One document matched: draft-ietf-sip-session-policy-framework-00.txt
SIPPING Working Group V. Hilt
Internet-Draft Bell Labs/Lucent Technologies
Expires: April 16, 2007 G. Camarillo
Ericsson
J. Rosenberg
Cisco Systems
October 13, 2006
A Framework for Session Initiation Protocol (SIP) Session Policies
draft-ietf-sip-session-policy-framework-00
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Copyright (C) The Internet Society (2006).
Abstract
Proxy servers play a central role as an intermediary in the Session
Initiation Protocol (SIP) as they define and impact policies on call
routing, rendezvous, and other call features. This document
specifies a framework for SIP session policies that provides a
standard mechanism by which a proxy can define or influence policies
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on sessions, such as the codecs or media types to be used. It
defines a model, an overall architecture and new protocol mechanisms
for session policies.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Session-Independent Policies . . . . . . . . . . . . . . . . . 5
3.1. Architecture and Overview . . . . . . . . . . . . . . . . 5
3.2. Policy Subscription . . . . . . . . . . . . . . . . . . . 6
4. Session-Specific Policies . . . . . . . . . . . . . . . . . . 7
4.1. Architecture . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3.1. Offer in Request . . . . . . . . . . . . . . . . . . . 10
4.3.2. Offer in Response . . . . . . . . . . . . . . . . . . 12
4.4. UA/Policy Server Rendezvous . . . . . . . . . . . . . . . 13
4.4.1. UAC Behavior . . . . . . . . . . . . . . . . . . . . . 13
4.4.2. Proxy Behavior . . . . . . . . . . . . . . . . . . . . 14
4.4.3. UAS Behavior . . . . . . . . . . . . . . . . . . . . . 15
4.4.4. Caching Policy Server URIs . . . . . . . . . . . . . . 15
4.4.5. Storing Policy Server URIs in a Dialog . . . . . . . . 16
4.4.6. Contacting the Policy Server . . . . . . . . . . . . . 17
4.4.7. Header Definition and Syntax . . . . . . . . . . . . . 18
4.5. Policy Subscription . . . . . . . . . . . . . . . . . . . 19
5. Security Considerations . . . . . . . . . . . . . . . . . . . 19
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
6.1. Registration of the "Policy-Id" Header . . . . . . . . . . 21
6.2. Registration of the "Policy-Contact" Header . . . . . . . 21
6.3. Registration of the "policy" SIP Option-Tag . . . . . . . 21
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 21
Appendix B. Session-Specific Policies - Call Flows . . . . . . . 21
B.1. Offer in Invite . . . . . . . . . . . . . . . . . . . . . 22
B.2. Offer in Response . . . . . . . . . . . . . . . . . . . . 24
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
7.1. Normative References . . . . . . . . . . . . . . . . . . . 25
7.2. Informative References . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
Intellectual Property and Copyright Statements . . . . . . . . . . 27
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1. Introduction
The Session Initiation Protocol (SIP) [6] is a signaling protocol for
creating, modifying and terminating multimedia sessions. A central
element in SIP is the proxy server. Proxy servers are intermediaries
that are responsible for request routing, rendezvous, authentication
and authorization, mobility, and other signaling services. However,
proxies are divorced from the actual sessions - audio, video, and
messaging - that SIP establishes. Details of the sessions are
carried in the payload of SIP messages, and are usually described
with the Session Description Protocol (SDP) [7]. Indeed, SIP
provides end-to-end encryption features using S/MIME, so that all
information about the sessions can be hidden from eavesdroppers and
proxies alike.
However, experience has shown that there is a need for SIP
intermediaries to impact aspects of a session. For example, SIP may
be used in a wireless network, which has limited resources for media
traffic. During periods of high activity, the wireless network
provider wants to restrict the amount of bandwidth available to each
individual user. With session policies, an intermediary in the
wireless network can inform the user agent about the bandwidth it can
currently count on. This information enables the user agent to make
an informed decision about the number of streams, the media types,
and the codecs it can successfully use in a session. Similarly, a
network provider may have a service level agreement with a user that
defines the set of media types a user can use. With session
policies, the network can convey the current set of policies to user
agents, enabling them to set up sessions without inadvertently
violating any of the network policies.
In another example, a SIP user agent is using a network which is
connected to the public Internet through a firewall or a network
border device. The network provider would like to tell the user
agent that it needs to send its media streams to a specific IP
address and port on the firewall or border device to reach the public
Internet. Knowing this policy enables the user agent to set up
sessions across the firewall or the network border. In contrast to
other methods for inserting a media intermediary, the use of session
policies does not require the inspection or modification of SIP
message bodies.
