SIPPING Working Group V. Hilt
Internet-Draft Bell Labs/Alcatel-Lucent
Intended status: Standards Track G. Camarillo
Expires: October 29, 2008 Ericsson
J. Rosenberg
Cisco
April 27, 2008
A Framework for Session Initiation Protocol (SIP) Session Policies
draft-ietf-sip-session-policy-framework-03
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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
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.
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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
3.2.1. UAC Behavior . . . . . . . . . . . . . . . . . . . . . 6
3.2.2. UAS Behavior . . . . . . . . . . . . . . . . . . . . . 7
4. Session-Specific Policies . . . . . . . . . . . . . . . . . . 7
4.1. Architecture . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3.1. Offer in Request . . . . . . . . . . . . . . . . . . . 11
4.3.2. Offer in Response . . . . . . . . . . . . . . . . . . 13
4.4. UA/Policy Server Rendezvous . . . . . . . . . . . . . . . 14
4.4.1. UAC Behavior . . . . . . . . . . . . . . . . . . . . . 15
4.4.2. Proxy Behavior . . . . . . . . . . . . . . . . . . . . 16
4.4.3. UAS Behavior . . . . . . . . . . . . . . . . . . . . . 19
4.4.4. Caching the Local Policy Server URI . . . . . . . . . 19
4.4.5. Header Definition and Syntax . . . . . . . . . . . . . 20
4.5. Policy Channel . . . . . . . . . . . . . . . . . . . . . . 22
4.5.1. Creation and Management . . . . . . . . . . . . . . . 22
4.5.2. Contacting the Policy Server . . . . . . . . . . . . . 23
4.5.3. Using Session Policies . . . . . . . . . . . . . . . . 25
5. Security Considerations . . . . . . . . . . . . . . . . . . . 25
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
6.1. Registration of the "Policy-Id" Header . . . . . . . . . . 27
6.2. Registration of the "Policy-Contact" Header . . . . . . . 27
6.3. Registration of the "policy" SIP Option-Tag . . . . . . . 27
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
7.1. Normative References . . . . . . . . . . . . . . . . . . . 28
7.2. Informative References . . . . . . . . . . . . . . . . . . 29
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 29
Appendix B. Session-Specific Policies - Call Flows . . . . . . . 29
B.1. Offer in Invite . . . . . . . . . . . . . . . . . . . . . 30
B.2. Offer in Response . . . . . . . . . . . . . . . . . . . . 32
B.3. Multiple Policy Servers for UAS . . . . . . . . . . . . . 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 34
Intellectual Property and Copyright Statements . . . . . . . . . . 35
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1. Introduction
The Session Initiation Protocol (SIP) [RFC3261] 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 session-mode 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)
[RFC4566].
Experience has shown that there is a need for SIP intermediaries to
impact aspects of a session. For example, SIP can be used in a
wireless network, which has limited resources for media traffic.
During periods of high activity, the wireless network provider may
want to restrict the amount of bandwidth available to each user.
With session policies, an intermediary in the wireless network can
inform the user agent about the bandwidth it has available. 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 can
have a service level agreement with a user that defines the set of
media types the 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 have the need to enforce the session policies they have
in place. For example, a domain might have a policy that disallows
the use of video and can have an enforcement mechanism that drops all
packets containing a video encoding. Unfortunately, these
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 can lead to a malfunctioning
of devices that is incomprehensible to the user. With session
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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 the proposed session description. It can then return a
policy for the session description that replaces the IP addresses and
ports of the UA with the ones opened in the firewall/NAT that are
reachable from external. 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 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. Since session-specific and session-independent
policies have different requirements, this specification defines two
different mechanisms to deliver them to user agents. These
mechanisms are independent of each other and, depending on whether
one or both types of session policies are needed, it is possible to
use the session-specific or the session-independent mechanism or both
to deliver policies to user agents.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in RFC 2119 [RFC2119].
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, it is possible for session-independent policies to change
over time. For example, a policy that defines a bandwidth limit for
a user can change during the day, defining a lower limit during peak
hours and allow more bandwidth off-peak. The policy server informs a
UA when session-independent policies change.
