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Holmberg 3 Internet-Draft Ericsson 4 Obsoletes: 4028 (if approved) S. Donovan 5 Intended status: Standards Track J. Rosenberg 6 Expires: January 28, 2022 Cisco Systems 7 July 27, 2021 9 Session Timers in the Session Initiation Protocol (SIP) 10 draft-ietf-sipcore-rfc4028bis-05 12 Abstract 14 This document defines an extension to the Session Initiation Protocol 15 (SIP). This extension allows for a periodic refresh of SIP sessions 16 through a re-INVITE or UPDATE request. The refresh allows both user 17 agents and proxies to determine whether the SIP session is still 18 active. The extension defines two new header fields: Session- 19 Expires, which conveys the lifetime of the session, and Min-SE, which 20 conveys the minimum allowed value for the session timer. 22 Status of This Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at https://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on January 28, 2022. 39 Copyright Notice 41 Copyright (c) 2021 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (https://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 57 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3 58 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 59 4. Overview of Operation . . . . . . . . . . . . . . . . . . . . 4 60 5. Session-Expires Header Field Definition . . . . . . . . . . . 6 61 6. Min-SE Header Field Definition . . . . . . . . . . . . . . . 8 62 7. 422 Response Code Definition . . . . . . . . . . . . . . . . 8 63 8. UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . . 9 64 9. Generating an Initial Session Refresh Request . . . . . . . . 9 65 10. Processing a 2xx Response . . . . . . . . . . . . . . . . . . 9 66 11. Processing a 422 Response . . . . . . . . . . . . . . . . . . 11 67 12. Generating Subsequent Session Refresh Requests . . . . . . . 11 68 13. Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . . 12 69 14. Processing of Requests . . . . . . . . . . . . . . . . . . . 13 70 15. Processing of Responses . . . . . . . . . . . . . . . . . . . 14 71 16. Session Expiration . . . . . . . . . . . . . . . . . . . . . 15 72 17. UAS Behavior . . . . . . . . . . . . . . . . . . . . . . . . 15 73 18. Performing Refreshes . . . . . . . . . . . . . . . . . . . . 17 74 19. Security Considerations . . . . . . . . . . . . . . . . . . . 18 75 20. Inside Attacks . . . . . . . . . . . . . . . . . . . . . . . 18 76 21. Outside Attacks . . . . . . . . . . . . . . . . . . . . . . . 19 77 22. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 78 23. IANA Registration of Min-SE and Session-Expires Header Fields 19 79 24. IANA Registration of the 422 (Session Interval Too Small) 80 Response Code . . . . . . . . . . . . . . . . . . . . . . . . 20 81 25. IANA Registration of the 'timer' Option Tag . . . . . . . . . 20 82 26. Example Call Flow . . . . . . . . . . . . . . . . . . . . . . 20 83 27. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25 84 28. References . . . . . . . . . . . . . . . . . . . . . . . . . 25 85 28.1. Normative References . . . . . . . . . . . . . . . . . . 25 86 28.2. Informative References . . . . . . . . . . . . . . . . . 25 87 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26 89 1. Introduction 91 The Session Initiation Protocol (SIP) [RFC3261] does not define a 92 keepalive mechanism for the sessions it establishes. Although the 93 user agents may be able to determine whether the session has timed 94 out by using session specific mechanisms, proxies will not be able to 95 do so. The result is that call stateful proxies will not always be 96 able to determine whether a session is still active. For instance, 97 when a user agent fails to send a BYE message at the end of a 98 session, or when the BYE message gets lost due to network problems, a 99 call stateful proxy will not know when the session has ended. In 100 this situation, the call stateful proxy will retain state for the 101 call and has no method to determine when the call state information 102 no longer applies. 104 To resolve this problem, this extension defines a keepalive mechanism 105 for SIP sessions. UAs send periodic re-INVITE or UPDATE [RFC3311] 106 requests (referred to as session refresh requests) to keep the 107 session alive. The interval for the session refresh requests is 108 determined through a negotiation mechanism defined here. If a 109 session refresh request is not received before the interval passes, 110 the session is considered terminated. Both UAs are supposed to send 111 a BYE, and call stateful proxies can remove any state for the call. 113 This refresh mechanism has additional applications. A user agent 114 would like to determine whether the session is still active for the 115 same reasons a call stateful proxy server would. This determination 116 can be made at a user agent without the use of SIP level mechanisms; 117 for audio sessions, periodic RTCP packets serve as an indication of 118 liveness [RFC3550]. However, it is desirable to separate indications 119 of SIP session liveness from the details of the particular session. 121 Another application of the session timer is in the construction of a 122 SIP Network Address Translator (NAT) Application Level Gateway (ALG) 123 [RFC2663]. The ALG embedded in a NAT will need to maintain state for 124 the duration of a call. This state must eventually be removed. 125 Relying on a BYE to trigger the removal of state, besides being 126 unreliable, introduces a potential denial of service attack. 128 This document provides an extension to SIP that defines a session 129 expiration mechanism. Periodic refreshes, through re-INVITEs or 130 UPDATEs, are used to keep the session active. The extension is 131 sufficiently backward compatible with SIP that it works as long as 132 either one of the two participants in a dialog understands the 133 extension. Two new header fields (Session-Expires and Min-SE) and a 134 new response code (422) are defined. Session-Expires conveys the 135 duration of the session, and Min-SE conveys the minimum allowed value 136 for the session expiration. The 422 response code indicates that the 137 session timer duration was too small. 139 2. Conventions 141 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 142 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 143 "OPTIONAL" in this document are to be interpreted as described in BCP 144 14 [RFC2119] [RFC8174] when, and only when, they appear in all 145 capitals, as shown here. 147 3. Terminology 149 Session Interval: The maximum amount of time that can occur between 150 session refresh requests in a dialog before the session will be 151 considered timed out. The session interval is conveyed in the 152 Session-Expires header field, which is defined here. The UAS 153 obtains this value from the Session-Expires header field in a 2xx 154 response to a session refresh request that it sends. Proxies and 155 UACs determine this value from the Session-Expires header field in 156 a 2xx response to a session refresh request that they receive. 