Domains often enforce the session policies they have in place. For
example, a domain might have a policy that disallows the use of video
and may enforce this policy by dropping all packets that contain a
video encoding. Unfortunately, enforcement mechanisms usually do not
inform the user about the policies they are enforcing. Instead, they
silently keep the user from doing anything against them. This may
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lead to a malfunctioning of devices that is incomprehensible to the
user. With session policies, the user knows about the current
network policies and can set up policy-compliant sessions or simply
connect to a domain with less stringent policies. Thus, session
policies provide an important combination of consent coupled with
enforcement. That is, the user becomes aware of the policy and needs
to act on it, but the provider still retains the right to enforce the
policy.
Two types of session policies exist: session-specific policies 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
examine the session description a UA is proposing and to return a
policy specifically 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. Session-specific policies are created on a
session-by-session basis at the time the session is established.
Session-independent policies on the other hand are policies that are
created independent of a session and generally apply to all SIP
sessions set up by a user agent. A session-independent policy can,
for example, be used to inform user agents about an existing
bandwidth limit or media type restrictions. Since these policies are
not based on a specific session description, they can be created
independent of an attempt to set up a session and only need to be
conveyed to the user agent when it initializes (e.g. at the time the
device is powered on) and when the policies are changed.
This specification defines a framework for SIP session policies. It
specifies a model, the overall architecture and new protocol
mechanisms that are needed for session-independent and session-
specific policies.
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.
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3. Session-Independent Policies
Session-independent policies are policies that are created
independent of a session and generally apply to all sessions a user
agent is setting up. They typically remain stable for a longer
period of time and apply to any session set up while they are valid.
However, session-independent policies may also change over time. For
example, a policy that defines a bandwidth limit for a user may
change during the day, defining a lower limit during peak hours and
allow more bandwidth off-peak.
3.1. Architecture and Overview
+-------------+
/------| policy |
+----+ / | server 1 |
| |---/ +-------------+
| UA | ...
| |---\ +-------------+
+----+ \ | policy |
\------| server n |
+-------------+
Figure 1
A SIP UA may receive session-independent policies from one or more
policy servers. In a typical configuration, a UA receives session-
independent policies from a policy server in the access or local
network domain (i.e. the domain from which the UA receives IP
service) and possibly the home network domain (i.e. the domain the UA
registers at). The local network may have policies that support the
access network infrastructure. For example, in a wireless network
where bandwidth is scarce, a provider may restrict the bandwidth
available to an individual user. The home network may have policies
that are needed to support services or policies that reflect the
service level agreement with the user. Thus, in most cases, a UA
will receive session-independent policies from one or two policy
servers.
Setting up session-independent policies involves the following steps:
1. A user agent requests session-independent policies from the
policy servers in the local network and home domain. A user
agent typically requests these policies when it starts up or
connects to a new network domain.
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2. The policy server selects the policies that apply to this user
agent. The policy server may have general policies that apply to
all users or maintain separate policies for each individual user.
The selected policies are returned to the user agent.
3. The policy server may update the policies, for example, when
network conditions change.
3.2. Policy Subscription
A UA requests session-independent policies by subscribing to session-
independent policies on the policy server in a domain. Subscriptions
to session-independent policies are established using the "ua-
profile" event package defined in the Framework for SIP User Agent
Profile Delivery [4].
The "ua-profile" event package [4] provides a mechanism to discover
policy servers in the local network and the home domain. The "local-
network" profile-type enables a UA to discover a policy server in the
local domain. The "user" profile type enables the discovery of a
policy server in the home domain. A UA compliant to this
specification SHOULD attempt to discover and subscribe to the policy
servers in these two domains.
A UA SHOULD (re-)subscribe to session-independent policies when the
following events occur:
o The UA registers a new address-of-record (AoR) or removes a AoR
from the set of AoRs it has registered. In these cases, the UA
SHOULD establish subscriptions for each new AoR using the "user"
and the "local-network" profile-types. The UA SHOULD terminate
all subscriptions for AoRs it has removed.
o The UA changes the domain it is connected to. The UA SHOULD
terminate all existing subscriptions for the "local-network"
profile-type. It SHOULD then create a new subscription for each
AoR using the "local-network" profile-type. This way, the UA
stops receiving policies from the previous local domain and starts
to receive the policies of the new local domain. The UA does not
need to change the subscriptions for "user" profiles.
If a subscriber is unable to establish a subscription, it SHOULD NOT
attempt to re-try this subscription, unless one of the above events
occurs again. This is to limit the number of SUBSCRIBE requests sent
within domains that do not support session-independent policies.
A UA compliant to this specification MUST support the User Agent
Profile Data Set for Media Policy [3]. To indicate that the UA wants
to receive session-independent policies, it includes the MIME type
"application/session-policy+xml" in the Accept header of a SUBSCRIBE
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request.
A policy server MAY send a notification to the subscriber every time
the session-independent policies covered by the subscription change.
The definition of what causes a policy to change is at the discretion
of the administrator. A change in the policy may be triggered, for
example, by a change in the network status, by the change in the time
of day or by an update of the service level agreement with the
customer. The session-independent policies contained in a
notification MUST represent a complete session-independent policy.
Deltas to previous policies or partial policies are not supported.
