SIPPING J. Rosenberg Internet-Draft dynamicsoft Expires: January 6, 2005 G. Camarillo Ericsson D. Willis dynamicsoft July 8, 2004 A Framework for Consent-Based Communications in the Session Initiation Protocol (SIP) draft-rosenberg-sipping-consent-framework-00.txt Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. 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 6, 2005. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract The Session Initiation Protocol (SIP) supports communications across many media types, including real-time audio, video, text, instant messaging, and presence. In its current form, it allows session invitations, instant messages, and other requests to be delivered from one party to another without requiring explicit consent of the recipient. Without such consent, it is possible for SIP to be used Rosenberg, et al. Expires January 6, 2005 [Page 1] Internet-Draft Consent Framework July 2004 for malicious purposes, including spam and denial-of-service attacks. This document identifies a framework for consent-based communications in SIP. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Relays . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Reference Architecture . . . . . . . . . . . . . . . . . . . 4 4. Structure of a Permission . . . . . . . . . . . . . . . . . 4 5. Attempting Communication . . . . . . . . . . . . . . . . . . 5 6. Requesting a Permission . . . . . . . . . . . . . . . . . . 6 7. Waiting for Permissions . . . . . . . . . . . . . . . . . . 6 8. Granting a Permission . . . . . . . . . . . . . . . . . . . 7 8.1 Permission Servers . . . . . . . . . . . . . . . . . . . . 7 9. Retrying the Original Request . . . . . . . . . . . . . . . 8 10. Permission Revocation . . . . . . . . . . . . . . . . . . . 8 11. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . 8 11.1 Basic Flow with No Permission Server . . . . . . . . . . 8 11.2 Basic Flow with a Permission Server . . . . . . . . . . 9 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 12.1 Normative References . . . . . . . . . . . . . . . . . . . 11 12.2 Informative References . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 11 Intellectual Property and Copyright Statements . . . . . . . 13 Rosenberg, et al. Expires January 6, 2005 [Page 2] Internet-Draft Consent Framework July 2004 1. Introduction The Session Initiation Protocol (SIP) [1] supports communications across many media types, including real-time audio, video, text, instant messaging and presence. This communication is established by the transmission of various SIP requests (such as INVITE and MESSAGE [2]) from an initiator to the recipient, with whom communication is desired. Although a recipient of such a SIP request can reject the request, and therefore decline the session, a SIP network will deliver a SIP request to the recipient without their explicit consent. Receipt of these requests without explicit consent can cause a number of problems in SIP networks. These include spam and DoS (Denial of Service) attacks. These problems are described in more detail in a companion requirements document [draft-rosenberg-sipping-consent-reqs-00.txt]. This specification defines a basic framework for adding consent-based communication to SIP. 2. Relays A central concept in this framework is that of a relay. A relay is defined as any SIP server, be it a proxy, back-to-back user agent or some hybrid, which receives a request and translates the request URI into one or more next hop URIs to which it then delivers a request. So, an essential aspect of a relay is that of translation. When a relay receives a request, it translates the request URI into one or more additional URIs. Or, more generally, it can create outgoing requests to one or more additional URIs. The translation operation is what creates the consent problem. Since the translation operation can result in more than one URI, it is the source of amplification. Servers that do not perform translations, such as outbound proxy servers, do not cause amplification. Since the translation operation is based on local policy or local data (such as registrations), it is the vehicle by which a request is delivered directly to an endpoint, when it would not otherwise be possible to. In other words, if a spammer has the address of a user, sip:user@example.com, it cannot deliver a MESSAGE request to the user agent of that user without having access to the registration data that maps sip:user@example.com to the UA on which that user is present. Thus, it is the usage of this registration data, and more generally, the translation logic, which must be authorized, in order to prevent undesired communications. Rosenberg, et al. Expires January 6, 2005 [Page 3] Internet-Draft Consent Framework July 2004 3. Reference Architecture The reference architecture is shown in Figure 1. In this architecture, a UAC wishes to send a message to a request URI representing a resource