SIP J. Rosenberg Internet-Draft dynamicsoft Expires: August 15, 2004 February 15, 2004 Obtaining and Using Globally Routable User Agent (UA) URIs (GRUU) in the Session Initiation Protocol (SIP) draft-ietf-sip-gruu-01 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 August 15, 2004. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract Several applications of the Session Initiation Protocol (SIP) require a user agent (UA) to construct and distribute a URI which can be used by anyone on the Internet to route a call to that specific UA instance. A URI which routes to a specific UA instance is called a Globally Routable UA URI (GRUU). This document describes an extension to SIP for obtaining a GRUU from a server, and for communicating a GRUU to a peer within a dialog. Rosenberg Expires August 15, 2004 [Page 1] Internet-Draft GRUU Mechanism February 2004 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Defining a GRUU . . . . . . . . . . . . . . . . . . . . . . 3 4. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . 4 4.1 REFER . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4.2 Conferencing . . . . . . . . . . . . . . . . . . . . . . . . 4 4.3 Presence . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. Overview of Operation . . . . . . . . . . . . . . . . . . . 5 6. User Agent Behavior . . . . . . . . . . . . . . . . . . . . 6 6.1 REGISTER Processing . . . . . . . . . . . . . . . . . . . . 6 6.2 Using the GRUU . . . . . . . . . . . . . . . . . . . . . . . 7 7. Registrar Behavior . . . . . . . . . . . . . . . . . . . . . 8 7.1 Creation and Maintenance of GRUUs . . . . . . . . . . . . . 8 7.2 Providing GRUUs to User Agents . . . . . . . . . . . . . . . 11 8. Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . . 12 9. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . 13 10. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 13 11. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 14 12. Security Considerations . . . . . . . . . . . . . . . . . . 17 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . 18 13.1 Header Field Parameter . . . . . . . . . . . . . . . . . . . 18 13.2 URI Parameter . . . . . . . . . . . . . . . . . . . . . . . 18 13.3 Media Feature Tag . . . . . . . . . . . . . . . . . . . . . 18 14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19 Normative References . . . . . . . . . . . . . . . . . . . . 19 Informative References . . . . . . . . . . . . . . . . . . . 20 Author's Address . . . . . . . . . . . . . . . . . . . . . . 21 Intellectual Property and Copyright Statements . . . . . . . 22 Rosenberg Expires August 15, 2004 [Page 2] Internet-Draft GRUU Mechanism February 2004 1. Introduction Several applications of the Session Initiation Protocol (SIP) [1] require a user agent (UA) to construct and distribute a URI which can be used by anyone on the Internet to route a call to that specific UA instance. An example of such an application is call transfer, based on the REFER method [4]. Another application is the usage of endpoint-hosted conferences within the conferencing framework [10]. We call these URIs Globally Routable UA URIs (GRUU). This specification provides a mechanism for obtaining and using GRUUs. 2. Terminology In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119 [2] and indicate requirement levels for compliant implementations. 3. Defining a GRUU A GRUU is a SIP URI which has a specific set of characteristics: Global: It can be used by any UAC connected to the Internet. In that regard, it is like an address-of-record (AOR) for a user. The address-of-record for a user, sip:joe@example.com, is meant to be used by anyone to call that user. The same is true for a GRUU. Temporally Scoped: It may be temporally scoped. In that regard, its not like an AOR for a user. The general assumption is that an AOR for a user is valid so long as the user resides within that domain (of course, policies can be imposed to limit its validity, but that is not the default case). However, a GRUU has a limited lifetime by default. It can never be valid for longer than the duration of the registration of the UA to which it is bound. For example, if my PC registers to the SIP network, a GRUU for my PC is only valid as long as my PC is registered. If the PC unregisters, the GRUU is invalid; calls to it would result in a 404. If the PC comes back, the GRUU will be valid once more. Furthermore, it will frequently be the case that the GRUU has a lifetime shorter than the duration of the registration. Instance Routing: It routes to a specific UA instance, and never forks. In that regard, it is unlike an address-of-record. When a call is made to a normal AOR which represents a user, routing logic is applied in proxies to deliver the call to one or more UAs. That logic can result in a different routing decision based on the time-of-day, or the identity of the caller. However, when a call is made to a GRUU, the routing logic is much more static. It has to cause the call to be delivered to a very specific UA instance. That UA instance has to be the same UA instance