Network Working Group R. Sparks Internet-Draft dynamicsoft Expires: January 15, 2003 July 17, 2002 The SIP Refer Method draft-ietf-sip-refer-06 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 January 15, 2003. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract This document defines the REFER method. This SIP extension requests that the recipient REFER to a resource provided in the request. It provides a mechanism allowing the party sending the REFER to be notified of the outcome of the referenced request. This can be used to enable many applications, including call transfer. In addition to the REFER method, this document defines the the refer event package and the Refer-To request header. Sparks Expires January 15, 2003 [Page 1] Internet-Draft The SIP Refer Method July 2002 Table of Contents 1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. The REFER Method . . . . . . . . . . . . . . . . . . . . . . 3 2.1 The Refer-To Header Field . . . . . . . . . . . . . . . . . 3 2.2 Header Field Support for the REFER Method . . . . . . . . . 4 2.3 Message Body Inclusion . . . . . . . . . . . . . . . . . . . 6 2.4 Behavior of SIP User Agents . . . . . . . . . . . . . . . . 6 2.4.1 Forming a REFER request . . . . . . . . . . . . . . . . . . 6 2.4.2 Processing a REFER request . . . . . . . . . . . . . . . . . 6 2.4.3 Accessing the Referred-to Resource . . . . . . . . . . . . . 7 2.4.4 Using SIP Events to Report the Results of the Reference . . 7 2.4.5 The Body of the NOTIFY . . . . . . . . . . . . . . . . . . . 8 2.4.6 Multiple REFER Requests in a Dialog . . . . . . . . . . . . 9 2.4.7 Using the Subscription-State Header Field with Event Refer . 9 2.5 Behavior of SIP Registrars/Redirect Servers . . . . . . . . 10 2.6 Behavior of SIP Proxies . . . . . . . . . . . . . . . . . . 10 3. Package Details: Event refer . . . . . . . . . . . . . . . . 10 3.1 Event Package Name . . . . . . . . . . . . . . . . . . . . . 10 3.2 Event Package Parameters . . . . . . . . . . . . . . . . . . 10 3.3 SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . . 10 3.4 Subscription Duration . . . . . . . . . . . . . . . . . . . 10 3.5 NOTIFY Bodies . . . . . . . . . . . . . . . . . . . . . . . 11 3.6 Notifier processing of SUBSCRIBE requests . . . . . . . . . 11 3.7 Notifier Generation of NOTIFY Requests . . . . . . . . . . . 11 3.8 Subscriber Processing of NOTIFY Requests . . . . . . . . . . 11 3.9 Handling of Forked Requests . . . . . . . . . . . . . . . . 11 3.10 Rate of Notifications . . . . . . . . . . . . . . . . . . . 11 3.11 State Agents . . . . . . . . . . . . . . . . . . . . . . . . 12 4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.1 Prototypical REFER callflow . . . . . . . . . . . . . . . . 12 4.2 Multiple REFERs in a dialog . . . . . . . . . . . . . . . . 14 5. Security Considerations . . . . . . . . . . . . . . . . . . 17 5.1 Circumventing Privacy . . . . . . . . . . . . . . . . . . . 17 5.2 Circumventing Security . . . . . . . . . . . . . . . . . . . 17 5.3 Limiting the Breach . . . . . . . . . . . . . . . . . . . . 18 6. Historic Material . . . . . . . . . . . . . . . . . . . . . 18 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 18 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 19 Normative References . . . . . . . . . . . . . . . . . . . . 19 Author's Address . . . . . . . . . . . . . . . . . . . . . . 20 Full Copyright Statement . . . . . . . . . . . . . . . . . . 21 Sparks Expires January 15, 2003 [Page 2] Internet-Draft The SIP Refer Method July 2002 1. Overview This document defines the REFER method. This SIP [1] extension requests that the recipient REFER to a resource provided in the request. This can be used to enable many applications, including Call Transfer. 2. The REFER Method REFER is a SIP method as defined by RFC3261 [1]. The REFER method indicates that the recipient (identified by the Request-URI) should contact a third party using the contact information provided in the request. Unless stated otherwise, the protocol for emitting and responding to a REFER request are identical to those for a BYE request in [1]. The behavior of SIP entities not implementing the REFER (or any other unknown) method is explicitly defined in [1]. A REFER request implicitly establishes a subscription to the refer event. Event subscriptions are defined in [2]. A REFER request MAY be placed outside the scope of a dialog created with an INVITE. REFER creates a dialog, and MAY be Record-Routed, hence MUST contain a single Contact header field value. REFERs occurring inside an existing dialog MUST follow the Route/Record- Route logic of that dialog. 