in domain A (sip:resource@A). This request may pass through a local outbound proxy (not shown), but eventually arrives at a server authoritative for domain A. This server, which acts as a relay, performs a translation operation, translating the request URI into one or more next hop URIs, which may or may not belong to domain A. This relay may be a proxy server of a URI-list service, for instance. +-------+ | | >| UAS 1 | +-------+ / | | | Rules | / +-------+ | DB | / +-------+ / | / V / +-----+ +-------+ / +-------+ | | | |/ | | | UAC |------>| Relay |-------->| UAS 2 | | | | |\ | | +-----+ +-------+ \ +-------+ \ \ [...] \ \ \ +-------+ \ | | >| UAS n | | | +-------+ Figure 1 4. Structure of a Permission This framework centers on the idea that a relay will only perform a translation if a permission is in place authorizing that translation. As such, the notion of a permission is another key part of this framework. A permission is an object, represented in XML, that contains several pieces of data: Rosenberg, et al. Expires January 6, 2005 [Page 4] Internet-Draft Consent Framework July 2004 Identity of the Sender: A URI representing the identity of the sender for whom permissions are granted. Identity of the Recipient: A URI representing the target of the translation. The permission grants ability for the sender to send requests, and for a relay receiving those requests to forward them to this URI. This is also called the recipient URI. Operations Permitted: A set of specific methods or qualifiers for which the permission applies. For example, the permission may only grant relaying for INVITE or MESSAGE, or for MESSAGE with specific MIME types. Signature: A digital signature over the rest of the permission, signed by an entity that can identify itself as the recipient URI. The signature is not always present. Permissions are installed on a resource by resource basis. That is, for each target URI to which a request is sent, there is a set of permissions installed for that URI. Each permission has the content described above. It is important to note that the permission itself does not depend on, or contain, the identity of a target URI (i.e., the input). As such, if a request is sent to sip:resource1@A and to sip:resource2@A, and for both targets, the same permission was installed, allowing requests from the sender to be relayed to sip:resource@B, that same permission would allow the translation to take place for both targets. A natural format for representing permissions appears to be the common policy format [4]. This format is also used for presence permissions. 5. Attempting Communication When a UA sends a request to a target resource, the request eventually arrives at a server that is authoritative for the domain in the request URI. The server may require, as part of its processing logic, the relaying of the request to one or more next hops. If such relaying is required, the server first authenticates the sender of the request. Such authentication can be done using the SIP identity mechansim [5]. Once the sender is authenticated, the server checks its permission database for that target resource. It looks for permissions containing senders whose URI matches the identity of the sender of the request. Of those that are found, the server checks to see if the permitted translated URI matches the URIs to which the server wishes to relay the request. Rosenberg, et al. Expires January 6, 2005 [Page 5] Internet-Draft Consent Framework July 2004 If at least one of the next hops to which the server wishes to relay have not been permitted, the server rejects the request with a 470 (Consent Needed) response. The 470 response code indicates that the request couldn't be relayed because at least one permission was not present. The error response can contain a body, which contains a list of URIs for the translations for which permissions have not yet been obtained. This is effectively an instruction for the sender to go off, and obtain permissions from those URIs. 6. Requesting a Permission If the attempt to communicate was rejected with a 470 (Consent Needed) response, the client knows that it must obtain some number of permissions in order for the communications to take place. The error response will include a list of URIs for which permission must be obtained. To obtain permission, the client sends a CONSENT request to each of the URIs it learned from the body of the error response. These URIs typically route to the relay, which will forward them on to the destinations whose permissions have not been obtained yet. The CONSENT request carries a Consent-Methods header field which indicates for which methods consent is being requested. When the CONSENT request arrives at the relay, the relay adds a Permission header field which contains a URI that the receiver can use to upload a permission (e.g., the receiver can use XCAP to upload an XML-based permission document). Then, the relay forwards the request towards its destination. If there are several relays between the sender and the final destination, those CONSENT requests may also fail if permissions have not yet been obtained, in which case the process recurses. Eventually, the client will have sent a request to all of the relays at the leaves of the translation tree between the sender and the final destinations. 