3.1. Architecture and Overview
+-------------+
/------| policy |
+----+ / | server 1 |
| |---/ +-------------+
| UA | ...
| |---\ +-------------+
+----+ \ | policy |
\------| server n |
+-------------+
Figure 1
A SIP UA can 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 local network domain
(i.e., the domain from which the UA receives IP service) and possibly
the SIP service provider domain (i.e., the domain the UA registers
at). The local network can have policies that support the access
network infrastructure. For example, in a wireless network where
bandwidth is scarce, a provider can restrict the bandwidth available
to an individual user. The SIP service provider can 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:
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1. A user agent requests session-independent policies from the
policy servers in the local network and SIP service provider
domain. A user agent typically requests these policies when it
starts up or connects to a new network domain.
2. The policy server selects the policies that apply to this user
agent. The policy server can 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 can update the policies, for example, when
network conditions change.
3.2. Policy Subscription
3.2.1. UAC Behavior
A UA that supports session-independent policies MUST attempt to
retrieve session-independent policies from the local network and the
SIP service provider domain, unless the UA knows (e.g., through
configuration) that a domain does not provide session-independent
policies. In this case, the UA SHOULD NOT retrieve session-
independent policies from this specific domain.
A UA MUST support the retrieval of session-independent policies from
the local network and the SIP service provider domain using the "ua-
profile" event package defined in the Framework for SIP User Agent
Profile Delivery [I-D.ietf-sipping-config-framework]. The UA MAY
support other methods of retrieving session-independent policies from
local network and SIP service provider domain.
The "ua-profile" event package [I-D.ietf-sipping-config-framework]
provides a mechanism to subscribe to session-independent policies. A
UA subscribes to the policy server in the local network domain using
the procedures defined for the "local-network" profile-type. The UA
uses the procedures defined for the "user" profile type to subscribe
to the policy server in the SIP service provider domain.
A UA (re-)subscribes 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
MUST establish subscriptions for each new AoR using the "user" and
the "local-network" profile-types. The UA MUST terminate all
subscriptions for AoRs it has removed.
o The UA changes the domain it is connected to. The UA MUST
terminate all existing subscriptions for the "local-network"
profile-type. The UA MUST then create a new subscription for each
AoR it maintains using the "local-network" profile-type. This
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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
[I-D.ietf-sipping-media-policy-dataset]. To indicate that the UA
wants to receive session-independent policies, it includes the MIME
type "application/media-policy-dataset+xml" in the Accept header of a
SUBSCRIBE request.
A UA MUST apply the session-independent policies it has received and
use these policies in the session descriptions it creates. If the UA
decides not to use the received policies, then the UA MUST NOT set up
a session unless it changes the domain that provided these policies.
A UA MAY try to connect to another local network and/or SIP service
provider domain with a different set of policies.
If a UA receives both session-independent and session-specific
policies, the UA MUST apply the session-independent policies to the
session description before the session description is sent to the
session-specific policy server (see Section 4). Thus, session-
independent policies are always applied before session-specific
policies are retrieved.
3.2.2. UAS Behavior
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 can 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.
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 can change during the course of the
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session. For example, a user can 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.
The role of the proxy is to provide a rendezvous mechanism for UAs
and policy servers. It ensures that each UA has the URI of the
policy server in its domain and knows where to retrieve policies
from. The proxy conveys the policy server URI to UAs in case they
have not yet received it (e.g., in a previous call or through
configuration). The proxy does not deliver the actual policies to
UAs.
The policy server is a separate logical entity that can 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 from the UA, 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 of the scope of this specification. A policy
server can, for example, query an external entity to get policies or
it can directly incorporate a policy decision point and generate
policies locally.
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A UA receives the URI of a policy server from a proxy. It uses this
URI to contact the policy server. It provides information about the
current session to the policy server and receives session policies in
response. The UA can also receive policy updates from the policy
server during the course of a session.
A network can 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 to a policy enforcement infrastructure.
In principle, each domain that is traversed by SIP signaling messages
can define session-specific policies for a session. Each domain
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
decouples signaling between endpoints from the policy exchange
between an endpoint and a policy server. This decoupling has 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 do
not travel along the signaling path, possibly crossing many domains.
Endpoints set up a separate policy channel to each policy server and
can disclose the information requested by the specific policy server
(e.g., offer or offer/answer). 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
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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 communication between UA and policy server on 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 only involve the disclosure of session
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
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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 (Not Acceptable Here) to UA A.