2.1 The Refer-To Header Field Refer-To is a request header field (request-header) as defined by [1]. It only appears in a REFER request. It provides a URL to reference. Refer-To = ("Refer-To" / "r") HCOLON ( name-addr / addr-spec ) * (SEMI generic-param) The following should be interpreted as if it appeared in Table 3 of RFC 3261. Header field where proxy ACK BYE CAN INV OPT REG ___________________________________________________________________ Refer-To R - - - - - - The Refer-To header field MAY be encrypted as part of end-to-end encryption. Sparks Expires January 15, 2003 [Page 3] Internet-Draft The SIP Refer Method July 2002 The Contact header field is an important part of the Route/Record-Route mechanism and is not available to be used to indicate the target of the reference. Examples Refer-To: sip:alice@atlanta.com Refer-To: Refer-To: Refer-To: Refer-To: http://www.ietf.org Long headers field values are line-wrapped here for clarity only. 2.2 Header Field Support for the REFER Method This table adds a column to tables 2 and 3 in [1], describing header field presence in a REFER method. See [1] for a key for the symbols used. A row for the Refer-To request-header should be inferred, mandatory for REFER. Refer-To is not applicable for any other methods. The proxy column in [1] applies to the REFER method unmodified. Header Where REFER Accept R o Accept 2xx - Accept 415 c Accept-Encoding R o Accept-Encoding 2xx - Accept-Encoding 415 c Accept-Language R o Accept-Language 2xx - Accept-Language 415 c Alert-Info - Allow Rr o Allow 405 m Authentication-Info 2xx o Authorization R o Call-ID c m Call-Info - Contact R m Contact 1xx - Sparks Expires January 15, 2003 [Page 4] Internet-Draft The SIP Refer Method July 2002 Contact 2xx m Contact 3-6xx o Content-Disposition o Content-Encoding o Content-Language o Content-Length o Content-Type * CSeq c m Date o Error-Info 3-6xx o Expires R o From c m In-Reply-To - Max-Forwards R m Min-Expires - MIME-Version o Organization o Priority R - Proxy-Authenticate 401 o Proxy-Authenticate 407 m Proxy-Authorization R o Proxy-Require R o Record-Route R o Record-Route 2xx,18x o Reply-To - Require c Retry-After 404,413,480,486 o Retry-After 500,503 o Retry-After 600,603 o Route R c Server r o Subject R - Supported R,2xx o Timestamp o To c(1) m Unsupported 420 o User-Agent o Via c(2) m Warning r o WWW-Authenticate 401 m WWW-Authenticate 407 o Table 1: Header Field Support Sparks Expires January 15, 2003 [Page 5] Internet-Draft The SIP Refer Method July 2002 2.3 Message Body Inclusion A REFER method MAY contain a body. This specification assigns no meaning to such a body. A receiving agent may choose to process the body according to its Content-Type. 2.4 Behavior of SIP User Agents 2.4.1 Forming a REFER request REFER is a SIP request and is constructed as defined in [1]. A REFER request MUST contain exactly one Refer-To header field value. 2.4.2 Processing a REFER request A UA accepting a well-formed REFER request SHOULD request approval from the user to proceed (this request could be satisfied with an interactive query or through accessing configured policy). If approval is granted, the UA MUST contact the resource identified by the URI in the Refer-To header field value as discussed in Section 2.4.3. If the approval sought above for a well-formed REFER request is immediately denied, the UA MAY decline the request. An agent responding to a REFER method MUST return a 400 (Bad Request) if the request contained zero or more than one Refer-To header field values. An agent (including proxies generating local responses) MAY return a 100 (Trying) or any appropriate 4xx-6xx class response as prescribed by [1]. Care should be taken when implementing the logic that determines whether or not to accept the REFER request. A UA not capable of accessing non-SIP URIs