157 Minimum Timer: Because of the processing load of mid-dialog requests, 159 all elements (proxy, UAC, UAS) can have a configured minimum value 160 for the session interval that they are willing to accept. This 161 value is called the minimum timer. 162 Session Expiration: The time at which an element will consider the 163 session timed out, if no successful session refresh transaction 164 occurs beforehand. 165 Session Refresh Request: An INVITE or UPDATE request processed 166 according to the rules of this specification. If the request 167 generates a 2xx response, the session expiration is increased to 168 the current time plus the session interval obtained from the 169 response. A session refresh request is not to be confused with a 170 target refresh request, defined in Section 6 of [RFC3261], which 171 is a request that can update the remote target of a dialog. 172 Initial Session Refresh Request: The first session refresh request 173 sent with a particular Call-ID value. 174 Subsequent Session Refresh Request: Any session refresh request sent 176 with a particular Call-ID after the initial session refresh 177 request. 179 Refresh: Same as a session refresh request. 181 4. Overview of Operation 183 This section provides a brief overview of the operation of the 184 extension. It is tutorial in nature and should not be considered 185 normative. 187 This extension has the property that it works even when only one UA 188 in a dialog supports it. The processing steps differ for handling 189 each of the four cases (the UAC does or doesn't support it, and the 190 UAS does or doesn't support it). For simplicity's sake, this section 191 will describe basic operation in the case where both sides support 192 the extension. 194 A UAC starts by sending an INVITE. This includes a Supported header 195 field with the option tag 'timer', indicating support for this 196 extension. 198 This request passes through proxies, any one of which may have an 199 interest in establishing a session timer. Each proxy can insert a 200 Session-Expires header field and a Min-SE header field into the 201 request (if none is already there) or alter the value of existing 202 Session-Expires and Min-SE header fields as described below. 204 The Min-SE header field establishes the lower bound for the session 205 refresh interval; i.e., the fastest rate any proxy servicing this 206 request will be allowed to require. The purpose of this header field 207 is to prevent hostile proxies from setting arbitrarily short refresh 208 intervals so that their neighbors are overloaded. Each proxy 209 processing the request can raise this lower bound (increase the 210 period between refreshes) but is not allowed to lower it. 212 The Session-Expires header field establishes the upper bound for the 213 session refresh interval; i.e., the time period after processing a 214 request for which any session-stateful proxy must retain its state 215 for this session. Any proxy servicing this request can lower this 216 value, but it is not allowed to decrease it below the value specified 217 in the Min-SE header field. 219 If the Session-Expires interval is too low for a proxy (i.e., lower 220 than the value of Min-SE that the proxy would wish to assert), the 221 proxy rejects the request with a 422 response. That response 222 contains a Min-SE header field identifying the minimum session 223 interval it is willing to support. The UAC will try again, this time 224 including the Min-SE header field in the request. The header field 225 contains the largest Min-SE header field it observed in all 422 226 responses previously received. This way, the minimum timer meets the 227 constraints of all proxies along the path. 229 After several INVITE/422 iterations, the request eventually arrives 230 at the UAS. The UAS can adjust the value of the session interval as 231 if it were a proxy; when done, it places the final session interval 232 into the Session-Expires header field in a 2xx response. The 233 Session-Expires header field also contains a 'refresher' parameter, 234 which indicates who is doing the refreshing -- the UA that is 235 currently the UAC, or the UA that is currently the UAS. As the 2xx 236 response travels back through the proxy chain, each proxy can observe 237 the final session interval but can't change it. 239 From the Session-Expires header field in the response, both UAs know 240 that a session timer is active, when it will expire, and who is 241 refreshing. At some point before the expiration, the currently 242 active refresher generates a session refresh request, which is a re- 243 INVITE or UPDATE [RFC3311] request. If the refresher never gets a 244 response to that session refresh request, it sends a BYE to terminate 245 the session. Similarly, if the other side never gets the session 246 refresh request before the session expires, it sends a BYE. 248 The refresh requests sent once the session is established are 249 processed identically to the initial requests, as described above. 250 This means that a successful session refresh request will extend the 251 session, as desired. 253 The extension introduces additional complications beyond this basic 254 flow to support cases where only one of the UAs supports it. One 255 such complication is that a proxy may need to insert the Session- 256 Expires header field into the response, in the event that the UAS 257 doesn't support the extension. The negotiation of the role of 258 refresher is also affected by this capability; it takes into 259 consideration which participants support the extension. 261 Note that the session timer refreshes the session, not the dialog 262 used to establish the session. Of course, the two are related. If 263 the session expires, a BYE is sent, which terminates the session and, 264 generally, the dialog. 266 5. Session-Expires Header Field Definition 268 The Session-Expires header field conveys the session interval for a 269 SIP session. It is placed only in INVITE or UPDATE requests, as well 270 as in any 2xx response to an INVITE or UPDATE. Like the SIP Expires 271 header field, it contains a delta-time. 273 The absolute minimum for the Session-Expires header field is 90 274 seconds. This value represents a bit more than twice the duration 275 that a SIP transaction can take in the event of a timeout. This 276 allows sufficient time for a UA to attempt a refresh at the halfpoint 277 of the session interval, and for that transaction to complete 278 normally before the session expires. However, 1800 seconds (30 279 minutes) is RECOMMENDED as the value for the Session-Expires header 280 field. In other words, SIP entities MUST be prepared to handle 281 Session-Expires header field values of any duration greater than 90 282 seconds, but entities that insert the Session-Expires header field 283 SHOULD NOT choose values of less than 30 minutes. 