Internet DRAFT - draft-hilt-sipping-session-spec-policy
draft-hilt-sipping-session-spec-policy
SIPPING Working Group V. Hilt
Internet-Draft Bell Labs/Lucent Technologies
Expires: January 13, 2006 G. Camarillo
Ericsson
J. Rosenberg
Cisco Systems
July 12, 2005
A Delivery Mechanism for Session-Specific Session Initiation Protocol
(SIP) Session Policies
draft-hilt-sipping-session-spec-policy-03
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This specification defines a delivery mechanism for session-specific
Session Initiation Protocol (SIP) sessions policies.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Overview of Operation . . . . . . . . . . . . . . . . . . . . 7
4.1 Offer in Request . . . . . . . . . . . . . . . . . . . . . 7
4.2 Offer in Response . . . . . . . . . . . . . . . . . . . . 9
5. UA/Policy Server Rendezvous . . . . . . . . . . . . . . . . . 10
5.1 UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . 10
5.2 UAS Behavior . . . . . . . . . . . . . . . . . . . . . . . 12
5.3 Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . 12
5.4 Header Definition and Syntax . . . . . . . . . . . . . . . 13
6. Policy Channel . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1 Session Information . . . . . . . . . . . . . . . . . . . 14
6.2 Session Policies . . . . . . . . . . . . . . . . . . . . . 15
6.2.1 Event Header Parameters . . . . . . . . . . . . . . . 15
6.2.2 The Use of URIs . . . . . . . . . . . . . . . . . . . 16
6.2.3 Subscriber Behavior . . . . . . . . . . . . . . . . . 16
6.2.4 Notifier Behavior . . . . . . . . . . . . . . . . . . 16
6.2.5 Example . . . . . . . . . . . . . . . . . . . . . . . 16
7. Updating Policies . . . . . . . . . . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.1 Normative References . . . . . . . . . . . . . . . . . . . 17
10.2 Informative References . . . . . . . . . . . . . . . . . . 18
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . 20
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1. Introduction
Some domains have policies in place, which impact the sessions
established using the Session Initiation Protocol (SIP) [4]. These
policies are often needed to support the network infrastructure or
the execution of services. For example, wireless networks usually
have limited resources for media traffic. During periods of high
activity, a wireless network provider wants to restrict the bandwidth
that is available in a session. With session policies, the user
agent is able to learn about the current bandwidth limit and can make
an informed decision about the number of streams, the media types,
and the codecs it can use in that session. Similarly, a user agent
can be informed that certain codecs or media types are disallowed and
may not be used in the current session.
In another example, a SIP user agent is using a network which
connects to the public Internet through a firewall or a network
border device. The provider would like to tell the user agent to
direct the media streams to the appropriate IP addresses and ports of
that firewall or border device. Knowing this policy enables the user
agent to setup sessions with other user agents across the firewall or
the network border.
In a third example, a domain wants to perform QoS marking and traffic
shaping on media streams. This functionality is implemented in a
media intermediary. With session policies, such a media intermediary
can be inserted into the media path. In contrast to other methods,
the use of session policies does not require the inspection or
modification of SIP message bodies by intermediaries (a discussion of
this and other design aspects can be found in [8]).
Domains sometimes enforce policies they have in place. For example,
a domain might have a configuration in which all packets containing a
certain audio codec are dropped. Unfortunately, enforcement
mechanisms usually do not inform the user about the policies they are
enforcing and silently keep the user from doing anything against
them. This may lead to the malfunctioning of devices that is
incomprehensible to the user. With session policies, the user knows
about the restricted codecs and can use a different codec or simply
connect to a domain with less stringent policies. 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.
Session-policies can be set up in two different ways: specifically
for a session or independent of a session. Session-specific policies
are created for one particular session, usually under consideration
of certain aspects of this session (e.g. the IP addresses and ports
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that are used for media). Since session-specific policies are
tailored to a session, they only apply to the session they are
created for. These policies require a delivery mechanism that
enables the exchange of session policy information at the time a
session is established. This document defines such a delivery
mechanism. It enables user agents to submit session details to a
policy server and allows the policy server to provide policies for
this session in response.
