SIPPING Working Group V. Hilt
Internet-Draft Bell Labs/Lucent Technologies
Expires: December 27, 2006 G. Camarillo
Ericsson
June 25, 2006
A Session Initiation Protocol (SIP) Event Package for Session-Specific
Session Policies.
draft-ietf-sipping-policy-package-01
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Copyright (C) The Internet Society (2006).
Abstract
This specification defines a Session Initiation Protocol (SIP) event
package for session-specific policies. This event package enables
user agents to subscribe to session policies for a SIP session and to
receive notifications if these policies change.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Event Package Formal Definition . . . . . . . . . . . . . . . 4
3.1. Event Package Name . . . . . . . . . . . . . . . . . . . . 4
3.2. Event Package Parameters . . . . . . . . . . . . . . . . . 4
3.3. SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . 5
3.4. Subscription Duration . . . . . . . . . . . . . . . . . . 6
3.5. NOTIFY Bodies . . . . . . . . . . . . . . . . . . . . . . 6
3.6. Subscriber generation of SUBSCRIBE requests . . . . . . . 7
3.7. Notifier processing of SUBSCRIBE requests . . . . . . . . 7
3.8. Notifier generation of NOTIFY requests . . . . . . . . . . 8
3.9. Subscriber processing of NOTIFY requests . . . . . . . . . 9
3.10. Handling of forked requests . . . . . . . . . . . . . . . 9
3.11. Rate of notifications . . . . . . . . . . . . . . . . . . 9
3.12. State Agents . . . . . . . . . . . . . . . . . . . . . . . 10
3.13. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 10
4. Security Considerations . . . . . . . . . . . . . . . . . . . 12
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
5.1. Event Package Name . . . . . . . . . . . . . . . . . . . . 13
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 14
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Normative References . . . . . . . . . . . . . . . . . . . 14
6.2. Informative References . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 16
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1. Introduction
The Framework for Session Initiation Protocol (SIP) [5] Session
Policies [6] specifies a protocol framework for session policies.
The framework enables a proxy to define and impact policies on
sessions such as the codecs or media types to be used. More details
on session policies can be found in [6].
Two types of session policies exist: 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 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 with external ones. Since session-
specific policies are tailored to a session, they only apply to the
session they are created for. A user agent 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 independent
of a session and generally apply to the SIP sessions set up by a user
agent (see [6]).
The Framework for SIP Session Policies [6] defines a mechanism that
enables UAs to discover the URIs of session-specific policy servers.
This specification defines a mechanism that enables UAs to contact
policy servers, provide information about the current session to the
policy server and to receive session policies and updates to these
policies in response. The mechanism is realized by enabling UAs to
subscribe to the session-specific policies on a policy server. This
specification defines a SIP event package [4] for subscriptions to
session-specific policies.
Subscribing to session-specific policies involves the following steps
(see [6]):
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
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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, for example, 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
abstract 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. This policy decision is the resource to
which the subscriber is subscribing. The notifier conveys the
initial policy decision to the subscriber in a NOTIFY request and all
changes to this decision in subsequent NOTIFY requests.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [1] and indicate requirement levels for
compliant implementations.
3. Event Package Formal Definition
This document provides the details for defining a SIP event package
as required by RFC 3265 [4].
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 optional event package parameter "local-
only". This parameter is only defined for NOTIFY requests and MUST
be ignored if received in a SUBSCRIBE request. The usage of the
"local-only" parameter is described in Section 3.3, Section 3.8 and
Section 3.9.
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3.3. SUBSCRIBE Bodies
A SUBSCRIBE for the session-specific policy package SHOULD contain a
body that describes a SIP session. The purpose of this body is to
enable the notifier to generate the policies the subscriber is
interested in. 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. 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/session-policy+xml" as defined in the User Agent Profile
Data Set for Media Policy [3]. The "application/session-policy+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.
