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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 SIPPING Working Group V. Hilt 3 Internet-Draft Bell Labs/Alcatel-Lucent 4 Intended status: Standards Track G. Camarillo 5 Expires: August 29, 2010 Ericsson 6 J. Rosenberg 7 jdrosen.net 8 February 25, 2010 10 A Framework for Session Initiation Protocol (SIP) Session Policies 11 draft-ietf-sip-session-policy-framework-07 13 Abstract 15 Proxy servers play a central role as an intermediary in the Session 16 Initiation Protocol (SIP) as they define and impact policies on call 17 routing, rendezvous, and other call features. This document 18 specifies a framework for SIP session policies that provides a 19 standard mechanism by which a proxy can define or influence policies 20 on sessions, such as the codecs or media types to be used. It 21 defines a model, an overall architecture and new protocol mechanisms 22 for session policies. 24 Status of this Memo 26 This Internet-Draft is submitted to IETF in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF), its areas, and its working groups. Note that 31 other groups may also distribute working documents as Internet- 32 Drafts. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 The list of current Internet-Drafts can be accessed at 40 http://www.ietf.org/ietf/1id-abstracts.txt. 42 The list of Internet-Draft Shadow Directories can be accessed at 43 http://www.ietf.org/shadow.html. 45 This Internet-Draft will expire on August 29, 2010. 47 Copyright Notice 48 Copyright (c) 2010 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (http://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the BSD License. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 64 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 65 3. Session-Independent Policies . . . . . . . . . . . . . . . . . 6 66 3.1. Architecture and Overview . . . . . . . . . . . . . . . . 6 67 3.2. Policy Subscription . . . . . . . . . . . . . . . . . . . 7 68 3.2.1. UAC Behavior . . . . . . . . . . . . . . . . . . . . . 7 69 3.2.2. UAS Behavior . . . . . . . . . . . . . . . . . . . . . 9 70 4. Session-Specific Policies . . . . . . . . . . . . . . . . . . 9 71 4.1. Architecture . . . . . . . . . . . . . . . . . . . . . . . 9 72 4.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 10 73 4.3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 12 74 4.3.1. Offer in Request . . . . . . . . . . . . . . . . . . . 12 75 4.3.2. Offer in Response . . . . . . . . . . . . . . . . . . 14 76 4.4. UA/Policy Server Rendezvous . . . . . . . . . . . . . . . 16 77 4.4.1. UAC Behavior . . . . . . . . . . . . . . . . . . . . . 16 78 4.4.2. Proxy Behavior . . . . . . . . . . . . . . . . . . . . 18 79 4.4.3. UAS Behavior . . . . . . . . . . . . . . . . . . . . . 20 80 4.4.4. Caching the Local Policy Server URI . . . . . . . . . 21 81 4.4.5. Header Field Definition and Syntax . . . . . . . . . . 22 82 4.5. Policy Channel . . . . . . . . . . . . . . . . . . . . . . 24 83 4.5.1. Creation and Management . . . . . . . . . . . . . . . 24 84 4.5.2. Contacting the Policy Server . . . . . . . . . . . . . 26 85 4.5.3. Using Session Policies . . . . . . . . . . . . . . . . 27 86 5. Security Considerations . . . . . . . . . . . . . . . . . . . 28 87 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 88 6.1. Registration of the "Policy-Id" Header Field . . . . . . . 29 89 6.2. Registration of the "Policy-Contact" Header Field . . . . 30 90 6.3. Registration of the "non-cacheable" Policy-Contact 91 Header Field Parameter . . . . . . . . . . . . . . . . . . 30 92 6.4. Registration of the "policy" SIP Option-Tag . . . . . . . 30 93 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31 94 7.1. Normative References . . . . . . . . . . . . . . . . . . . 31 95 7.2. Informative References . . . . . . . . . . . . . . . . . . 32 96 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 32 97 Appendix B. Session-Specific Policies - Call Flows . . . . . . . 32 98 B.1. Offer in Invite . . . . . . . . . . . . . . . . . . . . . 33 99 B.2. Offer in Response . . . . . . . . . . . . . . . . . . . . 35 100 B.3. Multiple Policy Servers for UAS . . . . . . . . . . . . . 36 101 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37 103 1. Introduction 105 The Session Initiation Protocol (SIP) [RFC3261] is a signaling 106 protocol for creating, modifying and terminating multimedia sessions. 107 A central element in SIP is the proxy server. Proxy servers are 108 intermediaries that are responsible for request routing, rendezvous, 109 authentication and authorization, mobility, and other signaling 110 services. However, proxies are divorced from the actual sessions - 111 audio, video, and session-mode messaging - that SIP establishes. 112 Details of the sessions are carried in the payload of SIP messages, 113 and are usually described with the Session Description Protocol (SDP) 114 [RFC4566]. 116 Experience has shown that there is a need for SIP intermediaries to 117 impact aspects of a session. For example, SIP can be used in a 118 wireless network, which has limited resources for media traffic. 119 During periods of high activity, the wireless network provider could 120 want to restrict the amount of bandwidth available to each user. 121 With session policies, an intermediary in the wireless network can 122 inform the user agent about the bandwidth it has available. This 123 information enables the user agent to make an informed decision about 124 the number of streams, the media types, and the codecs it can 125 successfully use in a session. Similarly, a network provider can 126 have a service level agreement with a user that defines the set of 127 media types the user can use. With session policies, the network can 128 convey the current set of policies to user agents, enabling them to 129 set up sessions without inadvertently violating any of the network 130 policies. 132 In another example, a SIP user agent is using a network which is 133 connected to the public Internet through a firewall or a network 134 border device. The network provider would like to tell the user 135 agent that it needs to send its media streams to a specific IP 136 address and port on the firewall or border device to reach the public 137 Internet. Knowing this policy enables the user agent to set up 138 sessions across the firewall or the network border. In contrast to 139 other methods for inserting a media intermediary, the use of session 140 policies does not require the inspection or modification of SIP 141 message bodies. 143 Domains often have the need to enforce the session policies they have 144 in place. For example, a domain might have a policy that disallows 145 the use of video and can have an enforcement mechanism that drops all 146 packets containing a video encoding. Unfortunately, these 147 enforcement mechanisms usually do not inform the user about the 148 policies they are enforcing. Instead, they silently keep the user 149 from doing anything against them. This can lead to a malfunctioning 150 of devices that is incomprehensible to the user. With session 151 policies, the user knows about the current network policies and can 152 set up policy-compliant sessions or simply connect to a domain with 153 less stringent policies. Thus, session policies provide an important 154 combination of consent coupled with enforcement. That is, the user 155 becomes aware of the policy and needs to act on it, but the provider 156 still retains the right to enforce the policy. 158 Two types of session policies exist: session-specific policies and 159 session-independent policies. Session-specific policies are policies 160 that are created for one particular session, based on the session 161 description of this session. They enable a network intermediary to 162 examine the session description a UA is proposing and to return a 163 policy specifically for this session description. For example, an 164 intermediary could open pinholes in a firewall/NAT for each media 165 stream in the proposed session description. It can then return a 166 policy for the session description that replaces the IP addresses and 167 ports of the UA with the ones opened in the firewall/NAT that are 168 reachable from external. Since session-specific policies are 169 tailored to a session, they only apply to the session they are 170 created for. Session-specific policies are created on a session-by- 171 session basis at the time the session is established. 173 Session-independent policies on the other hand are policies that are 174 created independent of a session and generally apply to all SIP 175 sessions set up by a user agent. A session-independent policy can, 176 for example, be used to inform user agents about an existing 177 bandwidth limit or media type restrictions. Since these policies are 178 not based on a specific session description, they can be created 179 independent of an attempt to set up a session and only need to be 180 conveyed to the user agent when it initializes (e.g., at the time the 181 device is powered on) and when policies are changed. 183 This specification defines a framework for SIP session policies. It 184 specifies a model, the overall architecture and new protocol 185 mechanisms that are needed for session-independent and session- 186 specific policies. Since session-specific and session-independent 187 policies have different requirements, this specification defines two 188 different mechanisms to deliver them to user agents. These 189 mechanisms are independent of each other and, depending on whether 190 one or both types of session policies are needed, it is possible to 191 use the session-specific or the session-independent mechanism or both 192 to deliver policies to user agents. 