7. Waiting for Permissions A CONSENT request is responded with a 202 (Accepted) response, which carries a URI in a Call-Info header field (wait-permission purpose) where the client can SUBSCRIBE to using the wait-permission event package. This event package models the state of the permission granted to the client for communicating with the target URI. When a permission is granted, the state changes, and the client receives a NOTIFY. This NOTIFY contains the permission(s) that have been granted for the sender. Usage of an event package has the benefit that the client can come back at any time and do a query SUBSCRIBE to see if permissions were Rosenberg, et al. Expires January 6, 2005 [Page 6] Internet-Draft Consent Framework July 2004 granted, or it can wait for them to be granted, and find out when. There is no requirement that the client use this event package to wait. For some requests, it may not be important for the sender to find out when permission is granted (e.g., a presence subscription). 8. Granting a Permission On reception of a CONSENT request, if the user wishes to grant a permission, XCAP is used, just as it is today in presence. The owner of the target resource would use contact the URI in the Permission header field of the CONSENT request and use XCAP to place the permission into a document containing the list of permissions for that target resource. The XCAP server needs to make sure that the entity uploading the permission document is the same as the destination of the CONSENT request. This is done by inserting a URI in the Permission header field of the CONSENT request which is long and random enough so that it cannot be guessed. In addition, the CONSENT request is delivered to the user using a SIPS URI. Then, the server inserting such a URI relies on the SIP routing infrastructure to deliver the CONSENT request to its proper destination. If the SIP routing infrastructure is compromised, it could route the CONSENT request to an attacker so that the attacker could authorize requests addressed to a victim. Nevertheless, if the SIP routing infrastructure gets compromised, many types of attacks much worse than this are possible. So, relaying on the SIP routing infrastructure seems like a sensible choice. Using XCAP to grant permissions will require the definition of a new application usage. We note that this usage appears to be a generalization of the presence rules usage currently defined [PRES-RULES]. 8.1 Permission Servers We have just described how a user agent that receives a CONSENT request can use XCAP to grant certain permissions. Nevertheless, users are not on-line all the time and, so, sometimes are not able to receive CONSENT requests. This issue is also found in presence, where a user's status is reported by a presence server instead of by the user's user agents, which can go on and off-line. Similarly, we define permission servers. Permission servers are network elements that act as SIP UAs and handle CONSENT requests for a user. Rosenberg, et al. Expires January 6, 2005 [Page 7] Internet-Draft Consent Framework July 2004 Permission servers inform users about new CONSENT requests using the "grant-permission" event package. The user associated with the target URI SUBSCRIBEs to the "grant-permission" event package at the permission server. This event package models the state of all pending CONSENT requests for a particular resource, for which permissions do not yet exist. When a new CONSENT request arrives for which permissions have not been granted, a NOTIFY is sent to the user. This informs them that permission is needed for a particular sender. The NOTIFY contains information on the operation which was requested. There is a strong similarity between the watcherinfo event package and the grant-permission event package. Indeed, the grant-permission package is effectively a superset of watcherinfo. Once in place, presentities could use the grant-permission event package for presence in addition to all other services for which opt-in is being provided. 9. Retrying the Original Request The sender learns about permissions through the wait-permission event package. Once it has obtained permissions for all of the resources that were identified in the 470 (Consent Needed) response, the client can retry the original request. 