SIP J. Rosenberg Internet-Draft dynamicsoft Expires: January 27, 2004 July 29, 2003 Requirements for Construction and Usage of Globally Routable User Agent (UA) URIs for the Session Initiation Protocol (SIP) draft-rosenberg-sipping-gruu-reqs-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 January 27, 2004. Copyright Notice Copyright (C) The Internet Society (2003). 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 presents some motivating use cases for GRUUs, and presents some requirements for their construction and usage. Rosenberg Expires January 27, 2004 [Page 1] Internet-Draft GRUU Requirements July 2003 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Defining a GRUU . . . . . . . . . . . . . . . . . . . . . . . 3 3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1 REFER . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.2 Conferencing . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.3 Presence . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.4 An Alternative to Dialog Reuse . . . . . . . . . . . . . . . . 5 4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. Security Requirements . . . . . . . . . . . . . . . . . . . . 7 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 Informative References . . . . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 8 Intellectual Property and Copyright Statements . . . . . . . . 9 Rosenberg Expires January 27, 2004 [Page 2] Internet-Draft GRUU Requirements July 2003 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 [8]. Another application is the usage of endpoint-hosted conferences within the conferencing framework [2]. We call these URIs Globally Routable UA URIs (GRUU). This document formally defines a GRUU, presents motivating use cases, and introduces requirements for their construction and usage. 2. 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 may or may not 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 throughout the lifetime of the GRUU. Rosenberg Expires January 27, 2004 [Page 3] Internet-Draft GRUU Requirements July 2003 3. Use Cases We have encountered several use cases for a GRUU. 3.1 REFER Consider a blind transfer application [6]. 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 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. 3.2 Conferencing A similar need arises in conferencing [2]. 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 [3] 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 conferencing features would fail. Therefore, it is a GRUU. 3.3 Presence Rosenberg Expires January 27, 2004 [Page 4] Internet-Draft GRUU Requirements July 2003 In a SIP-based presence [7] system, the presence agent (PA) generates notifications about the state of a user. This state is represented with the Presence Information Document Format (PIDF) [5]. 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, in this case, the GRUU needs to be constructed by a presence agent. This may be a server in the network, or may be on an end-device, such as a PC. 3.4 An Alternative to Dialog Reuse RFC 3261 describes a concept called dialog reuse. This feature allows a dialog, created by a SUBSCRIBE or INVITE request, to be reused for other methods which normally would create a dialog. The common use case is when there is an INVITE-initiated dialog between UA 1 and UA 2, and UA 1 wishes to SUBSCRIBE to the dialog event package [4] for UA 2. This subscription has to reach UA 2, not any of the other UAs representing the user. To do that, a SUBSCRIBE is sent on the dialog created by the INVITE. However, since the publication of RFC 3261, many problems have been discovered with dialog reuse. Lifecycle management of the dialogs becomes much more complex. The dialog needs to exist so long as there is some kind of "application state" still in usage on that dialog. This makes it hard to determine when the dialog has terminated. Secondly, it is confusing to entities, such as UAs, proxies, and B2BUAs, which had formally equated dialog state with call state. This has been a common assumption in many implementations, and would now be broken. GRUUs provide an alternative and easier means for achieving the same effect. Instead of launching the SUBSCRIBE on the same dialog, the SUBSCRIBE would be sent to the GRUU for UA 2. Thus, there are now separate dialogs for the session and for the subscription. We could deprecate dialog reuse. 4. Requirements Rosenberg Expires January 27, 2004 [Page 5] Internet-Draft GRUU Requirements July 2003 The following requirements pertain to the construction and usage of a GRUU: 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. 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. Rosenberg Expires January 27, 2004 [Page 6] Internet-Draft GRUU Requirements July 2003 5. Security Requirements When transferring a GRUU from one UA to another UA within the same dialog, a man-in-the-middle could modify the GRUU, resulting in subsequent calls (such as a transfer) to that user actually misrouting the call to the attacker. This means that mechanisms need to be in place for the recipient of a GRUU to verify the integrity and authenticity of the GRUU. This is covered in REQ 9. 6. Acknowledgements The author would like to thank Rohan Mahy, Paul Kyzivat, Alan Johnston, and Cullen Jennings for their contributions to this work. Informative 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] Rosenberg, J., "A Framework for Conferencing with the Session Initiation Protocol", draft-ietf-sipping-conferencing-framework-00 (work in progress), May 2003. [3] Rosenberg, J., "Indicating User Agent Capabilities in the Session Initiation Protocol (SIP)", draft-ietf-sip-callee-caps-00 (work in progress), June 2003. [4] Rosenberg, J. and H. Schulzrinne, "An INVITE Inititiated Dialog Event Package for the Session Initiation Protocol (SIP", draft-ietf-sipping-dialog-package-02 (work in progress), July 2003. [5] Fujimoto, S. and H. Sugano, "Presence Information Data Format (PIDF)", draft-ietf-impp-cpim-pidf-08 (work in progress), May 2003. [6] Sparks, R. and A. Johnston, "Session Initiation Protocol Call Control - Transfer", draft-ietf-sipping-cc-transfer-01 (work in progress), February 2003. [7] Rosenberg, J., "A Presence Event Package for the Session Initiation Protocol (SIP)", draft-ietf-simple-presence-10 (work in progress), January 2003. [8] Sparks, R., "The Session Initiation Protocol (SIP) Refer Method", RFC 3515, April 2003. Rosenberg Expires January 27, 2004 [Page 7] Internet-Draft GRUU Requirements July 2003 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 January 27, 2004 [Page 8] Internet-Draft GRUU Requirements July 2003 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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