P A includes the URI of PS A in the 488 (Not Acceptable Here)
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.
(1) UA A sends an INVITE without an offer to proxy P A and (2) P A
returns a 488 (Not Acceptable Here) response containing the URI of PS
A. (3),(4) UA A uses this policy server URI to set up the policy
channel. At this time, UA A does not disclose a session description
since it does not have the offer yet. (5) UA A re-sends the INVITE
request and includes a Policy-Id header to indicate that it has
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
discloses the offer to PS B. (7) UA B receives policies from PS B and
applies them to the offer. (8) UA B returns the updated offer the 200
OK. (9),(10) UA A contacts PS and discloses the offer and the answer
it is about to send. An important difference to the flow in the
previous example is that UA A performs steps (9) and (10) before
returning the answer in step (11). This enables UA A to return the
final answer in the ACK, which includes all applicable policies.
However, it requires that PS A immediately returns a policy to avoid
a delay in the transmission of the ACK. (12),(13) UA B also sends the
answer to PS B and applies the policies it receives to the answer
before using it.
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)
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| | | |
| | PS B | |
| | | |
| | | PolicyChannel |
| | | + InfoOffer |
| | |<-------------------| (6)
| | | PolicyChannel |
| | | + PolicyOffer |
| | |------------------->| (7)
| | | |
| | | |
| OK offer | OK offer | OK offer |
|<----------------|<---------------|<----------------| (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.
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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 [RFC3264] (e.g., INVITE, UPDATE and
PRACK requests). 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). A UA MAY
include the "policy" option tag into all requests.
A UAC can receive a 488 (Not Acceptable Here) 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 (Not Acceptable Here) response with this header
is generated by a proxy to convey the URI of the local policy server
to the UAC. After receiving a 488 (Not Acceptable Here) response
with a Policy-Contact header, a UAC compliant to this specification
needs to decide if it wants to continue with the session now knowing
that there is a policy server. If the UAC decides to continue, it
MUST use the policy server URI to contact the policy server using the
mechanism defined in Section 4.5. After receiving policies from the
policy server, the UAC decides if it wants to accept these policies
or not. If the UAC accepts these policies, the UAC MUST apply them
to the current request and resend the updated request. If no changes
are required by policies or no policies have been received, the
request can be resent without any policy-induced changes. If the UAC
decides that the list of policy servers or the received session
policies are unacceptable, then the UAC MUST NOT resend the request.
The UAC MAY resent the unchanged request if it cannot setup a policy
channel to the policy server, for example, because the policy server
is unreachable or returns an error condition that cannot be resolved
by the UAC (i.e., error conditions other than, for example, a 401
(Unauthorized) responses). This is to avoid that the failure of a
policy server prevents a UA from communicating.
To protect the integrity of the policy server URI in a Policy-Contact
header, the UAC SHOULD use a secured transport protocol such as TLS
between UAC and proxy.
The UAC MUST insert a Policy-Id header into requests for which it has
contacted a policy server and accepted the policies received. The
Policy-Id header is a new header that is defined in this
specification. The UA MUST create a Policy-Id header value for each
policy server it has contacted during the preparation of the request.
A Policy-Id header value contains two pieces of information: the
policy server URI and an optional token. The policy server URI is
the URI the UA has used to contact the policy server. The token is
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an opaque string the UAC can receive from the policy server. A token
can, for example, be contained in the policy document
[I-D.ietf-sipping-media-policy-dataset]. If the UAC has received a
token from the policy server the UAC MUST include the token in the
Policy-Id header. The format of the Policy-Id header is defined in
Section 4.4.5.
The main purpose of the Policy-Id header is to enable a proxy to
determine if the UAC already knows the URI of the local policy
server. If the policy server URI is not yet known to the UAC, the
proxy can convey this URI to the UAC by rejecting the request with a
488 (Not Acceptable Here) response.
In some cases, a request can traverse multiple domains with a
session-policy server. Each of these domains can return a 488 (Not
Acceptable Here) response containing a policy server URI. Since the
UAC contacts a policy server after receiving a 488 (Not Acceptable
Here) 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 setting up a session. To avoid the retransmission of
the local policy server URI in a 488 (Not Acceptable Here) for each
new request, a UA SHOULD maintain a cache that contains the URI of
the local policy server (see Section 4.4.4). The UAC SHOULD use the
cached policy server URI to contact the local policy server before
sending a request that initiates the offer/answer exchange for a new
session (e.g., an INVITE request).