SHOULD NOT accept REFER requests to them. If no final response has been generated according to the rules above, the UA MUST return a 202 Accepted response before the REFER transaction expires. If a REFER request is accepted (that is, a 2xx class response is returned), the recipient MUST create a subscription and send notifications of the status of the refer as described in Section 2.4.4. Sparks Expires January 15, 2003 [Page 6] Internet-Draft The SIP Refer Method July 2002 2.4.3 Accessing the Referred-to Resource The resource identified by the Refer-To URI is contacted using the normal mechanisms for that URI type. For example, if the URI is a SIP URI indicating INVITE (using a method=INVITE URI parameter for example), the UA would issue a new INVITE using all of the normal rules for sending an INVITE defined in [1]. 2.4.4 Using SIP Events to Report the Results of the Reference The NOTIFY mechanism defined in [2] MUST be used to inform the agent sending the REFER of the status of the reference. The dialog identifiers (To, From, and Call-ID) of each NOTIFY must match those of the REFER as they would if the REFER had been a SUBSCRIBE request. Each NOTIFY MUST contain an Event header field with a value of refer and possibly an id parameter (see Section 2.4.6). Each NOTIFY MUST contain a body of type "message/sipfrag" [3]. The creation of a subscription as defined by [1] always results in an immediate NOTIFY. Analogous to the case for SUBSCRIBE described in that document, the agent that issued the REFER MUST be prepared to receive a NOTIFY before the REFER transaction completes. The implicit subscription created by a REFER is the same as a subscription created with a SUBSCRIBE request. The agent issuing the REFER can terminate this subscription prematurely by unsubscribing using the mechanisms described in [2]. Terminating a subscription, either by explicitly unsubscribing or rejecting NOTIFY, is not an indication that the referenced request should be withdrawn or abandoned. In particular, an agent acting on a REFER request SHOULD NOT issue a CANCEL to any referenced SIP requests because the agent sending the REFER terminated its subscription to the refer event before the referenced request completes. The agent issuing the REFER may extend its subscription using the subscription refresh mechanisms described in [2]. REFER is the only mechanism that can create a subscription to event refer. If a SUBSCRIBE request for event refer is received for a subscription that does not already exist, it MUST be rejected with a 403. Notice that unlike SUBSCRIBE, the REFER transaction does not contain a duration for the subscription in either the request or the response. The lifetime of the state being subscribed to is determined by the progress of the referenced request. The duration Sparks Expires January 15, 2003 [Page 7] Internet-Draft The SIP Refer Method July 2002 of the subscription is chosen by the agent accepting the REFER and is communicated to the agent sending the REFER in the subscription's initial NOTIFY (using the Subscription-State expires header parameter). Note that agents accepting REFER and not wishing to hold subscription state can terminate the subscription with this initial NOTIFY. 2.4.5 The Body of the NOTIFY Each NOTIFY MUST contain a body of type "message/sipfrag" [3]. The body of a NOTIFY MUST begin with a SIP Response Status-Line as defined in [1]. The response class in this status line indicates the status of the referred action. The body MAY contain other SIP header fields to provide information about the outcome of the referenced action. This body provides a complete statement of the status of the referred action. The refer event package does not support state deltas. If a NOTIFY is generated when the subscription state is pending, its body should consist only of a status line containing a response code of 100. A minimal, but complete, implementation can respond with a single NOTIFY containing either the body: SIP/2.0 100 Trying if the subscription is pending, the body: SIP/2.0 200 OK if the reference was successful, the body: SIP/2.0 503 Service Unavailable if the reference failed, or the body: SIP/2.0 603 Declined if the REFER request was accepted before