OPEN ISSUE: this is a significant change from the previous version
of the draft where the SUBSCRIBE body contained a session
description in SDP format. Using an XML based policy format has a
number of advantages: i) it is more flexible and enables the
inclusion of information that can't be expressed via SDP (e.g. the
target URI), ii) it 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)
aligns the formats used for session-specific and session-
independent policies. However, a drawback is that it requires the
UA to generate these XML documents instead of simply inserting the
session description.
Note: the "application/session-policy+xml" format does not yet
support all functionality needed for the use in SUBSCRIBE bodies.
Subscriptions to the session-specific policy package are typically
created in conjunction with an SDP offer/answer exchange [7] during
the establishment of a session (see [6]). If used with an offer/
answer exchange, the subscriber SHOULD 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).
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
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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. The
subscriber MAY skip sending the remote session description to the
notifier if it has received a NOTIFY with the "local-only" parameter.
A notifier will typically include this parameter in a NOTIFY when it
has received the local session description and does not need to see
the remote session description.
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 (it may, of course, be terminated
earlier). A typical duration of a phone call is a few minutes.
Since the duration of a subscription to the session-specific policy
package is closely related to the lifetime of the corresponding
session, the value for the duration of a subscription is largely
irrelevant. However, it SHOULD be longer than the typical duration
of a session. The default subscription duration for this event
package is set to two hours.
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/session-policy+xml"
[3] 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/
session-policy+xml". If the header field is present, it MUST include
"application/session-policy+xml", and MAY include any other MIME type
capable of representing session-specific policies. As defined RFC
3265 [4], 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/
session-policy+xml"), the notifier can expect that the subscriber
supports all format extensions that were used in the SUBSCRIBE body.
However, the notifier cannot assume that the subscriber supports
other extensions beyond that. If the notifier uses other format
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extensions, it cannot count on the fact that they will be understood
by the subscriber. The rationale behind this is that the notifier
will often return a modified version of the document that was
submitted by the subscriber.
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 [4]. The subscriber SHOULD include enough
information in the SUBSCRIBE body to accurately describe the session
for which it seeks to receive session-specific policies. It SHOULD
use the most recent session description if multiple versions are
available.
OPEN ISSUE: is there a need to define a basic set of elements a
subscriber should try to include (if known/applicable)?
A user agent can, of course, 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 SHOULD
therefore refresh the subscription to session-specific policies every
time the session description of a session changes. It does so by
sending a SUBSCRIBE request, which contains the details of the
updated session descriptions.
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 TLS [2] and the SIPS URI
scheme. A subscriber SHOULD use SIPS URIs, if possible, when
subscribing to session-specific policies so that policies are
transmitted over TLS. If possible, subscribers SHOULD perform server
authentication, for example, via TLS or another transport mechanism.
3.7. Notifier processing of SUBSCRIBE requests
All subscriptions to session-specific policies SHOULD be
authenticated and authorized before approval. The notifier SHOULD
authenticate the subscriber using any of the techniques available
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through SIP, including digest, S/MIME, TLS or other transport
specific mechanisms. Administrators SHOULD use a SIPS URI as a
policy server URI.
The authorization policy is at the discretion of the administrator.
It is RECOMMENDED that all users are 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 and automata (e.g. a conference
server). In those cases, an authorization policy will typically be
provided for these applications.
Responding timely 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. This includes
minimizing the time needed to generate an initial policy decision. A
short response time is in particular 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 would delay the transmission of the
ACK (a more detailed discussion of this scenario can be found in
[6]).
3.8. Notifier generation of NOTIFY requests
A notifier sends a notification in response to SUBSCRIBE requests as
defined in RFC 3265 [4]. 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 change in the policy decision may
be triggered, for example, by a change in the network status, a
change in the services used by the user or by an update of the
service level agreement.
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 policy decision to reject a session is expressed by returning an
empty NOTIFY body. The notifier MAY terminate the subscription after
sending such a notification if it can be expected that this decision
will not change in the foreseeable future. The notifier SHOULD keep
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the subscription alive, if it expects that the session can be
admitted at a later point in time. A session is admitted by
returning a policy decision document that requires some or no changes
to the session. The decision to admit a session and possibly the
changes needed are expressed in the format negotiated for the NOTIFY
body (e.g. "application/session-policy+xml").