4. Session-Specific Policies
Session-specific policies are policies that are created specifically
for one particular session of a UA. Thus, session-specific policies
will typically be different for different sessions. The session-
specific policies for a session may change during the course of the
session. For example, a user may run out of credit during a session,
which will cause the network to disallow the transmission all media
streams from this point on.
4.1. Architecture
domain 1
+-----------+
/------| proxy |----...
+----+ / +-----------+
| |---/ +-----------+
| | | policy |
| UA |============| server |
| | +-----------+
| |**** +-----------+
+----+ * | policy |
*******|enforcement|****...
+-----------+
--- SIP Signaling
=== Policy Channel
*** Media
Figure 2
The following entities are needed for session-specific policies (see
Figure 2): a user agent (UA), a proxy, a policy server and possibly a
policy enforcement entity.
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The role of the proxy is to provide a rendezvous mechanism for UAs
and policy servers. It conveys the URI of the policy server in its
domain to UAs and ensures that each UA knows where to retrieve
policies from. It does not deliver the actual policies to UAs.
The policy server is a separate logical entity that may be physically
co-located with the proxy. The role of the policy server is to
deliver session policies to UAs. The policy server receives session
information, uses this information to determine the policies that
apply to the session and returns these policies to the UA. The
mechanism for generating policies (i.e. making policy decisions) is
outside the scope of this specification. A policy server may, for
example, query an external entity to get the policies that apply to a
session or it may directly incorporate a policy decision point and
generate policies locally.
A UA receives the URI of a policy server from a proxy. It uses this
URI to connect to the policy server. It provides information about
the current session to the policy server and receives session
policies in response. The UA may also receive policy updates from
the policy server during the course of a session.
A network may have a policy enforcement infrastructure in place.
However, this specification does not make any assumptions about the
enforcement of session policies and the mechanisms defined here are
orthogonal a policy enforcement infrastructure. Their goal is to
provide a mechanism to convey session information to a policy server
and to return the policies that apply to a session to the UA.
In principle, each domain that is traversed by SIP signaling messages
can define session-specific policies for a session. Each of these
domains needs to run a policy server and a proxy that is able to
rendezvous a UA with the policy server (as shown in Figure 2).
However, it is expected that session-specific policies will often
only be provided by the local domain of the user agent.
4.2. Overview
The protocol defined in this specification clearly separates SIP
signaling and the exchange of policies. SIP signaling is only used
to rendezvous the UA with the policy server. From this point on, UA
and policy server communicate directly with each other over a
separate policy channel. This is opposed to a piggyback model, where
the exchange of policy information between endpoint and a policy
server in the network is piggybacked onto the SIP signaling messages
that are exchanged between endpoints.
The main advantage of using a separate policy channel is that it
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decouples the exchange of signaling messages between endpoints from
the exchange of policies between endpoint and policy server. This
decoupling provides a number of desirable properties. It enables the
use of separate encryption mechanisms on the signaling path to secure
the communication between endpoints, and on the policy channel to
secure the communication between endpoint and policy server.
Policies can be submitted directly from the policy server to the
endpoint and never travel along the signaling path, possibly crossing
many domains. Endpoints set up a separate policy channel to each
policy server and can specifically decide which information they want
to disclose to which policy server. Finally, policy servers do not
need to rely on a SIP signaling message flowing by to send policies
or policy updates to an endpoint. A policy server can use the policy
channel at any time to update session policies as needed. A
disadvantage of the separate channel model is that it requires
additional messages for the exchange of policy information.
Following this model, signaling for session-specific policies
involves the following two fundamental tasks:
1. UA/policy server rendezvous: a UA setting up a session needs to
be able to discover the policy servers that are relevant to this
session.
2. Policy channel: once the UA has discovered the relevant policy
servers for a session, it needs to connect to these servers,
disclose session information and retrieve the policies that apply
to this session.
The setting up session-specific policies over the policy channel
involves the following steps:
1. A user agent submits information about the session it is trying
to establish to the policy server and asks whether a session
using these parameters is permissible.
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.
3. The policy server can update the policy decision at a later time.
A policy decision update can, for example, propose additional
changes to the session (e.g. change the available bandwidth) or
deny a previously accepted session (i.e. disallow the
continuation of a session).
In many cases, the mechanism for session-specific policies will be
used to disclose session information and return session policies.
However, some scenarios may only involve the disclosure of session
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information to a network intermediary. If an intermediary does not
intend to return a policy, it can simply accept the session as it was
proposed. Similarly, some session-specific policies only apply to
the offer (and therefore only require the disclosure of the offer)
whereas others apply to offer and answer. Both types of policies are
supported by session-specific policy mechanism.
4.3. Examples
This section provides two examples to illustrate the overall
operation of session-specific policies. The call flows depict the
rendezvous mechanism between UA and policy server and indicate the
points at which the UA exchanges policy information with the policy
server.