approval to follow the Sparks Expires January 15, 2003 [Page 8] Internet-Draft The SIP Refer Method July 2002 reference could be obtained and that approval was subsequently denied (see Section 2.4.7). An implementation MAY include more of a SIP message in that body to convey more information. Warning header field values received in responses to the referred action are good candidates. In fact, if the reference was to a SIP URI, the entire response to the referenced action could be returned (perhaps to assist with debugging). However, doing so could have grave security repercussions (see Section 5). Implementers must carefully consider what they choose to include. Note that if the reference was to a non-SIP URI, status in any NOTIFYs to the referrer must still be in the form of SIP Response Status-Lines. The minimal implementation discussed above is sufficient to provide a basic indication of success or failure. For example, if a client receives a REFER to a HTTP URL, and is successful in accessing the resource, its NOTIFY to the referrer can contain the message/sipfrag body of "SIP/2.0 200 OK". If the notifier wishes to return additional non-SIP protocol specific information about the status of the request, it may place it in the body of the sipfrag message. 2.4.6 Multiple REFER Requests in a Dialog A REFER creates an implicit subscription sharing the dialog identifers in the REFER request. If more than one REFER is issued in the same dialog (a second attempt at transferring a call for example), the dialog identifiers do not provide enough information to associate the resulting NOTIFYs with the proper REFER. Thus, for the second and subsequent REFER requests a UA receives in a given dialog, it MUST include an id parameter[2] in the Event header field of each NOTIFY containing the sequence number (the number from the CSeq header field value) of the REFER this NOTIFY is associated with. This id parameter MAY be included in NOTIFYs to the first REFER a UA receives in a given dialog. A SUBSCRIBE sent to refresh or terminate this subscription MUST contain this id parameter. 2.4.7 Using the Subscription-State Header Field with Event Refer Each NOTIFY must contain a Subscription-State header field as defined in [2]. The final NOTIFY sent in response to a REFER MUST indicate the subscription has been "terminated" with a reason of "noresource". (The resource being subscribed to is the state of the referenced request). If a NOTIFY indicates a reason that indicates a re-subscribe is Sparks Expires January 15, 2003 [Page 9] Internet-Draft The SIP Refer Method July 2002 appropriate according to [2], the agent sending the REFER is NOT obligated to re-subscribe. In the case where a REFER was accepted with a 202, but approval to follow the reference was subsequently denied, the reason and retry- after elements of the Subscription-State header field can be used to indicate if and when the REFER can be re-attempted (as described for SUBSCRIBE in [2]). 2.5 Behavior of SIP Registrars/Redirect Servers A registrar that is unaware of the definition of the REFER method will return a 501 response as defined in [1]. A registrar aware of the definition of REFER SHOULD return a 405 response. This specification places no requirements on redirect server behavior beyond those specified in [1]. Thus, it is possible for REFER requests to be redirected. 2.6 Behavior of SIP Proxies SIP proxies do not require modification to support the REFER method. Specifically, as required by [1], a proxy should process a REFER request the same way it processes an OPTIONS request. 3. Package Details: Event refer This document defines an event package as defined in [2]. 3.1 Event Package Name The name of this event package is "refer". 3.2 Event Package Parameters This package uses the "id" parameter defined in [2]. Its use in package is described in Section 2.4.6. 3.3 SUBSCRIBE Bodies SUBSCRIBE bodies have no special meaning for this event package. 