194 It is RECOMMENDED that UAs and intermediaries use the mechanisms 195 defined in this specification for signaling session policies to 196 endpoints. To ensure backwards compatibility with UAs that do not 197 support this specification, intermediaries may choose to resort to 198 mechanisms such as rejecting sessions that are not policy compliant 199 with a 488 response as a fallback solution if a UA does not indicate 200 support for session policies. As these techniques are known to have 201 many drawbacks it is RECOMMENDED that UAs and intermediaries use 202 explicit signaling of policies using the mechanisms defined in this 203 specification. 205 2. Terminology 207 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 208 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 209 document are to be interpreted as described in RFC 2119 [RFC2119]. 211 3. Session-Independent Policies 213 Session-independent policies are policies that are created 214 independent of a session and generally apply to all sessions a user 215 agent is setting up. They typically remain stable for a longer 216 period of time and apply to any session set up while they are valid. 217 However, it is possible for session-independent policies to change 218 over time. For example, a policy that defines a bandwidth limit for 219 a user can change during the day, defining a lower limit during peak 220 hours and allow more bandwidth off-peak. The policy server informs a 221 UA when session-independent policies change. 223 3.1. Architecture and Overview 225 +-------------+ 226 /------| policy | 227 +----+ / | server 1 | 228 | |---/ +-------------+ 229 | UA | ... 230 | |---\ +-------------+ 231 +----+ \ | policy | 232 \------| server n | 233 +-------------+ 235 Figure 1 237 A SIP UA can receive session-independent policies from one or more 238 policy servers. In a typical configuration, a UA receives session- 239 independent policies from a policy server in the local network domain 240 (i.e., the domain from which the UA receives IP service) and possibly 241 the SIP service provider domain (i.e., the domain the UA registers 242 at). The local network can have policies that support the access 243 network infrastructure. For example, in a wireless network where 244 bandwidth is scarce, a provider can restrict the bandwidth available 245 to an individual user. The SIP service provider can have policies 246 that are needed to support services or policies that reflect the 247 service level agreement with the user. Thus, in most cases, a UA 248 will receive session-independent policies from one or two policy 249 servers. 251 Setting up session-independent policies involves the following steps: 253 1. A user agent discovers session-independent policy servers in the 254 local network and SIP service provider domain 255 2. A user agent requests session-independent policies from the 256 discovered policy servers. A user agent typically requests these 257 policies when it starts up or connects to a new network domain. 258 3. The policy server selects the policies that apply to this user 259 agent. The policy server can have general policies that apply to 260 all users or maintain separate policies for each individual user. 261 The selected policies are returned to the user agent. 262 4. The policy server can update the policies, for example, when 263 network conditions change. 265 3.2. Policy Subscription 267 3.2.1. UAC Behavior 269 A UA that supports session-independent policies compliant to this 270 specification MUST attempt to retrieve session-independent policies 271 from the local network and the SIP service provider domain, unless 272 the UA knows (e.g., through configuration) that a domain does not 273 provide session-independent policies. In this case, the UA SHOULD 274 NOT retrieve session-independent policies from this specific domain. 276 A UA that supports session-independent policies compliant to this 277 specification MUST support the retrieval of session-independent 278 policies from the local network and the SIP service provider domain 279 using the "ua-profile" event package defined in the Framework for SIP 280 User Agent Profile Delivery [I-D.ietf-sipping-config-framework]. The 281 UA MAY support other methods of retrieving session-independent 282 policies from local network and SIP service provider domain. 284 The "ua-profile" event package [I-D.ietf-sipping-config-framework] 285 provides a mechanism to subscribe to session-independent policies. A 286 UA subscribes to the policy server in the local network domain using 287 the procedures defined for the "local-network" profile-type. The UA 288 uses the procedures defined for the "user" profile type to subscribe 289 to the policy server in the SIP service provider domain. 291 A UA (re-)subscribes to session-independent policies when the 292 following events occur: 294 o The UA registers a new address-of-record (AoR) or removes a AoR 295 from the set of AoRs it has registered. In these cases, the UA 296 MUST establish subscriptions for each new AoR using the "user" and 297 the "local-network" profile-types. The UA MUST terminate all 298 subscriptions for AoRs it has removed. 299 o The UA changes the domain it is connected to. The UA MUST 300 terminate all existing subscriptions for the "local-network" 301 profile-type. The UA MUST then create a new subscription for each 302 AoR it maintains using the "local-network" profile-type. This 303 way, the UA stops receiving policies from the previous local 304 domain and starts to receive the policies of the new local domain. 305 The UA does not need to change the subscriptions for "user" 306 profiles. 308 If a UA is unable to establish a subscription, the UA SHOULD NOT 309 attempt to re-try this subscription, unless one of the above events 310 occurs again. This is to limit the number of SUBSCRIBE requests sent 311 within domains that do not support session-independent policies. 312 However, a UA SHOULD retry the subscription with a longer time 313 interval (e.g., once every 24 hours). This enables UAs to detect new 314 policies that are deployed in a network that previously did not have 315 policies. 317 A UA that supports session-independent policies compliant to this 318 specification MUST support the User Agent Profile Data Set for Media 319 Policy [I-D.ietf-sipping-media-policy-dataset]. To indicate that the 320 UA wants to receive session-independent policies, the UA includes the 321 MIME type "application/media-policy-dataset+xml" in the Accept header 322 field of a SUBSCRIBE request. 324 A UA MUST apply the session-independent policies it has received and 325 use these policies in the session descriptions it creates. If the UA 326 decides not to use the received policies, then the UA MUST NOT set up 327 a session unless it changes the domain that provided these policies. 328 A UA MAY try to connect to another local network and/or SIP service 329 provider domain with a different set of policies. 331 If a UA receives both session-independent and session-specific 332 policies, the UA MUST apply the session-independent policies to the 333 session description before the session description is sent to the 334 session-specific policy server (see Section 4). Thus, session- 335 independent policies are always applied before session-specific 336 policies are retrieved. 338 3.2.2. UAS Behavior 340 A policy server MAY send a notification to the UA every time the 341 session-independent policies covered by the subscription change. The 342 definition of what causes a policy to change is at the discretion of 343 the administrator. A change in the policy can be triggered, for 344 example, by a change in the network status, by the change in the time 345 of day or by an update of the service level agreement with the 346 customer. 348 4. Session-Specific Policies 350 Session-specific policies are policies that are created specifically 351 for one particular session of a UA. Thus, session-specific policies 352 will typically be different for different sessions. The session- 353 specific policies for a session can change during the course of the 354 session. For example, a user can run out of credit during a session, 355 which will cause the network to disallow the transmission all media 356 streams from this point on. 358 4.1. Architecture 360 domain 1 361 +-----------+ 362 /------| proxy |----... 363 +----+ / +-----------+ 364 | |---/ +-----------+ 365 | | | policy | 366 | UA |============| server | 367 | | +-----------+ 368 | |**** +-----------+ 369 +----+ * | policy | 370 *******|enforcement|****... 371 +-----------+ 373 --- SIP Signaling 374 === Policy Channel 375 *** Media 377 Figure 2 379 The following entities are needed for session-specific policies (see 380 Figure 2): a user agent (UA), a proxy, a policy server and possibly a 381 policy enforcement entity. 383 The role of the proxy is to provide a rendezvous mechanism for UAs 384 and policy servers. It ensures that each UA has the URI of the 385 policy server in its domain and knows where to retrieve policies 386 from. The proxy conveys the policy server URI to UAs in case they 387 have not yet received it (e.g., in a previous call or through 388 configuration). The proxy does not deliver the actual policies to 389 UAs. 391 The policy server is a separate logical entity that can be physically 392 co-located with the proxy. The role of the policy server is to 393 deliver session policies to UAs. The policy server receives session 394 information from the UA, uses this information to determine the 395 policies that apply to the session and returns these policies to the 396 UA. The mechanism for generating policies (i.e., making policy 397 decisions) is outside of the scope of this specification. A policy 398 server can, for example, query an external entity to get policies or 399 it can directly incorporate a policy decision point and generate 400 policies locally. 