285 Small session intervals can be destructive to the network. They 286 cause excessive messaging traffic that affects both user agents and 287 proxy servers. They increase the possibility of 'glare' that can 288 occur when both user agents send a re-INVITE or UPDATE at the same 289 time. Since the primary purpose of the session timer is to provide a 290 means to time out state in SIP elements, very small values won't 291 generally be needed. 30 minutes was chosen because 95% of phone 292 calls are shorter than this duration. However, the 30 minute minimum 293 is listed as a SHOULD, and not as a MUST, since the exact value for 294 this number is dependent on many network factors, including network 295 bandwidths and latencies, computing power, memory availability, 296 network topology, and, of course, the application scenario. After 297 all, SIP can set up any kind of session, not just a phone call. At 298 the time of publication of this document, 30 minutes seems 299 appropriate. Advances in technologies may result in the number being 300 excessively large five years in the future. 302 The default value of the Session-Expires header field is undefined. 303 This means that the absence of the Session-Expires header field 304 implies no expiration of the session, using the mechanism defined in 305 this specification. Note that other mechanisms not defined in this 306 specification, such as locally configured timers, may apply. 308 The syntax of the Session-Expires header field is as follows: 310 Session-Expires = ("Session-Expires" / "x") HCOLON delta-seconds 311 *(SEMI se-params) 312 se-params = refresher-param / generic-param 313 refresher-param = "refresher" EQUAL ("uas" / "uac") 315 Note that a compact form, the letter x, has been reserved for 316 Session-Expires. The BNF for delta-seconds and generic-param is 317 defined in Section 25 of RFC 3261 [RFC3261]. 319 Table 1 is an extension of Tables 2 and 3 in [RFC3261] for the 320 Session-Expires and Min-SE header fields. The column 'PRA' is for 321 the PRACK method [RFC3262], 'UPD' is for the UPDATE method [RFC3311], 322 'SUB' is for the SUBSCRIBE method [RFC3265], and 'NOT' is for the 323 NOTIFY method [RFC3265]. 325 +--------+----+----+---+----+----+----+----+----+----+----+----+----+ 326 | Header | wh | pr | A | BY | CA | IN | OP | RE | PR | UP | SU | NO | 327 | | er | ox | C | E | N | V | T | G | A | D | B | T | 328 | | e | y | K | | | | | | | | | | 329 +--------+----+----+---+----+----+----+----+----+----+----+----+----+ 330 | Sessio | R | am | - | - | - | o | - | - | - | o | - | - | 331 | n-Expi | | r | | | | | | | | | | | 332 | res | | | | | | | | | | | | | 333 | Sessio | 2x | ar | - | - | - | o | - | - | - | o | - | - | 334 | n-Expi | x | | | | | | | | | | | | 335 | res | | | | | | | | | | | | | 336 | Min-SE | R | am | - | - | - | o | - | - | - | o | - | - | 337 | | | r | | | | | | | | | | | 338 | Min-SE | 42 | | - | - | - | m | - | - | - | m | - | - | 339 | | 2 | | | | | | | | | | | | 340 +--------+----+----+---+----+----+----+----+----+----+----+----+----+ 342 Table 1: Session-Expires and Min-SE Header Fields 344 6. Min-SE Header Field Definition 346 The Min-SE header field indicates the minimum value for the session 347 interval, in units of delta-seconds. When used in an INVITE or 348 UPDATE request, it indicates the smallest value of the session 349 interval that can be used for that session. When present in a 350 request or response, its value MUST NOT be less than 90 seconds. 352 When the header field is not present, its default value for is 90 353 seconds. 355 The Min-SE header field MUST NOT be used in responses except for 356 those with a 422 response code. It indicates the minimum value of 357 the session interval that the server is willing to accept. 359 The syntax of the Min-SE header field is as follows: 361 Min-SE = "Min-SE" HCOLON delta-seconds *(SEMI generic-param) 363 7. 422 Response Code Definition 365 This extension introduces the 422 (Session Interval Too Small) 366 response code. It is generated by a UAS or proxy when a request 367 contains a Session-Expires header field with a duration below the 368 minimum timer for the server. The 422 response MUST contain a Min-SE 369 header field with the minimum timer for that server. 371 8. UAC Behavior 373 9. Generating an Initial Session Refresh Request 375 A UAC that supports the session timer extension defined here MUST 376 include a Supported header field in each request (except ACK), 377 listing the option tag 'timer' [RFC3261]. It MUST do so even if the 378 UAC is not requesting usage of the session timer for this session. 380 The UAC MAY include a Require header field in the request with the 381 value 'timer' to indicate that the UAS must support the session timer 382 to participate in the session. This does not mean that the UAC is 383 requiring the UAS to perform the refreshes, only that it is requiring 384 the UAS to support the extension. In addition, the UAC MAY include a 385 Proxy-Require header field in the request with the value 'timer' to 386 indicate that proxies must support the session timer in order to 387 correctly process the request. However, usage of either Require or 388 Proxy-Require by the UAC is NOT RECOMMENDED. They are not needed, 389 since the extension works even when only the UAC supports the 390 extension. The Supported header field containing 'timer' MUST still 391 be included, even if the Require or Proxy-Require header fields are 392 present containing 'timer'. 394 A UAC MAY include the Min-SE header field in the initial INVITE 395 request. 397 A UAC MAY include a Session-Expires header field in an initial 398 session refresh request if it wants a session timer applied to the 399 session. The value of this header field indicates the session 400 interval desired by the UAC. If a Min-SE header is included in the 401 initial session refresh request, the value of the Session-Expires 402 MUST be greater than or equal to the value in Min-SE. 404 The UAC MAY include the refresher parameter with value 'uac' if it 405 wants to perform the refreshes. However, it is RECOMMENDED that the 406 parameter be omitted so that it can be selected by the negotiation 407 mechanisms described below. 409 10. Processing a 2xx Response 411 The session timer requires a UA to create and maintain state. This 412 state includes the session interval, the session expiration, and the 413 identity of the refresher. This state is associated with the dialog 414 on which the session has been negotiated. 416 When a 2xx response to a session refresh request arrives, it may or 417 may not contain a Require header field with the value 'timer'. If it 418 does, the UAC MUST look for the Session-Expires header field to 419 process the response. 421 If there was a Require header field in the response with the value 422 'timer', the Session-Expires header field will always be present. 