UAs can re-negotiate the session description during a session by
initiating a subsequent offer/answer exchange, e.g., in an INVITE,
UPDATE or PRACK request. When creating such a mid-dialog request, a
UA SHOULD contact all policy servers to which it has established a
policy channel during the initial offer/answer exchange (see
Section 4.5) before sending the request. This avoids the
retransmission of all policy server URIs in 488 (Not Acceptable Here)
responses for mid-dialog requests.
4.4.2. Proxy Behavior
A proxy provides rendezvous functionalities for UAs and 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 an offer/answer exchange initiating request contains a Supported
header field with the option tag "policy", the proxy MAY reject the
request with a 488 (Not Acceptable Here) response to provide the
local policy server URI to the UAC. Before rejecting a request, the
proxy MUST verify that the request does not contain a Policy-Id
header field with the local policy server URI as a value. If the
request does not contain 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 (Not Acceptable Here). The proxy MUST insert a Policy-
Contact header in the 488 (Not Acceptable Here) 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 present in a Policy-Id header value
of a request, then the proxy MUST NOT reject the request as described
above (it can still reject the request for other reasons). The proxy
SHOULD remove the Policy-Id header value of its associated policy
server from the Policy-Id header field before forwarding the request.
This value only increases message size and is not relevant to other
proxies on the path. It also would disclose the policy server URI to
subsequent proxies.
The Policy-Id header serves two main purposes: first and most
importantly, it enables the proxy to determine if a UAC already knows
the URI of the local policy server. The second purpose of the
Policy-Id header is to enable a domain to route all requests that
belong to the same session (i.e., the initial request and requests a
UA retransmits after contacting the policy server) to the same proxy
and policy server. This is important if a domain has multiple proxy/
policy server combinations (e.g., in a proxy/policy server farm that
receives requests through a load balancer), which create per-session
state in the network. An example for such a scenario is a policy
server that is associated with a session border device. The policy
server configures the session border device after receiving a session
description from the UAC via the policy channel. Retransmitted
requests for such a session need to be routed to the same proxy/
policy server as the initial request since this proxy/policy server
combination has configured the associated border device for the
session.
Routing all requests that belong to the same session to the same
proxy can be achieved by using the Policy-Id header token. It
requires that the policy server returns a token to the UAC that
uniquely identifies the specific proxy/policy server combination.
The UAC includes this token in the Policy-Id header and it can be
used (together with the policy server URI) by the proxies in this
domain to route the request along the desired path. The format of
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this token does not require standardization. The only requirement is
that the token provides sufficient information for proxies to route
the message inside a domain to the desired proxy/policy server. The
token can, for example, be a numeric identifier or an IP address.
Note: it has been proposed to use the Policy-Id header to provide
a hint for a proxy that the UAC has actually contacted the policy
server. This usage also requires the policy server to return a
token to the UA. In addition, the policy server needs to share
valid tokens with the proxy. After receiving a request with a
Policy-Id header, the proxy can determine if the token in the
Policy-Id header is valid. If it is valid, the proxy knows that
the UA has contacted the policy server for this session. However,
this token does not provide any proof that the UA has actually
used the policies it has received from the policy server. A
malicious UA can simply contact the policy server, discard all
policies it receives but still use the token in the Policy-Id
header.
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 after all existing values at the
end of the list. A proxy MUST NOT change the order of existing
Policy-Contact header values.
A proxy MUST use the Record-Route header mechanism [RFC3261] if its
associated policy server has session policies that apply to mid-
dialog requests. The Record-Route header enables a proxy to stay in
the signaling path and re-submit the policy server URIs to UAs during
mid-dialog requests that initiate an offer/answer exchange. Re-
submitting the policy server URI to UAs ensures that UAs keep
contacting the policy server for mid-dialog requests.
A proxy can find out if the UAS supports this extension by examining
the Supported header of responses. The proxy knows that the UAS
supports this extension if the Supported header of a response
contains the option tag "policy". A proxy can use this information
to determine if the UAS has understood the Policy-Contact header it
has inserted into the request.