402 A UA receives the URI of a policy server from a proxy. It uses this 403 URI to contact the policy server. It provides information about the 404 current session to the policy server and receives session policies in 405 response. The UA can also receive policy updates from the policy 406 server during the course of a session. 408 A network can have a policy enforcement infrastructure in place. 409 However, this specification does not make any assumptions about the 410 enforcement of session policies and the mechanisms defined here are 411 orthogonal to a policy enforcement infrastructure. 413 In principle, each domain that is traversed by SIP signaling messages 414 can define session-specific policies for a session. Each domain 415 needs to run a policy server and a proxy that is able to rendezvous a 416 UA with the policy server (as shown in Figure 2). However, it is 417 expected that session-specific policies will often only be provided 418 by the local domain of the user agent. 420 4.2. Overview 422 The protocol defined in this specification clearly separates SIP 423 signaling and the exchange of policies. SIP signaling is only used 424 to rendezvous the UA with the policy server. From this point on, UA 425 and policy server communicate directly with each other over a 426 separate policy channel. This is opposed to a piggyback model, where 427 the exchange of policy information between endpoint and a policy 428 server in the network is piggybacked onto the SIP signaling messages 429 that are exchanged between endpoints. 431 The main advantage of using a separate policy channel is that it 432 decouples signaling between endpoints from the policy exchange 433 between an endpoint and a policy server. This decoupling has a 434 number of desirable properties. It enables the use of separate 435 encryption mechanisms on the signaling path to secure the 436 communication between endpoints, and on the policy channel to secure 437 the communication between endpoint and policy server. Policies can 438 be submitted directly from the policy server to the endpoint and do 439 not travel along the signaling path, possibly crossing many domains. 440 Endpoints set up a separate policy channel to each policy server and 441 can disclose the information requested by the specific policy server 442 (e.g., offer or offer/answer). Finally, policy servers do not need 443 to rely on a SIP signaling message flowing by to send policies or 444 policy updates to an endpoint. A policy server can use the policy 445 channel at any time to update session policies as needed. A 446 disadvantage of the separate channel model is that it requires 447 additional messages for the exchange of policy information. 449 Following this model, signaling for session-specific policies 450 involves the following two fundamental tasks: 452 1. UA/policy server rendezvous: a UA setting up a session needs to 453 be able to discover the policy servers that are relevant to this 454 session. 455 2. Policy channel: once the UA has discovered the relevant policy 456 servers for a session, it needs to connect to these servers, 457 disclose session information and retrieve the policies that apply 458 to this session. 460 The communication between UA and policy server on the policy channel 461 involves the following steps: 463 1. A user agent submits information about the session it is trying 464 to establish to the policy server and asks whether a session 465 using these parameters is permissible. 466 2. The policy server generates a policy decision for this session 467 and returns the decision to the user agent. Possible policy 468 decisions are (1) to deny the session, (2) to propose changes to 469 the session parameters with which the session would be 470 acceptable, or (3) to accept the session as it was proposed. 471 3. The policy server can update the policy decision at a later time. 472 A policy decision update can, for example, propose additional 473 changes to the session (e.g., change the available bandwidth) or 474 deny a previously accepted session (i.e., disallow the 475 continuation of a session). 477 In many cases, the mechanism for session-specific policies will be 478 used to disclose session information and return session policies. 479 However, some scenarios only involve the disclosure of session 480 information to a network intermediary. If an intermediary does not 481 intend to return a policy, it can simply accept the session as it was 482 proposed. Similarly, some session-specific policies only apply to 483 the offer (and therefore only require the disclosure of the offer) 484 whereas others apply to offer and answer. Both types of policies are 485 supported by session-specific policy mechanism. 487 4.3. Examples 489 This section provides two examples to illustrate the overall 490 operation of session-specific policies. The call flows depict the 491 rendezvous mechanism between UA and policy server and indicate the 492 points at which the UA exchanges policy information with the policy 493 server. 495 The example is based on the following scenario: there are two domains 496 (domain A and domain B), which both have session-specific policies 497 for the UAs in their domain. Both domains do not provide policies to 498 the UAs outside of their domain. The two domains have a proxy (P A 499 and P B) and a policy server (PS A and PS B). The policies in both 500 domains involve the session description offer and answer. 502 4.3.1. Offer in Request 504 The first call flow shown in Figure 3 depicts an INVITE transaction 505 with the offer in the request. It is assumed that this is the first 506 INVITE request the UAC creates in this domain and that it therefore 507 does not have previous knowledge about the policy server URIs in this 508 domain. 510 (1) UA A sends an INVITE request to proxy P A. P A knows that 511 policies apply to this session and (2) returns a 488 (Not Acceptable 512 Here) response to UA A. P A includes the URI of PS A in the 488 (Not 513 Acceptable Here) response. This step is needed since the UAC has no 514 prior knowledge about the URI of PS A. (3) UA A uses the URI to 515 contact PS A, discloses the session description offer to PS A and (4) 516 receives policies for the offer. (5) UA A reformulates the INVITE 517 request under consideration of the received policies and includes a 518 Policy-Id header field to indicate that it has already contacted PS 519 A. P A does not reject the INVITE request this time and removes the 520 Policy-Id header field when forwarding the INVITE request. P B adds 521 a Policy-Contact header field containing the URI of PS B. (6) UA B 522 uses this URI to contact PS B and disclose the offer and the answer 523 it is about to send. (7) UA B receives policies from PS B and applies 524 them to the offer and answer respectively. (8) UA B returns the 525 updated answer in the 200 (OK) response. (9) UA A contacts PS A again 526 with the current offer and answer and (10) retrieves the policies for 527 both from PS A. 529 UA A P A P B UA B 530 | | | | 531 | INVITE offer | | | 532 |---------------->| | | (1) 533 | 488 | | | 534 | + Policy-Contact| | | 535 |<----------------| | | (2) 536 | ACK | | | 537 |---------------->| | | 538 | | PS A | | 539 | | | | 540 | PolicyChannel | | | 541 | + InfoOffer | | | 542 |------------------->| | | (3) 543 | PolicyChannel | | | 544 | + PolicyOffer | | | 545 |<-------------------| | | (4) 546 | | | | 547 | | | | 548 | INVITE offer' | INVITE offer' | INVITE offer' | 549 | + Policy-Id | | + Policy-Contact| 550 |---------------->|--------------->|---------------->| (5) 551 | | | | 552 | | PS B | | 553 | | | | 554 | | | PolicyChannel | 555 | | | + InfoOffer' | 556 | | | + InfoAnswer | 557 | | |<-------------------| (6) 558 | | | PolicyChannel | 559 | | | + PolicyOffer | 560 | | | + PolicyAnswer | 561 | | |------------------->| (7) 562 | | | | 563 | | | | 564 | OK answer' | OK answer' | OK answer' | 565 |<----------------|<---------------|<----------------| (8) 566 | ACK | 567 |--------------------------------------------------->| 568 | | | | 569 | | | | 570 | PolicyChannel | | | 571 | + InfoOffer' | | | 572 | + InfoAnswer' | | | 573 |------------------->| | | (9) 574 | PolicyChannel | | | 575 | + PolicyOffer | | | 576 | + PolicyAnswer | | | 577 |<-------------------| | | (10) 578 | | | | 580 Figure 3 582 4.3.2. Offer in Response 584 The call flow shown in Figure 4 depicts an INVITE transaction with 585 the offer in the response. 587 (1) UA A sends an INVITE request without an offer to proxy P A and 588 (2) P A returns a 488 (Not Acceptable Here) response containing the 589 URI of PS A. (3),(4) UA A uses this policy server URI to set up the 590 policy channel. At this time, UA A does not disclose a session 591 description since it does not have the offer yet. (5) UA A re-sends 592 the INVITE request and includes a Policy-Id header field to indicate 593 that it has contacted PS A. P A does not reject the INVITE request 594 this time and removes the Policy-Id header field when forwarding the 595 INVITE request. P B adds a Policy-Contact header field containing 596 the URI of PS B. (6) UA B uses this URI to discloses the offer to PS 597 B. (7) UA B receives policies from PS B and applies them to the 598 offer. (8) UA B returns the updated offer the 200 (OK) response. 599 (9),(10) UA A contacts PS and discloses the offer and the answer it 600 is about to send. An important difference to the flow in the 601 previous example is that UA A performs steps (9) and (10) before 602 returning the answer in step (11). This enables UA A to return the 603 final answer in the ACK request, which includes all applicable 604 policies. However, it requires that PS A immediately returns a 605 policy to avoid a delay in the transmission of the ACK request. 