for any request sent to that GRUU. This does not mean that a GRUU Rosenberg Expires August 15, 2004 [Page 3] Internet-Draft GRUU Mechanism February 2004 represents a fundamentally different type of URI; it only means that the logic a proxy applies to a GRUU is going to generally be simpler than that it applies to a normal AOR. 4. Use Cases We have encountered several use cases for a GRUU. 4.1 REFER Consider a blind transfer application [14]. User A is talking to user B. A wants to transfer the call to user C. So, it sends a REFER to user C. That REFER looks like, in part: REFER sip:C@example.com SIP/2.0 From: sip:A@example.com;tag=99asd To: sip:C@example.com Refer-To: (URI that identifiers B's UA) The Refer-To header needs to contain a URI that can be used by C to place a call to B. However, this call needs to route to the specific UA instace which B is using to talk to A. If it didn't, the transfer service would not execute. This URI is provided to A by B. Because B doesn't know who A will transfer the call to, the URI has to be usable by anyone. Therefore, it is a GRUU. 4.2 Conferencing A similar need arises in conferencing [10]. In that framework, a conference is described by a URI which identifies the focus of the conference. The focus is a SIP UA at the center of a conference. Each conference participant has a dialog with the focus. One case described in the framework is where a user A has made a call to B. They then put B on hold, and call C. Now, A has two separate dialogs for two separate calls - one to B, and one to C. A would like to conference them. One model is that A morphs itself into a focus. It sends a re-INVITE on each existing dialog, and provides both B and C with an updated URI that now holds the conference URI. It also has a callee capabilities [6] parameter which indicates that this URI is a conference URI. A proceeds to mix the media streams from B and C. This is called an ad-hoc conference. At this point, normal conferencing features can be applied. That means that B can send another user, D, the conference URI, perhaps in an email. D can send an INVITE to that URI, and join the conference. For this to work, the conference URI used by A in its re-INVITE has to be usable by anyone, and it has to route to the specific UA instance of A that is acting as the focus. If it didn't, basic Rosenberg Expires August 15, 2004 [Page 4] Internet-Draft GRUU Mechanism February 2004 conferencing features would fail. Therefore, it is a GRUU. 4.3 Presence In a SIP-based presence [15] system, the presence agent (PA) generates notifications about the state of a user. This state is represented with the Presence Information Document Format (PIDF) [13]. In a PIDF document, a user is represented by a series of tuples, each of which identifies the devices that the user has and provides information about them. Each tuple also has a contact URI, which is a SIP URI representing that device. A watcher can make a call to that URI, with the expectation that the call is routed to the device whose presence is represented in the tuple. The URI in the presence document therefore has to route to the specific UA instance whose presence was reported. Furthermore, since the presence document could be used by anyone who subscribes to the user, the URI has to be usable by anyone. As a result, it is a GRUU. It is interesting to note that the GRUU may need to be constructed by a presence agent, depending on how the presence document is computed by the server. 5. Overview of Operation This section is tutorial in nature, and does not specify any normative behavior. This extension allows a UA to obtain a GRUU, and to use a GRUU. These two mechanisms are separate, in that a UA can obtain a GRUU in any way it likes, and use the mechanisms in this specification to use them. Similarly, a UA can obtain a GRUU but never use it. A UA can obtain a GRUU by generating a normal REGISTER request, as specified in RFC 3261 [1]. This request contains a Supported header field with the value "gruu", indicating to the registrar that the UA supports this extension. The UA includes a "sip.instance" media feature tag in the Contact header field of each Contact for which a GRUU is desired. This media feature tag contains a globally unique ID that identifies the UA instance. If the domain that the user is registering against also supports GRUU, the REGISTER responses will contain the "gruu" parameter in each Contact header field. This parameter contains a GRUU which the domain guarantees will route to that UA instace. That GRUU is guaranteed to remain valid for the duration of the registration. The GRUU is bound to the UA instace. Should the client change its Contact URI, but indicate that it represents the same instance ID, the server would provide the same GRUU. Furthermore, if the registration for the Contact expires, and Rosenberg Expires August 15, 2004 [Page 5] Internet-Draft GRUU Mechanism February 2004 the UA registers the Contact at a later time with the same instance identifier, the server would provide the same GRUU. Since the