To protect the integrity of the policy server URI in a Policy-Contact
header, the proxy SHOULD use a secured transport protocol such as TLS
between proxy and UAs.
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4.4.3. UAS Behavior
A UAS can receive an INVITE, UPDATE or PRACK request (or another
request that can initiate offer/answer exchanges) that contains a
Policy-Contact header field with a list of policy server URIs. A UAS
that receives such a request needs to decide if it wants to accept
the session knowing that there are policy servers involved. If it
accepts, the UAS MUST 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 (i.e., the value that was inserted
first).
If a UAS decides that it does not want to accept a session because
there are policy servers involved or because one of the session
policies received from a policy server is not acceptable, the UAS
MUST reject the request with a 488 (Not Acceptable Here) response.
The UAS MAY accept a request and continue with setting up a session
if it cannot setup a policy channel to the policy server, for
example, because the policy server is unreachable or returns an error
condition that cannot be resolved by the UAS (i.e., error conditions
other than, for example, a 401 (Unauthorized) responses). This is to
avoid that the failure of a policy server prevents a UA from
communicating. Since this session may not be policy compliant
without the policy subscription, it may be blocked by policy
enforcement mechanisms if they are in place.
A UAS can receive a token from a policy server via the policy
channel. Since the UAS does not create a Policy-ID header, it can
simply ignore this token.
A UAS compliant to this specification MUST include a Supported header
field with the option tag "policy" into responses to requests that
can initiate an offer/answer exchange. It MAY include this option
tag in all responses. This way, a proxy that has inserted the
Policy-Contact header can know that the header was understood by the
UAS.
4.4.4. Caching the Local Policy Server URI
A UAC frequently needs to contact the policy server in the local
domain before setting up a session. To avoid the retransmission of
the local policy server URI for each session, a UA SHOULD maintain a
cache that contains the URI of the local policy server.
A UA can receive this URI in a Policy-Contact header of a request or
a 488 (Not Acceptable Here) response. The UA can also receive the
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local policy server URI through configuration, for example, via the
configuration framework [I-D.ietf-sipping-config-framework]. If a UA
has received a local policy server URI through configuration and
receives another local policy server URI in a Policy-Contact header,
the UA 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 policy server
as "non-cacheable". A UA MUST NOT cache a non-cacheable policy
server URI. The UA SHOULD remove the current URI from the cache when
receiving a local policy server URI that is marked as "non-
cacheable". 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 need not
be relevant for subsequent sessions, which can go to a different
destination, traversing different domains.
The UA MUST NOT cache tokens it has received from a policy server. A
token is only valid for one request.
4.4.5. Header Definition and Syntax
4.4.5.1. Policy-Id Header
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.
The value of a Policy-Id header consists of a policy server URI and
an optional token parameter. The token parameter contains a token
the UA may have received from the policy server.
The syntax of the Policy-Id header field is:
Policy-Id = "Policy-Id" HCOLON policyURI
*(COMMA policyURI)
policyURI = ( SIP-URI / SIPS-URI / absoluteURI )
[ SEMI token-param ] *( SEMI generic-param )
token-param = "token=" token
The BNF for SIP-URI, SIPS-URI, absoluteURI, token and generic-param
is defined in RFC3261 [RFC3261].
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4.4.5.2. Policy-Contact Header
The Policy-Contact header field can be inserted by a proxy into a 488
(Not Acceptable Here) response to INVITE, UPDATE or PRACK requests
(or other requests that initiate an offer/answer exchange). The
value of a Policy-Contact header consists of a policy server URI and
an optional "non-cacheable" parameter. The policy server URI
identifies the policy server that needs to be contacted by a UAC.
The "non-cacheable" parameter indicates that the policy server URI
should not be cached by the UAC.
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
topmost value of this list identifies the policy server that is
contacted first. New header field values are inserted at the end.
With this, the Policy-Contact header field effectively forms a fist-
in-first-out queue.
The syntax of the Policy-Contact header field is:
Policy-Contact = "Policy-Contact" HCOLON policyContactURI
*(COMMA policyContactURI)
policyContactURI = ( SIP-URI / SIPS-URI / absoluteURI )
[ SEMI "non-cacheable" ] *( SEMI generic-param )
The BNF for SIP-URI, SIPS-URI, absoluteURI and generic-param is
defined in RFC3261 [RFC3261].