The example is based on the following scenario: there are two domains
(domain A and domain B), which both have session-specific policies
for the UAs in their domain. Both domains do not provide policies to
the UAs outside of their domain. The two domains have a proxy (P A
and P B) and a policy server (PS A and PS B). The policies in both
domains involve the session description offer and answer.
4.3.1. Offer in Request
The first call flow shown in Figure 3 depicts an INVITE transaction
with the offer in the request. It is assumed that this is the first
INVITE request the UAC creates in this domain and that it therefore
does not have previous knowledge about the policy server URIs in this
domain.
(1) UA A sends an INVITE to proxy P A. P A knows that policies apply
to this session and (2) returns a 488 to UA A. P A includes the URI
of PS A in the 488 response. This step is needed since the UAC has
no prior knowledge about the URI of PS A. (3) UA A uses the URI to
contact PS A, discloses the session description offer to PS A and (4)
receives policies for the offer. (5) UA A reformulates the INVITE
request under consideration of the received policies and includes a
Policy-Id header to indicate that it has already contacted PS A. P A
does not reject the INVITE this time and removes the Policy-Id header
when forwarding the INVITE. P B adds a Policy-Contact header
containing the URI of PS B. (6) UA B uses this URI to contact PS B
and discloses the offer and the answer it is about to send. (7) UA B
receives policies from PS B and applies them to the offer and answer
respectively. (8) UA B returns the updated answer in the 200 OK. (9)
UA A contacts PS A with the answer and (10) retrieves answer policies
from PS A.
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UA A P A P B UA B
| | | |
| INVITE offer | | |
|---------------->| | | (1)
| 488 | | |
| + Policy-Contact| | |
|<----------------| | | (2)
| ACK | | |
|---------------->| | |
| | PS A | |
| | | |
| PolicyChannel | | |
| + InfoOffer | | |
|------------------->| | | (3)
| PolicyChannel | | |
| + PolicyOffer | | |
|<-------------------| | | (4)
| | | |
| | | |
| INVITE offer' | INVITE offer' | INVITE offer |
| + Policy-Id | | + Policy-Contact|
|---------------->|--------------->|---------------->| (5)
| | | |
| | PS B | |
| | | |
| | | PolicyChannel |
| | | + InfoOffer |
| | | + InfoAnswer |
| | |<-------------------| (6)
| | | PolicyChannel |
| | | + PolicyOffer |
| | | + PolicyAnswer |
| | |------------------->| (7)
| | | |
| | | |
| OK answer | OK answer | OK answer |
|<----------------|<---------------|<----------------| (8)
| ACK |
|--------------------------------------------------->|
| | | |
| | | |
| PolicyChannel | | |
| + InfoAnswer | | |
|------------------->| | | (9)
| PolicyChannel | | |
| + PolicyAnswer | | |
|<-------------------| | | (10)
| | | |
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Figure 3
4.3.2. Offer in Response
The call flow shown in Figure 4 depicts an INVITE transaction with
the offer in the response.
Steps (1) - (8) are analogous to steps (1) - (8) in the previous
flow. An important difference is that in steps (9) and (10) UA A
contacts PS A after receiving the offer in the 200 OK but before
returning the answer in step (11). This enables UA A to return the
final answer, which includes all applicable policies, in the ACK.
However, it requires that PS A immediately returns a policy to avoid
a delay in the transmission of the ACK. This is similar to Flow I in
[9].
UA A P A P B UA B
| | | |
| INVITE | | |
|---------------->| | | (1)
| 488 | | |
| + Policy-Contact| | |
|<----------------| | | (2)
| ACK | | |
|---------------->| | |
| | PS A | |
| | | |
| PolicyChannel | | |
|------------------->| | | (3)
| PolicyChannel | | |
|<-------------------| | | (4)
| | | |
| | | |
| INVITE | INVITE | INVITE |
| + Policy-Id | | + Policy-Contact|
|---------------->|--------------->|---------------->| (5)
| | | |
| | PS B | |
| | | |
| | | PolicyChannel |
| | | + InfoOffer |
| | |<-------------------| (6)
| | | PolicyChannel |
| | | + PolicyOffer |
| | |------------------->| (7)
| | | |
| | | |
| OK offer | OK offer | OK offer |
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|<----------------|<---------------|<----------------| (8)
| | | |
| | | |
| PolicyChannel | | |
| + InfoOffer | | |
| + InfoAnswer | | |
|------------------->| | | (9)
| PolicyChannel | | |
| + PolicyOffer | | |
| + PolicyAnswer | | |
|<-------------------| | | (10)
| | | |
| ACK answer |
|--------------------------------------------------->| (11)
| | | |
| | | |
| | | PolicyChannel |
| | | + InfoAnswer |
| | |<-------------------| (12)
| | | PolicyChannel |
| | | + PolicyAnswer |
| | |------------------->| (13)
| | | |
Figure 4
4.4. UA/Policy Server Rendezvous
The first step in setting up session-specific policies is to
rendezvous the UAs with the relevant policy servers. This is
achieved by providing the URIs of all policy servers relevant for a
session to the UAs.