Session-independent policies on the other hand are independent of a
specific session and generally apply to the sessions set up by a user
agent. An example is a policy which generally prohibits the use of
high-bandwidth codecs. In principle, these policies could also be
delivered to user agents individually for each session, using the
session-specific delivery mechanism. However, since these policies
apply to many sessions, it is more efficient to deliver them to user
agents only when the user agent is initialized or a policy changes.
The framework for session-independent policies [6] defines a delivery
mechanism for session-independent policies. It also defines a
minimal session policy format aimed at achieving interoperability
between different user agents and policy servers. This policy format
is independent of the policy delivery mechanism and can be used for
session-independent as well as for 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. Architecture
+-------------+
/------| Proxy |----...
+----+ / +-------------+
| |---/ +-------------+
| | | Policy |
| UA |============| Server |
| | +-------------+
| |**** +-------------+
+----+ * | Router w/ |
*******| Policy |****...
| Enforcement |
+-------------+
--- SIP Signaling
=== Policy Channel
*** Media
Figure 1
The following entities are involved in setting up session-specific
policies (see Figure 1): a user agent (UA), a proxy, a policy server
and possibly a router with policy enforcement functionality.
The proxy's role is to provide a rendezvous mechanism for UA and
policy server. It conveys the URI of the policy server in its domain
to UAs and ensures that UAs know 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. Each domain has at most one policy
server. The role of the policy server is to generate session
policies for a session. It receives session information from a UA,
generates a policy and returns that policy back to the UA. The way
policies are generated is outside the scope of this specification. A
policy server could, for example, use local rules, query external
sources for additional information or retrieve policies from a
separate policy infrastructure.
A UA receives the URI of a policy server from the proxy. It uses
this URI to establish a policy channel 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
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enforcement of session policies and the mechanisms defined here are
orthogonal a policy enforcement infrastructure. Their goal is to
provide a means for the UA to convey session information to a policy
server and to receive the policies that apply to this session in
response.
The protocol defined in this specification follows a separate channel
model. 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 session and
policy information between the user agent and the policy server is
piggybacked onto SIP signaling messages exchanged between the two
user agents.
A disadvantage of the separate channel model is that it requires
additional messages for the exchange of policy information. The
advantages of using a separate policy channel is that it decouples
the exchange of signaling messages between endpoints from the
exchange of policy information between endpoint and policy server.
This decoupling enables the use of encryption on the signaling path
(to secure the communication between endpoints) and on the policy
channel (to secure the communication between endpoint and policy
server). Existing schemes for authorization, authentication, signing
and encryption can be used on the policy channel. This is not
possible if policies are piggybacked onto the signaling messages.
Another advantage of the separate channel model is that policies do
not travel along the signaling path possibly crossing may domains.
If policy server and UA are in the same network, policy information
never leaves this network. In addition, endpoints can specifically
decide which aspects of a session they want to disclose to a certain
policy server. Finally, a policy server does not rely on a SIP
signaling message flowing by to provide a session policy to an
endpoint. A policy server can use the separate channel at any time
to update session policies as needed.
The communication on the policy channel between a UA and a policy
server involves two main operations:
1. The UA discloses information about the current session and the
offer/answer exchange to the policy server.
2. The policy server sends policy instructions to the UA.
Some types of policies do not involve sending policy instructions,
but only information disclosure. Still, a general session-specific
policy mechanism needs to support both operations.
The same way, some policy servers only need to inspect the offer, but
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not the answer. Nevertheless, a general mechanism needs to consider
policy servers which need to inspect both.
Finally, some policy servers need to update the session policies that
have been sent to a UA. Again, a general mechanism should provide
this capability.
4. Overview of Operation
This section provides example call flows to illustrate the
establishment of session-specific policies. It does not contain a
normative protocol definition.
In the following scenario, there are two domains (domain A and domain
B), which both have session-specific policies for UAs in their
domain. Both domains do not provide policies to 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.
4.1 Offer in Request
The first call flow depicts an INVITE transaction with the offer in
the request. It is assumed that the UAC does not have previous
knowledge about the policy server in its 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. (3) UA A contacts 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 2
4.2 Offer in Response
This call flow depicts an INVITE transaction with the offer in the
response.