Tables 1 and 2 are extensions of Tables 2 and 3 in RFC 3261
[RFC3261]. The column "INF" is for the INFO method [RFC2976], "PRA"
is for the PRACK method [RFC3262], "UPD" is for the UPDATE method
[RFC3311], "SUB" is for the SUBSCRIBE method [RFC3265], "NOT" is for
the NOTIFY method [RFC3265], "MSG" is for the MESSAGE method
[RFC3428], "REF" is for the REFER method [RFC3515], and "PUB" is for
the PUBLISH method [RFC3903].
Header field where proxy ACK BYE CAN INV OPT REG UPD
_______________________________________________________________
Policy-Id R rd - - - c - - c
Policy-Contact R a - - - c - - c
Policy-Contact 488 a - - - c - - c
Table 1: Policy-Id and Policy-Contact Header Fields
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Header field where proxy PRA PUB SUB NOT INF MSG REF
_______________________________________________________________
Policy-Id R rd c - - - - - -
Policy-Contact R a c - - - - - -
Policy-Contact 488 a c - - - - - -
Table 1: Policy-Id and Policy-Contact Header Fields
4.5. Policy Channel
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.
The Event Package for Session-Specific Session Policies
[I-D.ietf-sipping-policy-package] defines a SUBSCRIBE/NOTIFY-based
policy channel mechanism. A UA compliant to this specification MUST
support the Event Package for Session-Specific Session Policies
[I-D.ietf-sipping-policy-package]. This event package MUST be used
to contact a policy server if the policy server URI is a SIP-URI or
SIPS-URI. A UA MAY support other policy channel mechanisms.
4.5.1. Creation and Management
A UA discovers the list of policy servers relevant for a session
during the initial offer/answer exchange (see Section 4.4). A UA
compliant to this specification MUST set up a policy channel to each
of the discovered policy server. If the UA does not want to set up a
policy channel to one of the policy servers provided, the UA MUST NOT
continue with the session. This means that, the UA MUST cancel or
reject a pending INVITE transaction for the session or terminate the
session if it is already in progress.
If setting up a policy channel to one of the discovered policy
servers fails, the UA MAY continue with the initiation of a session
without contacting this policy server. Setting up a policy channel
can fail, for example, because the server is unreachable or returns
an error condition that cannot be resolved by the UAC (i.e., error
conditions other than, for example, a 401 (Unauthorized) responses).
The UA SHOULD continue an ongoing session if a policy server fails
after the session has been set up. The UA SHOULD consider the
policies it has previously received from the failed policy server.
This is to avoid that the failure of a policy server prevents a UA
from communicating.
A UA MUST maintain the policy channel to each discovered policy
server during the lifetime of a session, unless the policy channel is
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closed by the policy server or the UA discovers that the policy
server is no longer relevant for the session as described below.
A UAC can receive a 488 (Not Acceptable Here) with a Policy-Contact
header containing a new policy server URI in response to a mid-dialog
request. This indicates that the set of policy servers relevant for
the current session has changed. If this occurs, the UAC MUST retry
sending the request as if it was the first request in a dialog (i.e.,
without applying any policies except the policies from the local
policy server). This way, the UAC will re-discover the list of
policy servers for the current session. The UAC should compare the
new list with the list of policy servers to which it currently has
established a policy channel. The UAC MUST set up a policy channel
to each new policy server. The UAC SHOULD close policy channels to
policy server that are not listed any more. The UAC MUST contact
policy servers in the order in which they were discovered in the most
recent 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 of policy servers to which the UAS has currently established a
policy channel, then the UAS MUST set up a policy channel to all new
policy servers and contact them. The UAS SHOULD close policy
channels to servers that are not listed any more. The UAS MUST use
policy servers in the order in which they were contained in the most
recent Policy-Contact header.
A UA MUST inform the policy server when a session is terminated via
the policy channel, unless a policy server indicates via the policy
channel that it does not need to be contacted at the end of the
session. This enables a policy server to free all resources it has
allocated for this session.
4.5.2. Contacting the Policy Server
A UA MUST contact all policy servers to which it has established a
policy channel before sending or after receiving a mid-dialog
request. The UA MUST contact the policy server in the order in which
they were discovered most recently.