423 UACs MUST be prepared to receive a Session-Expires header field in a 424 response, even if none were present in the request. The 'refresher' 425 parameter will be present in the Session-Expires header field, 426 indicating who will perform the refreshes. The UAC MUST set the 427 identity of the refresher to the value of this parameter. If the 428 parameter contains the value 'uac', the UAC will perform them. It is 429 possible that the UAC requested the session timer (and thus included 430 a Session-Expires header field in the request) and that there was no 431 Require or Session-Expires header field in the 2xx response. This 432 will happen when the UAS doesn't support the session timer extension 433 and only the UAC has asked for a session timer (no proxies have 434 requested it). In this case, if the UAC still wishes to use the 435 session timer (which is purely for its benefit alone), it has to 436 perform them. To do this, the UAC follows the procedures defined in 437 this specification as if the Session-Expires header field were in the 438 2xx response, and its value was the same as that in the request, but 439 with a refresher parameter of 'uac'. 441 If the 2xx response did not contain a Session-Expires header field, 442 there is no session expiration. In this case, no refreshes need to 443 be sent. A 2xx without a Session-Expires can come for both initial 444 and subsequent session refresh requests. This means that the session 445 timer can be 'turned-off' in mid dialog by receiving a response 446 without a Session-Expires header field. 448 The UAC remembers the session interval for a session as the value of 449 the delta-time from the Session-Expires header field in the most 450 recent 2xx response to a session refresh request on a dialog. It is 451 explicitly allowed for there to be differing session intervals (or 452 none at all) on differing dialogs established as a result of a single 453 INVITE. The UAC also remembers whether it or its peer is the 454 refresher on for the session. 456 If the UAC must perform the refreshes, it computes the session 457 expiration for that session. The session expiration is the time of 458 reception of the last 2xx response to a session refresh request on 459 that dialog plus the session interval for that session. If the UA 460 seeks to continue with the session beyond the session expiration, it 461 MUST generate a refresh before the session expiration. It is 463 RECOMMENDED that this refresh be sent once half the session interval 464 has elapsed. Additional procedures for this refresh are described in 465 Section 18. 467 Similarly, a re-INVITE or UPDATE request sent within a dialog for 468 purposes other than session refreshes will also have the effect of 469 refreshing the session, and its processing will follow the procedures 470 defined in this specification. 472 11. Processing a 422 Response 474 If the response to a session refresh request is a 422 (Session 475 Interval Too Small) response message, then the UAC MAY retry the 476 request. The procedures for retrying are described in Section 12. 477 This new request constitutes a new transaction and SHOULD have the 478 same value as the Call-ID, To, and From of the previous request, but 479 the CSeq should contain a new sequence number that is one higher than 480 the previous. 482 12. Generating Subsequent Session Refresh Requests 484 The values of Supported, Require, and Proxy-Require used in the 485 initial Session refresh request MUST be used. 487 The UAC MUST insert the Min-SE header field into a session refresh 488 request for a particular dialog if it has ever received a 422 489 response to a previous session refresh request on the same dialog, or 490 if it has received a session refresh request on that dialog that 491 contained a Min-SE header field. Similarly, if no dialog has been 492 established yet, a UAC MUST insert the Min-SE header field into an 493 INVITE request if it has ever received a 422 response to a previous 494 INVITE request with the same Call-ID. 496 The value of the Min-SE header field present in a session refresh 497 request MUST be the largest value among all Min-SE header field 498 values returned in all 422 responses or received in session refresh 499 requests, on the same dialog, if a dialog has been established. If 500 no dialog has been established, the Min-SE header field value is set 501 to the largest value among all Min-SE header field values returned in 502 all 422 responses for an INVITE request with the same Call-ID. A 503 result of this rule is that the maximum value of the Min-SE is 504 effectively 'cleared' once the dialog is established, and from that 505 point on, only the values from proxies known to be on the proxy path 506 will end up being used. 508 The UAC may have its own opinions about the minimum session interval. 509 In that case, if the value above is too small, the UAC MAY increase 510 it. 512 In a session refresh request sent within a dialog with an active 513 session timer, the Session-Expires header field SHOULD be present. 514 When present, it SHOULD be equal to the maximum of the Min-SE header 515 field (recall that its default value when not present is 90 seconds) 516 and the current session interval. Inclusion of the Session-Expires 517 header field with this value avoids certain denial-of-service 518 attacks, as documented in Section 19. As such, a UA should only 519 ignore the SHOULD in unusual and singular cases where it is desirable 520 to change the session interval mid-dialog. 522 If the session refresh request is not the initial one, it is 523 RECOMMENDED that the refresher parameter be set to 'uac' if the 524 element sending the request is currently performing refreshes, and to 525 'uas' if its peer is performing the refreshes. This way, the role of 526 refresher does not change on each refresh. However, if it wishes to 527 explicitly change the roles, it MAY use a value of 'uas' if it knows 528 that the other side supports the session timer. It could know this 529 by having received a request from its peer with a Supported header 530 field containing the value 'timer'. If it seeks to reselect the 531 roles, it MAY omit the parameter. 533 A re-INVITE generated to refresh the session is a normal re-INVITE, 534 and an UPDATE generated to refresh a session is a normal UPDATE. If 535 a UAC knows that its peer supports the UPDATE method, it is 536 RECOMMENDED that UPDATE be used instead of a re-INVITE. A UA can 537 make this determination if it has seen an Allow header field from its 538 peer with the value 'UPDATE', or through a mid-dialog OPTIONS 539 request. It is RECOMMENDED that the UPDATE request not contain an 540 offer [RFC3264], but a re-INVITE SHOULD contain one, even if the 541 details of the session have not changed. In that case, the offer 542 MUST indicate that it has not changed. In the case of SDP, this is 543 accomplished by including the same value for the origin field as did 544 previous SDP messages to its peer. The same is true for an answer 545 exchanged as a result of a session refresh request; if it has not 546 changed, that MUST be indicated. 548 13. Proxy Behavior 550 Session timers are mostly of interest to call stateful proxy servers 551 (that is, to servers that maintain the state of calls and dialogs 552 established through them). However, a stateful proxy server (that 553 is, a server which is aware of transaction state but does not retain 554 call or dialog state) MAY also follow the rules described here. 555 Stateless proxies MUST NOT attempt to request session timers. 