GRUU is a URI like any other, it can be handed out by a UA by placing it in any header field which can contain a URI. A UA will normally place the GRUU into the Contact header field of dialog creating requests and responses it generates. However, it is important for the UA receiving the message to know whether the Contact URI is a GRUU or not. To make this determination, the UA looks for the presence of the Supported header field in the request or response. If it is present with a value of "gruu", it means that the Contact URI is a GRUU. When a UA uses a GRUU, it has the option of adding the "grid" URI parameter to the GRUU. This parameter is opaque to the proxy server handling the domain. However, when the server maps the GRUU to the corresponding Contact URI, the server will copy the grid parameter into the Contact URI. As a result, when the UA receives the request, the Request URI will contain the grid parameter it placed in the corresponding GRUU. 6. User Agent Behavior User agent behavior is divided into two separate parts - REGISTER processing, and GRUU usage. 6.1 REGISTER Processing When a UA wishes to obtain a GRUU within the domain of its AOR, when it generates a REGISTER request (initial or refresh), it MUST include the Supported header field in the request. The value of that header field MUST include "gruu" as one of the option tags. This alerts the registrar for the domain that the UA supports the GRUU mechanism. Furthermore, for each Contact for which the UA desires to obtain a GRUU, the UA MUST include a "sip.instance" media feature tag as a UA characteristic [6]. As described in [6], this media feature tag will be encoded in the Contact header field as the "+sip.instance" Contact header field parameter. The value of this parameter, as described in Section 13.3, MUST be a globally unique identifier, and SHOULD remain the same across all registrations generated from that particular UA instance. Besides the presence of the "gruu" option tag in the Supported header field and the "+sip.instance" Contact header field parameter, the REGISTER request is constructed identically to the case where this extension was not understood. Specifically, the Contact URI in the REGISTER request SHOULD NOT contain the gruu Contact header field Rosenberg Expires August 15, 2004 [Page 6] Internet-Draft GRUU Mechanism February 2004 parameter. Any such parameters are ignored by the registrar, as the UA cannot propose a GRUU for usage with the Contact URI. If a UA wishes to guarantee that the request is not processed unless the domain supports and uses this extension, it MAY include a Require header field in the request with a value that contains the "gruu" option tag. If the response is a 2xx, each Contact header that contained the "+sip.instance" Contact header field parameter may also contain a "gruu" parameter. This parameter contains a SIP URI that represents a GRUU corresponding to that UA instance. Any requests sent to the GRUU URI will be routed by the domain to the Contact URI bound currently bound to that instance ID. The GRUU will not change in subsequent 2xx responses to REGISTER. Indeed, even if the UA lets the contact expire, when it re-registers it at any later time, the registrar will normally provide the same GRUU for the same address-of-record and the UA instance ID. However, this property cannot be guaranteed, and a UA MUST be prepared to receive a different GRUU in a subsequent registration. 6.2 Using the GRUU A UA first obtains a GRUU using the procedures of Section 6.1, or by other means outside the scope of this specification. A UA can use the GRUU in the same way it would use any other SIP URI. However, a UA compliant to this specification MUST use a GRUU when populating the Contact header field of dialog-creating requests and responses. This includes the INVITE request and its 2xx response, the SUBSCRIBE [3] request, its 2xx response, and the NOTIFY request, and the REFER [4] request and its 2xx response. Similarly, in those requests and responses where the GRUU is used in the Contact header field, the UA MUST include a Supported header field that contains the option tag "gruu". However, it is not necessary for a UA to know whether or not its peer in the dialog uses a GRUU before inserting one into the Contact header field. When placing a GRUU into the Contact header field of a request or response, a UA MAY add the "grid" URI parameter to the GRUU. This parameter MAY take on any value permitted by the grammar for the parameter. Note that there are no limitations on the size of this parameter. When a UA sends a request to the GRUU, the proxy for the domain that owns the GRUU will translate the GRUU in the Request-URI, replacing it with the corresponding Contact URI. However, it will retain the "grid" parameter when this translation is performed. As a result, when the UA receives the request, the Request-URI will contain the "grid" created by the UA. This allows the UA to Rosenberg Expires August 15, 2004 [Page 7] Internet-Draft GRUU Mechanism February 2004 effectively manufacture an infinite supply of GRUU, each of which differs by the value of the "grid" parameter. When a UA receives a request that was sent to the GRUU, it will be able to tell which GRUU was invoked by the "grid" parameter. An implication of this behavior is that all mid-dialog requests will be routed through intermediate proxies. There will never be direct, UA to UA signaling. It is anticipated that this limitation will be addressed in future specifications. Once a UA knows that the Contact URI provided by its peer is a GRUU, it can use it in any application or SIP extension which requires a globally routable URI to operate. One such example is assisted call transfer. 