A Session Initiation Protocol (SIP) Event Package for

Session-Specific Policies

Abstract

This 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 © 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.

Table of Contents

   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

1. Introduction

The Framework for Session Initiation Protocol (SIP) [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.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

3. Event Package Formal Definition

This document provides the details for defining a SIP event package as required by RFC 6665 [RFC6665].

3.1. Event Package Name

The name of the event package defined in this specification is "session-spec-policy".

3.2. Event Package Parameters

This package defines the following two event package parameters:

   local-only:  The "local-only" parameter is optional and only defined
   
      for NOTIFY requests.  The "local-only" parameter indicates that
      the remote session description is not required by the notifier.
      It MUST be ignored if received in a SUBSCRIBE request.  The usage
      of the "local-only" parameter is described in Sections 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.

3.3. SUBSCRIBE Bodies

A SUBSCRIBE for this event package MUST contain a body that describes a SIP session. The purpose of this body is to enable the notifier to generate the policies in which the subscriber is interested. In this event package, the Request-URI, the event package name, and event parameters are not sufficient to determine the resource a subscription is for. However, with the session description in the SUBSCRIBE body, the notifier can generate the requested policy decision and create policy events for this resource.

All subscribers and notifiers MUST support the MIME type "application/media-policy-dataset+xml" as defined in "A User Agent Profile Data Set for Media Policy" [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.

3.4. Subscription Duration

A subscription to the session-specific policy package is usually established at the beginning of a session and terminated when the corresponding session ends. A typical duration of a phone call is a few minutes.

Since the duration of a subscription to the session-specific policy package is related to the lifetime of the corresponding session, the value for the duration of a subscription is largely irrelevant. However, the duration SHOULD be longer than the typical duration of a session. The default subscription duration for this event package is set to two hours.

A subscription MAY be terminated before a session ends by the notifier. For example, a notifier may terminate the subscription after the initial policy notification has been sent to the subscriber if it knows that these policies will not change during the session. A subscriber MUST NOT terminate a subscription unless it is terminating the session this subscription is for or discovers that the notifier has been removed from the list of policy servers relevant for this session (see the Session Policy Framework [RFC6794]). A subscriber MUST refresh a subscription with a SUBSCRIBE request before the last SUBSCRIBE request expires to avoid that the subscription times out.

3.5. NOTIFY Bodies

In this event package, the body of a notification contains the session policy requested by the subscriber. All subscribers and notifiers MUST support the format "application/ media-policy-dataset+xml" [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.

3.6. Subscriber Generation of SUBSCRIBE Requests

The subscriber follows the general rules for generating SUBSCRIBE requests defined in RFC 6665 [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.

3.7. Notifier Processing of SUBSCRIBE Requests

All subscriptions to session-specific policies SHOULD be authenticated and authorized before approval. However, 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. For details, see 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.

3.8. Notifier Generation of NOTIFY Requests

A notifier sends a notification in response to SUBSCRIBE requests as defined in RFC 6665 [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.

3.9. Subscriber Processing of NOTIFY Requests

A subscriber MUST apply the policy decision received in a NOTIFY request to the session associated with this subscription. If the UA decides not to apply the received policy decision, the UA MUST NOT set up the session and MUST terminate the session if the session is already in progress. If the UA has a pending INVITE transaction for this session, the UA MUST cancel or reject the INVITE request.

If the subscriber receives a NOTIFY request indicating that the session has been rejected, the subscriber MUST NOT attempt to establish this session. If the notifier has terminated the subscription after rejecting the session, the subscriber SHOULD NOT try to re-send the same SUBSCRIBE request again. The termination of the subscription by the notifier indicates that the policy decision for this session is final and will not change in the foreseeable future. The subscriber MAY try to re-subscribe for this session if at least one aspect of the session (e.g., a parameter in the session description or the target URI) has changed or if there is other reason to believe that re-trying the subscription will be successful (e.g., because time has progressed significantly since the last attempt).

The notifier may keep up the subscription after rejecting a session to indicate that it may send an updated policy decision for this session to the subscriber at a later time. This is useful, for example, if the session was rejected due to a temporary shortage of resources and the notifier expects that this problem to be resolved shortly. In another example, the session was rejected because it was attempted in a restricted period during the day but this period is going to end soon. In this case, the subscriber SHOULD not terminate the subscription to session-specific policies.

