SIPPING J. Rosenberg Internet-Draft Cisco Systems Expires: August 29, 2006 G. Camarillo, Ed. Ericsson D. Willis Cisco Systems February 25, 2006 A Framework for Consent-Based Communications in the Session Initiation Protocol (SIP) draft-ietf-sipping-consent-framework-04.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 29, 2006. Copyright Notice Copyright (C) The Internet Society (2006). 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 Rosenberg, et al. Expires August 29, 2006 [Page 1] Internet-Draft Consent Framework February 2006 from one party to another without requiring explicit consent of the recipient. Without such consent, it is possible for SIP to be used for malicious purposes, including amplification and DoS (Denial of Service) attacks. This document identifies a framework for consent- based communications in SIP. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Relays and Translations . . . . . . . . . . . . . . . . . . . 3 4. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.1. Permissions at a Relay . . . . . . . . . . . . . . . . . . 6 4.2. Consenting Manipulations on a Relay's Transaction Logic . 6 4.3. Permission Servers . . . . . . . . . . . . . . . . . . . . 7 4.4. Recipients Grant Permissions . . . . . . . . . . . . . . . 8 5. Overview of Operations . . . . . . . . . . . . . . . . . . . . 8 5.1. Amplification Avoidance . . . . . . . . . . . . . . . . . 10 5.2. Subscription to the Permission Status . . . . . . . . . . 11 5.3. Request for Permission . . . . . . . . . . . . . . . . . . 11 5.4. Permission Document Structure . . . . . . . . . . . . . . 11 5.5. Permission Requested Notification . . . . . . . . . . . . 12 5.6. Permission Upload . . . . . . . . . . . . . . . . . . . . 12 5.6.1. SIP Identity . . . . . . . . . . . . . . . . . . . . . 13 5.6.2. P-Asserted-Identity . . . . . . . . . . . . . . . . . 13 5.6.3. Return Routability . . . . . . . . . . . . . . . . . . 13 5.7. Permission Granted Notification . . . . . . . . . . . . . 14 5.8. Permission Revocation . . . . . . . . . . . . . . . . . . 14 5.9. Request-contained URI Lists . . . . . . . . . . . . . . . 15 5.10. Registrations . . . . . . . . . . . . . . . . . . . . . . 17 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 7. Security Considerations . . . . . . . . . . . . . . . . . . . 20 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 8.1. Normative References . . . . . . . . . . . . . . . . . . . 20 8.2. Informative References . . . . . . . . . . . . . . . . . . 22 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23 Intellectual Property and Copyright Statements . . . . . . . . . . 24 Rosenberg, et al. Expires August 29, 2006 [Page 2] Internet-Draft Consent Framework February 2006 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 [4]) 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 amplification and DoS (Denial of Service) attacks. These problems are described in more detail in a companion requirements document [17]. This specification defines a basic framework for adding consent-based communication to SIP. 2. Definitions Recipient URI: The request-URI of an outgoing request sent by an entity (e.g., a user agent or a proxy). The sending of such request may have been the result of a translation operation. Target URI: The request-URI of an incoming request that arrives to an entity (e.g., a proxy) that will perform a translation operation. Translation operation: Operation by which an entity (e.g., a proxy) translates the request URI of an incoming request (i.e., the target URI) into one or more URIs (i.e., recipient URIs) which are used as the request URIs of one or more outgoing requests. 3. Relays and Translations Relays play a key role in this framework. A relay is defined as any SIP server, be it a proxy, B2BUA (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. The request URI of the incoming request is referred to as 'target URI' and the destination URI of the outgoing requests is referred to as 'recipient URIs', as shown in Figure 1. Rosenberg, et al. Expires August 29, 2006 [Page 3] Internet-Draft Consent Framework February 2006 +---------------+ | | recipient URI | |----------------> target URI | Translation | -------------->| Operation | recipient URI | |----------------> | | +---------------+ Figure 1: Translation operation Thus, an essential aspect of a relay is that of translation. When a relay receives a request, it translates, following its translation logic, 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. Additionally, since the translation operation can result in more than one URI, it is also 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, 'user@example.com', it cannot deliver a MESSAGE request to