4.4.1. UAC Behavior
A UAC compliant to this specification MUST include a Supported header
field with the option tag "policy" into all requests that can
initiate an offer/answer exchange [8] (e.g. INVITE, UPDATE and PRACK
requests). Guidelines for the sets of messages in which offers and
answers can appear are defined in RFC3261 [6]. The UA MUST include
the "policy" option tag into these requests even if the particular
request does not contain an offer or answer (e.g. an INVITE request
without an offer).
The UAC may receive a 488 response that contains a Policy-Contact
header field. The Policy-Contact header is a new header defined in
this specification. It contains the URI of a policy server. A 488
response with this header is generated by a proxy to convey the URI
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of the local policy server to the UAC. A UAC SHOULD use this URI to
contact the policy server using mechanism defined in Section 4.5. It
SHOULD apply the policies received to the request and resend the
updated request. If no changes are required by policies or no
policies have been received, the request can be resend without any
policy-induced changes (headers etc. are still updated as needed for
the retransmission).
The UAC MUST insert a Policy-Id header into a request if it has
consulted a policy server for this request. The Policy-Id header
MUST include the URIs of all policy servers the UAC has contacted for
the request. The Policy-Id header enables a proxy to determine
whether the URI of its associated policy server is already known to
the UAC (and thus the request can be passed through) or whether the
URI still needs to be conveyed to the UAC in a 488 response.
In some cases, a request may traverse multiple domains with session-
policies in place. Each of these domains may return a 488 response
containing a policy server URI. Since the UAC contacts a policy
server after receiving a 488 response from a domain and before re-
sending the request, session policies are always applied to a request
in the order in which the request traverses through the domains. The
UAC MUST NOT change this implicit order among policy servers.
A UAC frequently needs to contact the policy server in the local
domain before sending a new request. To avoid the retransmission of
the local policy server URI in a 488 for each new request, a UA
SHOULD cache the URI of the local policy server (see Section 4.4.4).
It SHOULD use the cached policy server URI to contact the local
policy server before sending a request that initiates the first
offer/answer exchange in a dialog (e.g. an INVITE request).
A UAC may need to initiate subsequent offer/answer exchanges in a
dialog (e.g. using INVITE, UPDATE or PRACK requests) to re-negotiate
the session description. When creating such a mid-dialog request, a
UAC SHOULD contact the same policy servers it has contacted during
the initial offer/answer exchange in the dialog (see Section 4.4.5)
before sending the request. This avoids the retransmission of all
policy server URIs in 488 responses for mid-dialog requests.
4.4.2. Proxy Behavior
A proxy provides rendezvous functionality for UAs and a policy
server. This is achieved by conveying the URI of a policy server to
the UAC or the UAS (or both) when processing INVITE, UPDATE or PRACK
requests (or any other request that can initiate an offer/answer
exchange).
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If such a request contains a Supported header field with the option
tag "policy", the proxy MAY reject the request with a 488 response to
provide the local policy server URI to the UAC. Before rejecting a
request, the proxy MUST verify that the request does not have a
Policy-Id header field, which already contains the local policy
server URI. If the request does not have such a header or the local
policy server URI is not present in this header, then the proxy MAY
reject the request with a 488. The proxy MUST insert a Policy-
Contact header in the 488 response that contains the URI of its
associated policy server. The proxy MAY add the header field
parameter "non-cacheable" to prevent the UAC from caching this policy
server URI (see Section 4.4.4).
If the local policy server URI is already present in the Policy-Id
header of a request, the proxy MUST NOT reject the request as
described above. The proxy SHOULD remove this policy server URI from
the Policy-Id header field before forwarding the request. Keeping
this URI in the Policy-Id header would just consume space in the
message without providing any value and would disclose the URI to
subsequent proxies.
The proxy MAY insert a Policy-Contact header field into INVITE,
UPDATE or PRACK requests (or any other request that can initiate an
offer/answer exchange) in order to convey the policy server URI to
the UAS. If the request already contains a Policy-Contact header
field, the proxy MUST insert the URI ahead of all existing values at
the beginning of the list. A proxy MUST NOT change the order of
existing Policy-Contact header values.
4.4.3. UAS Behavior
A UAS may receive an INVITE, UPDATE or PRACK request (or another
request that can initiate offer/answer exchanges), which contains a
Policy-Contact header filed with a list of policy server URIs. A UAS
that receives such a request SHOULD contact all policy server URIs in
a Policy-Contact header. The UAS MUST contact the policy server URIs
in the order in which they were contained in the Policy-Contact
header, starting with the topmost value.
4.4.4. Caching Policy Server URIs
A UAC may frequently need to contact the policy server in the local
domain before sending a request. To avoid the retransmission of the
local policy server URI for each new request, each UA SHOULD cache
the URI of the local policy server. A UA may receive this URI in a
Policy-Contact header inserted by the local proxy into a 488 response
or a request. Alternatively, the UA may also have received the local
policy server URI through configuration or other means. If a UA has
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received a local policy server URI through configuration and receives
another one in a Policy-Contact header, it SHOULD overwrite the
configured URI with the most recent one received in a Policy-Contact
header.
