism May 2002 --my-boundary-9-- 8.2 Insecure REFER The flow for this example is the same as that of Section 8.1. Here, the referror has opted to not include a Referred-By token, and the refer target is willing to accept the referrenced request without one. F1 REFER sip:referree@referree.example SIP/2.0 Via: SIP/2.0/UDP referror.example;branch=z9hG4bK392039842 To: sip:referree@referree.example From: sip:referror@referror.example;tag=39092342 Call-ID: 2203900ef0299349d9209f023a CSeq: 1239930 REFER Max-Forwards: 70 Contact: Refer-To: sip:refertarget@target.example Referred-By: sip:referror@referror.example Content-Length: 0 F2 INVITE sip:refertarget@target.example SIP/2.0 Via: SIP/2.0/UDP referree.example;branch=z9hG4bKffe209934aac To: sip:refertarget@target.example From: sip:referree@referree.example;tag=2909034023 Call-ID: fe9023940-a3465@referree.example CSeq: 889823409 INVITE Max-Forwards: 70 Contact: Referred-By: sip:referror@referror.example Content-Type: application/sdp Content-Length: (appropriate value) v=0 o=referree 2890844526 2890844526 IN IP4 referree.example s=Session SDP c=IN IP4 referree.example t=0 0 m=audio 49172 RTP/AVP 0 a=rtpmap:0 PCMU/8000 Sparks Expires November 20, 2002 [Page 12] Internet-Draft The Referred-By Mechanism May 2002 8.3 Requiring Referror Identity In contrast to the example in Section 8.2, the refer target requires a Referred-By token to accept the referrenced request. The referror choses to provide an encrypted token (note that the block surrounded by asterisks represents encrypted content). F1 and F2 are identical to the messages detailed in Section 8.2. Referror Referree Refer Target | F1 REFER | | |-------------------------->| | | 202 Accepted | | |<--------------------------| | | | F2 INVITE | | |--------------------------->| | | F3 429 Provide Referror Identity | |<---------------------------| | | ACK | | F4 NOTIFY |--------------------------->| |<--------------------------| | | 200 OK | | |-------------------------->| | | F5 REFER | | |-------------------------->| | | 202 Accepted | | |<--------------------------| | | | F6 INVITE | | |--------------------------->| | | 200 OK | | |<---------------------------| | | ACK | | NOTIFY |--------------------------->| |<--------------------------| | | 200 OK | | |-------------------------->| | | | | F3 SIP/2.0 429 Provide Referror Identity Via: SIP/2.0/UDP referree.example;branch=z9hG4bKffe209934aac To: sip:refertarget@target.example;tag=392093422302334 From: sip:referree@referree.example;tag=2909034023 Call-ID: fe9023940-a3465@referree.example CSeq: 889823409 INVITE Content-Length: 0 F4 NOTIFY sip:referror@referror.example SIP/2.0 Via: SIP/2.0/UDP referree.example;branch=z9hG4bK2934209da390 Sparks Expires November 20, 2002 [Page 13] Internet-Draft The Referred-By Mechanism May 2002 To: sip:referror@referror.example;tag=39092342 From: sip:referree@referree.example;tag=199949923 Call-ID: 2203900ef0299349d9209f023a CSeq: 3920390 NOTIFY Event: refer;id=1239930 Subscription-State: terminated Content-Type: message/sipfrag Content-Length: (appropriate value) SIP/2.0 429 Provide Referror Identity F5 REFER sip:referree@referree.example SIP/2.0 Via: SIP/2.0/UDP referror.example;branch=z9hG4bK98823423 To: sip:referree@referree.example From: sip:referror@referror.example;tag=39092342 Call-ID: 2203900ef0299349d9209f023a CSeq: 1239931 REFER Max-Forwards: 70 Contact: Refer-To: sip:refertarget@target.example Referred-By: sip:referror@referror.example ;cid=%3C20342EFXEI.390sdefn2@referror.example%3E Content-Type: multipart/mixed; boundary=unique-boundary-1 Content-Length: (appropriate value) --unique-boundary-1 Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha1; boundary=boundary42 Content-ID: <20342EFXEI.390sdefn2@referror.example> Content-Length: (appropriate value) --boundary42 Content-Type: application/pkcs7-mime; smime-type=enveloped-data; name=smime.p7m Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename=smime.p7m handling=required Content-Length: (appropriate value) *********************************************************** * Content-Type: message/sipfrag * * Content-Disposition: auth-id; handling=optional * * * * From: sip:referror@referror.example * * Call-ID: 2203900ef0299349d9209f023a * Sparks Expires November 20, 2002 [Page 14] Internet-Draft The Referred-By Mechanism May 2002 * Date: Thu, 21 Feb 2002 13:02:03 GMT * * Refer-To: sip:refertarget@target.example * * Referred-By: sip:referror@referror.example * * ;cid=%3C20342EFXEI.390sdefn2@referror.example%3E * *********************************************************** --boundary42 Content-Type: application/pkcs7-signature; name=smime.p7s Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename=smime.p7s; handling=required (appropriate signature) --boundary42-- F6 INVITE sip:refertarget@target.example SIP/2.0 Via: SIP/2.0/UDP referree.example;branch=z9hG4bK3920390423 To: sip:refertarget@target.example From: sip:referree@referree.example;tag=1342093482342 Call-ID: 23499234-9239842993@referree.example CSeq: 19309423 INVITE Max-Forwards: 70 Referred-By: sip:referror@referror.example ;cid=%3C20342EFXEI.390sdefn2@referror.example%3E Contact: Content-Type: multipart/mixed; boundary=my-boundary-9 Content-Length: (appropriate value) --my-boundary-9 Content-Type: application/sdp Content-Length: (appropriate value) v=0 o=referree 2890844526 2890844526 IN IP4 referree.example s=Session SDP c=IN IP4 referree.example t=0 0 m=audio 49172 RTP/AVP 0 a=rtpmap:0 PCMU/8000 --my-boundary-9 Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha1; boundary=boundary42 Sparks Expires November 20, 2002 [Page 15] Internet-Draft The Referred-By Mechanism May 2002 Content-ID: <20342EFXEI.390sdefn2@referror.example> Content-Length: (appropriate value) --boundary42 Content-Type: application/pkcs7-mime; smime-type=enveloped-data; name=smime.p7m Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename=smime.p7m handling=required Content-Length: (appropriate value) *********************************************************** * Content-Type: message/sipfrag * * Content-Disposition: auth-id; handling=optional * * * * From: sip:referror@referror.example * * Call-ID: 2203900ef0299349d9209f023a * * Date: Thu, 21 Feb 2002 13:02:03 GMT * * Refer-To: sip:refertarget@target.example * * Referred-By: sip:referror@referror.example * * ;cid=%3C20342EFXEI.390sdefn2@referror.example%3E * *********************************************************** --boundary42 Content-Type: application/pkcs7-signature; name=smime.p7s Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename=smime.p7s; handling=required (appropriate signature) --boundary42-- --my-boundary-9-- 8.4 Nested REFER The Refer-To URI may be a SIP URI indicating the REFER method. Consider The following URI which A uses to refer B to send a REFER request to C which refers C to send an INVITE to D. Note that A provides a Referred-By token which gets passed through B and C to D. In particular, B does not provide its own Referred-By token to C. Also note that A is notified of the outcome of the request it triggered at B (the REFER), not at C (the INVITE). Sparks Expires November 20, 2002 [Page 16] Internet-Draft The Referred-By Mechanism May 2002 Refer-To: This reference would result in the following flow: A B C D | F1 REFER | | | |------------------>| | | | 202 Accepted | | | |<------------------| | | | | F2 REFER | | | |------------------>| | | | 202 Accepted | | | |<------------------| | | F3 NOTIFY | | F4 INVITE | |<------------------| |------------------>| | 200 OK | | 200 OK | |------------------>| |<------------------| | | | ACK | | | |------------------>| | | NOTIFY | | | |<------------------| | | | 200 OK | | | |------------------>| | | | | | F1 REFER sip:B SIP/2.0 Via: SIP/2.0/UDP A;branch=z9hG4bK3802394232 To: sip:B From: sip:A;tag=23490234 Call-ID: 2304098023@A CSeq: 2342093 REFER Max-Forwards: 70 Contact: Refer-To: Referred-By: ;cid=%3C23094202342.10123091233@A%3E Content-Type: multipart/mixed; boundary=unique-boundary-1 Content-Length: (appropriate value) --unique-boundary-1 Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha1; boundary=dragons39 Content-ID: <23094202342.10123091233@A> Content-Length: (appropriate value) --dragons39 Sparks Expires November 20, 2002 [Page 17] Internet-Draft The Referred-By Mechanism May 2002 Content-Type: message/sipfrag Content-Disposition: auth-id; handling=optional From: sip:A Call-ID: 2304098023@A Date: Thu, 21 Feb 2002 13:02:03 GMT Refer-To: Referred-By: ;cid=%3C23094202342.101230912342A%3E --dragons39 Content-Type: application/pkcs7-signature; name=smime.p7s Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename=smime.p7s; handling=required (appropriate signature goes here) --dragons39-- --unique-boundary-1-- F2 REFER sip:C SIP/2.0 Via: SIP/2.0/UDP B;branch=z9hG4bK00239842 To: sip:C From: sip:B;tag=2934u23 Call-ID: 203942834@B CSeq: 8321039 REFER Max-Forwards: 70 Contact: Refer-To: Referred-By: ;cid=%3C23094202342.10123091233@A%3E Content-Type: multipart/mixed; boundary=unique-boundary-1 Content-Length: (appropriate value) --unique-boundary-1 Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha1; boundary=dragons39 Content-ID: <23094202342.10123091233@A> Content-Length: (appropriate value) --dragons39 Content-Type: message/sipfrag Content-Disposition: auth-id; handling=optional From: sip:A Call-ID: 2304098023@A Sparks Expires November 20, 2002 [Page 18] Internet-Draft The