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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.
1.
Introduction
2.
Terminology
3.
Session-Independent Policies
3.1.
Architecture and Overview
3.2.
Policy Subscription
3.2.1.
UAC Behavior
3.2.2.
UAS Behavior
4.
Session-Specific Policies
4.1.
Architecture
4.2.
Overview
4.3.
Examples
4.3.1.
Offer in Request
4.3.2.
Offer in Response
4.4.
UA/Policy Server Rendezvous
4.4.1.
UAC Behavior
4.4.2.
Proxy Behavior
4.4.3.
UAS Behavior
4.4.4.
Caching the Local Policy Server URI
4.4.5.
Header Field Definition and Syntax
4.5.
Policy Channel
4.5.1.
Creation and Management
4.5.2.
Contacting the Policy Server
4.5.3.
Using Session Policies
5.
Security Considerations
6.
IANA Considerations
6.1.
Registration of the "Policy-Id" Header Field
6.2.
Registration of the "Policy-Contact" Header Field
6.3.
Registration of the "policy" SIP Option-Tag
7.
References
7.1.
Normative References
7.2.
Informative References
Appendix A.
Acknowledgements
Appendix B.
Session-Specific Policies - Call Flows
B.1.
Offer in Invite
B.2.
Offer in Response
B.3.
Multiple Policy Servers for UAS
§
Authors' Addresses
§
Intellectual Property and Copyright Statements
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The Session Initiation Protocol (SIP) [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) 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] (Handley, M., Jacobson, V., and C. Perkins, “SDP: Session Description Protocol,” July 2006.).
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 could 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 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.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].
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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.
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+-------------+
/------| 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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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 (Channabasappa, S., “A Framework for Session Initiation Protocol User Agent Profile Delivery,” February 2010.) [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] (Channabasappa, S., “A Framework for Session Initiation Protocol User Agent Profile Delivery,” February 2010.) 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:
If a subscriber is unable to establish a subscription, the subscriber 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] (Hilt, V., Worley, D., Camarillo, G., and J. Rosenberg, “A User Agent Profile Data Set for Media Policy,” March 2010.). 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 field 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 (Session-Specific Policies)). Thus, session-independent policies are always applied before session-specific policies are retrieved.
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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.
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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 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.
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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.
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.
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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 involves the following two fundamental tasks:
The communication between UA and policy server on the policy channel involves the following steps:
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.
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This section provides two examples to illustrate the overall operation of session-specific policies. The call flows depict the rendezvous mechanism between UA and policy server and indicate the points at which the UA exchanges policy information with the policy server.
The example is based on the following scenario: there are two domains (domain A and domain B), which both have session-specific policies for the UAs in their domain. Both domains do not provide policies to the UAs outside of their domain. The two domains have a proxy (P A and P B) and a policy server (PS A and PS B). The policies in both domains involve the session description offer and answer.
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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 request to proxy P A. P A knows that policies apply to this session and (2) returns a 488 (Not Acceptable Here) response 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 field to indicate that it has already contacted PS A. P A does not reject the INVITE request this time and removes the Policy-Id header field when forwarding the INVITE request. P B adds a Policy-Contact header field 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) response. (9) UA A contacts PS A with the answer and (10) retrieves answer policies from PS A.
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) | | | |
| Figure 3 |
| TOC |
The call flow shown in Figure 4 depicts an INVITE transaction with the offer in the response.
(1) UA A sends an INVITE request 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 field to indicate that it has contacted PS A. P A does not reject the INVITE request this time and removes the Policy-Id header field when forwarding the INVITE request. P B adds a Policy-Contact header field 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) response. (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 request, 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 request. (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) | | | | | | 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 |
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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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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] (Rosenberg, J. and H. Schulzrinne, “An Offer/Answer Model with Session Description Protocol (SDP),” June 2002.) (e.g., INVITE, UPDATE [RFC3311] (Rosenberg, J., “The Session Initiation Protocol (SIP) UPDATE Method,” October 2002.) and PRACK [RFC3262] (Rosenberg, J. and H. Schulzrinne, “Reliability of Provisional Responses in Session Initiation Protocol (SIP),” June 2002.) 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 field is a new header field defined in this specification. It contains the URI of a policy server. A 488 (Not Acceptable Here) response with this header field 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 field, 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 (Policy Channel). 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 resend 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 field, the UAC SHOULD use a secured transport protocol such as TLS between UAC and proxy.
The UAC MUST insert a Policy-Id header field into requests for which it has contacted a policy server and accepted the policies received. The Policy-Id header field is a new header field that is defined in this specification. The UA MUST create a Policy-Id header field value for each policy server it has contacted during the preparation of the request. A Policy-Id header field 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 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] (Hilt, V., Worley, D., Camarillo, G., and J. Rosenberg, “A User Agent Profile Data Set for Media Policy,” March 2010.). If the UAC has received a token from the policy server the UAC MUST include the token in the Policy-Id header field. The format of the Policy-Id header field is defined in Section 4.4.5 (Header Field Definition and Syntax).