Steps (1) - (8) are analogous to steps (1) - (8) in the above 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 [7].
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 3
5. 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.
5.1 UAC Behavior
When a UA compliant to this specification generates an INVITE or
UPDATE request, it MUST include a Supported header field with the
option tag "policy" in the request.
A UAC may receive a 488 in response to an INVITE or UPDATE request,
which contains a Policy-Contact header field. This is a new header
that contains the URI of a policy server. A 488 response with this
header is generated by a proxy to convey the URI of the local policy
server to the UAC. The UAC SHOULD contact this URI to retrieve the
session policies that apply to the current request. If the UAC
decides to accept the received policies, it SHOULD apply them to the
request and resend the updated request.
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If the UAC has applied session policies to a request, it MUST insert
a Policy-Id header into that request. The Policy-Id header MUST
include the URIs of all policy servers the UAC has contacted during
the processing of that request. The Policy-Id header enables a proxy
to determine whether the URI of its 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 the policy
server URI received in a 488 response before it resends the request,
session policies are always applied to a session in the order in
which the request traverses through these domains. Policies of the
local network are applied first (since the local proxy is the first
proxy that responds with a 488 response), policies of the first
policy-enabled transit network are applied next, and so on. The
order in which policies are applied to a session may be significant,
for example, if a policy inserts media intermediaries into the media
path.
Session policies may apply to the offer, the answer or both session
descriptions. Depending on the requirements of the policy, a UAC may
need to contact the policy server with the offer and with the answer.
A UAC MUST always contact the same policy servers for the offer and
the answer. If the UAC receives an answer in the response to an
INVITE request (i.e. the request contained the offer), it MUST send
the ACK before retrieving the policies for the answer from the policy
server. If the UAC receives a response with an offer (i.e. the
INVITE request did not contain an offer), the UAC MUST first contact
the policy server to retrieve session policies and apply these
policies before sending the answer in the ACK. The answer in the ACK
will therefore already consider the relevant policies.
This approach 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 [7]).
A UAC SHOULD cache the URI of the local policy server. It receives
this URI in a 488 from the proxy in the local domain. The UAC SHOULD
use this URI to retrieve session policies for a new INVITE or UPDATE
request before it is sent. Caching the local policy server URI
avoids the retransmission of this URI for each new INVITE or UPDATE
request. Some domains may want to prevent the UAC from caching the
local policy server URI. For example, if the policy server does not
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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". The UA SHOULD NOT cache a non-cacheable
policy server URI and SHOULD remove the current URI from its cache
when receiving such a URI.
The UAC 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 and may traverse different domains.
The UAC SHOULD maintain a list of policy server URIs for each dialog.
This list SHOULD include all policy server URIs that were contacted
for the initial INVITE that created the dialog. The UAC should keep
this list until the dialog is terminated. The UAC SHOULD contact the
policy server URIs in this list before sending an INVITE or UPDATE
request within that dialog. This avoids the retransmission of policy
server URIs for mid-dialog requests. Contacting policy servers for
mid-dialog INVITE or UPDATE requests is needed to enable policy
servers to keep track of the session description and to update
policies accordingly.
5.2 UAS Behavior
An incoming INVITE or UPDATE request may contain a Policy-Contact
header with a list of policy server URIs. The UAS SHOULD use these
URIs to retrieve session policies. The UAS MUST use the policy
server URIs in the order in which they were contained in the Policy-
Contact header, starting with the topmost value.
If the UAS receives an ACK with an answer, it may need to contact the
policy servers again depending on the policy. In this case, it MUST
contact the same policy servers it has contacted for the offer.
5.3 Proxy Behavior
A proxy may provide the URI of the local policy server to the UAC or
the UAS when processing an INVITE or UPDATE request.
If an INVITE or UPDATE 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 check whether the request has a
Policy-Id header field that already contains this policy server URI.
If the request does not have such a header or the local policy server
URI is not present in that 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 the local policy server.
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The proxy MAY add the header field parameter "non-cacheable" to
prevent the UAC from caching this policy server URI.
If the local policy server URI is already present in the Policy-Id
header of an INVITE or UPDATE 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.