A UA that receives a SIP message containing an offer or answer SHOULD
completely process the message (e.g., according to RFC3261 [RFC3261])
before contacting the policy server. The SIP processing of the
message includes, for example, updating dialog state and timers as
well as creating ACK or PRACK requests as necessary. This ensures
that contacting a policy server does not interfere with SIP message
processing and timing (e.g., by inadvertently causing timers to
expire). This implies, for example, that a UAC which has received a
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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 discussed in the next
paragraph.
In some cases, a UA needs to use the offer/answer it has received in
a SIP message to create an ACK or PRACK response for this message,
i.e., it needs to use the offer/answer before finishing the SIP
machinery for 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 answer before it can send the answer in 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 can always respond
immediately to a policy request and does not require manual
intervention to create a policy. This will be the case for most
policy servers. If, however, a policy server cannot respond with
a policy right away, it can return a policy that temporarily
denies the session and update this policy as the actual policy
decision becomes available. A delay in the response from the
policy server to the UA would delay the transmission of the ACK
and could trigger retransmissions of the INVITE response (also see
the recommendations for Flow I in RFC3725 [RFC3725]).
The case of multiple policy servers providing policies to the same UA
requires additional considerations. A policy returned by one policy
server can contain information that needs to be shared with the other
policy servers. For example, two policy servers may have the policy
to insert a media intermediary by modifying the IP addresses and
ports of media streams. In order for media streams to pass through
both intermediaries, each intermediary needs to know the IP address
and port on which the other media intermediary is expecting the
stream to arrive. If media streams are flowing in both directions,
this means that each intermediary needs to know IP addresses and
ports of the other intermediary.
UAs usually contact a policy server twice during an offer/answer
exchange (unless a policy server indicates that it only needs to be
contacted once). Therefore the case of multiple policy servers
providing policies to a single UA does not require additional steps
in most cases. However, a UAS usually contacts each policy server
only once (see Figure 4). If a session policy returned by one of the
policy servers requires that information is shared between multiple
servers and the UAS receives policies from more than one policy
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server, the UAS MUST contact all policy servers a second time after
contacting all servers the first time. Whether or not this second
round is required is determined by the type of information returned
by the policy server. The data format for session policies must
explicitly state if a second round is needed for a particular data
element. If a UAS supports this data format and receives such an
element, it knows that it must contact policy servers a second time.
If such a data element is modified during a mid-call offer/answer
exchange and multiple policy servers are providing policies to a UA
then all UAs MUST contact policy servers in a first and second round.
An example call flow for the UAS is shown in Appendix B.3.
4.5.3. Using Session Policies
A UA MUST disclose the session description(s) for the current session
to policy servers through the policy channel. The UA MUST apply
session policies it receives to the offer and, if one is received, to
the answer before using the offer/answer. If these policies are
unacceptable, the UA MUST NOT continue with the session. This means
that, the UA MUST cancel or reject a pending INVITE transaction for
the session or terminate the session if it is already in progress.
If the UA receives an unacceptable policy in an INVITE response, the
UA MUST complete the INVITE transaction and then terminate the
session.
When a UA receives a notification about a change in the current
policies, the UA MUST apply the updated policies to the current
session or the UA MUST terminate the session. If the policy update
causes a change in the session description of a session, then the UA
needs 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.
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
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 causes the UA to terminate the session.
A user agent transmits session information to a policy server for
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session-specific policies. This session information can contain
sensitive data the user does not want an eavesdropper or an
unauthorized policy server to see. Vice versa, session policies can
contain sensitive information about the network or service level
agreements the service provider does 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. authentication of the policy server and, if needed, the user
agent,
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 the 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, can 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.
Policy servers SHOULD authenticate UAs to protect the information
that is contained in a session policy. However, a policy server can
also 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.
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 4.4.5 of this document
6.2. Registration of the "Policy-Contact" Header
Name of Header: Policy-Contact
Short form: none
Normative description: Section 4.4.5 of this document
6.3. Registration of the "policy" SIP Option-Tag
This specification registers a new SIP option tag, as per the
guidelines in Section 27.1 of RFC3261 [RFC3261].
This document defines the SIP option tag "policy".