556 Proxies that ask for session timers SHOULD record-route, as they 557 won't receive refreshes if they don't. 559 The proxy processing rules require the proxy to remember 560 information between the request and response, ruling out stateless 561 proxies. 563 14. Processing of Requests 565 Processing of requests is identical for all session refresh requests. 567 To request a session timer for a session, a proxy makes sure that a 568 Session-Expires header field is present in a session refresh request 569 for that session. A proxy MAY insert a Session-Expires header field 570 in the request before forwarding it if none was present in the 571 request. This Session-Expires header field may contain any desired 572 expiration time the proxy would like, but not with a duration lower 573 than the value in the Min-SE header field in the request, if it is 574 present. The proxy MUST NOT include a refresher parameter in the 575 header field value. 577 If the request already had a Session-Expires header field, the proxy 578 MAY reduce its value but MUST NOT set it to a duration lower than the 579 value in the Min-SE header field in the request, if it is present. 580 If the value of the Session-Expires header field is greater than or 581 equal to the value in the Min-SE header field (recall that the 582 default is 90 seconds when the Min-SE header field is not present), 583 the proxy MUST NOT increase the value of the Session-Expires header 584 field. If the value of the Session-Expires header field is lower 585 than the value of the Min-SE header field (possibly because the proxy 586 increased the value of the Min-SE header field, as described below), 587 the proxy MUST increase the value of the Session-Expires header field 588 to make it equal to Min-SE header field value. The proxy MUST NOT 589 insert or modify the value of the 'refresher' parameter in the 590 Session-Expires header field. 592 If the request contains a Supported header field with a value 593 'timer', the proxy MAY reject the INVITE request with a 422 (Session 594 Interval Too Small) response if the session interval in the Session- 595 Expires header field is smaller than the minimum interval defined by 596 the proxy's local policy. When sending the 422 response, the proxy 597 MUST include a Min-SE header field with the value of its minimum 598 interval. That minimum MUST NOT be lower than 90 seconds. 600 If the request doesn't indicate support for the session timer but 601 contains a session interval that is too small, the proxy cannot 602 usefully reject the request, as this would result in a call failure. 603 Rather, the proxy SHOULD insert a Min-SE header field containing its 604 minimum interval. If a Min-SE header field is already present, the 605 proxy SHOULD increase (but MUST NOT decrease) the value to its 606 minimum interval. The proxy MUST then increase the Session-Expires 607 header field value to be equal to the value in the Min-SE header 608 field, as described above. A proxy MUST NOT insert a Min-SE header 609 field or modify the value of an existing header field in a proxied 610 request if that request contains a Supported header field with the 611 value 'timer'. This is needed to protect against certain denial of 612 service attacks, described in Section 19. 614 Assuming that the proxy has requested a session timer (and thus has 615 possibly inserted the Session-Expires header field or reduced it), 616 the proxy MUST remember that it is using a session timer, and also 617 remember the value of the Session-Expires header field from the 618 proxied request. This MUST be remembered for the duration of the 619 transaction. 621 The proxy MUST remember, for the duration of the transaction, whether 622 the request contained the Supported header field with the value 623 'timer'. If the request did not contain a Supported header field 624 with the value 'timer', the proxy MAY insert a Require header field 625 with the value 'timer' into the request. However, this is NOT 626 RECOMMENDED. This allows the proxy to insist on a session timer for 627 the session. This header field is not needed if a Supported header 628 field was in the request; in this case, the proxy would already be 629 sure the session timer can be used for the session. 631 15. Processing of Responses 633 When the final response to the request arrives, it is examined by the 634 proxy. 636 If the response does not contain a Session-Expires header field but 637 the proxy remembers that it requested a session timer in the request 638 (by inserting, modifying, or examining and accepting the Session- 639 Expires header field in the proxied request), this means that the UAS 640 did not support the session timer. If the proxy remembers that the 641 UAC did not support the session timer either, the proxy forwards the 642 response upstream normally. There is no session expiration for this 643 session. If, however, the proxy remembers that the UAC did support 644 the session timer, additional processing is needed. 646 Because there is no Session-Expires or Require header field in the 647 response, the proxy knows that it is the first session-timer-aware 648 proxy to receive the response. This proxy MUST insert a Session- 649 Expires header field into the response with the value it remembered 650 from the forwarded request. It MUST set the value of the 'refresher' 651 parameter to 'uac'. The proxy MUST add the 'timer' 653 option tag to any Require header field in the response, and if none 654 was present, add the Require header field with that value before 655 forwarding it upstream. 657 If the received response contains a Session-Expires header field, no 658 modification of the response is needed. 660 In all cases, if the 2xx response forwarded upstream by the proxy 661 contains a Session-Expires header field, its value represents the 662 session interval for the session associated with that response. The 663 proxy computes the session expiration as the time when the 2xx 664 response is forwarded upstream, plus the session interval. This 665 session expiration MUST update any existing session expiration for 666 the session. The refresher parameter in the Session-Expires header 667 field in the 2xx response forwarded upstream will be present, and it 668 indicates which UA is performing the refreshes. There can be 669 multiple 2xx responses to a single INVITE, each representing a 670 different dialog, resulting in multiple session expirations, one for 671 each session associated with each dialog. 673 The proxy MUST NOT modify the value of the Session-Expires header 674 field received in the response (assuming one was present) before 675 forwarding it upstream. 677 16. Session Expiration 679 When the current time equals or passes the session expiration for a 680 session, the proxy MAY remove associated call state, and MAY free any 681 resources associated with the call. Unlike the UA, it MUST NOT send 682 a BYE. 684 17. UAS Behavior 686 The UAS must respond to a request for a session timer by the UAC or a 687 proxy in the path of the request, or it may request that a session 688 timer be used itself. 