Domains can prevent a UA from caching the local policy server URI.
This is useful, for example, if the policy server does not need to be
involved in all sessions or the policy server URI changes from
session to session. A proxy can mark the URI of such a local policy
server as "non-cacheable". A UA MUST NOT cache a non-cacheable
policy server URI. It SHOULD remove the current URI from the cache
when receiving a "non-cacheable" URI. This is to avoid the use of
policy server URIs that are outdated.
The UA SHOULD NOT cache policy server URIs it has received from
proxies outside of the local domain. These policy servers may not be
relevant for subsequent sessions, which may go to a different
destination, traversing different domains.
4.4.5. Storing Policy Server URIs in a Dialog
A UA discovers the list of policy servers relevant for a dialog
during the initial offer/answer exchange. It SHOULD store this list
of policy server URIs for a dialog, as part of the dialog state. The
UA SHOULD maintain this list until the dialog is terminated. It
SHOULD store policy server URIs in this list even if they are marked
as "non-cacheable". The non-cacheable parameter only refers to
caching policy server URIs for re-use between dialogs.
If a UAC has contacted all stored policy servers before sending a
mid-dialog request and receives a 488 in response to this request
with a Policy-Contact header containing a new policy server URI, it
MUST discard the stored policy server URI list for the current
dialog. Receiving a 488 response at this point indicates that the
set of policy servers relevant for the current dialog has changed.
The UAC SHOULD retry sending the request as if it was the first
request in a dialog (i.e. without applying any policies except
policies from the local policy server). This way, the UAC will re-
discover the list of policy server URIs relevant for the current
request.
If a UAS receives a mid-dialog request with a Policy-Contact header
containing a list of policy server URIs that is different from the
list stored for the dialog, then the UAS SHOULD replace the stored
list with the one received in the Policy-Contact header field.
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4.4.6. Contacting the Policy Server
A UA compliant to this specification SHOULD contact the discovered
policy servers and apply session policies to an offer or answer
before using the offer or answer.
Some session policies only apply to the offer whereas other policies
apply to the offer as well as the answer. A UA that contacts a
policy server UA SHOULD disclose the offer to the policy server. A
UA SHOULD disclose the answer to the policy server, unless the policy
server has indicated on the policy channel (when processing the
offer) that the disclosure of the answer is not needed for this
session. When disclosing the answer to the policy servers, the UA
MUST contact the same policy servers it has contacted for the offer.
A UA that receives a SIP message containing an offer or answer SHOULD
completely process the message (e.g. according to [6]) before
contacting the policy server. The SIP processing of the message
includes, for example, updating dialog state and timers as well as
creating an ACK or PRACK request as necessary. This ensures that
contacting a policy server does not interfere with SIP message
processing (e.g. by inadvertently causing timers to expire). It
implies, for example, that a UAC which has received a response to an
INVITE request SHOULD finish the processing of the response including
transmitting the ACK before it contacts the policy server. An
important exception to this rule is explained in the next paragraph.
In some cases, a UA needs to use the offer/answer it has received in
a SIP message to complete SIP processing of this message. For
example, a UAC that has received an offer in the response to an
INVITE request needs to apply policies to the offer and the resulting
answer before it can insert the answer into an ACK. In these cases,
a UA SHOULD contact the policy server even if this is during the
processing of a SIP message. This implies that a UA, which has
received an offer in the response of an INVITE request, SHOULD
contact the policy server and apply session policies before sending
the answer in the ACK.
Note: this assumes that the policy server immediately responds to
a policy request and does not require manual intervention to
create a policy. A delay in the response from the policy server
would delay the transmission of the ACK and could trigger
retransmissions of the INVITE response (also see the
recommendations for Flow I in [9]).
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4.4.7. Header Definition and Syntax
The Policy-Id header field is inserted by the UAC into INVITE, UPDATE
or PRACK requests (or any other request that can be used to initiate
an offer/answer exchange). The Policy-Id header identifies all
policy servers the UAC has contacted for this request. A Policy-Id
header value is the URI of a policy server.
The syntax of the Policy-Id header field is:
Policy-Id = "Policy-Id" HCOLON policyURI
*(COMMA policyURI)
policyURI = ( SIP-URI / SIPS-URI ) [ SEMI generic-param ]
The Policy-Contact header field can be inserted by a proxy into a 488
response to INVITE, UPDATE or PRACK requests (or other requests that
initiate an offer/answer exchange). It contains a policy server URI
that needs to be contacted by the UAC. A proxy MAY add the "non-
cacheable" header field parameter to indicate that a UA MUST NOT
cache this policy server URI.
The Policy-Contact header field can also be inserted by a proxy into
INVITE, UPDATE and PRACK requests (or other requests that can be used
to initiate an offer/answer exchange). It contains an ordered list
of policy server URIs that need to be contacted by the UAS. The UAS
starts to process the header field at the topmost value of this list.