the UA (User Agent) of that user without having access to the registration data that maps 'user@example.com' to the user agent 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. (Of course, if the spammer knows the address of the user agent, it will be able to deliver requests directly to it.) Figure 2 shows a relay that performs translations. The user agent client (UAC) in the figure sends a SIP request to a URI representing a resource in the domain 'example.com' (resource@example.com). This request may pass through a local outbound proxy (not shown), but eventually arrives at a server authoritative for the domain 'example.com'. This server, which acts as a relay, performs a translation operation, translating the target URI into one or more recipient URIs, which may or may not belong to the domain 'example.com'. This relay may be, for instance, a proxy server or a URI-list service [18]. Rosenberg, et al. Expires August 29, 2006 [Page 4] Internet-Draft Consent Framework February 2006 +-------+ | | >| UAS | / | | / +-------+ / / +-----------------------+ / | | / +-----+ | Relay | / +-------+ | | | |/ | | | UAC |------>| |-------->| Proxy | | | |+---------------------+|\ | | +-----+ || Translation || \ +-------+ || Logic || \ |+---------------------+| \ [...] +-----------------------+ \ \ \ +-------+ \ | | >| B2BUA | | | +-------+ Figure 2: Relay performing a translation This framework allows potential recipients of a translation to agree to be actual recipients by giving permission to the relay performing the translation to send them traffic. 4. Architecture Figure 3 shows the architectural elements of this framework. Section 4.1 describes the role of permissions at a relay. Section 4.2 discusses the actions taken by a relay when its translation logic is manipulated by a client. Section 4.3 introduces permission servers and their functionality. Section 4.4 describes how potential recipients can grant permissions to a relay to add them to the relay's translation logic. Rosenberg, et al. Expires August 29, 2006 [Page 5] Internet-Draft Consent Framework February 2006 +-----------------------+ Permission +------------+ | | Request | | +--------+ | Relay |----------->| Permission | | | | | | Server | | Client | | | | | | | |+-------+ +-----------+| +------------+ +--------+ ||Transl.| |Permissions|| | | ||Logic | | || Permission | | |+-------+ +-----------+| Request | | +-----------------------+ V | ^ ^ +------------+ | Manipulation | | Permission Grant | | +---------------+ +-------------------| Recipient | | | +------------+ Figure 3: Reference Architecture 4.1. Permissions at a Relay Relays implementing this framework need to obtain and store permissions associated to their translation logics. These permissions indicate if a particular recipient has agreed to receive traffic or not at any given time. Recipients that have not given permission to the relay to send them traffic are simply ignored by the relay when performing a translation. Permissions are valid as long as the context where they were granted is valid. For example, the permissions obtained by a URI-list SIP service that distributes MESSAGE requests to a set of recipients will be valid as long as the URI-list SIP service exists. 4.2. Consenting Manipulations on a Relay's Transaction Logic This framework aims to ensure that any particular Relay only performs translations towards destinations that have given permission to the Relay to perform such a translation. Consequently, when the translation logic of a relay is manipulated (e.g., a new recipient URI is added), the relay needs to obtain permission from the new recipient in order to install the new translation logic. Relays ask recipients for permission using CONSENT [10] requests. For example, the relay hosting the URI-list service at 'friends@example.com' performs a translation from that URI to a set of recipient URIs. When a client (e.g., the administrator of that URI-list service) adds 'bob@example.org' as a new recipient URI, the relay sends a CONSENT request to 'bob@example.org' asking whether or not it is OK to perform the translation from 'friends@example.com' to Rosenberg, et al. Expires August 29, 2006 [Page 6] Internet-Draft Consent Framework February 2006 'bob@example.org' (CONSENT requests carry in their message bodies a permission document that describes the translation for which permissions are being requested). If the answer is positive, the new translation logic is installed at the relay. That is, the new recipient URI is added. Note that the mechanism to be used to manipulate the translation logic of a particular relay depends on the relay. One possible mechanism to manipulate translation logic is XCAP [15]. Section 5.9 and Section 5.10 describe how to add recipients to a translation using request-contained URI lists and REGISTER requests respectively. In any case, manipulation mechanisms implementing this framework need to have a means to indicate that a particular recipient URI is in the 'Permission Pending' state and to provide the URI where the REFER request needs to be sent to. 4.3. Permission Servers When a CONSENT request sent by a relay arrives to the recipient URI to which it was sent, the receiving user can grant or deny the permission needed to perform the translation. 