690 If an incoming request contains a Supported header field with a value 691 'timer' and a Session Expires header field, the UAS MAY reject the 692 INVITE request with a 422 (Session Interval Too Small) response if 693 the session interval in the Session-Expires header field is smaller 694 than the minimum interval defined by the UAS' local policy. When 695 sending the 422 response, the UAS MUST include a Min-SE header field 696 with the value of its minimum interval. This minimum interval MUST 697 NOT be lower than 90 seconds. 699 If the UAS wishes to accept the request, it copies the value of the 700 Session-Expires header field from the request into the 2xx response. 702 The UAS response MAY reduce its value but MUST NOT set it to a 703 duration lower than the value in the Min-SE header field in the 704 request, if it is present; otherwise the UAS MAY reduce its value but 705 MUST NOT set it to a duration lower than 90 seconds. The UAS MUST 706 NOT increase the value of the Session-Expires header field. 708 If the incoming request contains a Supported header field with a 709 value 'timer' but does not contain a Session-Expires header, it means 710 that the UAS is indicating support for timers but is not requesting 711 one. The UAS may request a session timer in the 2XX response by 712 including a Session-Expires header field. The value MUST NOT be set 713 to a duration lower than the value in the Min-SE header field in the 714 request, if it is present. 716 The UAS MUST set the value of the refresher parameter in the Session- 717 Expires header field in the 2xx response. This value specifies who 718 will perform refreshes for the dialog. The value is based on the 719 value of this parameter in the request, and on whether the UAC 720 supports the session timer extension. The UAC supports the extension 721 if the 'timer' option tag was present in a Supported header field in 722 the request. Table 2 defines how the value in the response is set. 723 A value of 'none' in the 2nd column means that there was no refresher 724 parameter in the request. A value of 'NA' in the third column means 725 that this particular combination shouldn't happen, as it is 726 disallowed by the protocol. 728 UAC supports? refresher parameter refresher parameter 729 in request in response 730 ------------------------------------------------------- 731 N none uas 732 N uac NA 733 N uas NA 734 Y none uas or uac 735 Y uac uac 736 Y uas uas 738 Figure 1: UAS Behavior 740 The fourth row of Table 2 describes a case where both the UAC and UAS 741 support the session timer extension, and where the UAC did not select 742 who will perform refreshes. This allows the UAS to decide whether it 743 or the UAC will perform the refreshes. However, as the table 744 indicates, the UAS cannot override the UAC's choice of refresher, if 745 it made one. 747 If the refresher parameter in the Session-Expires header field in the 748 2xx response has a value of 'uac', the UAS MUST place a Require 749 header field into the response with the value 'timer'. This is 750 because the uac is performing refreshes and the response has to be 751 processed for the UAC to know this. If the refresher parameter in 752 the 2xx response has a value of 'uas' and the Supported header field 753 in the request contained the value 'timer', the UAS SHOULD place a 754 Require header field into the response with the value 'timer'. In 755 this case, the UAC is not refreshing, but it is supposed to send a 756 BYE if it never receives a refresh. Since the call will still 757 succeed without the UAC sending a BYE, insertion of the Require is a 758 SHOULD here, and not a MUST. 760 Just like the UAC, the UAS stores state for the session timer. This 761 state includes the session interval, the session expiration, and the 762 identity of the refresher. This state is bound to the dialog used to 763 set up the session. The session interval is set to the value of the 764 delta-time from the Session-Expires header field in the most recent 765 2xx response to a session refresh request on that dialog. It also 766 remembers whether it or its peer is the refresher on the dialog, 767 based on the value of the refresher parameter from the most recent 768 2xx response to a session refresh request on that dialog. If the 769 most recent 2xx response had no Session-Expires header field, there 770 is no session expiration, and no refreshes have to be performed. 772 If the UAS must refresh the session, it computes the session 773 expiration. The session expiration is the time of transmission of 774 the last 2xx response to a session refresh request on that dialog 775 plus the session interval. If UA wishes to continue with the session 776 beyond the session expiration, it MUST generate a refresh before the 777 session expiration. It is RECOMMENDED that this refresh be sent once 778 half the session interval has elapsed. Additional procedures for 779 this refresh are described in Section 18. 781 18. Performing Refreshes 783 The side generating a refresh does so according to the UAC procedures 784 defined in Section 8. Note that only a 2xx response to a session 785 refresh request extends the session expiration. This means that a UA 786 could attempt a refresh and receive a 422 response with a Min-SE 787 header field that contains a value much larger than the current 788 session interval. The UA will still have to send a session refresh 789 request before the session expiration (which has not changed), even 790 though this request will contain a value of the Session-Expires that 791 is much larger than the current session interval. 793 If the session refresh request transaction times out or generates a 794 408 or 481 response, then the UAC sends a BYE request as per 795 Section 12.2.1.2 of RFC 3261 [RFC3261]. If the session refresh 796 request does not generate a 2xx response (and, as a result, the 797 session is not refreshed), and a response other than 408 or 481 is 798 received, the UAC SHOULD follow the rules specific to that response 799 code and retry if possible. For example, if the response is a 401, 800 the UAC would retry the request with new credentials. However, the 801 UAC SHOULD NOT continuously retry the request if the server indicates 802 the same error response. 804 Similarly, if the side not performing refreshes does not receive a 805 session refresh request before the session expiration, it SHOULD send 806 a BYE to terminate the session, slightly before the session 807 expiration. The minimum of 32 seconds and one third of the session 808 interval is RECOMMENDED. 810 Firewalls and NAT ALGs may be very unforgiving about allowing SIP 811 traffic to pass after the expiration time of the session. This is 812 why the BYE should be sent before the expiration. 