New header field values are inserted at the top. The Policy-Contact
header field effectively forms a stack. The "non-cacheable" header
field parameter MUST NOT be used in a request.
The syntax of the Policy-Contact header field is:
Policy-Contact = "Policy-Contact" HCOLON policyContactURI
*(COMMA policyContactURI)
policyContactURI = ( SIP-URI / SIPS-URI )
[ SEMI "non-cacheable" / generic-param ]
The BNF for SIP-URI, IPS-URI and generic-param is defined in [6].
Table 1 is an extension of Tables 2 and 3 in [6]. The column 'UPD'
is for the UPDATE method [5].
Header field where proxy ACK BYE CAN INV OPT REG UPD
_______________________________________________________________
Policy-Id R rd - - - o - - o
Policy-Contact R a - - - o - - o
Policy-Contact 488 a - - - o - - o
Table 1: Policy-Id and Policy-Contact Header Fields
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4.5. Policy Subscription
The rendezvous mechanism described in the previous section enables
proxies to deliver the URIs of policy servers to the UAC and UAS.
This section describes the mechanism for the policy channel, i.e. the
protocol UAs use to contact the policy servers. The main task of the
policy channel is to enable a UA to submit information about the
session it is trying to establish (i.e. the offer and the answer) to
a policy server and to receive the resulting session-specific
policies and possible updates to these policies in response.
A UA compliant to this specification MUST implement the Event Package
for Session-Specific Session Policies [2]. It contacts a policy
server by subscribing to this event package.
When subscribing to session-specific policies, the UA discloses
information about the session it is trying to establish to the policy
server as described in [2]. This information is used by the policy
server to determine the session-specific policy for this session.
The policy server returns the policies that apply to this session in
NOTIFY messages. It returns an initial set of policies when the
subscription is established and may notify the UA when there are
updates to these policies. Complete call flow examples for session-
specific policies that include policy channel messages can be found
in Appendix B.
A UA SHOULD use the policies it has received from the policy server
in the current session (i.e. the session the subscription is for).
When a UA receives a notification about a change in the current
policies, it SHOULD apply the updated policies to the current
session. If this update causes a change in the session description
of a session, the UA may need to re-negotiate the modified session
description with its peer UA, for example, using a re-INVITE or
UPDATE request. For example, if a policy update disallows the use of
video and video is part of the current session description, then the
UA will need to create an new session description offer without
video. After receiving this offer, the peer UA knows that video
can't be used any more and responds with the corresponding answer.
The re-INVITE or UPDATE message need to be generated in accordance to
Section 4.4.1.
5. Security Considerations
Session policies can significantly change the behavior of a user
agent and can be used by an attacker to compromise a user agent. For
example, session policies can be used to prevent a user agent from
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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 will cause the UA to terminate the session.
A user agent transmits session information to a policy server for
session-specific policies. 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.
It is important to secure the communication between the proxy and the
user agent (for session-specific policies) as well as the user agent
and the policy server. The following four discrete attributes need
to be protected:
1. integrity of the policy server URI (for session-specific
policies),
2. mutual authentication between the user agent and the policy
server,
3. confidentiality of the messages exchanged between the user agent
and the policy server and
4. ensuring that private information is not exchanged between the
two parties, even over an confidentiality-assured and
authenticated session.
To protect the integrity of the policy server URI, a UA SHOULD use a
secured transport protocol such as TLS between proxies and the UA.
Protecting the integrity of the policy server URI is important since
an attacker could intercept SIP messages between the UA and proxy and
remove the policy headers needed for session-specific policies. This
would impede the rendezvous between UA and policy server and, since
the UA would not contact the policy server, may prevent a UA from
setting up a session.
Instead of removing a policy server URI, an attacker can also modify
the policy server URI and point the UA to a compromised policy
server. To prevent such an attack from being effective, it is
RECOMMENDED that a UA authenticates policy servers.
It is RECOMMENDED that administrators use SIPS URIs as policy server
URIs so that subscriptions to session policies are transmitted over
TLS.
The above security attributes are important to protect the
communication between the user agent and policy server. This
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document does not define the protocol used for the communication
between user agent and policy server and merely refers to other
specifications for this purpose. The security considerations of
these specifications need to address the above security aspects.
6. IANA Considerations
6.1. Registration of the "Policy-Id" Header
Name of Header: Policy-Id
Short form: none
Normative description: Section 4.4.7 of this document
6.2. Registration of the "Policy-Contact" Header
Name of Header: Policy-Contact
Short form: none
Normative description: Section 4.4.7 of this document
6.3. Registration of the "policy" SIP Option-Tag
Name of option: policy
Description: Support for the Policy-Contact and Policy-Id headers.
SIP headers defined: Policy-Contact, Policy-Id
Normative description: This document
Appendix A. Acknowledgements
Many thanks to Allison Mankin for the discussions and the suggestions
for this draft and to Roni Even, Bob Penfield, Mary Barnes and Shida
Schubert for reviewing the draft and providing feedback. Many thanks
to Vijay Gurbani for the comments and feedback.