606 (12),(13) UA B again sends the current offer and answer to PS B and 607 applies the policies it receives to both before using them. 609 UA A P A P B UA B 610 | | | | 611 | INVITE | | | 612 |---------------->| | | (1) 613 | 488 | | | 614 | + Policy-Contact| | | 615 |<----------------| | | (2) 616 | ACK | | | 617 |---------------->| | | 618 | | PS A | | 619 | | | | 620 | PolicyChannel | | | 621 |------------------->| | | (3) 622 | PolicyChannel | | | 623 |<-------------------| | | (4) 624 | | | | 625 | | | | 626 | INVITE | INVITE | INVITE | 627 | + Policy-Id | | + Policy-Contact| 628 |---------------->|--------------->|---------------->| (5) 629 | | | | 630 | | PS B | | 631 | | | | 632 | | | PolicyChannel | 633 | | | + InfoOffer | 634 | | |<-------------------| (6) 635 | | | PolicyChannel | 636 | | | + PolicyOffer | 637 | | |------------------->| (7) 638 | | | | 639 | | | | 640 | OK offer' | OK offer' | OK offer' | 641 |<----------------|<---------------|<----------------| (8) 642 | | | | 643 | | | | 644 | PolicyChannel | | | 645 | + InfoOffer' | | | 646 | + InfoAnswer | | | 647 |------------------->| | | (9) 648 | PolicyChannel | | | 649 | + PolicyOffer | | | 650 | + PolicyAnswer | | | 651 |<-------------------| | | (10) 652 | | | | 653 | ACK answer' | 654 |--------------------------------------------------->| (11) 655 | | | | 656 | | | | 657 | | | PolicyChannel | 658 | | | + InfoOffer' | 659 | | | + InfoAnswer' | 660 | | |<-------------------| (12) 661 | | | PolicyChannel | 662 | | | + PolicyOffer | 663 | | | + PolicyAnswer | 664 | | |------------------->| (13) 665 | | | | 667 Figure 4 669 4.4. UA/Policy Server Rendezvous 671 The first step in setting up session-specific policies is to 672 rendezvous the UAs with the relevant policy servers. This is 673 achieved by providing the URIs of all policy servers relevant for a 674 session to the UAs. 676 4.4.1. UAC Behavior 678 A UAC compliant to this specification MUST include a Supported header 679 field with the option tag "policy" into all requests that can 680 initiate an offer/answer exchange [RFC3264] (e.g., INVITE, UPDATE 681 [RFC3311] and PRACK [RFC3262] requests). The UAC MUST include the 682 "policy" option tag into these requests even if the particular 683 request does not contain an offer or answer (e.g., an INVITE request 684 without an offer). A UAC MAY include the "policy" option tag into 685 all requests. 687 A UAC can receive a 488 (Not Acceptable Here) response that contains 688 a Policy-Contact header field. The Policy-Contact header field is a 689 new header field defined in this specification. It contains one (or 690 multiple alternative) URIs for a policy server. A 488 (Not 691 Acceptable Here) response with this header field is generated by a 692 proxy to convey an URI of the local policy server to the UAC. After 693 receiving a 488 (Not Acceptable Here) response with a Policy-Contact 694 header field, a UAC compliant to this specification needs to decide 695 if it wants to continue with the session now knowing that there is a 696 policy server. If the UAC decides to continue, the UAC MUST use one 697 of the policy server URIs to contact the policy server using the 698 mechanism defined in Section 4.5. 700 The Policy-Contact header can contain multiple URIs each with a 701 different URI scheme and containing an "alt-uri" parameter with 702 identical values. These URIs represent alternative policy channel 703 mechanisms for obtaining the same policy. The UAC chooses one of the 704 alternative URIs to use to obtain the policy. The UAC MAY take as a 705 hint the order of the alternative URIs as indicating a preference as 706 to which URI scheme to use. The topmost URI schemes in the list 707 might be more preferred by the domain of the proxy for use to obtain 708 the policy. 710 After receiving policies from the policy server, the UAC decides if 711 it wants to accept these policies or not. If the UAC accepts these 712 policies, the UAC MUST apply them to the current request and resend 713 the updated request. If no changes are required by policies or no 714 policies have been received, the request can be resent without any 715 policy-induced changes. If the UAC decides that the list of policy 716 servers or the received session policies are unacceptable, then the 717 UAC MUST NOT resend the request. 719 The UAC MAY resend the unchanged request if it cannot setup a policy 720 channel to the policy server, for example, because the policy server 721 is unreachable or returns an error condition that cannot be resolved 722 by the UAC (i.e., error conditions other than, for example, a 401 723 (Unauthorized) responses). This is to avoid that the failure of a 724 policy server prevents a UA from communicating. 726 To protect the integrity of the policy server URI in a Policy-Contact 727 header field, the UAC SHOULD use a secured transport protocol such as 728 TLS between UAC and proxy. 730 The UAC MUST insert a Policy-Id header field into requests for which 731 it has contacted a policy server and accepted the policies received. 732 The Policy-Id header field is a new header field that is defined in 733 this specification. The UA MUST create a Policy-Id header field 734 value for each policy server it has contacted during the preparation 735 of the request. A Policy-Id header field value contains two pieces 736 of information: the policy server URI and an optional token. The 737 policy server URI is the URI the UA has used to contact the policy 738 server. The token is an opaque string the UAC can receive from the 739 policy server. A token can, for example, be contained in the policy 740 document [I-D.ietf-sipping-media-policy-dataset]. If the UAC has 741 received a token from the policy server the UAC MUST include the 742 token in the Policy-Id header field. The format of the Policy-Id 743 header field is defined in Section 4.4.5. 745 The main purpose of the Policy-Id header field is to enable a proxy 746 to determine if the UAC already knows an URI of the local policy 747 server. If the policy server URI is not yet known to the UAC, the 748 proxy can convey this URI to the UAC by rejecting the request with a 749 488 (Not Acceptable Here) response. 751 In some cases, a request can traverse multiple domains with a 752 session-policy server. Each of these domains can return a 488 (Not 753 Acceptable Here) response containing a policy server URI. Since the 754 UAC contacts a policy server after receiving a 488 (Not Acceptable 755 Here) response from a domain and before re-sending the request, 756 session policies are always applied to a request in the order in 757 which the request traverses through the domains. The UAC MUST NOT 758 change this implicit order among policy servers. 760 A UAC frequently needs to contact the policy server in the local 761 domain before setting up a session. To avoid the retransmission of 762 the local policy server URI in a 488 (Not Acceptable Here) response 763 for each new request, a UA SHOULD maintain a cache that contains the 764 URI of the policy server in the local domain (see Section 4.4.4). 766 The UAC SHOULD use the cached policy server URI to contact the local 767 policy server before sending a request that initiates the offer/ 768 answer exchange for a new session (e.g., an INVITE request). The UAC 769 SHOULD NOT cache a policy server URI that is in a different domain 770 than the UAC even if it is the first policy server URI returned. The 771 first policy server URI returned can be from another domain if the 772 local domain does not have a policy server. 774 UAs can re-negotiate the session description during a session by 775 initiating a subsequent offer/answer exchange, e.g., in an INVITE, 776 UPDATE or PRACK request. When creating such a mid-dialog request, a 777 UA SHOULD contact all policy servers to which it has established a 778 policy channel during the initial offer/answer exchange (see 779 Section 4.5) before sending the request. This avoids the 780 retransmission of all policy server URIs in 488 (Not Acceptable Here) 781 responses for mid-dialog requests. 783 4.4.2. Proxy Behavior 785 A proxy provides rendezvous functionalities for UAs and policy 786 server. This is achieved by conveying the URI of a policy server to 787 the UAC or the UAS (or both) when processing INVITE, UPDATE or PRACK 788 requests (or any other request that can initiate an offer/answer 789 exchange). 791 If an offer/answer exchange initiating request contains a Supported 792 header field with the option tag "policy", the proxy MAY reject the 793 request with a 488 (Not Acceptable Here) response to provide the 794 local policy server URI to the UAC. Before rejecting a request, the 795 proxy MUST verify that the request does not contain a Policy-Id 796 header field with the local policy server URI as a value. If the 797 request does not contain such a header field or a local policy server 798 URI is not present in this header field, then the proxy MAY reject 799 the request with a 488 (Not Acceptable Here) response. The proxy 800 MUST insert a Policy-Contact header field in the 488 (Not Acceptable 801 Here) response that contains one (or multiple) URIs of its associated 802 policy server. The proxy MAY add the header field parameter "non- 803 cacheable" to prevent the UAC from caching this policy server URI 804 (see Section 4.4.4). 