3.4 Subscription Duration The duration of an implicit subscription created by a REFER request is initially determined by the agent accepting the REFER and communicated to the subscribing agent in the Subscription-State header field's expire parameter in the first NOTIFY sent in the Sparks Expires January 15, 2003 [Page 10] Internet-Draft The SIP Refer Method July 2002 subscription. Reasonable choices for this initial duration depend on the type of request indicated in the Refer-To URI. The duration SHOULD be chosen to be longer than the time the referenced request will be given to complete. For example, if the Refer-To URI is a SIP INVITE URI, the subscription interval should be longer than the Expire value in the INVITE. Additional time MAY be included to account for time needed to authorize the subscription. The subscribing agent MAY extend the subscription by refreshing it, or terminate it by unsubscribing. As described in Section 2.4.7, the agent accepting the REFER will terminate the subscription when it reports the final result of the reference, indicating that termination in the Subscription-State header field. 3.5 NOTIFY Bodies The bodies of NOTIFY requests for event refer are discussed in Section 2.4.5. 3.6 Notifier processing of SUBSCRIBE requests Notifier processing of SUBSCRIBE requests is discussed in Section 2.4.4. 3.7 Notifier Generation of NOTIFY Requests Notifier generation of NOTIFY requests is discussed in Section 2.4.4. 3.8 Subscriber Processing of NOTIFY Requests Subscriber processing of NOTIFY requests is discussed in Section 2.4.4. 3.9 Handling of Forked Requests A REFER sent within the scope of an existing dialog will not fork. A REFER sent outside the context of a dialog MAY fork, and if it is accepted by multiple agents, MAY create multiple subscriptions. These subscriptions are created and managed as per "Handling of Forked Requests" in [2] as if the REFER had been a SUBSCRIBE. The agent sending the REFER manages the state associated with each subscription separately. It does NOT merge the state from the separate subscriptions. The state is the status of the referenced request at each of the accepting agents. 3.10 Rate of Notifications An event refer NOTIFY might be generated each time new knowledge of the status of a referenced requests becomes available. For instance, Sparks Expires January 15, 2003 [Page 11] Internet-Draft The SIP Refer Method July 2002 if the REFER was to a SIP INVITE, NOTIFYs might be generated with each provisional response and the final response to the INVITE. Alternatively, the subscription might only result in two NOTIFY requests, the immediate NOTIFY and the NOTIFY carrying the final result of the reference. NOTIFYs to event refer SHOULD NOT be sent more frequently than once per second. 3.11 State Agents Separate state agents are not defined for event refer. 4. Examples 4.1 Prototypical REFER callflow Agent A Agent B | | | F1 REFER | |----------------------->| | F2 202 Accepted | |<-----------------------| | F3 NOTIFY | |<-----------------------| | F4 200 OK | |----------------------->| | | | | | |-------> | | (whatever) | |<------ | | | F5 NOTIFY | |<-----------------------| | F6 200 OK | |----------------------->| | | | | Here are examples of what the four messages between Agent A and Agent B might look like if the reference to (whatever) that Agent B makes is successful. The details of this flow indicate this particular REFER occurs outside a session (there is no To tag in the REFER request). If the REFER occurs inside a session, there would be a non-empty To tag in the request. Message One (F1) Sparks Expires January 15, 2003 [Page 12] Internet-Draft The SIP Refer Method July 2002 REFER sip:b@agentland SIP/2.0 Via: SIP/2.0/UDP agenta.agentland;branch=z9hG4bK2293940223 To: From: ;tag=193402342 Call-ID: 898234234@agenta.agentland CSeq: 93809823 REFER Max-Forwards: 70 Refer-To: (whatever URI) Contact: sip:a@agentland Content-Length: 0 Message Two (F2) SIP/2.0 202 Accepted Via: SIP/2.0/UDP agenta.agentland;branch=z9hG4bK2293940223 To: ;tag=4992881234 From: ;tag=193402342 Call-ID: 898234234@agenta.agentland CSeq: 93809823 REFER Contact: sip:b@agentland Content-Length: 0 Message Three (F3) NOTIFY sip:a@agentland SIP/2.0 Via: SIP/2.0/UDP agentb.agentland;branch=z9hG4bK9922ef992-25 To: ;tag=193402342 From: ;tag=4992881234 Call-ID: 898234234@agenta.agentland CSeq: 1993402 NOTIFY Max-Forwards: 70 Event: refer Subscription-State: active;expires=(depends on Refer-To URI) Contact: sip:b@agentland