The main purpose of the Policy-Id header field 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) response for each new request, a UA SHOULD maintain a cache that contains the URI of the local policy server (see Section 4.4.4 (Caching the Local Policy Server URI)). 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 (Policy Channel)) before sending the request. This avoids the retransmission of all policy server URIs in 488 (Not Acceptable Here) responses for mid-dialog requests.
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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).
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 field or the local policy server URI is not present in this header field, then the proxy MAY reject the request with a 488 (Not Acceptable Here) response. The proxy MUST insert a Policy-Contact header field 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 (Caching the Local Policy Server URI)).
If the local policy server URI is present in a Policy-Id header field 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 field 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 field 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 field 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 field 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 field 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 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 field 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 field, the proxy can determine if the token in the Policy-Id header field 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 field.
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 field values.
A proxy MUST use the Record-Route mechanism [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) if its associated policy server has session policies that apply to mid-dialog requests. The Record-Route header field 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 field of responses. The proxy knows that the UAS supports this extension if the Supported header field 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 field it has inserted into the request.
To protect the integrity of the policy server URI in a Policy-Contact header field, the proxy SHOULD use a secured transport protocol such as TLS between proxy and UAs.
| TOC |
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 field. The UAS MUST contact the policy server URIs in the order in which they were contained in the Policy-Contact header field, 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) response). This is to avoid that the failure of a policy server prevents a UA from communicating. Since this session might not be policy compliant without the policy subscription, it can 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 field, 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. The UAS MAY include this option tag in all responses. This way, a proxy that has inserted the Policy-Contact header field can know that the header field was understood by the UAS.
| TOC |
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 field of a request or a 488 (Not Acceptable Here) response. The UA can also receive the local policy server URI through configuration, for example, via the configuration framework [I‑D.ietf‑sipping‑config‑framework] (Channabasappa, S., “A Framework for Session Initiation Protocol User Agent Profile Delivery,” February 2010.). If a UA has received a local policy server URI through configuration and receives another local policy server URI in a Policy-Contact header field, the UA SHOULD overwrite the configured URI with the most recent one received in a Policy-Contact header field.
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.
| TOC |
| TOC |
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 field identifies all policy servers the UAC has contacted for this request.
The value of a Policy-Id header field consists of a policy server URI and an optional token parameter. The token parameter contains a token the UA might have received from the policy server.
The syntax of the Policy-Id header field is described below in ABNF, according to RFC5234 (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.) [RFC5234], as an extension to the ABNF for SIP in RFC3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [RFC3261]:
Policy-Id = "Policy-Id" HCOLON policyURI
*(COMMA policyURI)
policyURI = ( SIP-URI / SIPS-URI / absoluteURI )
[ SEMI token-param ] *( SEMI generic-param )
token-param = "token=" token
| TOC |
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 field 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 is not intended to 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 described below in ABNF, according to RFC5234 (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.) [RFC5234], as an extension to the ABNF for SIP in RFC3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [RFC3261]:
Policy-Contact = "Policy-Contact" HCOLON policyContactURI
*(COMMA policyContactURI)
policyContactURI = ( SIP-URI / SIPS-URI / absoluteURI )
[ SEMI "non-cacheable" ] *( SEMI generic-param )
Tables 1 and 2 are extensions of Tables 2 and 3 in RFC 3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [RFC3261]. The column "INF" is for the INFO method [RFC2976] (Donovan, S., “The SIP INFO Method,” October 2000.), "PRA" is for the PRACK method [RFC3262] (Rosenberg, J. and H. Schulzrinne, “Reliability of Provisional Responses in Session Initiation Protocol (SIP),” June 2002.), "UPD" is for the UPDATE method [RFC3311] (Rosenberg, J., “The Session Initiation Protocol (SIP) UPDATE Method,” October 2002.), "SUB" is for the SUBSCRIBE method [RFC3265] (Roach, A., “Session Initiation Protocol (SIP)-Specific Event Notification,” June 2002.), "NOT" is for the NOTIFY method [RFC3265] (Roach, A., “Session Initiation Protocol (SIP)-Specific Event Notification,” June 2002.), "MSG" is for the MESSAGE method [RFC3428] (Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C., and D. Gurle, “Session Initiation Protocol (SIP) Extension for Instant Messaging,” December 2002.), "REF" is for the REFER method [RFC3515] (Sparks, R., “The Session Initiation Protocol (SIP) Refer Method,” April 2003.), and "PUB" is for the PUBLISH method [RFC3903] (Niemi, A., “Session Initiation Protocol (SIP) Extension for Event State Publication,” October 2004.).
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
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
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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] (Hilt, V. and G. Camarillo, “A Session Initiation Protocol (SIP) Event Package for Session-Specific Session Policies.,” March 2010.) defines a SUBSCRIBE/NOTIFY-based [RFC3265] (Roach, A., “Session Initiation Protocol (SIP)-Specific Event Notification,” June 2002.) 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] (Hilt, V. and G. Camarillo, “A Session Initiation Protocol (SIP) Event Package for Session-Specific Session Policies.,” March 2010.). 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.