806 More than one URI for the policy server using different URI schemes 807 MAY be provided by the proxy as alternative URIs to contact the 808 policy. If a proxy includes multiple URIs for the same policy, the 809 proxy MUST include an "alt-uri" parameter for all policy server URIs 810 that are alternatives for obtaining the same policy. The "alt-uri" 811 parameter MUST contain either the domain name of the domain for which 812 all the alternative policy server URIs relate to or a FQDN (e.g., the 813 hostname of a policy server). All URIs that are alternatives for the 814 same policy MUST have the same value for the "alt-uri" parameter. 815 The value used for the "alt-uri" parameter MUST be such that the same 816 value will not be included with other policy server URIs that a UA 817 needs to contact by any other proxy within the same domain or another 818 domain. A proxy MAY hint to a UA at a preference as to which URI 819 scheme to use by including the more preferred URI scheme higher in 820 the list than the other alternative URIs. A SIP or SIPS URI MUST be 821 included even if other URI schemes are defined and used in the 822 future. 824 If a local policy server URI is present in a Policy-Id header field 825 value of a request, then the proxy MUST NOT reject the request as 826 described above (it can still reject the request for other reasons). 827 The proxy SHOULD remove the Policy-Id header field value of its 828 associated policy server from the Policy-Id header field before 829 forwarding the request. This value only increases message size and 830 is not relevant to other proxies on the path. It also would disclose 831 the policy server URI to subsequent proxies. 833 The Policy-Id header field serves two main purposes: first and most 834 importantly, it enables the proxy to determine if a UAC already knows 835 the URI of the local policy server. The second purpose of the 836 Policy-Id header field is to enable a domain to route all requests 837 that belong to the same session (i.e., the initial request and 838 requests a UA retransmits after contacting the policy server) to the 839 same proxy and policy server. This is important if a domain has 840 multiple proxy/policy server combinations (e.g., in a proxy/policy 841 server farm that receives requests through a load balancer), which 842 create per-session state in the network. An example for such a 843 scenario is a policy server that is associated with a session border 844 device. The policy server configures the session border device after 845 receiving a session description from the UAC via the policy channel. 846 Retransmitted requests for such a session need to be routed to the 847 same proxy/policy server as the initial request since this proxy/ 848 policy server combination has configured the associated border device 849 for the session. 851 Routing all requests that belong to the same session to the same 852 proxy can be achieved by using the Policy-Id header field token. It 853 requires that the policy server returns a token to the UAC that 854 uniquely identifies the specific proxy/policy server combination. 855 The UAC includes this token in the Policy-Id header field and it can 856 be used (together with the policy server URI) by the proxies in this 857 domain to route the request along the desired path. The format of 858 this token does not require standardization. The only requirement is 859 that the token provides sufficient information for proxies to route 860 the message inside a domain to the desired proxy/policy server. The 861 token can, for example, be a numeric identifier or an IP address. 863 Note: it has been proposed to use the Policy-Id header field to 864 provide a hint for a proxy that the UAC has actually contacted the 865 policy server. This usage also requires the policy server to 866 return a token to the UA. In addition, the policy server needs to 867 share valid tokens with the proxy. After receiving a request with 868 a Policy-Id header field, the proxy can determine if the token in 869 the Policy-Id header field is valid. If it is valid, the proxy 870 knows that the UA has contacted the policy server for this 871 session. However, this token does not provide any proof that the 872 UA has actually used the policies it has received from the policy 873 server. A malicious UA can simply contact the policy server, 874 discard all policies it receives but still use the token in the 875 Policy-Id header field. 877 The proxy MAY insert a Policy-Contact header field into INVITE, 878 UPDATE or PRACK requests (or any other request that can initiate an 879 offer/answer exchange) in order to convey the policy server URI to 880 the UAS. If the request already contains a Policy-Contact header 881 field, the proxy MUST insert the URI after all existing values at the 882 end of the list. A proxy MUST NOT change the order of existing 883 Policy-Contact header field values. 885 A proxy MUST use the Record-Route mechanism [RFC3261] if its 886 associated policy server has session policies that apply to mid- 887 dialog requests. The Record-Route header field enables a proxy to 888 stay in the signaling path and re-submit the policy server URIs to 889 UAs during mid-dialog requests that initiate an offer/answer 890 exchange. Re-submitting the policy server URI to UAs ensures that 891 UAs keep contacting the policy server for mid-dialog requests. 893 A proxy can find out if the UAS supports this extension by examining 894 the Supported header field of responses. The proxy knows that the 895 UAS supports this extension if the Supported header field of a 896 response contains the option tag "policy". A proxy can use this 897 information to determine if the UAS has understood the Policy-Contact 898 header field it has inserted into the request. 900 To protect the integrity of the policy server URI in a Policy-Contact 901 header field, the proxy SHOULD use a secured transport protocol such 902 as TLS between proxy and UAs. 904 4.4.3. UAS Behavior 906 A UAS can receive an INVITE, UPDATE or PRACK request (or another 907 request that can initiate offer/answer exchanges) that contains a 908 Policy-Contact header field with a list of policy server URIs. A UAS 909 that receives such a request needs to decide if it wants to accept 910 the session knowing that there are policy servers involved. If the 911 Policy-Contact header contains multiple URIs each with a different 912 URI scheme and containing an "alt-uri" parameter with identical 913 values these URI schemes represent alternative policy channel 914 mechanisms for obtaining the same policy. If the UAS accepts the 915 session, the UAS chooses one of any alternative URIs to use to obtain 916 the policy. The UAS MAY take as a hint the order of the alternative 917 URIs as indicating a preference as to which URI scheme to use. The 918 topmost URI schemes in the list might be more preferred by the domain 919 of the proxy for use to obtain the policy. The UAS MUST contact all 920 policy server URIs in a Policy-Contact header field that do not 921 contain an "alt-uri" parameter with identical values. The UAS MUST 922 contact these policy server URIs in the order in which they were 923 contained in the Policy-Contact header field, starting with the 924 topmost value (i.e., the value that was inserted first). 926 If a UAS decides that it does not want to accept a session because 927 there are policy servers involved or because one of the session 928 policies received from a policy server is not acceptable, the UAS 929 MUST reject the request with a 488 (Not Acceptable Here) response. 931 The UAS MAY accept a request and continue with setting up a session 932 if it cannot setup a policy channel to the policy server, for 933 example, because the policy server is unreachable or returns an error 934 condition that cannot be resolved by the UAS (i.e., error conditions 935 other than, for example, a 401 (Unauthorized) response). This is to 936 avoid that the failure of a policy server prevents a UA from 937 communicating. Since this session might not be policy compliant 938 without the policy subscription, it can be blocked by policy 939 enforcement mechanisms if they are in place. 941 A UAS can receive a token from a policy server via the policy 942 channel. Since the UAS does not create a Policy-ID header field, it 943 can simply ignore this token. 945 A UAS compliant to this specification MUST include a Supported header 946 field with the option tag "policy" into responses to requests that 947 can initiate an offer/answer exchange. The UAS MAY include this 948 option tag in all responses. This way, a proxy that has inserted the 949 Policy-Contact header field can know that the header field was 950 understood by the UAS. 952 4.4.4. Caching the Local Policy Server URI 954 A UAC frequently needs to contact the policy server in the local 955 domain before setting up a session. To avoid the retransmission of 956 the local policy server URI for each session, a UA SHOULD maintain a 957 cache that contains the URI of the local policy server. 959 A UA can receive this URI in a Policy-Contact header field of a 960 request or a 488 (Not Acceptable Here) response. The UA can also 961 receive the local policy server URI through configuration, for 962 example, via the configuration framework 963 [I-D.ietf-sipping-config-framework]. If a UA has received a local 964 policy server URI through configuration and receives another local 965 policy server URI in a Policy-Contact header field, the UA SHOULD 966 overwrite the configured URI with the most recent one received in a 967 Policy-Contact header field. 969 Domains can prevent a UA from caching the local policy server URI. 970 This is useful, for example, if the policy server does not need to be 971 involved in all sessions or the policy server URI changes from 972 session to session. A proxy can mark the URI of such a policy server 973 as "non-cacheable". A UA MUST NOT cache a non-cacheable policy 974 server URI. The UA SHOULD remove the current URI from the cache when 975 receiving a local policy server URI that is marked as "non- 976 cacheable". This is to avoid the use of policy server URIs that are 977 outdated. 