Internet Engineering Task Force (IETF) V. Hilt Request for Comments: 6795 Bell Labs/Alcatel-Lucent Category: Standards Track G. Camarillo ISSN: 2070-1721 Ericsson December 2012 A Session Initiation Protocol (SIP) Event Package for Session-Specific Policies
AbstractThis specification defines a Session Initiation Protocol (SIP) event package for session-specific policies. This event package enables user agents (UAs) to subscribe to session policies for a SIP session and to receive notifications if these policies change. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6795. Copyright Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
1. Introduction ....................................................2 2. Terminology .....................................................3 3. Event Package Formal Definition .................................3 3.1. Event Package Name .........................................4 3.2. Event Package Parameters ...................................4 3.3. SUBSCRIBE Bodies ...........................................4 3.4. Subscription Duration ......................................5 3.5. NOTIFY Bodies ..............................................5 3.6. Subscriber Generation of SUBSCRIBE Requests ................6 3.7. Notifier Processing of SUBSCRIBE Requests ..................8 3.8. Notifier Generation of NOTIFY Requests .....................9 3.9. Subscriber Processing of NOTIFY Requests ..................10 3.10. Handling of Forked Requests ..............................11 3.11. Rate of Notifications ....................................11 3.12. State Agents .............................................11 3.13. Examples .................................................11 4. Security Considerations ........................................14 5. IANA Considerations ............................................16 5.1. Event Package Name ........................................16 6. References .....................................................16 6.1. Normative References ......................................16 6.2. Informative References ....................................17 Appendix A. Acknowledgements ......................................18 RFC3261] Session Policies [RFC6794] defines a protocol framework that enables a proxy to define and impact policies on sessions such as the codecs or media types to be used. This framework identifies two types of session policies: session-specific and session-independent policies. Session-specific policies are policies that are created for one particular session, based on the session description of this session. They enable a network intermediary to inspect the session description that a UA is proposing and to return a policy specifically generated for this session description. For example, an intermediary could open pinholes in a firewall/NAT for each media stream in a session and return a policy that replaces the internal IP addresses and ports in the session description with external ones. Since session- specific policies are tailored to a session, they only apply to the session for which they are created. A UA requests session-specific policies on a session-by-session basis at the time a session is created and the session description is known. Session-independent policies, on the other hand, are policies that are created independently of a session and generally apply to all the SIP sessions set up by a user agent.
"A Framework for Session Initiation Protocol (SIP) Session Policies" [RFC6794] defines a mechanism that enables UAs to discover the URIs of session-specific policy servers. This specification defines a SIP event package [RFC6665] that enables UAs to subscribe to session- specific policies on a policy server. Subscribing to session- specific policies involves the following steps (see the Session Policy Framework [RFC6794]): 1. A user agent submits the details of the session it is trying to establish to the policy server and asks whether a session using these parameters is permissible. For example, a user agent might propose a session that contains the media types audio and video. 2. The policy server generates a policy decision for this session and returns the decision to the user agent. Possible policy decisions are (1) to deny the session, (2) to propose changes to the session parameters with which the session would be acceptable, or (3) to accept the session as it was proposed. An example for a policy decision is to disallow the use of video but agree to all other aspects of the proposed session. 3. The policy server can update the policy decision at a later time. A policy decision update can require additional changes to the session (e.g., because the available bandwidth has changed) or deny a previously accepted session (i.e., disallow the continuation of a session). The event package for session-specific policies enables a user agent to subscribe to the policies for a SIP session following the above model. The subscriber initiates a subscription by submitting the details of the session it is trying to establish to the notifier (i.e., the policy server) in the body of a SUBSCRIBE request. The notifier uses this information to determine the policy decision for this session. It conveys the initial policy decision to the subscriber in a NOTIFY request and all changes to this decision in subsequent NOTIFY requests. RFC2119]. RFC6665].
