Internet DRAFT - draft-ietf-sipping-policy-package
draft-ietf-sipping-policy-package
SIPPING Working Group V. Hilt
Internet-Draft Bell Labs/Alcatel-Lucent
Intended status: Standards Track G. Camarillo
Expires: September 24, 2010 Ericsson
March 23, 2010
A Session Initiation Protocol (SIP) Event Package for Session-Specific
Session Policies.
draft-ietf-sipping-policy-package-08
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.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 24, 2010.
Copyright Notice
Copyright (c) 2010 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
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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 . . . . . . . . . . . . . . . . . . 5
3.5. NOTIFY Bodies . . . . . . . . . . . . . . . . . . . . . . 6
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 . . . . . . . . . . . . . . . . . . 12
3.12. State Agents . . . . . . . . . . . . . . . . . . . . . . . 12
3.13. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 12
4. Security Considerations . . . . . . . . . . . . . . . . . . . 15
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
5.1. Event Package Name . . . . . . . . . . . . . . . . . . . . 17
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.1. Normative References . . . . . . . . . . . . . . . . . . . 17
6.2. Informative References . . . . . . . . . . . . . . . . . . 18
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Introduction
The Framework for Session Initiation Protocol (SIP) [RFC3261] Session
Policies [I-D.ietf-sip-session-policy-framework] 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 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 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 all the SIP sessions set up by a user agent.
The Framework for SIP Session Policies
[I-D.ietf-sip-session-policy-framework] defines a mechanism that
enables UAs to discover the URIs of session-specific policy servers.
This specification defines a SIP event package [RFC3265] 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
[I-D.ietf-sip-session-policy-framework]):
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).
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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 and all changes to this decision in subsequent
NOTIFYs.
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 3265 [RFC3265].
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 Section 3.6,
Section 3.8 and Section 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 Section 3.7,
Section 3.8 and Section 3.9.
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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 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.
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 the User Agent
Profile Data Set for Media Policy
[I-D.ietf-sipping-media-policy-dataset]. 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 SDP [RFC4566] instead
of encoding the session descriptions in the Media Policy
[I-D.ietf-sipping-media-policy-dataset] 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) it 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
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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
[I-D.ietf-sip-session-policy-framework]). 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" [I-D.ietf-sipping-media-policy-dataset] 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 MAY include any
other MIME type capable of representing session-specific policies.
As defined in RFC 3265 [RFC3265], 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 3265 [RFC3265]. The subscriber MUST provide
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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
[I-D.ietf-sipping-media-policy-dataset] 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
[I-D.ietf-sip-session-policy-framework]). 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
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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 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 TLS [RFC5246] and the
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
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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.
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 the Session Policy Framework
[I-D.ietf-sip-session-policy-framework].
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 3265 [RFC3265]. 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
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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 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 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
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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 which 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 users 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 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 [RFC3265].
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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
[I-D.ietf-sip-session-policy-framework]) 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 | |
|------------------>| |
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|(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)]
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(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)]
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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
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policy server, the policy server will present it a 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
[I-D.ietf-sip-session-policy-framework] 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
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5.1. Event Package Name
This specification registers an event package, based on the
registration procedures defined in RFC 3265 [RFC3265]. The following
is the information required for such a registration:
Package Name: session-spec-policy
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.
6. References
6.1. Normative References
[I-D.ietf-sip-session-policy-framework]
Hilt, V., Camarillo, G., and J. Rosenberg, "A Framework
for Session Initiation Protocol (SIP) Session Policies",
draft-ietf-sip-session-policy-framework-07 (work in
progress), February 2010.
[I-D.ietf-sipping-media-policy-dataset]
Hilt, V., Worley, D., Camarillo, G., and J. Rosenberg, "A
User Agent Profile Data Set for Media Policy",
draft-ietf-sipping-media-policy-dataset-09 (work in
progress), March 2010.
[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.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
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Internet-Draft Session Policy Event Package March 2010
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
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 draft. Many thanks to Roni Even, Bob Penfield, Mary Barnes,
Shida Schubert and Jon Peterson for reviewing the draft and to Vijay
Gurbani for the contributions to the security considerations section.
Authors' Addresses
Volker Hilt
Bell Labs/Alcatel-Lucent
791 Holmdel-Keyport 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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