979 The UA SHOULD NOT cache policy server URIs it has received from 980 proxies outside of the local domain. These policy servers need not 981 be relevant for subsequent sessions, which can go to a different 982 destination, traversing different domains. 984 The UA MUST NOT cache tokens it has received from a policy server. A 985 token is only valid for one request. 987 4.4.5. Header Field Definition and Syntax 989 4.4.5.1. Policy-Id Header Field 991 The Policy-Id header field is inserted by the UAC into INVITE, UPDATE 992 or PRACK requests (or any other request that can be used to initiate 993 an offer/answer exchange). The Policy-Id header field identifies all 994 policy servers the UAC has contacted for this request. 996 The value of a Policy-Id header field consists of a policy server URI 997 and an optional token parameter. The token parameter contains a 998 token the UA might have received from the policy server. 1000 The syntax of the Policy-Id header field is described below in ABNF, 1001 according to RFC5234 [RFC5234], as an extension to the ABNF for SIP 1002 in RFC3261 [RFC3261]: 1004 Policy-Id = "Policy-Id" HCOLON policyURI 1005 *(COMMA policyURI) 1006 policyURI = ( SIP-URI / SIPS-URI / absoluteURI ) 1007 [ SEMI token-param ] *( SEMI generic-param ) 1008 token-param = "token=" token 1010 4.4.5.2. Policy-Contact Header Field 1012 The Policy-Contact header field can be inserted by a proxy into a 488 1013 (Not Acceptable Here) response to INVITE, UPDATE or PRACK requests 1014 (or other requests that initiate an offer/answer exchange). The 1015 value of a Policy-Contact header field consists of a policy server 1016 URI and an optional "non-cacheable" header field parameter. The 1017 policy server URI identifies the policy server that needs to be 1018 contacted by a UAC. The "non-cacheable" header field parameter 1019 indicates that the policy server URI is not intended to be cached by 1020 the UAC. 1022 The Policy-Contact header field can also be inserted by a proxy into 1023 INVITE, UPDATE and PRACK requests (or other requests that can be used 1024 to initiate an offer/answer exchange). It contains an ordered list 1025 of policy server URIs that need to be contacted by the UAS. The 1026 topmost value of this list identifies the policy server that is 1027 contacted first. New header field values are inserted at the end. 1028 With this, the Policy-Contact header field effectively forms a fist- 1029 in-first-out queue. 1031 The syntax of the Policy-Contact header field is described below in 1032 ABNF, according to RFC5234 [RFC5234], as an extension to the ABNF for 1033 SIP in RFC3261 [RFC3261]: 1035 Policy-Contact = "Policy-Contact" HCOLON 1036 *(COMMA policyContact-info) 1038 policyContact-info = LAQUOT policyContact-uri RAQUOT 1039 *( SEMI policyContact-param ) 1041 policyContact-uri = ( SIP-URI / SIPS-URI / absoluteURI ) 1043 policyContact-param = ( "non-cacheable" / policyContact-alt-uri 1044 / generic-param ) 1046 policyContact-alt-uri = "alt-uri" EQUAL hostname 1048 Tables 1 and 2 are extensions of Tables 2 and 3 in RFC 3261 1049 [RFC3261]. The column "INF" is for the INFO method [RFC2976], "PRA" 1050 is for the PRACK method [RFC3262], "UPD" is for the UPDATE method 1051 [RFC3311], "SUB" is for the SUBSCRIBE method [RFC3265], "NOT" is for 1052 the NOTIFY method [RFC3265], "MSG" is for the MESSAGE method 1053 [RFC3428], "REF" is for the REFER method [RFC3515], and "PUB" is for 1054 the PUBLISH method [RFC3903]. 1056 Header field where proxy ACK BYE CAN INV OPT REG UPD 1057 _______________________________________________________________ 1058 Policy-Id R rd - - - c - - c 1059 Policy-Contact R a - - - c - - c 1060 Policy-Contact 488 a - - - c - - c 1061 Table 1: Policy-Id and Policy-Contact Header Fields 1063 Header field where proxy PRA PUB SUB NOT INF MSG REF 1064 _______________________________________________________________ 1065 Policy-Id R rd c - - - - - - 1066 Policy-Contact R a c - - - - - - 1067 Policy-Contact 488 a c - - - - - - 1068 Table 1: Policy-Id and Policy-Contact Header Fields 1070 4.5. Policy Channel 1072 The main task of the policy channel is to enable a UA to submit 1073 information about the session it is trying to establish (i.e., the 1074 offer and the answer) to a policy server and to receive the resulting 1075 session-specific policies and possible updates to these policies in 1076 response. 1078 The Event Package for Session-Specific Session Policies 1079 [I-D.ietf-sipping-policy-package] defines a SUBSCRIBE/NOTIFY-based 1080 [RFC3265] policy channel mechanism. A UA compliant to this 1081 specification MUST support the Event Package for Session-Specific 1082 Session Policies [I-D.ietf-sipping-policy-package]. The UA MUST use 1083 this event package to contact a policy server if the policy server 1084 URI is a SIP-URI or SIPS-URI. A UA MAY support other policy channel 1085 mechanisms. 1087 4.5.1. Creation and Management 1089 A UA discovers the list of policy servers relevant for a session 1090 during the initial offer/answer exchange (see Section 4.4). A UA 1091 compliant to this specification MUST set up a policy channel to each 1092 of the discovered policy server. If the UA does not want to set up a 1093 policy channel to one of the policy servers provided, the UA MUST 1094 cancel or reject a pending INVITE transaction for the session or 1095 terminate the session if it is already in progress. 1097 If setting up a policy channel to one of the discovered policy 1098 servers fails, the UA MAY continue with the initiation of a session 1099 without contacting this policy server. Setting up a policy channel 1100 can fail, for example, because the server is unreachable or returns 1101 an error condition that cannot be resolved by the UAC (i.e., error 1102 conditions other than, for example, a 401 (Unauthorized) responses). 1103 The UA SHOULD continue an ongoing session if a policy server fails 1104 after the session has been set up. The UA SHOULD consider the 1105 policies it has previously received from the failed policy server. 1106 This is to avoid that the failure of a policy server prevents a UA 1107 from communicating. 1109 A UA MUST maintain the policy channel to each discovered policy 1110 server during the lifetime of a session, unless the policy channel is 1111 closed by the policy server or the UA discovers that the policy 1112 server is no longer relevant for the session as described below. 1114 A UAC can receive a 488 (Not Acceptable Here) response with a Policy- 1115 Contact header field containing a new policy server URI in response 1116 to a mid-dialog request. This indicates that the set of policy 1117 servers relevant for the current session has changed. If this 1118 occurs, the UAC MUST retry sending the request as if it was the first 1119 request in a dialog (i.e., without applying any policies except the 1120 policies from the local policy server). This way, the UAC will re- 1121 discover the list of policy servers for the current session. The UAC 1122 MUST set up a policy channel to each new policy server. The UAC 1123 SHOULD close policy channels to policy server that are not listed any 1124 more. If the policy channel to these servers is not closed, the UAC 1125 can receive policies that do not apply to the session any more. The 1126 UAC MUST contact policy servers in the order in which they were 1127 discovered in the most recent request. 1129 If a UAS receives a mid-dialog request with a Policy-Contact header 1130 field containing a list of policy server URIs that is different from 1131 the list of policy servers to which the UAS has currently established 1132 a policy channel, then the UAS MUST set up a policy channel to all 1133 new policy servers and contact them. The UAS SHOULD close policy 1134 channels to servers that are not listed any more. If the policy 1135 channel to these servers is not closed, the UAS can receive policies 1136 that do not apply to the session any more. The UAS MUST use policy 1137 servers in the order in which they were contained in the most recent 1138 Policy-Contact header field. 1140 A UA MUST inform the policy server when a session is terminated via 1141 the policy channel, unless a policy server indicates via the policy 1142 channel that it does not need to be contacted at the end of the 1143 session. This enables a policy server to free all resources it has 1144 allocated for this session. 1146 4.5.2. Contacting the Policy Server 1148 A UA MUST contact all policy servers to which it has established a 1149 policy channel before sending or after receiving a mid-dialog 1150 request. The UA MUST contact the policy servers in the order in 1151 which they were discovered most recently. 1153 A UA that receives a SIP message containing an offer or answer SHOULD 1154 completely process the message (e.g., according to RFC3261 [RFC3261]) 1155 before contacting the policy server. The SIP processing of the 1156 message includes, for example, updating dialog state and timers as 1157 well as creating ACK or PRACK requests as necessary. This ensures 1158 that contacting a policy server does not interfere with SIP message 1159 processing and timing (e.g., by inadvertently causing timers to 1160 expire). This implies, for example, that a UAC which has received a 1161 response to an INVITE request would normally finish the processing of 1162 the response including transmitting the ACK request before it 1163 contacts the policy server. An important exception to this rule is 1164 discussed in the next paragraph. 