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, which are a key element of this framework. Permission servers are network elements that act as SIP user agents and handle CONSENT requests for a user. Permission servers inform users about new CONSENT requests using the 'grant-permission' event package [12]. Figure 4 illustrates this point. The user associated with the recipient URI for which the relay will ask for permission subscribes [2] (1) 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. When a new CONSENT request (3) arrives to the permission server, a NOTIFY (5) is sent to the user. This informs them that permission is needed for a particular sender. The NOTIFY contains the permission document received in the CONSENT request. This permission document is a description of the translation for which permissions are being requested. Rosenberg, et al. Expires August 29, 2006 [Page 7] Internet-Draft Consent Framework February 2006 There is a strong similarity between the 'winfo' event template- package [19] 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. Relay B's Permission B Server | |(1) SUBSCRIBE | | |Event: grant-permission | |<------------------| | |(2) 200 OK | | |------------------>| | |(3) NOTIFY | | |------------------>| | |(4) 200 OK | | |<------------------| |(5) CONSENT B@example | |------------------>| | |(6) 202 Accepted | | |<------------------| | | |(7) NOTIFY | | |------------------>| | |(8) 200 OK | | |<------------------| Figure 4: Permission server operation 4.4. Recipients Grant Permissions Recipients provide relays with permissions using SIP PUBLISH requests. These requests contain a permission document that describes the translation for which permissions are being granted. 5. Overview of Operations This section provides an overview of this framework using an example of the prototypical call flow. The elements described in Section 4 (i.e., relays, translations, and permission servers) play an essential role in this call flow. Figure Figure 5 shows the complete process to add a recipient URI ('B@example.com') to the translation logic of a relay. The call flow starts with user B subscribing to the permission server using the 'grant-permission' event package [12]. User B will be informed about Rosenberg, et al. Expires August 29, 2006 [Page 8] Internet-Draft Consent Framework February 2006 the arrival of CONSENT [10] requests addressed to 'B@example.com'. User A attempts to add 'B@example.com' as a new recipient URI to the translation logic of the relay (5). User A uses XCAP [15] and the XML (Extensible Markup Language) format for representing resource lists [16] as extended by [14] to perform this addition. Since the relay does not have permission from 'B@example.com' to perform translations towards that URI, the relay places 'B@example.com' in the 'Pending' state [14] and informs user A (6). A@example.com Relay B's Permission B@example.com Server | | |(1) SUBSCRIBE | | | |Event: grant-permission | | |<---------------| | | |(2) 200 OK | | | |--------------->| | | |(3) NOTIFY | | | |--------------->| | | |(4) 200 OK | | | |<---------------| |(5) Add Recipient B@example.com | | |--------------->| | | |(6) Permission Pending | | |<---------------| | | |(7) REFER | | | |Refer-To: B@example.com?method=CONSENT | |--------------->| | | |(8) 200 OK | | | |<---------------| | | |(9) SUBSCRIBE | | | |Event: list-state | | |--------------->| | | |(10) 200 OK | | | |<---------------| | | |(11) NOTIFY | | | |<---------------| | | |(12) 200 OK | | | |--------------->| | | | |(13) CONSENT B@example | | |Permission-Upload: uri-up | | |Permission Document | | |--------------->| | | |(14) 202 Accepted | | |<---------------| | | | |(15) NOTIFY | | | |uri-up | Rosenberg, et al. Expires August 29, 2006 [Page 9] Internet-Draft Consent Framework February 2006 | | |Permission Document | | |--------------->| | | |(16) 200 OK | | | |<---------------| | |(17) PUBLISH uri-up | | |Permission Document | | |<--------------------------------| | |(18) 200 OK | | | |-------------------------------->| |(19) NOTIFY | | | |<---------------| | | |(20) 200 OK | | | |--------------->| | | Figure 