10. Permission Revocation At any time, if a client wants to revoke any permission, it uses the XCAP URI that received in the CONSENT message or through the grant-permission event package. If a client lost this URI for some reason, it would need to wait until it received a new request and respond with a 470 (Consent Needed) response. The client would get the URI in a new CONSENT request. OPEN ISSUE: if we defined the Permission header field so that it can be present in any request, and not only in CONSENT requests, the relay could add this header field to every request directed to the user which used SIPS. 11. Use Cases The following use cases exhibit how the framework works. 11.1 Basic Flow with No Permission Server A Relay B Rosenberg, et al. Expires January 6, 2005 [Page 8] Internet-Draft Consent Framework July 2004 | MESSAGE list@relay | | |-------------------------->| | | 470 | | | xyz@relay | | |<--------------------------| | | | | | CONSENT xyz@relay | CONSENT B | | Consent-methods: MESSAGE | Consent-methods: MESSAGE | |-------------------------->| Permission: xcap-uri | | |-------------------------->| | 202 Accepted | | | Call-Info: 123@relay; | 202 Accepted | | purpose: wait-permission |<--------------------------| |<--------------------------| | | | | | SUBSCRIBE 123@relay | | |-------------------------->| | | 200 OK | | |<--------------------------| | | | | | NOTIFY (no permission) | | |<--------------------------| | | 200 OK | | |-------------------------->| | | | | | | XCAP xcap-uri | | | Permission Grant | | |<--------------------------| | | 200 OK | | NOTIFY (permission) |-------------------------->| |<--------------------------| | | 200 OK | | |-------------------------->| | | | | | MESSAGE list@relay | | |-------------------------->| MESSAGE B | | |-------------------------->| | | | Figure 2 Alternatively, the Call-Info header field could have been inserted by B directly. In this case, A would SUBSCRIBE to B, instead of subscribing to the Relay. 11.2 Basic Flow with a Permission Server Rosenberg, et al. Expires January 6, 2005 [Page 9] Internet-Draft Consent Framework July 2004 A Relay B's Permission B Server | MESSAGE list@relay | | | |-------------------------->| | | | 470 | | | | xyz@relay | | | |<--------------------------| | | | | | | | CONSENT xyz@relay | CONSENT B | | | Consent-methods: MESSAGE | Consent-methods: MESSAGE | |-------------------------->| Permission: xcap-uri | | |----------------->| | | 202 Accepted | | | | Call-Info: 123@relay; | 202 Accepted | | | purpose: wait-permission |<-----------------| | |<--------------------------| | | | | | | | SUBSCRIBE 123@relay | | | |-------------------------->| | | | 200 OK | | | |<--------------------------| | | | | | | | NOTIFY (no permission) | | [B goes on-line] | |<--------------------------| | | | 200 OK | | | |-------------------------->| | SUBSCRIBE | | | | grant-permission | | | |<------------------| | | | 200 OK | | | |------------------>| | | | | | | | NOTIFY | | | | xcap-uri | | | |------------------>| | | | 200 OK | | | |<------------------| | | XCAP xcap-uri | | | | Permission Grant| | | |<-------------------------------------| | | 200 OK | | | NOTIFY (permission) |--------------------------------------| |<--------------------------| | | | 200 OK | | | |-------------------------->| | | | | | | | MESSAGE list@relay | | | |-------------------------->| | | Rosenberg, et al. Expires January 6, 2005 [Page 10] Internet-Draft Consent Framework July 2004 | | MESSAGE B | | | |------------------------------------->| | | | | Figure 3 12. References 12.1 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] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and D. Gurle, "Session Initiation Protocol (SIP) Extension for Instant Messaging", RFC 3428, December 2002. 12.2 Informative References [3] Rosenberg, J., "A Presence Event Package for the Session Initiation Protocol (SIP)", draft-ietf-simple-presence-10 (work in progress), January 2003. [4] Schulzrinne, H., "Common Policy", draft-ietf-geopriv-common-policy-00 (work in progress), February 2004. [5] Peterson, J., "SIP Authenticated Identity Body (AIB) Format", draft-ietf-sip-authid-body-02 (work in progress), July 2003. Authors' Addresses 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, et al. Expires January 6, 2005 [Page 11] Internet-Draft Consent Framework July 2004 Gonzalo Camarillo Ericsson Hirsalantie 11 Jorvas 02420 Finland EMail: Gonzalo.Camarillo@ericsson.com Dean Willis dynamicsoft 5100 Tennyson Parkway Suite 1200 Plano, TX 75028 USA EMail: dean.willis@softarmor.com Rosenberg, et al. Expires January 6, 2005 [Page 12] Internet-Draft Consent Framework July 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights 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; nor does it represent that it has made any independent effort to identify any such rights. Information on the IETF's procedures with respect to rights in IETF Documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. 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Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Rosenberg, et al. Expires January 6, 2005 [Page 13]