1166 In some cases, a UA needs to use the offer/answer it has received in 1167 a SIP message to create an ACK or PRACK request for this message, 1168 i.e., it needs to use the offer/answer before finishing the SIP 1169 machinery for this message. For example, a UAC that has received an 1170 offer in the response to an INVITE request needs to apply policies to 1171 the offer and the answer before it can send the answer in an ACK 1172 request. In these cases, a UA SHOULD contact the policy server even 1173 if this is during the processing of a SIP message. This implies that 1174 a UA, which has received an offer in the response of an INVITE 1175 request, would normally contact the policy server and apply session 1176 policies before sending the answer in the ACK request. 1178 Note: this assumes that the policy server can always respond 1179 immediately to a policy request and does not require manual 1180 intervention to create a policy. This will be the case for most 1181 policy servers. If, however, a policy server cannot respond with 1182 a policy right away, it can return a policy that temporarily 1183 denies the session and update this policy as the actual policy 1184 decision becomes available. A delay in the response from the 1185 policy server to the UA would delay the transmission of the ACK 1186 request and could trigger retransmissions of the INVITE response 1187 (also see the recommendations for Flow I in RFC3725 [RFC3725]). 1189 The case of multiple policy servers providing policies to the same UA 1190 requires additional considerations. A policy returned by one policy 1191 server can contain information that needs to be shared with the other 1192 policy servers. For example, two policy servers can have the policy 1193 to insert a media intermediary by modifying the IP addresses and 1194 ports of media streams. In order for media streams to pass through 1195 both intermediaries, each intermediary needs to know the IP address 1196 and port on which the other media intermediary is expecting the 1197 stream to arrive. If media streams are flowing in both directions, 1198 this means that each intermediary needs to know IP addresses and 1199 ports of the other intermediary. 1201 UACs usually contact a policy server twice during an offer/answer 1202 exchange (unless a policy server indicates that it only needs to be 1203 contacted once). Therefore the case of multiple policy servers 1204 providing policies to a single UAC does not require additional steps 1205 in most cases. However, a UAS usually contacts each policy server 1206 only once (see Figure 4). If a session policy returned by one of the 1207 policy servers requires that information is shared between multiple 1208 servers and the UAS receives policies from more than one policy 1209 server, then the UAS MUST contact all policy servers a second time 1210 after contacting all servers the first time. Whether or not a second 1211 round is required is determined by the type of information returned 1212 by the policy server. The data format for session policies 1213 explicitly states if a second round is needed for a particular data 1214 element. If a UA supports this data format and receives such an 1215 element, it knows that is expected to contact policy servers a second 1216 time. If such a data element is modified during a mid-call offer/ 1217 answer exchange and multiple policy servers are providing policies to 1218 a UA then all UAs MUST contact policy servers in a first and second 1219 round. An example call flow is shown in Appendix B.3. 1221 4.5.3. Using Session Policies 1223 A UA MUST disclose the session description(s) for the current session 1224 to policy servers through the policy channel. The UA MUST apply 1225 session policies it receives to the offer and, if one is received, to 1226 the answer before using the offer/answer. If these policies are 1227 unacceptable, the UA MUST NOT continue with the session. This means 1228 that, the UA MUST cancel or reject a pending INVITE transaction for 1229 the session or terminate the session if it is already in progress. 1230 If the UA receives an unacceptable policy in an INVITE response, the 1231 UA MUST complete the INVITE transaction and then terminate the 1232 session. 1234 When a UA receives a notification about a change in the current 1235 policies, the UA MUST apply the updated policies to the current 1236 session or the UA MUST terminate the session. If the policy update 1237 causes a change in the session description of a session, then the UA 1238 needs to re-negotiate the modified session description with its peer 1239 UA, for example, using a re-INVITE or UPDATE request. For example, 1240 if a policy update disallows the use of video and video is part of 1241 the current session description, then the UA will need to create an 1242 new session description offer without video. After receiving this 1243 offer, the peer UA knows that video can't be used any more and 1244 responds with the corresponding answer. 1246 5. Security Considerations 1248 Policy enforcement mechanisms can prevent a UA from communicating 1249 with another UA if the UAs are not aware of the policies that are 1250 enforced. Policy enforcement mechanisms without policy signaling can 1251 therefore create a denial of service condition for UAs. This 1252 specification provides a mechanism for intermediaries to signal the 1253 policies that are enforced to UAs. It enables UAs to establish 1254 sessions that are conform and pass through policy enforcement. 1256 Session policies can significantly change the behavior of a UA and 1257 can be used by an attacker to compromise a UA. For example, session 1258 policies can be used to prevent a UA from successfully establishing a 1259 session (e.g., by setting the available bandwidth to zero). Such a 1260 policy can be submitted to the UA during a session, which causes the 1261 UA to terminate the session. 1263 A UA transmits session information to a policy server for session- 1264 specific policies. This session information can contain sensitive 1265 data the user does not want an eavesdropper or an unauthorized policy 1266 server to see. Vice versa, session policies can contain sensitive 1267 information about the network or service level agreements the service 1268 provider does not want to disclose to an eavesdropper or an 1269 unauthorized UA. 1271 It is important to secure the communication between the proxy and the 1272 UA (for session-specific policies) as well as the UA and the policy 1273 server. The following four discrete attributes need to be protected: 1275 1. integrity of the policy server URI (for session-specific 1276 policies), 1277 2. authentication of the policy server and, if needed, the user 1278 agent, 1279 3. confidentiality of the messages exchanged between the user agent 1280 and the policy server and 1281 4. ensuring that private information is not exchanged between the 1282 two parties, even over an confidentiality-assured and 1283 authenticated session. 1285 To protect the integrity of the policy server URI, a UA SHOULD use a 1286 secured transport protocol such as TLS between proxies and the UA. 1287 Protecting the integrity of the policy server URI is important since 1288 an attacker could intercept SIP messages between the UA and the proxy 1289 and remove the policy header fields needed for session-specific 1290 policies. This would impede the rendezvous between UA and policy 1291 server and, since the UA would not contact the policy server, can 1292 prevent a UA from setting up a session. 1294 Instead of removing a policy server URI, an attacker can also modify 1295 the policy server URI and point the UA to a compromised policy 1296 server. To prevent such an attack from being effective, it is 1297 RECOMMENDED that a UA authenticates policy servers. 1299 It is RECOMMENDED that the UA only accepts session-independent 1300 policies from trustworthy policy servers as these policies affect all 1301 sessions of a UA. A list of trustworthy session-independent policy 1302 servers can be provided to the UA through configuration. As SIP 1303 messages can be affected by any proxy on a path and session-specific 1304 policies only apply to a single session, a UA MAY choose to accept 1305 session-specific policies from other policy servers as well. 1307 Policy servers SHOULD authenticate UAs to protect the information 1308 that is contained in a session policy. However, a policy server can 1309 also frequently encounter UAs it cannot authenticate. In these 1310 cases, the policy server MAY provide a generic policy that does not 1311 reveal sensitive information to these UAs. 1313 It is RECOMMENDED that administrators use SIPS URIs as policy server 1314 URIs so that subscriptions to session policies are transmitted over 1315 TLS. 1317 The above security attributes are important to protect the 1318 communication between the UA and policy server. This document does 1319 not define the protocol used for the communication between UA and 1320 policy server and merely refers to other specifications for this 1321 purpose. The security considerations of these specifications need to 1322 address the above security aspects. 1324 6. IANA Considerations 1326 6.1. Registration of the "Policy-Id" Header Field 1328 Name of Header Field: Policy-Id 1330 Short form: none 1332 Normative description: Section 4.4.5 of this document 1334 6.2. Registration of the "Policy-Contact" Header Field 1336 Name of Header Field: Policy-Contact 1338 Short form: none 1340 Normative description: Section 4.4.5 of this document 1342 6.3. Registration of the "non-cacheable" Policy-Contact Header Field 1343 Parameter 1345 Registry Name: Header Field Parameters and Parameter Values 1346 Reference: RFC3968 [RFC3968] 1348 Registry: 1350 Header Field Parameter Name Predefined Reference 1351 Values 1352 _____________________________________________________________________ 1353 Policy-Contact non-cacheable Yes this document 1355 6.4. Registration of the "policy" SIP Option-Tag 1357 This specification registers a new SIP option tag, as per the 1358 guidelines in Section 27.1 of RFC3261 [RFC3261]. 