Content-Type: message/sipfrag;version=2.0 Content-Length: 20 SIP/2.0 100 Trying Message Four (F4) Sparks Expires January 15, 2003 [Page 13] Internet-Draft The SIP Refer Method July 2002 SIP/2.0 200 OK Via: SIP/2.0/UDP agentb.agentland;branch=z9hG4bK9922ef992-25 To: ;tag=193402342 From: ;tag=4992881234 Call-ID: 898234234@agenta.agentland CSeq: 1993402 NOTIFY Contact: sip:a@agentland Content-Length: 0 Message Five (F5) NOTIFY sip:a@agentland SIP/2.0 Via: SIP/2.0/UDP agentb.agentland;branch=z9hG4bK9323394234 To: ;tag=193402342 From: ;tag=4992881234 Call-ID: 898234234@agenta.agentland CSeq: 1993403 NOTIFY Max-Forwards: 70 Event: refer Subscription-State: terminated;reason=noresource Contact: sip:b@agentland Content-Type: message/sipfrag;version=2.0 Content-Length: 16 SIP/2.0 200 OK Message Six (F6) SIP/2.0 200 OK Via: SIP/2.0/UDP agentb.agentland;branch=z9hG4bK9323394234 To: ;tag=193402342 From: ;tag=4992881234 Call-ID: 898234234@agenta.agentland CSeq: 1993403 NOTIFY Contact: sip:a@agentland Content-Length: 0 4.2 Multiple REFERs in a dialog Message One above brings an implicit subscription dialog into existence. Suppose Agent A issued a second REFER inside that dialog: Sparks Expires January 15, 2003 [Page 14] Internet-Draft The SIP Refer Method July 2002 Agent A Agent B | | | F7 REFER | |----------------------->| | F8 202 Accepted | |<-----------------------| | F9 NOTIFY | |<-----------------------| | F10 200 OK | |----------------------->| | |-------> | | (something different) | |<------ | | | F11 NOTIFY | |<-----------------------| | F12 200 OK | |----------------------->| | | | | Message Seven (F7) REFER sip:b@agentland SIP/2.0 Via: SIP/2.0/UDP agenta.agentland;branch=z9hG4bK9390399231 To: ;tag=4992881234 From: ;tag=193402342 Call-ID: 898234234@agenta.agentland CSeq: 93809824 REFER Max-Forwards: 70 Refer-To: (some different URI) Contact: sip:a@agentland Content-Length: 0 Message Eight (F8) SIP/2.0 202 Accepted Via: SIP/2.0/UDP agenta.agentland;branch=z9hG4bK9390399231 To: ;tag=4992881234 From: ;tag=193402342 Call-ID: 898234234@agenta.agentland CSeq: 93809824 REFER Contact: sip:b@agentland Content-Length: 0 Message Nine (F9) Sparks Expires January 15, 2003 [Page 15] Internet-Draft The SIP Refer Method July 2002 NOTIFY sip:a@agentland SIP/2.0 Via: SIP/2.0/UDP agentb.agentland;branch=z9hG4bK9320394238995 To: ;tag=193402342 From: ;tag=4992881234 Call-ID: 898234234@agenta.agentland CSeq: 1993404 NOTIFY Max-Forwards: 70 Event: refer Subscription-State: active;expires=(depends on Refer-To URI) Contact: sip:b@agentland Content-Type: message/sipfrag;version=2.0 Content-Length: 20 SIP/2.0 100 Trying Message Ten (F10) SIP/2.0 200 OK Via: SIP/2.0/UDP agentb.agentland;branch=z9hG4bK9320394238995 To: ;tag=193402342 From: ;tag=4992881234 Call-ID: 898234234@agenta.agentland CSeq: 1993404 NOTIFY Contact: sip:a@agentland Content-Length: 0 Message Eleven (F11) NOTIFY sip:a@agentland SIP/2.0 Via: SIP/2.0/UDP agentb.agentland;branch=z9hG4bK2994a93eb-fe To: ;tag=193402342 From: ;tag=4992881234 Call-ID: 898234234@agenta.agentland CSeq: 1993405 NOTIFY Max-Forwards: 70 Event: refer;id=93809824 Subscription-State: terminated;reason=noresource Contact: sip:b@agentland Content-Type: message/sipfrag;version=2.0 Content-Length: 16 SIP/2.0 200 OK Message Twelve (F12) Sparks Expires January 15, 2003 [Page 16] Internet-Draft The SIP Refer Method July 2002 SIP/2.0 200 OK Via: SIP/2.0/UDP agentb.agentland;branch=z9hG4bK2994a93eb-fe To: ;tag=193402342 From: ;tag=4992881234 Call-ID: 898234234@agenta.agentland CSeq: 1993405 NOTIFY Contact: sip:a@agentland Content-Length: 0 5. Security Considerations The security considerations of [1] apply to the REFER method. This mechanism relies on providing contact information for the referred-to resource to the party being referred. Care should be taken to provide a suitably restricted URI if the referred to resource should be protected. Using message/sipfrag bodies to return the progress and results of a REFER request is extremely powerful. Careless use of that capability will compromise security and privacy. Here are a couple of simple, somewhat contrived, examples to demonstrate the potential for harm. 5.1 Circumventing Privacy Suppose Alice has a user agent