The proxy MAY insert a Policy-Contact header field into an INVITE or
UPDATE request 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 before of all existing values at the
beginning of the list. A proxy MUST NOT change the order of existing
Policy-Contact header values.
5.4 Header Definition and Syntax
The Policy-Id header field is inserted into an INVITE or UPDATE
request by the UAC. It identifies all policy servers the UAC has
contacted for the 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 absoluteURI
*(COMMA absoluteURI)
The Policy-Contact header field can be inserted into INVITE and
UPDATE requests by a proxy. 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 Policy-Contact header field can also be inserted into a 488
response to an INVITE or UPDATE request by a proxy. 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 the
UAC should not cache the policy server URI.
The syntax of the Policy-Contact header field is:
Policy-Contact = "Policy-Contact" HCOLON policyURI
*(COMMA policyURI)
policyURI = absoluteURI [ SEMI "non-cacheable" ]
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The BNF for absoluteURI is defined in [4].
Table 1 is an extension of Tables 2 and 3 in [4]. The column 'UPD'
is for the UPDATE method [3].
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
Figure 6
6. Policy Channel
The policy channel is set up between the UA and the policy server.
This channel is needed to accomplish two tasks: first, to convey
information about the current session from the UA to the policy
server and, second, to return the policies for that session from the
policy server back to the UA.
6.1 Session Information
OPEN ISSUE: Which method should be used to convey session information
from the UA to the policy server? Use cases for session-specific
policies that may help resolving this issue are discussed in [6].
The following proposals have been made:
1. SIP SUBSCRIBE: session information is conveyed to the policy
server in the body of SUBSCRIBE requests. The UA subscribes to
session policies for each session. Semantically, session
information is a filter criteria that selects the policies, which
apply to the current session from the pool of all available
policies. This semantics may not be applicable to all policies,
in particular, if they are generated dynamically based on session
information. Also, session information can only be provided by
the subscriber and not by a third party.
2. SIP PUBLISH: the UA submits session information to the policy
server in the body of a SIP PUBLISH request. The policy server
uses this information to generate policies and makes these
policies available for subscriptions. The UA can subscribe to
these policies and will receive all policies (new or updated) via
NOTIFY requests. The subscription can be established at the same
time the PUBLISH request is sent. The UA may even use a single
subscription to receive the policies for all sessions it sets up.
In this case, each NOTIFY would cover all policies from that
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server (content indirection may be used).
3. HTTP: Similar to PUBLISH. Instead of using a PUBLISH request,
the UA submits session information in the body of a HTTP request.
The policies are received through a subscription. The UA needs
two policy server URIs: a SIP URI (for the subscription) and a
HTTP URI (to upload session information).
4. XCAP: Similar to HTTP. Instead of using plain HTTP, XCAP is used
to upload session information.
OPEN ISSUE: Which information should be disclosed to the policy
server. Is this policy specific? Or should the UA generally
disclose the session description?
6.2 Session Policies
The UA accesses the policies that apply to the current session
through the policy server URIs it has received during session
establishment (see Section 5). The UA subscribes to these URIs and
receives the current session policies. The policies for a session
may change while the session is in progress. The UA is notified
about updates to policies through the subscription.
The session policy documents may be contained directly in the body of
a NOTIFY message or they may be retrieved from an URI contained in
the NOTIFY via content indirection.
The subscription to session-specific policies is based on the
Framework for SIP User Agent Profile Delivery [2]. It uses the new
profile-type 'policy', which is defined in this document. Defining a
new profile type for session-policies enables the decoupling of
session-specific policies from other sources of profile information,
such as user, device, or local profiles. 'Policy' profiles are
provided by domains that have session-specific policies in place.
6.2.1 Event Header Parameters
The new token 'policy' is defined for the 'profile-type' event header
parameter. This extends the syntax of the profile-type event header
parameter [2] as follows:
profile-types = "device" / "user" / "application" /
"local" / "policy"
A SUBSCRIBE request for the policy profile-type may contain the
'network-user' parameter with the user's AoR. This parameter may be
needed since the subscription URI does not reveal the user's AOR.
Knowing the user's AOR may help a policy server to decide whether or
not to accept a subscription and to determine which policies are
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applicable.