7. Registrar Behavior A registrar compliant to this specification is responsible for the creation and maintenance of GRUUs, and for providing those GRUU's to a UA in response to a REGISTER request. 7.1 Creation and Maintenance of GRUUs A domain is responsible for creation and maintenance of a GRUU, along with its association to instance IDs, AORs and Contact URIs. These associations are modeled in the UML diagram in Figure 2. +-------------+ | | | | | GRUU |----------------------+ | | | | | | +-------------+ | | 1 | | | | associated-with | | | | | | 1 | +----------------+ | | | | +--------| instance ID/ |------+ | | | AOR Pair | | | | | | | | | +----------------+ | | | | | Rosenberg Expires August 15, 2004 [Page 8] Internet-Draft GRUU Mechanism February 2004 | | | | | |translates V V |to +--------------+ +-----------+ | | | | | | | instance | | AOR | | | ID | | | | | | +-----------+ | +--------------+ | | ^ | | | | | | | | | |is-bound-to | | +----------------+ | | | | | | | | | | | | +--------| Contact URI |<-----+ | | | 0..* | | | | +----------------+ | 0..1 ^ | | | +-----------------------------+ Figure 2 The combination of a UA instance ID and an AOR is referred to as an instance ID/AOR pair. There is a one-to-one mapping between such a pair and a GRUU; the GRUU is said to be associated with the pair, and the pair is associated with the GRUU. As a result, if two instance ID/AOR pairs are different, they each must be associated with a different GRUU. If two GRUUs are different, they each must be associated with a different instance ID/AOR pair. It is important to understand that this uniqueness is over the instance ID/AOR pair, not just the instance ID. For example, if a user registered the Contact sip:ua@pc.example.com;+sip.instance="1", representing a device with instance ID 1, to the AOR sip:user@example.com, and also registered the same Contact, representing the same instance ID - sip:ua@pc.example.com;+sip.instance="1" to a second AOR, say sip:boss@example.com, each of those UA instances would have a different GRUU, since they belong to different AORs. A GRUU translates to zero or one Contact URIs. If the instance ID associated with the GRUU is the instance ID of a Contact URI currently bound to the AOR associated with that GRUU, then the GRUU translates to that Contact URI. If, however, the instance ID associated with the GRUU is not an instance ID of a Contact URI currently bound to the AOR associated with the GRUU (possibly because Rosenberg Expires August 15, 2004 [Page 9] Internet-Draft GRUU Mechanism February 2004 there are no Contact URIs bound to the AOR), the GRUU maps to no Contact URI, and the GRUU is said to be invalid. A registrar MAY create a GRUU for a particular instance ID/AOR pair at any time. Of course, if a UA requests a GRUU in a registration, and the registrar has not yet created one, it will need to do so in order to respond to the registration request. However, the registrar can create the GRUU in advance of any request from a UA. This specification does not mandate a particular mechanism for construction of the GRUU. However, the GRUU MUST exhibit the following properties: o The domain part of the URI is an IP address present on the public Internet, or, if it is a host name, exists in the global DNS and corresponds to an IP address present on the public Internet. o When a request is sent to this URI, it routes to a proxy server in the same domain as that of the registrar. o A proxy server in the domain can determine that the URI is a GRUU. o When a proxy server in this domain receives a request sent to a URI that is a GRUU, that URI MUST be translated to the Contact URI currently bound to the AOR associated with that GRUU whose instance ID is the one associated with the GRUU. In many cases, it will be desirable to construct the GRUU in such a way that it will not be possible, based on inspection of the URI, to determine the Contact URI that the GRUU translates to. It may also be desirable to construct it so that it will not be possible to determine the instance ID/AOR pair associated with the GRUU. Whether or not a GRUU should be constructed with this property is a local policy decision. With these rules, it is possible, though not required, to construct a GRUU without requiring the maintenance of any additional state. To do that, the URI would be constructed in the following fashion: user-part = "GRUU" + BASE64(E(K, (salt + instance ID + AOR))) Where E(K,X) represents a suitable encryption function (such as AES with 128 bit keys) with key K applied to data block X, and the "+" operator implies concatenation. Salt represents a random string that prevents a client from obtaining pairs of known plaintext and ciphertext. A good choice would be at least 128 bits of randomness in the salt. The benefit of this mechanism is that a server need not store additional information on mapping a GRUU to its corresponding Contact URI. The user part of the GRUU contains the instance ID and AOR. Assuming that the domain stores registrations in a database indexed by the AOR, the proxy processing the GRUU would look up the AOR, Rosenberg Expires August 15, 2004 [Page 10] Internet-Draft GRUU Mechanism February 2004 extract the currently registered Contacts, and find the one matching the instance ID encoded in the request URI. The Contact URI whose instance ID is that instance ID is then used as the translated version of the URI. Encryption is needed to prevent attacks whereby the server is sent requests with faked GRUU, causing the server to direct requests to any named URI. Even with encryption, the proxy should validate the user part after decryption. In particular, the AOR should be one managed by the proxy in that domain. Should a UA send a request with a fake GRUU, the proxy would decrypt and then discard it because there would be no URI or an invalid URI inside. Once an association from an instance ID/AOR to a GRUU is created, that mapping MUST remain in existence, and valid, as long as there exists any Contact bound to that AOR whose instance ID is that instance ID. If, through a de-registration or expiration, there is no longer any Contact bound to that AOR whose instance ID is that instance ID, the registrar MUST remove the mapping, and invalidate the GRUU. However, at any time in the future, should a UA register a Contact to that same AOR indicating that it represents that same instance ID, the registrar SHOULD provide the UA the same GRUU provided previously. Indeed, this requirement would ideally be a MUST if it was achieveable, but even with the stateless algorithm described above, key rotation or server failures may cause the GRUU associated with an instance ID/AOR pair to change. The value of associatig the GRUU with an instance ID/AOR pair, as opposed to a Contact URI/AOR pair, is that the association can transcend registrations. As a result, registrars SHOULD make every effort possible to maintain the association for as long as possible. 7.2 Providing GRUUs to User Agents When a registrar compliant to this specification receives a REGISTER request, it checks for the presence of the Require header field in the request. If present, and if it contains the "gruu" option tag, the registrar MUST follow the procedures in the next paragraph for inclusion of the "gruu" parameter in a 2xx response to REGISTER. If not present, but a Supported header field was present with the "gruu" option tag, the registrar SHOULD follow the procedures in the next paragraph for inclusion of the "gruu" parameter in a 2xx response to REGISTER. If the Supported header field was not present, or it if was present but did not contain the value "gruu", the registrar SHOULD NOT follow the procedures of the next paragraph for inclusion of the "gruu" parameter in a 2xx response to REGISTER. If the register request contained any "gruu" Contact header field parameters, these MUST be ignored by the registrar. A UA cannot suggest or otherwise provide a GRUU to the registrar. Rosenberg Expires August 15, 2004 [Page 11] Internet-Draft GRUU Mechanism February 2004 A GRUU is provided to a UA by including it in the "gruu" Contact header field parameter for each Contact URI that contains a "+sip.instance" Contact header field parameter. The value of the gruu parameter is a quoted string containing the URI that is the GRUU for the associated instance ID/AOR pair. If the server does not currently have a GRUU associated with the instance ID/AOR, one is created according to the procedures of Section 7.1. Otherwise, if a GRUU already exists for that instance ID/AOR pair, the GRUU associated with that pair MUST be placed into the "gruu" Contact header field parameter of the REGISTER response. Inclusion of a GRUU in the "gruu" Contact header field parameter of a REGISTER response is separate from the computation and storage of the GRUU. It is possible that the registrar has computed a GRUU for one UA, but a different UA that queries for the current set of registrations doesn't understand GRUU. In that case, the REGISTER response sent to that second UA would not contain the "gruu" Contact header field parameter, even though the UA has a GRUU for that Contact. 