that accepts REFER requests to SIP INVITE URIs, and NOTIFYs the referrer of the progress of the INVITE by copying each response to the INVITE into the body of a NOTIFY. Suppose further that Carol has a reason to avoid Mallory and has configured her system at her proxy to only accept calls from a certain set of people she trusts (including Alice), so that Mallory doesn't learn when she's around, or what user agent she's actually using. Mallory can send a REFER to Alice, with a Refer-To URI indicating Carol. If Alice can reach Carol, the 200 OK Carol sends gets returned to Mallory in a NOTIFY, letting him know not only that Carol is around, but also the IP address of the agent she's using. 5.2 Circumventing Security Suppose Alice, with the same user agent as above, is working at a company that is working on the greatest SIP device ever invented - the SIP FOO. The company has been working for months building the device and the marketing materials, carefully keeping the idea, even the name of the idea secret (since a FOO is one of those things that Sparks Expires January 15, 2003 [Page 17] Internet-Draft The SIP Refer Method July 2002 anybody could do if they'd just had the idea first). FOO is up and running, and anyone at the company can use it, but it's not available outside the company firewall. Mallory has heard rumor that Alice's company is onto something big, and has even managed to get his hands on a URI that he suspects might have something to do with it. He sends a REFER to ALICE with the mysterious URI and as Alice connects to the FOO, Mallory gets NOTIFYs with bodies containing Server: FOO/v0.9.7 5.3 Limiting the Breach For each of these cases, and in general, returning a carefully selected subset of the information available about the progress of the reference through the NOTIFYs mitigates risk. The minimal implementation described in Section 2.4.5 exposes the least information about what the agent operating on the REFER request has done, and is least likely to be a useful tool for malicious users. 6. Historic Material This method was initially motivated by the call-transfer application. Starting as TRANSFER, and later generalizing to REFER, this method improved on the BYE/Also concept of the expired draft-ietf-sip-cc-01 by disassociating transfers from the processing of BYE. These changes facilitate recovery of failed transfers and clarify state management in the participating entities. Early versions of this work required the agent responding to REFER to wait until the referred action completed before sending a final response to the REFER. That final response reflected the success or failure of the referred action. This was infeasible due to the transaction timeout rules defined for non-INVITE requests in [1]. A REFER must always receive an immediate (within the lifetime of a non- INVITE transaction) final response. 7. IANA Considerations (Note to RFC Editor: Please fill in all occurences of XXXX in this section with the RFC number of this specification). This document defines a new SIP method name (REFER), a new SIP header field name with a compact form (Refer-To and r respectively), and an event package (refer). Sparks Expires January 15, 2003 [Page 18] Internet-Draft The SIP Refer Method July 2002 The following row should be added to the method sub-registry under http://www.iana.org/assignments/sip-parameters. REFER [RFCXXXX] The following information should be added to the header sub-registry under http://www.iana.org/assignmemts/sip-parameters. Header Name: Refer-To Compact Form: r Reference: RFC XXXX This specification registers an event package, based on the registration procedures defined in [2]. The following is the information required for such a registration: Package Name: refer Package or Package-Template: This is a package. Published Specification: RFC XXXX Person to Contact: Robert Sparks, rsparks@dynamicsoft.com 8. Acknowledgments This draft is a collaborative product of the SIP working group. Normative References [1] 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. [2] Roach, A., "Session Initiation Protocol (SIP)-Specific Event Notification", RFC 3265, June 2002. [3] Sparks, R., "Internet Media Type message/sipfrag", Internet- Draft