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A UA discovers the list of policy servers relevant for a session during the initial offer/answer exchange (see Section 4.4 (UA/Policy Server Rendezvous)). 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 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) response with a Policy-Contact header field 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 field 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 field.
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.
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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 servers 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 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [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 response to an INVITE request should finish the processing of the response including transmitting the ACK request 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 request 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 request. 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 request.
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 request and could trigger retransmissions of the INVITE response (also see the recommendations for Flow I in 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),” April 2004.) [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 can 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 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 explicitly states 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 is expected to 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 (Multiple Policy Servers for UAS).
| TOC |
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.
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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 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:
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 header fields 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 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.
| TOC |
| TOC |
Name of Header Field: Policy-Id
Short form: none
Normative description: Section 4.4.5 (Header Field Definition and Syntax) of this document
| TOC |
Name of Header Field: Policy-Contact
Short form: none
Normative description: Section 4.4.5 (Header Field Definition and Syntax) of this document
| TOC |
This specification registers a new SIP option tag, as per the guidelines in Section 27.1 of RFC3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [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 header fields.
SIP header fields defined: Policy-Contact, Policy-Id
Normative description: This document
| TOC |
| TOC |
| [I-D.ietf-sipping-config-framework] | Channabasappa, S., “A Framework for Session Initiation Protocol User Agent Profile Delivery,” draft-ietf-sipping-config-framework-17 (work in progress), February 2010 (TXT). |
| [I-D.ietf-sipping-media-policy-dataset] | Hilt, V., Worley, D., Camarillo, G., and J. Rosenberg, “A User Agent Profile Data Set for Media Policy,” draft-ietf-sipping-media-policy-dataset-09 (work in progress), March 2010 (TXT). |
| [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-08 (work in progress), March 2010 (TXT). |
| [RFC2119] | Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML). |
| [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 (TXT). |
| [RFC3262] | Rosenberg, J. and H. Schulzrinne, “Reliability of Provisional Responses in Session Initiation Protocol (SIP),” RFC 3262, June 2002 (TXT). |
| [RFC3264] | Rosenberg, J. and H. Schulzrinne, “An Offer/Answer Model with Session Description Protocol (SDP),” RFC 3264, June 2002 (TXT). |
| [RFC3265] | Roach, A., “Session Initiation Protocol (SIP)-Specific Event Notification,” RFC 3265, June 2002 (TXT). |
| [RFC3311] | Rosenberg, J., “The Session Initiation Protocol (SIP) UPDATE Method,” RFC 3311, October 2002 (TXT). |
| [RFC5234] | Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” STD 68, RFC 5234, January 2008 (TXT). |
| TOC |
| [RFC2976] | Donovan, S., “The SIP INFO Method,” RFC 2976, October 2000 (TXT). |
| [RFC3428] | Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C., and D. Gurle, “Session Initiation Protocol (SIP) Extension for Instant Messaging,” RFC 3428, December 2002 (TXT). |
| [RFC3515] | Sparks, R., “The Session Initiation Protocol (SIP) Refer Method,” RFC 3515, April 2003 (TXT). |
| [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 (TXT). |
| [RFC3903] | Niemi, A., “Session Initiation Protocol (SIP) Extension for Event State Publication,” RFC 3903, October 2004 (TXT). |
| [RFC4566] | Handley, M., Jacobson, V., and C. Perkins, “SDP: Session Description Protocol,” RFC 4566, July 2006 (TXT). |
| TOC |
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.
| TOC |
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 (Hilt, V. and G. Camarillo, “A Session Initiation Protocol (SIP) Event Package for Session-Specific Session Policies.,” March 2010.) [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
pa: answer policy
ps uri: policy server URI (in Policy-Contact header field)
ps id: policy server id (in Policy-Id header field)
| TOC |
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 | | | | |------------------>| | | | | | | | | | | | | | | |
| TOC |
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'> | | | | |------------------------------------------------>| | | | |(23) SUBSCRIBE <o', a'> | | | |<------------------| | | | |(24) 200 OK | | | | |------------------>| | | | |(25) NOTIFY <po, pa> | | | |------------------>| | | | |(26) 200 OK | | | | |<------------------| | | | | | | | | | | | |
| TOC |
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"> | | | |<------------------| | | | |(17) 200 OK | | | | |------------------>| | | | |(18) NOTIFY <po, pa> | | | |------------------>| | | | |(19) 200 OK | | | | |<------------------| | | | | |(20) 200 OK <a"> | | | | |<--------| | |(21) 200 OK <a"> | | | | |<----------------------------| | |(22) 200 OK <a"> | | | | |<--------| | | | | |(23) ACK | | | | | |------------------------------------------------>| | | | | | | | | | | | |
| TOC |
| 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 |
| TOC |
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