8. Proxy Behavior When a proxy server receives a request, and the proxy owns the domain in the Request URI, and the proxy is supposed to access a Location Service in order to compute request targets (as specified in Section 16.5 of RFC 3261 [1]), the proxy MUST check if the Request URI is a GRUU created by that domain. If the URI is a GRUU, the proxy MUST determine if there is still a Contact URI bound to AOR associated with the GRUU, whose instance ID is the instance ID associated with the GRUU. If that AOR no longer has any contacts bound to it, or if it does have contacts bound to it, but none of them have an instance ID equal to the instance ID associated with the GRUU, the proxy MUST generate a 404 (Not Found) response to the request. Otherwise, the proxy MUST populate the target set with a single URI. This URI MUST be equal to the Contact URI that is translated from the GRUU. Furthermore, if the GRUU contained a "grid" URI parameter, the URI in the target set MUST also contain the same parameter with the same value. A proxy MAY apply other processing to the request, such as execution of called party features. In particular, it is RECOMMENDED that non-routing called party features, such as call logging and screening, that are associated with the AOR are also applied to requests for all GRUUs associated with that AOR. Rosenberg Expires August 15, 2004 [Page 12] Internet-Draft GRUU Mechanism February 2004 In many cases, a proxy will record-route an initial INVITE request, and the user agents will insert a GRUU into the Contact header field. When this happens, a mid-dialog request will arrive at the proxy with a Route header field that was inserted by the proxy, and a Request-URI that represents a GRUU. Proxies follow normal processing in this case; they will strip the Route header field, and then process the Request URI as described above. The procedures of RFC 3261 are then followed to proxy the request. The request SHOULD NOT be redirected in this case. In many instances, a GRUU is used by a UA in order to assist in the traversal of NATs, and a redirection may prevent such a case from working. 9. Grammar This specification defines two new Contact header field parameters, gruu and +sip.instance, and a new URI parameter, grid. The grammar for string-value is obtained from [6]. contact-params = c-p-q / c-p-expires / c-p-gruu / cp-instance / contact-extension c-p-gruu = "gruu" EQUAL DQUOTE SIP-URI DQUOTE cp-instance = "+sip.instance" EQUAL LDQUOT string-value RDQUOT uri-parameter = transport-param / user-param / method-param / ttl-param / maddr-param / lr-param / grid-param / other-param grid-param = "grid=" pvalue 10. Requirements This specification was created in order to meet the following requirements: REQ 1: When a UA invokes a GRUU, it MUST cause the request to be routed to the specific UA instance to which the GRUU refers. REQ 2: It MUST be possible for a GRUU to be invoked from anywhere on the Internet, and still cause the request to be routed appropriately. That is, a GRUU MUST NOT be restricted to use within a specific addressing realm. REQ 3: It MUST be possible for a GRUU to be constructed without requiring the network to store additional state. REQ 4: It MUST be possible for a UA to obtain a multiplicity of GRUUs, each one of which routes to that UA instance. This is needed to support ad-hoc conferencing, for example, where a a UA instance needs a different URI for each conference it is hosting. Rosenberg Expires August 15, 2004 [Page 13] Internet-Draft GRUU Mechanism February 2004 REQ 5: When a UA receives a request sent to a GRUU, it MUST be possible for the UA to know the GRUU which was used to invoke the request. This is necessary as a consequence of requirement 4. REQ 6: It MUST be possible for a UA to add opaque content to a GRUU, which is not interpreted or altered by the network, and used only by the UA instance to whom the GRUU refers. This provides a basic cookie type of functionality, allowing a UA to build a GRUU with state embedded within it. REQ 7: It MUST be possible for a proxy to execute services and features on behalf of a UA instace represented by a GRUU. As an example, if a user has call blocking features, a proxy may want to apply those call blocking features to calls made to the GRUU in addition to calls made to the user's AOR. REQ 8: It MUST be possible for a UA in a dialog to inform its peer of its GRUU, and for the peer to know that the URI represents a GRUU. This is needed for the conferencing and dialog reuse applications of GRUUs, where the URIs are transferred within a dialog. REQ 9: When transferring a GRUU per requirement 8, it MUST be possible for the UA receiving the GRUU to be assured of its integrity and authenticity. REQ 10: It MUST be possible for a server, authoritative for a domain, to construct a GRUU which routes to a UA instace bound to an AOR in that domain. In other words, the proxy can construct a GRUU too. This is needed for the presence application. 