5: Prototypical call flow 5.1. Amplification Avoidance Once 'B@example.com' is in the 'Permission Pending' state, the relay needs to ask user B for permission by sending a CONSENT request to 'B@example.com'. However, the relay needs to ensure that it is not used as an amplifier to launch amplification attacks. In such an attack, the attacker would add a large number of recipient URIs to the translation logic of a relay. The relay would then send a CONSENT request to each of those URIs. The bandwidth generated by the relay would be much higher than the bandwidth used by the attacker to add those URIs to the translation logic of the relay. This framework uses a credit-based authorization mechanism to avoid the attack just described. It requires users adding new recipient URIs to a translation to generate an amount of bandwidth that is comparable to the bandwidth the relay will generate when sending CONSENT requests towards those recipient URIs. This requirement is met by having users generate REFER requests [5] towards the relay. Each REFER request triggers the sending of a CONSENT request by the relay. So, the relay sends user A the URI (6) where user A needs to send a REFER request. User A generates such a REFER request (7) and sends it to the relay. User A uses the 'norefersub' extension [7], which supresses the implicit subscription that is associated with REFER transactions. This is because user A is not interested in the result of the CONSENT transaction, but in whether or not user B will authorize the translation by providing the requested permission. The relay provides a URI (6) where user A can subscribe to obtain information on whether or not user B provides the requested Rosenberg, et al. Expires August 29, 2006 [Page 10] Internet-Draft Consent Framework February 2006 permission. User A subscribes to that URI using the 'list-state' [14] event package (9). 5.2. Subscription to the Permission Status After sending the REFER (7) user A subscribes to the 'list-state' event package at the relay. This subscription keeps user A informed about the status of the permissions (e.g., granted or denied) the relay will request on receiving the REFER request (7). 5.3. Request for Permission On receiving the REFER request (7), the relay generates a CONSENT request (13) towards 'B@example.com'. This CONSENT request carries a permission document, which describes the translation that needs to be authorized, and a URI where to upload the permission for that translation. User B will authorize the translation by uploading the permission document received in the CONSENT request into this URI, as described in Section 5.6. When the permission document is uploaded to the URI provided by the relay (17), the relay needs to make sure that the permission document received was generated by user B and not by an attacker. The relay can use three methods to authenticate the permission document: SIP identity, P-Asserted-Identity [3], or a return routability test. These methods are described in Section 5.6. Relays using a return routability test to perform this authentication need to send the CONSENT request to a SIPS URI. 5.4. Permission Document Structure A permission document is the XML representation of a permission. A permission document contains several pieces of data: Identity of the Sender: A URI representing the identity of the sender for whom permissions are granted. Identity of the Original Recipient: A URI representing the identity of the original recipient, which is used as the input for the translation operation. This is also called the target URI. Identity of the Final Recipient: A URI representing the result of the translation. The permission grants ability for the sender to send requests to the target URI, and for a relay receiving those requests to forward them to this URI. This is also called the recipient URI. Rosenberg, et al. Expires August 29, 2006 [Page 11] Internet-Draft Consent Framework February 2006 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. Permission documents may contain wildcards. For example, a permission document may authorize any relay to forward requests coming from a particular sender to a particular recipient. Such a permission document would apply to any target URI. That is, the field containing the identity of the original recipient would match any URI. The format for permission documents is defined in [11]. The permission document in the CONSENT request (13) sent by the relay contains the following values: Identity of the Sender: Any. Identity of the Original Recipient: friends@example.com Identity of the Final Recipient: B@example.com It is expected that the Sender field often contains a wildcard. However, scenarios involving request-contained URI lists, such as the one described in Section 5.9, may require permission documents that apply to a specific sender. Of course, in cases where the identity of the sender matters, it is essential that relays authenticate senders. 