The subscriber may receive a NOTIFY request that contains an "insufficient-info" event package parameter to indicate that the SUBSCRIBE request did not contain enough information. The subscriber

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.

3.10. Handling of Forked Requests

This event package allows the creation of only one dialog as a result of an initial SUBSCRIBE request. The techniques to achieve this behavior are described in [RFC6665].

3.11. Rate of Notifications

It is anticipated that the rate of policy changes will be very low. In any case, notifications SHOULD NOT be generated at a rate of more than once every five seconds.

3.12. State Agents

State agents play no role in this package.

3.13. Examples

The following message flow illustrates how a user agent (Alice's phone) can subscribe to session-specific policies when establishing a call (here to Bob's phone). The flow assumes that the user agent has already received the policy server URI (e.g., through configuration or as described in the Session Policy Framework [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:policy@biloxi.example.com SIP/2.0
        Via: SIP/2.0/TLS pc.biloxi.example.com:5061
         ;branch=z9hG4bK74bf
        Max-Forwards: 70
        From: Alice <sips:alice@biloxi.example.com>;tag=8675309
        To: PS <sips:policy@biloxi.example.com>
        Call-ID: rt4353gs2egg@pc.biloxi.example.com
        CSeq: 1 SUBSCRIBE
        Contact: <sips:alice@pc.biloxi.example.com>
        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:alice@pc.biloxi.example.com SIP/2.0
        Via: SIP/2.0/TLS srvr.biloxi.example.com:5061
         ;branch=z9hG4bK74br
        Max-Forwards: 70
        From: PS <sips:policy@biloxi.example.com>;tag=31451098
        To: Alice <sips:alice@biloxi.example.com>;tag=8675309
        Call-ID: rt4353gs2egg@pc.biloxi.example.com
        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:policy@biloxi.example.com SIP/2.0
        Via: SIP/2.0/TLS pc.biloxi.example.com:5061
         ;branch=z9hG4bKna998sl
        Max-Forwards: 70
        From: Alice <sips:alice@biloxi.example.com>;tag=8675309
        To: PS <sips:policy@biloxi.example.com>;tag=31451098
        Call-ID: rt4353gs2egg@pc.biloxi.example.com
        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:alice@pc.biloxi.example.com SIP/2.0
        Via: SIP/2.0/TLS srvr.biloxi.example.com:5061
         ;branch=z9hG4bKna998sk
        Max-Forwards: 70
        From: PS <sips:policy@biloxi.example.com>;tag=31451098
        To: Alice <sips:alice@biloxi.example.com>;tag=8675309
        Call-ID: rt4353gs2egg@pc.biloxi.example.com
        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

4. Security Considerations

Session policies can significantly change the behavior of a user agent and can therefore be used by an attacker to compromise a user agent. For example, session policies can be used to prevent a user agent from successfully establishing a session (e.g., by setting the available bandwidth to zero). Such a policy can be submitted to the user agent during a session, which may cause the UA to terminate the session.

A user agent transmits session information to a policy server. This information may contain sensitive data the user may not want an eavesdropper or an unauthorized policy server to see. For example, the session information may contain the encryption keys for media streams. Vice versa, session policies may also contain sensitive information about the network or service level agreements the service provider may not want to disclose to an eavesdropper or an unauthorized user agent.

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.

5. IANA Considerations

5.1. Event Package Name

This specification registers an event package as follows, based on the registration procedures defined in RFC 6665 [RFC6665].

   Package Name: session-spec-policy

Package or Template-Package: This is a package.

Published Document: RFC 6795.

Person to Contact: Volker Hilt, volker.hilt@bell-labs.com.

6. References

6.1. Normative References

   [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.

6.2. Informative References

   [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.

Appendix A. Acknowledgements

Many thanks to Jonathan Rosenberg for the discussions and suggestions for this document. Many thanks to Roni Even, Bob Penfield, Mary Barnes, Shida Schubert and Jon Peterson for reviewing the document and to Vijay Gurbani for the contributions to the Security Considerations section.

Authors' Addresses

Volker Hilt
Bell Labs/Alcatel-Lucent
Lorenzstrasse 10
70435 Stuttgart
Germany

   EMail: volker.hilt@bell-labs.com
   
   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland
   
   EMail: Gonzalo.Camarillo@ericsson.com