11. Examples The following call flow shows a basic registration and call setup, followed by a subscription directed to the GRUU. Rosenberg Expires August 15, 2004 [Page 14] Internet-Draft GRUU Mechanism February 2004 Caller Proxy Callee | |(1) REGISTER | | |<--------------------| | |(2) 200 OK | | |-------------------->| |(3) INVITE | | |-------------------->| | | |(4) INVITE | | |-------------------->| | |(5) 200 OK | | |<--------------------| |(6) 200 OK | | |<--------------------| | |(7) ACK | | |-------------------->| | | |(8) ACK | | |-------------------->| |(9) SUBSCRIBE | | |-------------------->| | | |(10) SUBSCRIBE | | |-------------------->| | |(11) 200 OK | | |<--------------------| |(12) 200 OK | | |<--------------------| | | |(13) NOTIFY | | |<--------------------| |(14) NOTIFY | | |<--------------------| | |(15) 200 OK | | |-------------------->| | | |(16) 200 OK | | |-------------------->| The Callee supports the GRUU extension. As such, its REGISTER (1) looks like: REGISTER sip:example.com SIP/2.0 Via: SIP/2.0/UDP client.example.com;branch=z9hG4bKnashds7 Max-Forwards: 70 From: Callee ;tag=a73kszlfl Supported: gruu To: Callee Call-ID: 1j9FpLxk3uxtm8tn@client.example.com CSeq: 1 REGISTER Contact: ;+sip.instance="" Content-Length: 0 Rosenberg Expires August 15, 2004 [Page 15] Internet-Draft GRUU Mechanism February 2004 The REGISTER response would look like: SIP/2.0 200 OK Via: SIP/2.0/UDP client.example.com;branch=z9hG4bKnashds7 From: Callee ;tag=a73kszlfl To: Callee ;tag=b88sn Call-ID: 1j9FpLxk3uxtm8tn@client.example.com CSeq: 1 REGISTER Contact: ;gruu="sip:hha9s8d=-999a@example.com" ;+sip.instance="" Content-Length: 0 Note how the Contact header field in the REGISTER response contains the gruu parameter with the URI sip:hha9s8d=-999a@example.com. This represents a GRUU that translates to the Contact URI sip:callee@client.example.com. The INVITE from the caller is a normal SIP INVITE. The 200 OK generated by the callee, however, now contains a GRUU in the Contact header field. The UA has also chosen to include a grid URI parameter into the GRUU. SIP/2.0 200 OK Via: SIP/2.0/UDP proxy.example.com;branch=z9hG4bKnaa8 Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK99a From: Caller ;tag=n88ah To: Callee ;tag=a0z8 Call-ID: 1j9FpLxk3uxtma7@host.example.com CSeq: 1 INVITE Supported: gruu Allow: INVITE, OPTIONS, CANCEL, BYE, ACK Contact: Content-Length: -- Content-Type: application/sdp [SDP Not shown] At some point later in the call, the caller decides to subscribe to the dialog event package [11] at that specific UA. To do that, it generates a SUBSCRIBE request (message 9), but directs it towards the GRUU contained in the Contact header field. Rosenberg Expires August 15, 2004 [Page 16] Internet-Draft GRUU Mechanism February 2004 SUBSCRIBE sip:hha9s8d=-999a@example.com;grid=99a SIP/2.0 Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK9zz8 From: Caller ;tag=kkaz- To: Callee Call-ID: faif9a@host.example.com CSeq: 2 SUBSCRIBE Supported: gruu Event: dialog Allow: INVITE, OPTIONS, CANCEL, BYE, ACK Contact: Content-Length: 0 In this example, the caller itself supports the GRUU extension, and is using its own GRUU to populate the Contact header field of the SUBSCRIBE. This request is routed to the proxy, which proceeds to perform a location lookup on the request URI. It is translated into the Contact URI of that GRUU, and then proxied there (message 10). Note how the grid parameter is maintained. SUBSCRIBE sip:callee@client.example.com;grid=99a SIP/2.0 Via: SIP/2.0/UDP proxy.example.com;branch=z9hG4bK9555 Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK9zz8 From: Caller ;tag=kkaz- To: Callee Call-ID: faif9a@host.example.com CSeq: 2 SUBSCRIBE Supported: gruu Event: dialog Allow: INVITE, OPTIONS, CANCEL, BYE, ACK Contact: Content-Length: 0 12. Security Considerations Since GRUUs do not reveal information about the identity of the associated address-of-record or Contact URI, they provide routability without identity. However, GRUUs do not provide a complete or reliable solution for privacy. In particular, since the GRUU does not change during the lifetime of a registration, an attacker could correlate two calls as coming from the same source, which in and of itself reveals information about the caller. Furthermore, GRUUs do not address other aspects of privacy, such as the addresses used for media transport. For a discussion of how privacy services are provided in SIP, see RFC 3323 [9]. Rosenberg Expires August 15, 2004 [Page 17] Internet-Draft GRUU Mechanism February 2004 It is important for a UA to be assured of the integrity of a GRUU when it is given one in a REGISTER response. If the GRUU is tampered with by an attacker, the result could be denial of service to the UA. As a result, it is RECOMMENDED that a UA use the SIPS URI scheme when registering. 13. IANA Considerations This specification defines a new Contact header field parameter, URI parameter and media feature tag. 13.1 Header Field Parameter This specification defines a new header field parameter, as per the registry created by [7]. The required information is as follows: Header field in which the parameter can appear: Contact Name of the Parameter gruu RFC Reference RFC XXXX [[NOTE TO IANA: Please replace XXXX with the RFC number of this specification.]] 13.2 URI Parameter This specification defines a new SIP URI parameter, as per the registry created by [8]. Name of the Parameter grid RFC Reference RFC XXXX [[NOTE TO IANA: Please replace XXXX with the RFC number of this specification.]] 