3.6, 3.8 and 3.9. insufficient-info: The "insufficient-info" parameter is optional and only defined for NOTIFY requests. It is used by the notifier to indicate that a policy decision could not be made due to insufficient information. The "insufficient-info" parameter MUST be ignored if received in a SUBSCRIBE request. The usage of the "insufficient-info" parameter is described in Sections 3.7, 3.8 and 3.9. RFC6796]. The "application/ media-policy-dataset+xml" format is the default format for SUBSCRIBE bodies in this event package. Subscribers and notifiers MAY negotiate the use of other formats capable of representing a session. Note: It has been proposed to directly use Session Description Protocol (SDP) [RFC4566] instead of encoding the session descriptions in the Media Policy [RFC6796] format. However, using a separate format such as the Media Policy format has a number of advantages over the direct use of SDP: i) the Media Policy format is more flexible and allows the inclusion of information that
can't be expressed in SDP (e.g., the target URI), ii) the Media Policy format enables the encoding of local and remote session descriptions in a single document (not requiring the use of MIME multipart and new content disposition types), and iii) the Media Policy format aligns the formats used for session-specific and session-independent policies. A drawback is that it requires the UA to encode SDP and session information in Media Policy documents. RFC6794]). A subscriber MUST refresh a subscription with a SUBSCRIBE request before the last SUBSCRIBE request expires to avoid that the subscription times out. RFC6796] as a format for NOTIFY bodies. The SUBSCRIBE request MAY contain an Accept header field. If no such header field is present, it has a default value of "application/ media-policy-dataset+xml". If the header field is present, it MUST include "application/media-policy-dataset+xml", and it MAY include any other MIME type capable of representing session-specific
policies. As defined in RFC 6665 [RFC6665], the body of notifications MUST be in one of the formats defined in the Accept header of the SUBSCRIBE request or in the default format. If the notifier uses the same format in NOTIFY bodies that was used by the subscriber in the SUBSCRIBE body (e.g., "application/ media-policy-dataset+xml"), the notifier can expect that the subscriber supports all format extensions that were used in the SUBSCRIBE body. The notifier cannot assume that the subscriber supports other extensions beyond that and SHOULD NOT use such extensions. If the SUBSCRIBE request contained a representation of the local session description and the subscription was accepted, then the NOTIFY body MUST contain a policy for the local session description. If the SUBSCRIBE request of an accepted subscription contained the local and the remote session description, then the NOTIFY body MUST contain two policies: one for the local and one for the remote session description. RFC6665]. The subscriber MUST provide sufficient information in the SUBSCRIBE body to fully describe the session for which it seeks to receive session-specific policies. The subscriber MUST use the most recent session description as a basis for this information. If the "application/media-policy-dataset+xml" format is used in SUBSCRIBE bodies, the subscriber MUST provide a value for each field that is defined for session information documents [RFC6796] and for which the subscriber has information available. In other words, the subscriber MUST fill in the elements of a session information document as complete as possible. If the subscriber supports extensions of the "application/media-policy-dataset+xml" format, the subscriber MUST also provide a value for each field defined by this extension for session information documents, if possible. Providing as much information as possible avoids that a session is rejected due to a lack of session information and the negotiation of the information to be disclosed between notifier and subscriber. Subscriptions to this event package are typically created in conjunction with an SDP offer/answer exchange [RFC3264] during the establishment of a session (see the Session Policy Framework [RFC6794]). If used with an offer/answer exchange, the subscriber MUST insert the representation of the local session description in the SUBSCRIBE body. The local session description is the one that
was created by the subscriber (e.g., the offer if the subscriber has initiated the offer/answer exchange). Under certain circumstances, a UA may not have a session description when subscribing to session- specific policies, for example, when it is composing an empty INVITE request (i.e., an INVITE request that does not contain an offer). In these cases, a UA SHOULD establish a subscription without including a representation of the local session description. The UA MUST refresh the subscription with a SUBSCRIBE request that contains this session description as soon as the session description becomes available, for example, when the UA receives a 200 OK to an empty INVITE request. A policy server can choose to admit a session only after the UA has disclosed the session descriptions. The subscriber SHOULD also include a representation of the remote session description in the SUBSCRIBE body. The remote session description is the one the subscriber has received (i.e., the answer if the subscriber has initiated the offer/answer exchange). In some scenarios, the remote session description is not available to the subscriber at the time the subscription to session-specific policies is established. In this case, the initial SUBSCRIBE message SHOULD only contain a representation of the local session description. When the remote description becomes available, the subscriber SHOULD refresh the subscription by sending another SUBSCRIBE request, which then contains the local and the remote session description, unless the subscriber has received a NOTIFY request with the "local-only" parameter. This parameter indicates that the notifier does not need to see the remote session description. A user agent can change the session description of an ongoing session. A change in the session description will typically affect the policy decisions for this session. A subscriber MUST refresh the subscription to session-specific policies every time the session description of a session changes. It does this by sending a SUBSCRIBE request, which contains the details of the updated session descriptions. A subscriber may receive an error that indicates a server failure in response to a SUBSCRIBE request. In this case, the subscriber SHOULD try to locate an alternative server, for example, using the procedures described in [RFC3263]. If no alternative server can be located, the subscriber MAY continue with the session for which it wanted to receive session-specific policies without subscribing to session-specific policies. This is to avoid that a failed policy server prevents a UA from setting up or continuing with a session. Since the sessions created by the UA may not be policy compliant without this subscription, they may be blocked by policy enforcement mechanisms if they are in place.