5.5. Permission Requested Notification On receiving the CONSENT request (13), B's permission server sends a NOTIFY request (15) to user B, who had previously subscribed to the grant-permission event package (1). This NOTIFY request contains, the permission document, which describes the translation that needs to be authorized, and a URI where to upload the permission for that translation. Both the permission document and the URI to upload the permission are copied from the CONSENT request (13) into the NOTIFY request (15). 5.6. Permission Upload On receiving the NOTIFY request (15), user B authorizes the translation described in the permission document received by uploading this permission document to the relay. User B uses a PUBLISH request (17) to upload the permission document to the URI received in the NOTIFY request. Rosenberg, et al. Expires August 29, 2006 [Page 12] Internet-Draft Consent Framework February 2006 When the permission document is uploaded to the URI provided by the relay (17), the relay needs to make sure that the permission document received was generated by user B and not by an attacker. The relay can use three methods to authenticate the permission document: SIP identity, P-Asserted-Identity [3], or a return routability test. 5.6.1. SIP Identity The SIP identity [8] mechanism can be used to authenticate the sender of the PUBLISH request uploading the permission document. The relay checks that the originator of the PUBLISH request is the owner of the recipient URI in the permission document. Otherwise, the permission document is discarded. 5.6.2. P-Asserted-Identity The P-Asserted-Identity [3] mechanism can be used to authenticate the sender of the PUBLISH request uploading the permission document. However, as discussed in RFC 3325 [3], this mechanism should only be used within networks of trusted SIP servers. That is, the use of this mechanism is only applicable inside an administrative domain with previously agreed-upon policies. The relay checks that the originator of the PUBLISH request is the owner of the recipient URI in the permission document. Otherwise, the permission document is discarded. 5.6.3. Return Routability SIP identity provides a good authentication mechanism for this type of scenario. Nevertheless, SIP identity is not widely available on the public Internet yet. That is why an authentication mechanism that can already be used at this point is needed. Return routability tests do not provide the same level of security as SIP identity, but they provide a good-enough security level in architectures where the SIP identity mechanism is not available (e.g., the current Internet). The relay generates an unguessable URI (e.g., with a long and random-looking user part) and places it in the CONSENT request (13). The recipient needs to upload the permission document to that URI. Relays using a return routability test to perform this authentication need to send the CONSENT request to a SIPS URI. This ensures that attackers do not get access to the (unguessable) URI. Thus, the only user able to upload the permission document to the (unguessable) URI is the receiver of the CONSENT request. Rosenberg, et al. Expires August 29, 2006 [Page 13] Internet-Draft Consent Framework February 2006 Relays can transition from return routability tests to SIP identity by simply requiring the use of SIP identity for incoming PUBLISH requests. That is, such a relay would reject PUBLISH requests that did not use SIP identity. 5.7. Permission Granted Notification On receiving the PUBLISH request (17), the relay sends a NOTIFY request (19) to inform user A that the permission for the translation has been received that the translation logic at the relay has been updated. That is, 'B@example.com' has been added as a recipient URI. 5.8. Permission Revocation At any time, if a client wants to revoke any permission, it uses the same URI as before to upload, using a PUBLISH request, a new permission document that does not authorize the translation at the relay any longer. If a client loses this URI for some reason, it needs to wait until it receives a new request product of the translation. Such a request will contain a Trigger-Consent header field with a URI. That URI will have an escaped Refer-To header field identifying the client (i.e., the recipient of the translation). The client needs to send a REFER request to the URI in the Trigger-Consent header field in order to receive a