Referred-By Mechanism May 2002 Date: Thu, 21 Feb 2002 13:02:03 GMT Refer-To: Referred-By: ;cid=%3C23094202342.101230912342A%3E --dragons39 Content-Type: application/pkcs7-signature; name=smime.p7s Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename=smime.p7s; handling=required (appropriate signature goes here) --dragons39-- --unique-boundary-1-- F3 NOTIFY sip:A SIP/2.0 Via: SIP/2.0/UDP A;branch=z9hG4bK3802394232 To: sip:A;tag=23490234 From: sip:B;tag=5923020 Call-ID: 2304098023@A CSeq: 29420342 NOTIFY Event: refer;id=2342093 Subscription-State: terminated Max-Forwards: 70 Contact: Content-Type: message/sipfrag Content-Length: (appropriate value) SIP/2.0 202 Accepted F4 INVITE sip:D SIP/2.0 Via: SIP/2.0/UDP C;branch=z9hG4bK29348234 To: sip:D From: sip:C;tag=023942334 Call-ID: 23489020352@C CSeq: 1230934 INVITE Max-Forwards: 70 Contact: Referred-By: ;cid=%3C23094202342.10123091233@A%3E Content-Type: multipart/mixed; boundary=unique-boundary-1 Content-Length: (appropriate value) --unique-boundary-1 Content-Type: application/sdp Content-Length: (appropriate value) Sparks Expires November 20, 2002 [Page 19] Internet-Draft The Referred-By Mechanism May 2002 v=0 o=C 2890844526 2890844526 IN IP4 C s=Session SDP c=IN IP4 C t=0 0 m=audio 49172 RTP/AVP 0 a=rtpmap:0 PCMU/8000 --unique-boundary-1 Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha1; boundary=dragons39 Content-ID: <23094202342.10123091233@A> Content-Length: (appropriate value) --dragons39 Content-Type: message/sipfrag Content-Disposition: auth-id; handling=optional From: sip:A Call-ID: 2304098023@A Date: Thu, 21 Feb 2002 13:02:03 GMT Refer-To: Referred-By: ;cid=%3C23094202342.101230912342A%3E --dragons39 Content-Type: application/pkcs7-signature; name=smime.p7s Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename=smime.p7s; handling=required (appropriate signature goes here) --dragons39-- --unique-boundary-1-- 9. IANA Considerations (Note to RFC Editor: Please fill in all occurances of XXXX in this section with the RFC number of this specification). This document defines a new SIP header field name with a compact form (Referred-By and b respectively). It also defines an new SIP client error response code (429). Sparks Expires November 20, 2002 [Page 20] Internet-Draft The Referred-By Mechanism May 2002 The following changes should be made to http:///www.iana.org/ assignments/sip-parameters The following row should be added to the header field section (replacing any existing row for Referred-By). Header Name Compact Form Reference Referred-By b [RFCXXXX] The following row should be added to the response code section under the Request Failure 4xx heading 429 Provide Referror Identity [RFCXXXX] 10. Open Issues 1. Should referrees be allowed to change the content-id of a Referred-By token as they copy it into the referrenced request? See Section 2.2.1. 2. Can we remove the inherently incomplete header support table in favor of short descriptive text. See Section 3.1. 3. This mechanism proves to the target that the referror sent a REFER with this particular Refer-To and Referred-By header field values. It DOES NOT prove to the target that the referror sent that REFER to this particular referree (which may enable an intercept/cut-paste attack). Including the REFER start line (the Request-URI in particular) is not sufficient to tightne this up - location services may arbitrarily retarget the REFER and the target will generally have no way to reconcile the REFER Request- URI with the actual identity of the referree. Do we need to tighten this? If so, I beleive the solution needs to lie in the peterson-identity mechanism. The same attack applies to that mechanism in general, resulting in theft of identity. References [1] PlaceHolder, A., "Placeholder", RFC 3261, placeholder 2002. [2] PlaceHolder, A., "Placeholder", draft-ietf-sip-refer (work in progress), placeholder 2002. [3] PlaceHolder, A., "Placeholder", draft-sparks-sip-mimetypes (work in progress), placeholder 2002. [4] PlaceHolder, A., "Placeholder", draft-ietf-sipping-cc-transfer- Sparks Expires November 20, 2002 [Page 21] Internet-Draft The Referred-By Mechanism May 2002 00 (work in progress), placeholder 2002. [5] PlaceHolder, A., "Placeholder", draft-peterson-sip-identity-00 (work in progress), placeholder 2002. [6] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. Author's Address Robert J. Sparks dynamicsoft 5100 Tennyson Parkway Suite 1200 Plano, TX 75024 EMail: rsparks@dynamicsoft.com Sparks Expires November 20, 2002 [Page 22] Internet-Draft The Referred-By Mechanism May 2002 Full Copyright Statement Copyright (C) The Internet Society (2002). All Rights Reserved. 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