Session policies can contain sensitive information. Moreover, policy decisions can significantly impact the behavior of a user agent. A user agent should therefore verify the identity of a policy server and make sure that policies have not been altered in transit. All implementations of this package MUST support Transport Layer Security (TLS) [RFC5246] and the Session Initiation Protocol Secure (SIPS) URI scheme. A subscriber SHOULD use SIPS URIs when subscribing to session-specific policies so that policies are transmitted over TLS. See Section 4. Section 4. The authorization policy is at the discretion of the administrator. In general, all users SHOULD be allowed to subscribe to the session- specific policies of their sessions. A subscription to this event package will typically be established by a device that needs to know about the policies for its sessions. However, subscriptions may also be established by applications (e.g., a conference server). In those cases, an authorization policy will typically be provided for these applications. Responding in a timely manner to a SUBSCRIBE request is crucial for this event package. A notifier must minimize the time needed for processing SUBSCRIBE requests and generating the initial NOTIFY request. This includes minimizing the time needed to generate an initial policy decision. In particular, a short response time is important for this event package since it minimizes the delay for fetching policies during an INVITE transaction and therefore reduces call setup time. In addition, subscriptions to session-specific policies can be established while the subscriber is in an INVITE transaction at a point where it has received the 200 OK but before sending the ACK. Delaying the creation of the initial NOTIFY request would delay the transmission of the ACK. A more detailed discussion of this scenario can be found in the Session Policy Framework [RFC6794]. A subscriber may not have disclosed enough information in the SUBSCRIBE request to enable the notifier to generate a policy decision. For example, a UA may have subscribed to session-specific policies without including the representation of a session description. The policy server SHOULD accept such a subscription.
The policy server SHOULD generate a NOTIFY request that includes the "insufficient-info" event package parameter. A NOTIFY request with this parameter indicates that a policy decision could not be made due to insufficient information. The body of such a NOTIFY request can either be empty or contain a policy decision document that provides hints about which information was missing. RFC6665]. In addition, a notifier MAY send a notification at any time during the subscription. Typically, it will send one every time the policy decision this subscription is for has changed. When and why a policy decision changes is entirely at the discretion of the administrator. A policy decision can change for many reasons. For example, a network may become congested due to an increase in traffic and reduce the bandwidth available to an individual user. Another example is a session that has been started during "business hours" and continues into "evening hours" where more bandwidth or video sessions are available to the user according to the service level agreement. Policy decisions are expressed in the format negotiated for the NOTIFY body (e.g., "application/media-policy-dataset+xml"). The policy document in a NOTIFY body MUST represent a complete policy decision. Notifications that contain the deltas to previous policy decisions or partial policy decisions are not supported in this event package. The notifier SHOULD terminate the subscription if the policy decision is to reject a session and if it can be expected that this decision will not change in the foreseeable future. The notifier SHOULD keep the subscription alive, if it rejects a session but expects that the session can be admitted soon. For example, if the session was rejected due to a temporary shortage of resources and the notifier expects that these resources will become available again shortly it should keep the subscription alive. The decision to reject a session is expressed in the policy decision document. A session is admitted by returning a policy decision document that requires some or no changes to the session. If the notifier has not received enough information to make a policy decision from the subscriber (e.g., because it did not receive a session description), the notifier SHOULD NOT terminate the subscription since it can be expected that the UA refreshes the subscription with a SUBSCRIBE request that contains more information. The notifier SHOULD generate a NOTIFY request with the "insufficient- info" event package parameter to indicate that a policy decision
could not be made due to insufficient information. This NOTIFY request can contain an empty body or a body that contains a policy decision document indicating which information was missing. Some session-specific policies do not require the disclosure of the remote session description to the notifier. If a notifier determines that this is the case after receiving a SUBSCRIBE request, the notifier SHOULD include the "local-only" event parameter in NOTIFY requests.
