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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
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 (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.
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RFC 6795 Session Policy Event Package December 2012
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.
Hilt & Camarillo Standards Track [Page 2]
RFC 6795 Session Policy Event Package December 2012
"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].
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RFC 6795 Session Policy Event Package December 2012
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
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RFC 6795 Session Policy Event Package December 2012
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
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RFC 6795 Session Policy Event Package December 2012
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
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RFC 6795 Session Policy Event Package December 2012
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.
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RFC 6795 Session Policy Event Package December 2012
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.
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RFC 6795 Session Policy Event Package December 2012
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
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RFC 6795 Session Policy Event Package December 2012
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
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RFC 6795 Session Policy Event Package December 2012
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.
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RFC 6795 Session Policy Event Package December 2012
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
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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: ...
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[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.
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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
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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.
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[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.
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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
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