CONSENT request from the relay. Such a CONSENT request will contain the permission document that was uploaded to the relay at some point and the URI where the user can upload a new permission document that does not authorize the translation at the relay any longer. Figure 6 shows an example of a user revoking permissions at a relay. The user rejects an incoming INVITE (5) request, which contains a Trigger-Consent header field. Using the URI in the that header field, the user sends a REFER request (8) to the relay. On receiving the REFER request (8), the relay generates a CONSENT request (8) towards the user. Finally, the user revokes the permissions by sending a PUBLISH request (14) to the relay. Rosenberg, et al. Expires August 29, 2006 [Page 14] Internet-Draft Consent Framework February 2006 Relay B's Permission B@example.com Server | |(1) SUBSCRIBE | | |Event: grant-permission | |<---------------| | |(2) 200 OK | | |--------------->| | |(3) NOTIFY | | |--------------->| | |(4) 200 OK | | |<---------------| |(5) INVITE | | |Trigger-Consent: <123@relay.example.com> | ?Refer-To= |-------------------------------->| |(6) 603 Decline | | |<--------------------------------| |(7) ACK | | |-------------------------------->| |(8) REFER 123@relay.example.com | |Refer-To: B@example.com | |<--------------------------------| |(9) 200 OK | | |-------------------------------->| |(10) CONSENT B@example | |Permission-Upload: uri-up | |Permission Document | |--------------->| | |(11) 202 Accepted | |<---------------| | | |(12) NOTIFY | | |uri-up | | |Permission Document | |--------------->| | |(13) 200 OK | | |<---------------| |(14) PUBLISH uri-up | |Permission Document Revoking Permissions |<--------------------------------| |(15) 200 OK | | |-------------------------------->| Figure 6: Permission Revocation 5.9. Request-contained URI Lists In the scenarios described so far, a user adds recipient URIs to the translation logic of a relay. However, the relay does not perform Rosenberg, et al. Expires August 29, 2006 [Page 15] Internet-Draft Consent Framework February 2006 translations towards those URIs until permissions are obtained. URI-list services using request-contained URI lists are a special case because the selection of recipient URIs is performed at the same time as the communication attempt. A user places a set of recipient URIs in a request and sends it to a relay so that the relay sends a similar request to all those recipient URIs. This type of URI-list services maintain a list of recipient URIs from which permission have been received. This list is manipulated in the same way as described in Section 5 and represents the set of URIs that will be accepted if a request containing a URI-list arrives to the relay. Additionally, Figure 7 shows another way to add entries to that list. If the relay receives a request (1) that contains URIs for which the relay does not have permission, the relay rejects the request with a 470 (Consent Needed) response (2). Such a response contains a Trigger-Consent header field with a URI for each recipient for which there is no permission, as shown in Figure 7. Each URI entry in the Trigger-Consent header field contains an escaped Refer-To header field with the URI of the recipient. The user needs to send REFER requests to the URIs in the Trigger-Consent header field. Additionally, the response also contains a Call-Info header field with a URI where the user can subscribe in order to be informed on whether or not the relay receives permission from user B. The value of the purpose parameter for this entry is 'list-state'. OPEN ISSUE: do we need to provide that URI in a Call-Info header field (or in a new header field) or can we assume that the sender has a relationship with the relay and will know that URI already? The rest of the process is similar to the one described in Section 5. Note, however, that for simplicity, Figure 7 does not show the split between user B's permission server and user agent. Rosenberg, et al. Expires August 29, 2006 [Page 16] Internet-Draft Consent Framework February 2006 A@example.com Relay B@example.com |(1) INVITE | | |B@example.com | | |C@example.com | | |------------------>| | |(2) 470 Consent Needed | |Trigger-Consent: <123@relay.example.com> | ?Refer-To= |Call-Info: 456@Relay;purpose=list-state |<------------------| | |(3) ACK | | |------------------>| | |(4) SUBSCRIBE 456@Relay | |Event: list-state | |------------------>| | |(5) 200 OK | | |<------------------| | |(6) NOTIFY | | |<------------------| | |(7) 200 OK | | |------------------>| | |(8) REFER 123@Relay| | |Refer-To: B@example.com | |------------------>| | |(9) 200 OK | | |<------------------| | | |(10) CONSENT B@example | |Permission-Upload: uri-up-relay | |Permission Document| | |------------------>| | |(11) 202 Accepted | | |<------------------| | |(12) PUBLISH uri-up-relay | |Permission Document| | |<------------------| | |(13) 200 OK | | |------------------>| |(14) NOTIFY | | |<------------------| | |(15) 200 OK | | |------------------>| | Figure 7: INVITE with a URI list in its body 5.10. Registrations Registrations are a special type of translations. The user registering has a trust relationship with the registrar in its home Rosenberg, et al. Expires August 29, 2006 [Page 17] Internet-Draft Consent Framework February 2006 domain. This is not the case when a user gives any type of permissions to a relay in a different domain. Traditionally, REGISTER transactions have performed two operations at the same time: setting up a translation and authorizing the use of that translation. For example, a user registering its current contact URI is giving permission to the registrar to forward traffic sent to the user's AoR (Address of Records) to the registered contact URI. This works fine when the entity registering is the same as the one that will be receiving traffic at a later point (e.g., the entity receives traffic over the same connection used for the registration as described in [9]). However, this schema creates some potential attacks which relate to third-party registrations. An attacker binds, via a registration, his or her AoR with the contact URI of a victim. Now, the victim will receive unsolicited traffic that was originally addressed to the attacker. The process of authorizing a registration is shown in Figure 8. User A performs a third-party registration (1) and receives a 200 (OK) response (2) with a Trigger-Consent header field. This header field contains the URI for which there is no permission in an escaped Refer-To header field. That is, the URI the user is attempting to register. A REFER request sent to the URI in the Trigger-Consent header field will trigger the registrar to send a CONSENT request to the URI being registered. The user sends a REFER request (7) to the URI received in the Trigger-Consent header field. In order to know whether or not the registrar receives the permission needed, the user subscribes (3) to the 'reg-event' event package [6], which can report consent-related information using the extension defined in [13]. The rest of the process is similar to the one described in Section 5. Rosenberg, et al. Expires August 29, 2006 [Page 18] Internet-Draft Consent Framework February 2006 A@example.com Registrar a@ws123.example.com |(1) REGISTER | | |Contact: a@ws123.example.com | |Supported: consent-reg | |------------------>| | |(2) 200 OK | | |Required: consent-reg | |Trigger-Consent: <123@relay.example.com> | ?Refer-To= |<------------------| | |(3) SUBSCRIBE | | |Event: reg-event | | |------------------>| | |(4) 200 OK | | |<------------------| | |(5) NOTIFY | | |<------------------| | |(6) 200 OK | | |------------------>| | |(7) REFER 123@Registrar | |Refer-To: a@ws123.example.com | |------------------>| | |(8) 200 OK | | |<------------------| | | |(9) CONSENT a@ws123.example | |Permission-Upload: uri-up | |Permission Document| | |------------------>| | |(10) 202 Accepted | | |<------------------| | |(11) PUBLISH uri-up| | |Permission Document| | |<------------------| | |(12) 200 OK | | |------------------>| |(13) NOTIFY | | |<------------------| | |(14) 200 OK | | |------------------>| | Figure 8: Registration Permission documents used to authorize registrations are very general. For example, one such document may authorize the registrar to forward any request from any sender to a particular recipient URI. This is the type of granularity that this framework intends to provide for registrations. Users who want to define how incoming requests are treated with a finer granularity (e.g., requests from Rosenberg, et al. Expires August 29, 2006 [Page 19] Internet-Draft Consent Framework February 2006 user A should only be accepted between 9:00 and 11:00) should use other mechanisms such as CPL [20]. Note that, as indicated previously, user agents using the same connection to register and to receive traffic from the registrar, as described in [9] do not need to use the mechanism described in this section. A user agent being registered by a third party may not be able to use the SIP Identity or P-Asserted-Identity mechanisms to prove to the registrar that the user agent is the owner of the URI being registered (e.g., sip:user@192.0.2.1), which is the recipient URI of the translation. In this case, return routability needs to be used. 6. IANA Considerations This document does not require the IANA to take any actions. 