The following row has been added to the "Option Tags" section of the
SIP Parameter Registry:
+------------+------------------------------------------+-----------+
| Name | Description | Reference |
+------------+------------------------------------------+-----------+
| policy | This option tag is used to indicate that | this |
| | a UA can process policy server URIs for | document |
| | and subscribe to session-specific | |
| | policies. | |
+------------+------------------------------------------+-----------+
Name of option: policy
Description: Support for the Policy-Contact and Policy-Id headers.
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SIP headers defined: Policy-Contact, Policy-Id
Normative description: This document
7. References
7.1. Normative References
[I-D.ietf-sipping-config-framework]
Channabasappa, S., "A Framework for Session Initiation
Protocol User Agent Profile Delivery",
draft-ietf-sipping-config-framework-15 (work in progress),
February 2008.
[I-D.ietf-sipping-media-policy-dataset]
Hilt, V., "A User Agent Profile Data Set for Media
Policy", draft-ietf-sipping-media-policy-dataset-05 (work
in progress), November 2007.
[I-D.ietf-sipping-policy-package]
Hilt, V. and G. Camarillo, "A Session Initiation Protocol
(SIP) Event Package for Session-Specific Session
Policies", draft-ietf-sipping-policy-package-04 (work in
progress), August 2007.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2976] Donovan, S., "The SIP INFO Method", RFC 2976,
October 2000.
[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.
[RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
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[RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP)
UPDATE Method", RFC 3311, October 2002.
7.2. Informative References
[RFC3428] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C.,
and D. Gurle, "Session Initiation Protocol (SIP) Extension
for Instant Messaging", RFC 3428, December 2002.
[RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[RFC3725] 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.
[RFC3903] Niemi, A., "Session Initiation Protocol (SIP) Extension
for Event State Publication", RFC 3903, October 2004.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
Appendix A. Acknowledgements
Many thanks to Allison Mankin for the discussions and the suggestions
for this draft. Many thanks to Roni Even, Bob Penfield, Mary Barnes,
Shida Schubert and Keith Drage for reviewing the draft and to Vijay
Gurbani for the contributions to the security considerations.
Appendix B. Session-Specific Policies - Call Flows
The following call flows illustrate the overall operation of session-
specific policies including the policy channel protocol as defined in
the SIP Event Package for Session-Specific Session Policies
[I-D.ietf-sipping-policy-package].
The following abbreviations are used:
o: offer
o': offer modified by a policy
po: offer policy
a: answer
a': answer modified by a policy
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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 <po, 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 |
| | | |<------------------|
| | | | | |
| | | | | |
B.3. Multiple Policy Servers for UAS
UA A P A PS A PS B P B UA B
| | | | | |
| | | | | |
| | | | | |
|(1) INV <o> | | | |
|-------->| | | | |
| |(2) INV <o, uri PSA> | |
| |---------------------------->| |
| | | | |(3) INV <o, uri PSA, uri PSB>
| | | | |-------->|
| | |(4) SUBSCRIBE <o, a> |
| | |<----------------------------|
| | |(5) 200 OK | |
| | |---------------------------->|
| | |(6) NOTIFY <po, pa>| |
| | |---------------------------->|
| | |(7) 200 OK | |
| | |<----------------------------|
| | | |(8) SUBSCRIBE <o', a'>
| | | |<------------------|
| | | |(9) 200 OK |
| | | |------------------>|
| | | |(10) NOTIFY <po, pa>
| | | |------------------>|
| | | |(11) 200 OK |
| | | |<------------------|
| | |(12) SUBSCRIBE <o", a"> |
| | |<----------------------------|
| | |(13) 200 OK | |
| | |---------------------------->|
| | |(14) NOTIFY <po, pa> |
| | |---------------------------->|
| | |(15) 200 OK | |
| | |<----------------------------|
| | | |(16) SUBSCRIBE <o", a">
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| | | |<------------------|
| | | |(17) 200 OK |
| | | |------------------>|
| | | |(18) NOTIFY <po, pa>
| | | |------------------>|
| | | |(19) 200 OK |
| | | |<------------------|
| | | | |(20) 200 OK <a">
| | | | |<--------|
| |(21) 200 OK <a"> | | |
| |<----------------------------| |
|(22) 200 OK <a"> | | | |
|<--------| | | | |
|(23) ACK | | | | |
|------------------------------------------------>|
| | | | | |
| | | | | |
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
Jonathan Rosenberg
Cisco
Edison, NJ
USA
Email: jdrosen@cisco.com
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