6.2.2 The Use of URIs
The SUBSCRIBE request URI for the 'policy' profile is a policy server
URI the user agent has received through mechanisms described in
Section 5.
A policy server URI MAY contain a 'document' URI parameter when it is
received by the UA. This parameters can be used to identify a
specific document on the policy server, to which the UA should
subscribe. If this parameter is present in a policy server URI, it
MUST be copied into the 'document' event header parameter of the
SUBSCRIBE request. The 'document' parameter MUST be removed from the
policy server URI before it is used in the SUBSCRIBE request URI.
6.2.3 Subscriber Behavior
The 'policy' profile SHOULD be used when subscribing to a policy
server URI. The UA SHOULD establish a separate subscription to each
policy server URI it has received. It may receive session-specific
policies through each of these subscriptions. The subscriber SHOULD
include the 'network-user' parameter in the SUBSCRIBE request.
6.2.4 Notifier Behavior
A notifier (i.e. a policy server) MUST immediately respond to
SUBSCRIBE requests and MUST immediately send a NOTIFY in case it
accepts the subscription. If the notifier cannot respond with a
session policy right away, it must send an empty policy document and
later update this policy. Note that updating a policy may require
the subscriber to re-negotiate session parameters and should be
avoided if possible.
The timely transmission of session policies is in particular
important if the UA (i.e. the subscriber) is requesting policies
for an offer in an INVITE response (see Section 4.2).
The policy server SHOULD authenticate the user before submitting a
policy that grants additional privileges to the user.
6.2.5 Example
The following example contains a policy server URI and a SUBSCRIBE
message.
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Policy Server URI:
sip:policy@ps.example.com;document=session-id/48ei48rj474k
SUBSCRIBE sip:policy@ps.example.com SIP/2.0
Via: SIP/2.0/TCP terminal.example.com;branch=z9hG4bK6d6d35b6
Event: sip-profile;profile-type="policy";
document="session-id/48ei48rj474k";
vendor="vendor.example.com";model="Z100";version="1.2.3";
network-user="alice@example.com"
To: sip:policy@ps.example.com
From: sip:alice@example.com;tag=1234
Call-ID: ue8K743jRhr83@terminal.example.com
CSeq: 1 SUBSCRIBE
Contact: <sip:alice@terminal.example.com>
Accept: message/external-body, application/session-policy+xml
Content-Length: 0
7. Updating Policies
A UA may receive policy updates through a policy channel. The UA
SHOULD apply these policies to the current session. It MUST generate
a re-INVITE or UPDATE request if the updated policies modify aspects
of the session that need to be communicated to the peer UA.
8. Security Considerations
In particular authentication and authorization are critical issues
that need to be addressed here.
[TBD.]
9. IANA Considerations
[TBD.]
10. References
10.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Petrie, D., "A Framework for Session Initiation Protocol User
Agent Profile Delivery", draft-ietf-sipping-config-framework-06
(work in progress), February 2005.
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[3] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
Method", RFC 3311, October 2002.
[4] 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.
10.2 Informative References
[5] Hilt, V. and G. Camarillo, "Use Cases for Session-Specific
Session Initiation Protocol (SIP) Session Policies",
draft-hilt-sipping-policy-usecases-00 (work in progress),
June 2005.
[6] Hilt, V., Camarillo, G., and J. Rosenberg, "Session Initiation
Protocol (SIP) Session Policies - Document Format and Session-
Independent Delivery Mechanism",
draft-ietf-sipping-session-indep-policy-03 (work in progress),
June 2005.
[7] 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.
[8] Rosenberg, J., "Requirements for Session Policy for the Session
Initiation Protocol (SIP)",
draft-ietf-sipping-session-policy-req-02 (work in progress),
July 2004.
Authors' Addresses
Volker Hilt
Bell Labs/Lucent Technologies
101 Crawfords Corner Rd
Holmdel, NJ 07733
USA
Email: volkerh@bell-labs.com
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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
Appendix A. Acknowledgements
Many thanks to Allison Mankin for the discussions and the suggestions
for this draft. A big thanks also to everyone who contributed by
providing feedback on the mailing list and in IETF meetings.
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