814 19. Security Considerations 816 The session timer introduces the capability of a proxy or UA element 817 to force compliant UAs to send refreshes at a rate of the element's 818 choosing. This introduces the possibility of denial-of-service 819 attacks with significant amplification properties. These attacks can 820 be launched from 'outsiders' (elements that attempt to modify 821 messages in transit) or by 'insiders' (elements that are legitimately 822 in the request path but are intent on doing harm). Fortunately, both 823 cases are adequately handled by this specification. 825 20. Inside Attacks 827 This introduces the possibility of rogue proxies or UAs introducing 828 denial-of-service attacks. However, the mechanisms in this 829 specification prevent that from happening. 831 First, consider the case of a rogue UAC that wishes to force a UAS to 832 generate refreshes at a rapid rate. To do so, it inserts a Session- 833 Expires header field into an INVITE with a low duration and a 834 refresher parameter equal to uas. Assume it places a Supported 835 header field into the request. The UAS or any proxy that objects to 836 this low timer will reject the request with a 422, thereby preventing 837 the attack. If no Supported header field was present, the proxies 838 will insert a Min-SE header field into the request before forwarding 839 it. As a result, the UAS will not choose a session timer lower than 840 the minimum allowed by all elements on the path. This too prevents 841 the attack. 843 Next, consider the case of a rogue UAS that wishes to force a UAC to 844 generate refreshes at a rapid rate. In that case, the UAC has to 845 support session timer. The initial INVITE arrives at the rogue UAS, 846 which returns a 2xx with a very small session interval. The UAC uses 847 this timer and quickly sends a refresh. Section 12 requires that the 848 UAC copy the current session interval into the Session-Expires header 849 field in the request. This enables the proxies to see the current 850 value. The proxies will reject this request and provide a Min-SE 851 with a higher minimum, which the UAC will then use. Note, that if 852 the proxies did not reject the request, but rather proxied the 853 request with a Min-SE header field, an attack would still be 854 possible. The UAS could discard this header field in a 2xx response 855 and force the UAC to continue to generate rapid requests. 857 In a similar fashion, a rogue proxy cannot force either the UAC or 858 UAS to generate refreshes unless the proxy remains on the signaling 859 path and sees every request and response. 861 21. Outside Attacks 863 An element that can observe and modify a request or response in 864 transit can force rapid session refreshes. To prevent this, requests 865 and responses have to be protected by message integrity. Since the 866 session timer header fields are not end-to-end and are manipulated by 867 proxies, the SIP S/MIME capabilities are not suitable for this task. 868 Rather, integrity has to be protected by using hop-by-hop mechanisms. 869 As a result, it is RECOMMENDED that an element send a request with a 870 Session-Expires header field or a Supported header field with the 871 value 'timer' by using TLS. As adequate protection is obtained only 872 if security is applied on each hop, it is RECOMMENDED that the SIPS 873 URI scheme be used in conjunction with this extension. This means 874 that proxies that record-route and request session timer SHOULD 875 record-route with a SIPS URI. A UA that inserts a Session-Expires 876 header into a request or response SHOULD include a Contact URI that 877 is a SIPS URI. 879 22. IANA Considerations 881 This extension defines two new header fields, a new response code, 882 and a new option tag. SIP [RFC3261] defines IANA procedures for 883 registering these. 885 23. IANA Registration of Min-SE and Session-Expires Header Fields 887 The following is the registration for the Min-SE header field: 889 RFC Number: RFC 4028 890 Header Name: Min-SE 891 Compact Form: none 893 The following is the registration for the Session-Expires header 894 field: 896 RFC Number: RFC 4028 897 Header Name: Session-Expires 898 Compact Form: x 900 24. IANA Registration of the 422 (Session Interval Too Small) Response 901 Code 903 The following is the registration for the 422 (Session Interval Too 904 Small) response code: 906 Response Code: 422 907 Default Reason Phrase: Session Interval Too Small 908 RFC Number: RFC 4028 910 25. IANA Registration of the 'timer' Option Tag 912 The following is the registration for the 'timer' option tag: 914 Name: timer 915 Description: This option tag is for support of the session timer 916 extension. Inclusion in a Supported header field in a request or 917 response indicates that the UA can perform refreshes according to 918 that specification. Inclusion in a Require header in a request 919 means that the UAS must understand the session timer extension to 920 process the request. Inclusion in a Require header field in a 921 response indicates that the UAC must look for the Session-Expires 922 header field in the response and process it accordingly. 924 26. Example Call Flow 926 Example Session Timer Flow 928 Alice Proxy P1 Proxy P2 Bob 929 |(1) INVITE | | | 930 |SE: 50 | | | 931 |----------->| | | 932 |(2) 422 | | | 933 |MSE: 3600 | | | 934 |<-----------| | | 935 |(3) ACK | | | 936 |----------->| | | 937 |(4) INVITE | | | 938 |SE:3600 | | | 939 |MSE:3600 | | | 940 |----------->| | | 942 | |(5) INVITE | | 943 | |SE:3600 | | 944 | |MSE:3600 | | 945 | |----------->| | 946 | |(6) 422 | | 947 | |MSE:4000 | | 948 | |<-----------| | 949 | |(7) ACK | | 950 | |----------->| | 951 |(8) 422 | | | 952 |MSE:4000 | | | 953 |<-----------| | | 954 |(9) ACK | | | 955 |----------->| | | 956 |(10) INVITE | | | 957 |SE:4000 | | | 958 |MSE:4000 | | | 959 |----------->| | | 960 | |(11) INVITE | | 961 | |SE:4000 | | 962 | |MSE:4000 | | 963 | |----------->| | 964 | | |(12) INVITE | 965 | | |SE:4000 | 966 | | |MSE:4000 | 967 | | |----------->| 968 | | |(13) 200 OK | 969 | | |SE:4000 | 970 | | |<-----------| 971 | |(14) 200 OK | | 972 | |SE:4000 | | 973 | |<-----------| | 974 |(15) 200 OK | | | 975 |SE:4000 | | | 976 |<-----------| | | 977 |(16) ACK | | | 978 |----------->| | | 979 | |(17) ACK | | 980 | |------------------------>| 981 |(18) UPDATE | | | 982 |SE:4000 | | | 983 |----------->| | | 984 | |(19) UPDATE | | 985 | |SE:4000 | | 986 | |------------------------>| 987 | |(20) 200 OK | | 988 | |SE:4000 | | 989 | |<------------------------| 991 |(21) 200 OK | | | 992 |SE:4000 | | | 993 |<-----------| | | 994 | |(22) BYE | | 995 | |<------------------------| 996 |(23) BYE | | | 997 |<-----------| | | 998 | |(24) 408 | | 999 | |------------------------>| 1001 Figure 2: Example Session Timer Flow 1003 Figure 1 gives an example of a call flow that makes use of the 1004 session timer. In this example, both the UAC and UAS support the 1005 session timer extension. The initial INVITE request generated by the 1006 UAC, Alice (message 1), might look like this: 1008 INVITE sips:bob@biloxi.example.com SIP/2.0 1009 Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8 1010 Supported: timer 1011 Session-Expires: 50 1012 Max-Forwards: 70 1013 To: Bob 1014 From: Alice ;tag=1928301774 1015 Call-ID: a84b4c76e66710 1016 CSeq: 314159 INVITE 1017 Contact: 1018 Content-Type: application/sdp 1019 Content-Length: 142 1021 (Alice's SDP not shown) 1023 This request indicates that Alice supports the session timer, and is 1024 requesting session refreshes every 50 seconds. This arrives at the 1025 first proxy, P1. This session interval is below the minimum allowed 1026 value of 3600. So P1 rejects the request with a 422 (message 2): 1028 SIP/2.0 422 Session Interval Too Small 1029 Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8 1030 ;received=192.0.2.1 1031 Min-SE: 3600 1032 To: Bob ;tag=9a8kz From: Alice 1033 ;tag=1928301774 Call-ID: 1034 a84b4c76e66710 CSeq: 314159 INVITE 1036 This response contains a Min-SE header field with the value 3600. 