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, it MAY
include the "local-only" event parameter in NOTIFY requests.
3.9. Subscriber processing of NOTIFY requests
A subscriber SHOULD apply the policy decision received in a NOTIFY to
the session associated with this subscription.
If the subscriber receives a notification with an empty body, the
session has been rejected. The subscriber SHOULD NOT attempt to
establish this session. However, the subscriber MAY still keep up
the subscription to session-specific policies for this session since
the policy decision may change and the session may be admitted at a
later time. If the notifier has terminated the subscription, the
subscriber SHOULD NOT try to re-subscribe for the same session.
A subscriber may receive an update to a policy decision for a session
that is already established. The subscriber SHOULD apply the new
policy decision to this session. It may need 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.
If the subscriber receives a NOTIFY that contains the "local-only"
event parameter, it MAY stop inserting the remote session description
in SUBSCRIBE requests within this subscription. It MAY skip
refreshing the subscription in order to convey information about the
remote session description to the notifier.
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 [4].
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.
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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 [6]) and it does not show messages for authentication
on 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
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(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/session-policy+xml
Content-Type: application/session-policy+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/session-policy+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
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(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/session-policy+xml
Content-Type: application/session-policy+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/session-policy+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
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agent and can therefore be used by an attacker to compromise a user
agent. For example, session policies can be used to set up a user
agent so that it is unable to successfully establish 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 will cause the UA
to terminate the session.
A user agent transmits session information to a policy server. This
session information may contain sensitive data the user may not want
an eavesdropper or an unauthorized policy server to see. In
particular, 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.
To prevent these attacks, a subscriber using this event package
SHOULD authenticate the notifier (i.e. the policy server) before
disclosing session information or accepting a session policy. This
requires the subscriber to perform server authentication which can be
done, for example, via TLS or another transport mechanism. A
subscriber SHOULD use SIPS URIs, if possible, when subscribing to
session-specific policy events so that policies are transmitted over
TLS.
Similarly, notifiers SHOULD authenticate subscribers using any of the
techniques available through SIP, including digest, S/MIME, TLS or
other transport specific mechanisms. Administrators SHOULD use SIPS
URIs as policy server URIs.
A session description may contain sensitive information a subscriber
does not want to share with the notifier. For example, a user agent
may not want to share the media encryption keys with the policy
server. The subscriber should therefore ensure that it is only
sending session information to the notifier that it is willing to
disclose.
5. IANA Considerations
5.1. Event Package Name
This specification registers an event package, based on the
registration procedures defined in RFC 3265 [2]. The following is
the information required for such a registration:
Package Name: session-spec-policy
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Package or Template-Package: This is a package.
Published Document: RFC XXXX (Note to RFC Editor: Please fill in XXXX
with the RFC number of this specification).
Person to Contact: Volker Hilt, volkerh@bell-labs.com.
Appendix A. Acknowledgements
Many thanks to Jonathan Rosenberg for the many discussions and
suggestions for this draft.
6. References
6.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[3] Hilt, V., Camarillo, G., and J. Rosenberg, "A User Agent Profile
Data Set for Media Policy",
draft-ietf-sipping-media-policy-dataset-01 (work in progress),
March 2006.
[4] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[5] 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.
6.2. Informative References
[6] Hilt, V., Camarillo, G., and J. Rosenberg, "A Framework for
Session Initiation Protocol (SIP) Session Policies",
draft-ietf-sipping-session-policy-framework-01 (work in
progress), March 2006.
[7] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
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Authors' Addresses
Volker Hilt
Bell Labs/Lucent Technologies
101 Crawfords Corner Rd
Holmdel, NJ 07733
USA
Email: volkerh@bell-labs.com
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Gonzalo.Camarillo@ericsson.com
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Intellectual Property Statement
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Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
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Hilt & Camarillo Expires December 27, 2006 [Page 16]