7. Security Considerations TBD. Editor's note: we have to avoid that attackers provide permissions for translations that apply to other users (e.g., allow everyone to send traffic to a victim) and that attackers provide permissions for a translation that apply to them but routes to a victim (e.g., 3rd party registration that binds attacker@relay to victim@somewhere). For the former we need authentication (e.g., SIP identity) and for the latter we relay on the routing infrastructure to route CONSENTs to the same place the traffic will be sent to once permissions are obtained (i.e., a return routability test). 8. References 8.1. 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] Roach, A., "Session Initiation Protocol (SIP)-Specific Event Notification", RFC 3265, June 2002. [3] Jennings, C., Peterson, J., and M. Watson, "Private Extensions to the Session Initiation Protocol (SIP) for Asserted Identity Rosenberg, et al. Expires August 29, 2006 [Page 20] Internet-Draft Consent Framework February 2006 within Trusted Networks", RFC 3325, November 2002. [4] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C., and D. Gurle, "Session Initiation Protocol (SIP) Extension for Instant Messaging", RFC 3428, December 2002. [5] Sparks, R., "The Session Initiation Protocol (SIP) Refer Method", RFC 3515, April 2003. [6] Rosenberg, J., "A Session Initiation Protocol (SIP) Event Package for Registrations", RFC 3680, March 2004. [7] Levin, O., "Suppression of Session Initiation Protocol REFER Method Implicit Subscription", draft-ietf-sip-refer-with-norefersub-04 (work in progress), January 2006. [8] Peterson, J. and C. Jennings, "Enhancements for Authenticated Identity Management in the Session Initiation Protocol (SIP)", draft-ietf-sip-identity-06 (work in progress), October 2005. [9] Jennings, C. and R. Mahy, "Managing Client Initiated Connections in the Session Initiation Protocol (SIP)", draft-ietf-sip-outbound-01 (work in progress), October 2005. [10] Camarillo, G., "A Document Format for Expressing Consent", draft-camarillo-sip-consent-method-00 (work in progress), February 2006. [11] Camarillo, G., "A Document Format for Expressing Consent", draft-camarillo-sipping-consent-format-00 (work in progress), February 2006. [12] Camarillo, G., "A Document Format for Expressing Consent", draft-camarillo-sipping-grant-permission-00 (work in progress), February 2006. [13] Camarillo, G., "Session Initiation Protocol (SIP) Registration Event Package Extension for Consent-Based Communications", draft-camarillo-sipping-consent-reg-event-00 (work in progress), February 2006. [14] Camarillo, G., "The Session Initiation Protocol (SIP) List State Event Package", draft-camarillo-sipping-list-state-00 (work in progress), February 2006. [15] Rosenberg, J., "The Extensible Markup Language (XML) Configuration Access Protocol (XCAP)", Rosenberg, et al. Expires August 29, 2006 [Page 21] Internet-Draft Consent Framework February 2006 draft-ietf-simple-xcap-08 (work in progress), October 2005. [16] Rosenberg, J., "Extensible Markup Language (XML) Formats for Representing Resource Lists", draft-ietf-simple-xcap-list-usage-05 (work in progress), February 2005. [17] Rosenberg, J., "Requirements for Consent-Based Communications in the Session Initiation Protocol (SIP)", draft-ietf-sipping-consent-reqs-04 (work in progress), January 2006. [18] Camarillo, G. and A. Roach, "Framework and Security Considerations for Session Initiation Protocol (SIP) Uniform Resource Identifier (URI)-List Services", draft-ietf-sipping-uri-services-05 (work in progress), January 2006. 8.2. Informative References [19] Rosenberg, J., "A Watcher Information Event Template-Package for the Session Initiation Protocol (SIP)", RFC 3857, August 2004. [20] Lennox, J., Wu, X., and H. Schulzrinne, "Call Processing Language (CPL): A Language for User Control of Internet Telephony Services", RFC 3880, October 2004. Rosenberg, et al. Expires August 29, 2006 [Page 22] Internet-Draft Consent Framework February 2006 Authors' Addresses Jonathan Rosenberg Cisco Systems 600 Lanidex Plaza Parsippany, NJ 07054 US Phone: +1 973 952-5000 Email: jdrosen@cisco.com URI: http://www.jdrosen.net Gonzalo Camarillo (editor) Ericsson Hirsalantie 11 Jorvas 02420 Finland Email: Gonzalo.Camarillo@ericsson.com Dean Willis Cisco Systems 2200 E. Pres. George Bush Turnpike Richardson, TX 75082 USA Email: dean.willis@softarmor.com Rosenberg, et al. Expires August 29, 2006 [Page 23] Internet-Draft Consent Framework February 2006 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 procedures with respect to rights in RFC 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. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Rosenberg, et al. Expires August 29, 2006 [Page 24]