13.3 Media Feature Tag This section registers a new media feature tag, per the procedures defined in RFC 2506 [5]. The tag is placed into the sip tree, which is defined in [6]. Media feature tag name: sip.instance ASN.1 Identifier: New assignment by IANA. Summary of the media feature indicated by this tag: This feature tag contains a string that indicates a unique identifier associated with the UA instance registering the Contact. This identifier is globally unique, and remains bound to the UA instance for as long as is achievable. For UA instances that are implemented as hardware, such as an IP phone, the instance ID would ideally be burned into firmware when the device is manufactured. For software, the instance ID would generally be randomly created at installation time. Values appropriate for use with this feature tag: String. Rosenberg Expires August 15, 2004 [Page 18] Internet-Draft GRUU Mechanism February 2004 The feature tag is intended primarily for use in the following applications, protocols, services, or negotiation mechanisms: This feature tag is most useful in a communications application, for describing the capabilities of a device, such as a phone or PDA. Examples of typical use: Routing a call to a specific device. Related standards or documents: RFC XXXX [[Note to IANA: Please replace XXXX with the RFC number of this specification.]] Security Considerations: This media feature tag can be used in ways which affect application behaviors. For example, the SIP caller preferences extension [12] allows for call routing decisions to be based on the values of these parameters. Therefore, if an attacker can modify the values of this tag, they may be able to affect the behavior of applications. As a result of this, applications which utilize this media feature tag SHOULD provide a means for ensuring its integrity. Similarly, this feature tag should only be trusted as valid when it comes from the user or user agent described by the tag. As a result, protocols for conveying this feature tag SHOULD provide a mechanism for guaranteeing authenticity. 14. Acknowledgements The author would like to thank Rohan Mahy, Paul Kyzivat, Alan Johnston, and Cullen Jennings for their contributions to this work. 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] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [3] Roach, A., "Session Initiation Protocol (SIP)-Specific Event Notification", RFC 3265, June 2002. [4] Sparks, R., "The Session Initiation Protocol (SIP) Refer Method", RFC 3515, April 2003. [5] Holtman, K., Mutz, A. and T. Hardie, "Media Feature Tag Registration Procedure", BCP 31, RFC 2506, March 1999. [6] Rosenberg, J., "Indicating User Agent Capabilities in the Session Initiation Protocol (SIP)", draft-ietf-sip-callee-caps-03 (work in progress), January 2004. [7] Camarillo, G., "The Internet Assigned Number Authority Header Field Parameter Registry for the Session Initiation Protocol", Rosenberg Expires August 15, 2004 [Page 19] Internet-Draft GRUU Mechanism February 2004 draft-ietf-sip-parameter-registry-01 (work in progress), November 2003. [8] Camarillo, G., "The Internet Assigned Number Authority Universal Resource Identifier Parameter Registry for the Session Initiation Protocol", draft-ietf-sip-uri-parameter-reg-01 (work in progress), November 2003. Informative References [9] Peterson, J., "A Privacy Mechanism for the Session Initiation Protocol (SIP)", RFC 3323, November 2002. [10] Rosenberg, J., "A Framework for Conferencing with the Session Initiation Protocol", draft-ietf-sipping-conferencing-framework-01 (work in progress), October 2003. [11] Rosenberg, J. and H. Schulzrinne, "An INVITE Inititiated Dialog Event Package for the Session Initiation Protocol (SIP)", draft-ietf-sipping-dialog-package-03 (work in progress), October 2003. [12] Rosenberg, J., Schulzrinne, H. and P. Kyzivat, "Caller Preferences for the Session Initiation Protocol (SIP)", draft-ietf-sip-callerprefs-10 (work in progress), October 2003. [13] Sugano, H. and S. Fujimoto, "Presence Information Data Format (PIDF)", draft-ietf-impp-cpim-pidf-08 (work in progress), May 2003. [14] Sparks, R. and A. Johnston, "Session Initiation Protocol Call Control - Transfer", draft-ietf-sipping-cc-transfer-01 (work in progress), February 2003. [15] Rosenberg, J., "A Presence Event Package for the Session Initiation Protocol (SIP)", draft-ietf-simple-presence-10 (work in progress), January 2003. Rosenberg Expires August 15, 2004 [Page 20] Internet-Draft GRUU Mechanism February 2004 Author's Address Jonathan Rosenberg dynamicsoft 600 Lanidex Plaza Parsippany, NJ 07054 US Phone: +1 973 952-5000 EMail: jdrosen@dynamicsoft.com URI: http://www.jdrosen.net Rosenberg Expires August 15, 2004 [Page 21] Internet-Draft GRUU Mechanism February 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property 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; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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