1360 This document defines the SIP option tag "policy". 1362 The following row has been added to the "Option Tags" section of the 1363 SIP Parameter Registry: 1365 +------------+------------------------------------------+-----------+ 1366 | Name | Description | Reference | 1367 +------------+------------------------------------------+-----------+ 1368 | policy | This option tag is used to indicate that | this | 1369 | | a UA can process policy server URIs for | document | 1370 | | and subscribe to session-specific | | 1371 | | policies. | | 1372 +------------+------------------------------------------+-----------+ 1374 Name of option: policy 1376 Description: Support for the Policy-Contact and Policy-Id header 1377 fields. 1379 SIP header fields defined: Policy-Contact, Policy-Id 1380 Normative description: This document 1382 7. References 1384 7.1. Normative References 1386 [I-D.ietf-sipping-config-framework] 1387 Channabasappa, S., "A Framework for Session Initiation 1388 Protocol User Agent Profile Delivery", 1389 draft-ietf-sipping-config-framework-17 (work in progress), 1390 February 2010. 1392 [I-D.ietf-sipping-media-policy-dataset] 1393 Hilt, V., Worley, D., Camarillo, G., and J. Rosenberg, "A 1394 User Agent Profile Data Set for Media Policy", 1395 draft-ietf-sipping-media-policy-dataset-08 (work in 1396 progress), January 2010. 1398 [I-D.ietf-sipping-policy-package] 1399 Hilt, V. and G. Camarillo, "A Session Initiation Protocol 1400 (SIP) Event Package for Session-Specific Session 1401 Policies.", draft-ietf-sipping-policy-package-05 (work in 1402 progress), July 2008. 1404 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1405 Requirement Levels", BCP 14, RFC 2119, March 1997. 1407 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 1408 A., Peterson, J., Sparks, R., Handley, M., and E. 1409 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 1410 June 2002. 1412 [RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of 1413 Provisional Responses in Session Initiation Protocol 1414 (SIP)", RFC 3262, June 2002. 1416 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1417 with Session Description Protocol (SDP)", RFC 3264, 1418 June 2002. 1420 [RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific 1421 Event Notification", RFC 3265, June 2002. 1423 [RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP) 1424 UPDATE Method", RFC 3311, October 2002. 1426 [RFC3968] Camarillo, G., "The Internet Assigned Number Authority 1427 (IANA) Header Field Parameter Registry for the Session 1428 Initiation Protocol (SIP)", BCP 98, RFC 3968, 1429 December 2004. 1431 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1432 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1434 7.2. Informative References 1436 [RFC2976] Donovan, S., "The SIP INFO Method", RFC 2976, 1437 October 2000. 1439 [RFC3428] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C., 1440 and D. Gurle, "Session Initiation Protocol (SIP) Extension 1441 for Instant Messaging", RFC 3428, December 2002. 1443 [RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer 1444 Method", RFC 3515, April 2003. 1446 [RFC3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. 1447 Camarillo, "Best Current Practices for Third Party Call 1448 Control (3pcc) in the Session Initiation Protocol (SIP)", 1449 BCP 85, RFC 3725, April 2004. 1451 [RFC3903] Niemi, A., "Session Initiation Protocol (SIP) Extension 1452 for Event State Publication", RFC 3903, October 2004. 1454 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1455 Description Protocol", RFC 4566, July 2006. 1457 Appendix A. Acknowledgements 1459 Many thanks to Allison Mankin, Andrew Allen, Cullen Jennings and 1460 Vijay Gurbani for their contributions to this draft. Many thanks to 1461 Roni Even, Bob Penfield, Mary Barnes, Shida Schubert, Keith Drage, 1462 Lisa Dusseault, Tim Polk and Pasi Eronen for their reviews and 1463 suggestions. 1465 Appendix B. Session-Specific Policies - Call Flows 1467 The following call flows illustrate the overall operation of session- 1468 specific policies including the policy channel protocol as defined in 1469 the SIP Event Package for Session-Specific Session Policies 1470 [I-D.ietf-sipping-policy-package]. 1472 The following abbreviations are used: 1474 o: offer 1475 o': offer modified by a policy 1476 po: offer policy 1477 a: answer 1478 a': answer modified by a policy 1479 pa: answer policy 1480 ps uri: policy server URI (in Policy-Contact header field) 1481 ps id: policy server id (in Policy-Id header field) 1483 B.1. Offer in Invite 1484 UA A P A PS A PS B P B UA B 1485 | | | | | | 1486 |(1) INV | | | | 1487 |-------->| | | | | 1488 |(2) 488 | | | | 1489 |<--------| | | | | 1490 |(3) ACK | | | | | 1491 |-------->| | | | | 1492 |(4) SUBSCRIBE | | | | 1493 |------------------>| | | | 1494 |(5) 200 OK | | | | 1495 |<------------------| | | | 1496 |(6) NOTIFY | | | | 1497 |<------------------| | | | 1498 |(7) 200 OK | | | | 1499 |------------------>| | | | 1500 |(8) INV | | | | 1501 |-------->| | | | | 1502 | |(9) INV | | | 1503 | |---------------------------->| | 1504 | | | | |(10) INV 1505 | | | | |-------->| 1506 | | | |(11) SUBSCRIBE 1507 | | | |<------------------| 1508 | | | |(12) 200 OK | 1509 | | | |------------------>| 1510 | | | |(13) NOTIFY 1511 | | | |------------------>| 1512 | | | |(14) 200 OK | 1513 | | | |<------------------| 1514 | | | | |(15) 200 OK 1515 | | | | |<--------| 1516 | |(16) 200 OK | | | 1517 | |<----------------------------| | 1518 |(17) 200 OK | | | | 1519 |<--------| | | | | 1520 |(18) ACK | | | | | 1521 |------------------------------------------------>| 1522 |(19) SUBSCRIBE | | | 1523 |------------------>| | | | 1524 |(20) 200 OK | | | | 1525 |<------------------| | | | 1526 |(21) NOTIFY | | | 1527 |<------------------| | | | 1528 |(22) 200 OK | | | | 1529 |------------------>| | | | 1530 | | | | | | 1531 | | | | | | 1533 B.2. Offer in Response 1535 UA A P A PS A PS B P B UA B 1536 | | | | | | 1537 |(1) INV | | | | | 1538 |-------->| | | | | 1539 |(2) 488 | | | | 1540 |<--------| | | | | 1541 |(3) ACK | | | | | 1542 |-------->| | | | | 1543 |(4) SUBSCRIBE | | | | 1544 |------------------>| | | | 1545 |(5) 200 OK | | | | 1546 |<------------------| | | | 1547 |(6) NOTIFY | | | | 1548 |<------------------| | | | 1549 |(7) 200 OK | | | | 1550 |------------------>| | | | 1551 |(8) INV | | | | 1552 |-------->| | | | | 1553 | |(9) INV | | | | 1554 | |---------------------------->| | 1555 | | | | |(10) INV 1556 | | | | |-------->| 1557 | | | |(11) SUBSCRIBE | 1558 | | | |<------------------| 1559 | | | |(12) 200 OK | 1560 | | | |------------------>| 1561 | | | |(13) NOTIFY | 1562 | | | |------------------>| 1563 | | | |(14) 200 OK | 1564 | | | |<------------------| 1565 | | | | |(15) 200 OK 1566 | | | | |<--------| 1567 | |(16) 200 OK | | | 1568 | |<----------------------------| | 1569 |(17) 200 OK | | | | 1570 |<--------| | | | | 1571 |(18) SUBSCRIBE | | | 1572 |------------------>| | | | 1573 |(19) 200 OK | | | | 1574 |<------------------| | | | 1575 |(20) NOTIFY | | | 1576 |<------------------| | | | 1577 |(21) 200 OK | | | | 1578 |------------------>| | | | 1579 |(22) ACK | | | | 1580 |------------------------------------------------>| 1581 | | | |(23) SUBSCRIBE 1582 | | | |<------------------| 1583 | | | |(24) 200 OK | 1584 | | | |------------------>| 1585 | | | |(25) NOTIFY 1586 | | | |------------------>| 1587 | | | |(26) 200 OK | 1588 | | | |<------------------| 1589 | | | | | | 1590 | | | | | | 1592 B.3. Multiple Policy Servers for UAS 1594 UA A P A PS A PS B P B UA B 1595 | | | | | | 1596 | | | | | | 1597 | | | | | | 1598 |(1) INV | | | | 1599 |-------->| | | | | 1600 | |(2) INV | | 1601 | |---------------------------->| | 1602 | | | | |(3) INV 1603 | | | | |-------->| 1604 | | |(4) SUBSCRIBE | 1605 | | |<----------------------------| 1606 | | |(5) 200 OK | | 1607 | | |---------------------------->| 1608 | | |(6) NOTIFY | | 1609 | | |---------------------------->| 1610 | | |(7) 200 OK | | 1611 | | |<----------------------------| 1612 | | | |(8) SUBSCRIBE 1613 | | | |<------------------| 1614 | | | |(9) 200 OK | 1615 | | | |------------------>| 1616 | | | |(10) NOTIFY 1617 | | | |------------------>| 1618 | | | |(11) 200 OK | 1619 | | | |<------------------| 1620 | | |(12) SUBSCRIBE | 1621 | | |<----------------------------| 1622 | | |(13) 200 OK | | 1623 | | |---------------------------->| 1624 | | |(14) NOTIFY | 1625 | | |---------------------------->| 1626 | | |(15) 200 OK | | 1627 | | |<----------------------------| 1628 | | | |(16) SUBSCRIBE 1629 | | | |<------------------| 1630 | | | |(17) 200 OK | 1631 | | | |------------------>| 1632 | | | |(18) NOTIFY 1633 | | | |------------------>| 1634 | | | |(19) 200 OK | 1635 | | | |<------------------| 1636 | | | | |(20) 200 OK 1637 | | | | |<--------| 1638 | |(21) 200 OK | | | 1639 | |<----------------------------| | 1640 |(22) 200 OK | | | | 1641 |<--------| | | | | 1642 |(23) ACK | | | | | 1643 |------------------------------------------------>| 1644 | | | | | | 1645 | | | | | | 1647 Authors' Addresses 1649 Volker Hilt 1650 Bell Labs/Alcatel-Lucent 1651 791 Holmdel-Keyport Rd 1652 Holmdel, NJ 07733 1653 USA 1655 Email: volkerh@bell-labs.com 1657 Gonzalo Camarillo 1658 Ericsson 1659 Hirsalantie 11 1660 Jorvas 02420 1661 Finland 1663 Email: Gonzalo.Camarillo@ericsson.com 1665 Jonathan Rosenberg 1666 jdrosen.net 1667 Monmouth, NJ 1668 USA 1670 Email: jdrosen@jdrosen.net