Appendix B. Session-Specific Policies - Call Flows
The following call flows illustrate the overall operation of session-
specific policies. The call flows contain all messages needed for
UA/policy server rendezvous and the policy subscription.
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The following abbreviations are used:
o: offer
o': offer modified by a policy
po: offer policy
a: answer
a': answer modified by a policy
pa: answer policy
ps uri: policy server URI (in Policy-Contact header)
ps id: policy server id (in Policy-Id header)
B.1. Offer in Invite
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UA A P A PS A PS B P B UA B
| | | | | |
|(1) INV <o> | | | |
|-------->| | | | |
|(2) 488 <ps uri> | | | |
|<--------| | | | |
|(3) ACK | | | | |
|-------->| | | | |
|(4) SUBSCRIBE <o> | | | |
|------------------>| | | |
|(5) 200 OK | | | |
|<------------------| | | |
|(6) NOTIFY <po> | | | |
|<------------------| | | |
|(7) 200 OK | | | |
|------------------>| | | |
|(8) INV <ps id, o'>| | | |
|-------->| | | | |
| |(9) INV <o'> | | |
| |---------------------------->| |
| | | | |(10) INV <o', ps uri>
| | | | |-------->|
| | | |(11) SUBSCRIBE <o', a>
| | | |<------------------|
| | | |(12) 200 OK |
| | | |------------------>|
| | | |(13) NOTIFY <po, pa>
| | | |------------------>|
| | | |(14) 200 OK |
| | | |<------------------|
| | | | |(15) 200 OK <a'>
| | | | |<--------|
| |(16) 200 OK <a'> | | |
| |<----------------------------| |
|(17) 200 OK <a'> | | | |
|<--------| | | | |
|(18) ACK | | | | |
|------------------------------------------------>|
|(19) SUBSCRIBE <o', a'> | | |
|------------------>| | | |
|(20) 200 OK | | | |
|<------------------| | | |
|(21) NOTIFY <pa> | | | |
|<------------------| | | |
|(22) 200 OK | | | |
|------------------>| | | |
| | | | | |
| | | | | |
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B.2. Offer in Response
UA A P A PS A PS B P B UA B
| | | | | |
|(1) INV | | | | |
|-------->| | | | |
|(2) 488 <ps uri> | | | |
|<--------| | | | |
|(3) ACK | | | | |
|-------->| | | | |
|(4) SUBSCRIBE | | | |
|------------------>| | | |
|(5) 200 OK | | | |
|<------------------| | | |
|(6) NOTIFY | | | |
|<------------------| | | |
|(7) 200 OK | | | |
|------------------>| | | |
|(8) INV <ps id> | | | |
|-------->| | | | |
| |(9) INV | | | |
| |---------------------------->| |
| | | | |(10) INV <ps uri>
| | | | |-------->|
| | | |(11) SUBSCRIBE <o> |
| | | |<------------------|
| | | |(12) 200 OK |
| | | |------------------>|
| | | |(13) NOTIFY <po> |
| | | |------------------>|
| | | |(14) 200 OK |
| | | |<------------------|
| | | | |(15) 200 OK <o'>
| | | | |<--------|
| |(16) 200 OK <o'> | | |
| |<----------------------------| |
|(17) 200 OK <o'> | | | |
|<--------| | | | |
|(18) SUBSCRIBE <o', a> | | |
|------------------>| | | |
|(19) 200 OK | | | |
|<------------------| | | |
|(20) NOTIFY <po, pa> | | |
|<------------------| | | |
|(21) 200 OK | | | |
|------------------>| | | |
|(22) ACK <a'> | | | |
|------------------------------------------------>|
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| | | |(23) SUBSCRIBE <o', a'>
| | | |<------------------|
| | | |(24) 200 OK |
| | | |------------------>|
| | | |(25) NOTIFY <po, pa>
| | | |------------------>|
| | | |(26) 200 OK |
| | | |<------------------|
| | | | | |
| | | | | |
7. References
7.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Hilt, V. and G. Camarillo, "A Session Initiation Protocol (SIP)
Event Package for Session-Specific Session Policies.",
draft-ietf-sipping-policy-package-01 (work in progress),
April 2006.
[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] Petrie, D., "A Framework for Session Initiation Protocol User
Agent Profile Delivery", draft-ietf-sipping-config-framework-09
(work in progress), October 2006.
[5] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
Method", RFC 3311, October 2002.
[6] 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.
7.2. Informative References
[7] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[8] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
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[9] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
"Best Current Practices for Third Party Call Control (3pcc) in
the Session Initiation Protocol (SIP)", BCP 85, RFC 3725,
April 2004.
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
Jonathan Rosenberg
Cisco Systems
600 Lanidex Plaza
Parsippany, NJ 07054
USA
Email: jdrosen@cisco.com
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