SHOULD refresh the subscription with more complete information as soon as the missing information (e.g., the session description) is available. A subscriber may receive an update to a policy decision for a session that is already established. The subscriber MUST apply the new policy decision to this session. If a UA decides that it does not want to apply the new policy decision, the UA MUST terminate the session. An updated policy decision may require the UA to generate a re-INVITE or UPDATE request in this session if the session description has changed or it may need to terminate this session. A policy update that requires a UA to terminate a session can, for example, be triggered by the user's account running out of credit or the detection of an emergency that requires the termination of non- emergency calls. If the subscriber receives a NOTIFY request that contains the "local- only" event parameter, the subscriber SHOULD NOT include the remote session description in subsequent SUBSCRIBE requests within this subscription. RFC6665]. RFC6794]), and it does not show messages for authentication on a transport or SIP level. These call flow examples are informative and not normative.
Implementers should consult the main text of this document for exact protocol details. Policy Server Alice Bob | | | |(1) SUBSCRIBE | | |<------------------| | |(2) 200 OK | | |------------------>| | |(3) NOTIFY | | |------------------>| | |(4) 200 OK | | |<------------------| | | |(5) INVITE | | |------------------>| | | | | |(6) 200 OK | | |<------------------| | |(7) ACK | | |------------------>| |(8) SUBSCRIBE | | |<------------------| | |(9) 200 OK | | |------------------>| | |(10) NOTIFY | | |------------------>| | |(11) 200 OK | | |<------------------| | | | | Message Details (1) SUBSCRIBE Alice -> Policy Server SUBSCRIBE sips:firstname.lastname@example.org SIP/2.0 Via: SIP/2.0/TLS pc.biloxi.example.com:5061 ;branch=z9hG4bK74bf Max-Forwards: 70 From: Alice <sips:email@example.com>;tag=8675309 To: PS <sips:firstname.lastname@example.org> Call-ID: email@example.com CSeq: 1 SUBSCRIBE Contact: <sips:firstname.lastname@example.org> Expires: 7200 Event: session-spec-policy Accept: application/media-policy-dataset+xml Content-Type: application/media-policy-dataset+xml
Content-Length: ... [Local session description (offer)] (2) 200 OK Policy Server -> Alice (3) NOTIFY Policy Server -> Alice NOTIFY sips:email@example.com SIP/2.0 Via: SIP/2.0/TLS srvr.biloxi.example.com:5061 ;branch=z9hG4bK74br Max-Forwards: 70 From: PS <sips:firstname.lastname@example.org>;tag=31451098 To: Alice <sips:email@example.com>;tag=8675309 Call-ID: firstname.lastname@example.org CSeq: 1 NOTIFY Event: session-spec-policy Subscription-State: active;expires=7200 Content-Type: application/media-policy-dataset+xml Content-Length: ... [Policy for local session description (offer)] (4) 200 OK Alice -> Policy Server (5) INVITE Alice -> Bob (6) 200 OK Bob -> Alice (7) ACK Alice -> Bob (8) SUBSCRIBE Alice -> Policy Server SUBSCRIBE sips:email@example.com SIP/2.0 Via: SIP/2.0/TLS pc.biloxi.example.com:5061 ;branch=z9hG4bKna998sl Max-Forwards: 70 From: Alice <sips:firstname.lastname@example.org>;tag=8675309 To: PS <sips:email@example.com>;tag=31451098 Call-ID: firstname.lastname@example.org CSeq: 2 SUBSCRIBE Expires: 7200 Event: session-spec-policy Accept: application/media-policy-dataset+xml Content-Type: application/media-policy-dataset+xml Content-Length: ...