1037 Alice then retries the request. This time, the request contains a 1038 Min-SE header, as Alice has received a 422 for other INVITE requests 1039 with the same Call-ID. The new request (message 4) might look like 1040 this: 1042 INVITE sips:bob@biloxi.example.com SIP/2.0 1043 Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds9 1044 Supported: timer 1045 Session-Expires: 3600 1046 Min-SE: 3600 1047 Max-Forwards: 70 1048 To: Bob 1049 From: Alice ;tag=1928301774 1050 Call-ID: a84b4c76e66710 1051 CSeq: 314160 INVITE 1052 Contact: 1053 Content-Type: application/sdp 1054 Content-Length: 142 1056 (Alice's SDP not shown) 1058 Proxy P1 record-routes. Since the session interval is now acceptable 1059 to it, it forwards the request to P2 (message 5). However, the 1060 session interval is below its minimum configured amount of 4000. So 1061 it rejects the request with a 422 response code (message 6) and 1062 includes a Min-SE header field with the value of 4000. Once more, 1063 Alice retries the INVITE. This time, the Min-SE header field in her 1064 INVITE is the maximum of all Min-SE she has received (3600 and 4000). 1065 Message 10 might look like this: 1067 INVITE sips:bob@biloxi.example.com SIP/2.0 1068 Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds10 1069 Supported: timer 1070 Session-Expires: 4000 1071 Min-SE: 4000 1072 Max-Forwards: 70 1073 To: Bob 1074 From: Alice ;tag=1928301774 1075 Call-ID: a84b4c76e66710 1076 CSeq: 314161 INVITE 1077 Contact: 1078 Content-Type: application/sdp 1079 Content-Length: 142 1081 (Alice's SDP not shown) 1083 P1 record-routes once again, but P2 does not (this wouldn't normally 1084 happen; presumably, if it asked for session timer, it would record- 1085 route the subsequent request). The UAS receives the request. It 1086 copies the Session-Expires header from the request to the response 1087 and adds a refresher parameter with value 'uac'. This 200 OK is 1088 forwarded back to Alice. The response she receives (message 15) 1089 might look like this: 1091 SIP/2.0 200 OK 1092 Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds10 1093 ;received=192.0.2.1 1094 Require: timer 1095 Supported: timer Record-Route: sips:p1.atlanta.example.com;lr 1096 Session-Expires: 4000;refresher=uac To: Bob 1097 ;tag=9as888nd From: Alice 1098 ;tag=1928301774 Call-ID: 1099 a84b4c76e66710 CSeq: 314161 INVITE Contact: 1100 Content-Type: application/sdp Content-Length: 142 1102 (Bob's SDP not shown) 1104 Alice generates an ACK (message 16), which is routed through P1 and 1105 then to Bob. Since Alice is the refresher, around 2000 seconds later 1106 Alice sends an UPDATE request to refresh the session. Because this 1107 request is part of an established dialog and Alice has not received 1108 any 422 responses or requests on that dialog, there is no Min-SE 1109 header field in her request (message 18): 1111 UPDATE sips:bob@192.0.2.4 SIP/2.0 1112 Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds12 1113 Route: sips:p1.atlanta.example.com;lr 1114 Supported: timer 1115 Session-Expires: 4000;refresher=uac 1116 Max-Forwards: 70 1117 To: Bob ;tag=9as888nd 1118 From: Alice ;tag=1928301774 1119 Call-ID: a84b4c76e66710 1120 CSeq: 314162 UPDATE 1121 Contact: 1123 This is forwarded through P1 to Bob. Bob generates a 200 OK, copying 1124 the Session-Expires header field into the response. This is 1125 forwarded through P1 and arrives at Alice. The response she receives 1126 (message 21) might look like this: 1128 SIP/2.0 200 OK 1129 Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds12 1130 ;received=192.0.2.1 1131 Require: timer 1132 Session-Expires: 4000;refresher=uac To: Bob 1133 ;tag=9as888nd From: Alice 1134 ;tag=1928301774 Call-ID: 1135 a84b4c76e66710 CSeq: 314162 UPDATE Contact: 1137 Shortly afterward, Alice's UA crashes. As a result, she never sends 1138 a session refresh request. 3968 seconds later, Bob times out and 1139 sends a BYE request (message 22). This is sent to P1. P1 attempts 1140 to deliver it but fails (because Alice's UA has crashed). P1 then 1141 returns a 408 (Request Timeout) to Bob. 1143 27. Acknowledgements 1145 The authors wish to thank Brett Tate for his contributions to this 1146 work. Brian Rosen completed the editing of the document. 1148 28. References 1150 28.1. Normative References 1152 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1153 Requirement Levels", BCP 14, RFC 2119, 1154 DOI 10.17487/RFC2119, March 1997, 1155 . 1157 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 1158 A., Peterson, J., Sparks, R., Handley, M., and E. 1159 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 1160 DOI 10.17487/RFC3261, June 2002, 1161 . 1163 [RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP) 1164 UPDATE Method", RFC 3311, DOI 10.17487/RFC3311, October 1165 2002, . 1167 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1168 with Session Description Protocol (SDP)", RFC 3264, 1169 DOI 10.17487/RFC3264, June 2002, 1170 . 1172 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1173 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1174 May 2017, . 1176 28.2. Informative References 1178 [RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address 1179 Translator (NAT) Terminology and Considerations", 1180 RFC 2663, DOI 10.17487/RFC2663, August 1999, 1181 . 1183 [RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of 1184 Provisional Responses in Session Initiation Protocol 1185 (SIP)", RFC 3262, DOI 10.17487/RFC3262, June 2002, 1186 . 1188 [RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific 1189 Event Notification", RFC 3265, DOI 10.17487/RFC3265, June 1190 2002, . 1192 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1193 Jacobson, "RTP: A Transport Protocol for Real-Time 1194 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 1195 July 2003, . 1197 Authors' Addresses 1199 Christer Holmberg 1200 Ericsson 1201 Hirsalantie 11 1202 Jorvas 02420 1203 Finland 1205 Email: christer.holmberg@ericsson.com 1207 Steve Donovan 1208 Cisco Systems, Inc. 1209 2200 E. President George Bush Turnpike 1210 Richardson, Texas 75082 1211 US 1213 Email: srd@cisco.com 1215 Jonathan Rosenberg 1216 Cisco Systems, Inc. 1217 600 Lanidex Plaza 1218 Parsippany, New Jersey 07054 1219 US 1221 Email: jdrosen@cisco.com