[Local session description (offer)] [Remote session description (answer)] (9) 200 OK Policy Server -> Alice (10) NOTIFY Policy Server -> Alice NOTIFY sips:email@example.com SIP/2.0 Via: SIP/2.0/TLS srvr.biloxi.example.com:5061 ;branch=z9hG4bKna998sk Max-Forwards: 70 From: PS <sips:firstname.lastname@example.org>;tag=31451098 To: Alice <sips:email@example.com>;tag=8675309 Call-ID: firstname.lastname@example.org CSeq: 2 NOTIFY Event: session-spec-policy Subscription-State: active;expires=7200 Content-Type: application/media-policy-dataset+xml Content-Length: ... [Policy for local session description (offer)] [Policy for remote session description (answer)] F6 200 OK Alice -> Policy Server
It is therefore important to secure the communication between the user agent and the policy server. The following three discrete attributes need to be protected: 1. authentication of the policy server and, if needed, the user agent, 2. confidentiality of the messages exchanged between the user agent and the policy server, and 3. ensuring that private information is not exchanged between the two parties, even over a confidentiality-assured and authenticated session. Authentication of the peers and protecting the confidentiality of the policies in transit is achieved by existing SIP security mechanisms (the use of TLS and SIPS URI scheme [RFC3261], [RFC5630]). Accordingly, policy servers SHOULD be addressable only through a SIPS URI. Policy server and user agent MUST support TLS. The confidentiality of the communication between the policy server and the user agent will be assured as long as the policy server supports TLS and is reached through a SIPS URI. Authenticating the two parties can be performed using X.509 certificates exchanged through TLS and other techniques such as HTTP Digest. When the user agent establishes a TLS session with the policy server, the policy server will present it with an X.509 certificate. The user agent SHOULD ensure that the identity of the policy server encoded in the certificate matches the URI of the policy server the user agent has received either using the Session Policy Framework [RFC6794] or other means such as configuration. When a policy server receives a new subscription (as opposed to a refresh subscription), the policy server SHOULD try to authenticate the user agent using any means at its disposal. If the user agent has an X.509 certificate suitable for use with TLS, the identity of the user agent SHOULD be contained in the certificate, or, if the user agent does not possess a certificate, the policy server SHOULD challenge the user agent using HTTP Digest. A policy server may frequently encounter UAs it cannot authenticate. In these cases, the policy server MAY provide a generic policy that does not reveal sensitive information to these UAs. If the subscriber and notifier desire to protect the integrity of the policy exchange in an end-to-end manner, they MAY use S/MIME to protect the session policies. However, RFC3261 cautions that "[i]mplementers should note, however, that there may be rare network
intermediaries (not typical proxy servers) that rely on viewing or modifying the bodies of SIP messages (especially SDP), and that secure MIME may prevent these sorts of intermediaries from functioning" [RFC3261]. And finally, the fact that the user agent and the policy server have successfully authenticated each other and have established a secure TLS session does not absolve either one from ensuring that they do not communicate sensitive information. For example, a session description may contain sensitive information -- session keys, for example -- that the user agent may not want to share with the policy server; and indeed, the policy server does not need such information to effectively formulate a policy. Thus, the user agent should not insert such sensitive information in a session information document that it sends to the policy server. Likewise, the policy server may have information that is sensitive and of no use to the user agent -- network service level agreements, or network statistics, for example. Thus, the policy server should refrain from transmitting such information to the user agent. RFC6665]. Package Name: session-spec-policy Package or Template-Package: This is a package. Published Document: RFC 6795. Person to Contact: Volker Hilt, email@example.com. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3261] 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.
[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol (SIP): Locating SIP Servers", RFC 3263, June 2002. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008. [RFC6665] Roach, A., "SIP-Specific Event Notification", RFC 6665, July 2012. [RFC6794] Hilt, V., Camarillo, G., and J. Rosenberg, "A Framework for Session Initiation Protocol (SIP) Session Policies", RFC 6794, December 2012. [RFC6796] Hilt, V., Camarillo, G., Rosenberg, J., and D. Worley, "A User Agent Profile Data Set for Media Policy", RFC 6796, December 2012. [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [RFC5630] Audet, F., "The Use of the SIPS URI Scheme in the Session Initiation Protocol (SIP)", RFC 5630, October 2009.