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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Obsolete informational reference (is this intentional?): RFC 4474 (Obsoleted by RFC 8224) Summary: 0 errors (**), 0 flaws (~~), 2 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 ECRIT H. Schulzrinne 3 Internet-Draft Columbia University 4 Intended status: Standards Track H. Tschofenig 5 Expires: March 30, 2014 Nokia Solutions and Networks 6 C. Holmberg 7 Ericsson 8 M. Patel 9 InterDigital Communications 10 September 26, 2013 12 Public Safety Answering Point (PSAP) Callback 13 draft-ietf-ecrit-psap-callback-11.txt 15 Abstract 17 After an emergency call is completed (either prematurely terminated 18 by the emergency caller or normally by the call taker) it is possible 19 that the call taker feels the need for further communication. For 20 example, the call may have been dropped by accident without the call 21 taker having sufficient information about the current situation of a 22 wounded person. A call taker may trigger a callback towards the 23 emergency caller using the contact information provided with the 24 initial emergency call. This callback could, under certain 25 circumstances, be treated like any other call and as a consequence it 26 may get blocked by authorization policies or may get forwarded to an 27 answering machine. 29 The IETF emergency services architecture specification already offers 30 a solution approach for allowing PSAP callbacks to bypass 31 authorization policies to reach the caller without unnecessary 32 delays. Unfortunately, the specified mechanism only supports limited 33 scenarios. This document discusses shortcomings of the current 34 mechanisms and illustrates additional scenarios where better-than- 35 normal call treatment behavior would be desirable. A solution based 36 on a new header field value, called "psap-callback", for the SIP 37 Priority header field is specified to accomplish the PSAP callback 38 marking. 40 Status of This Memo 42 This Internet-Draft is submitted in full conformance with the 43 provisions of BCP 78 and BCP 79. 45 Internet-Drafts are working documents of the Internet Engineering 46 Task Force (IETF). Note that other groups may also distribute 47 working documents as Internet-Drafts. The list of current Internet- 48 Drafts is at http://datatracker.ietf.org/drafts/current/. 50 Internet-Drafts are draft documents valid for a maximum of six months 51 and may be updated, replaced, or obsoleted by other documents at any 52 time. It is inappropriate to use Internet-Drafts as reference 53 material or to cite them other than as "work in progress." 55 This Internet-Draft will expire on March 30, 2014. 57 Copyright Notice 59 Copyright (c) 2013 IETF Trust and the persons identified as the 60 document authors. All rights reserved. 62 This document is subject to BCP 78 and the IETF Trust's Legal 63 Provisions Relating to IETF Documents 64 (http://trustee.ietf.org/license-info) in effect on the date of 65 publication of this document. Please review these documents 66 carefully, as they describe your rights and restrictions with respect 67 to this document. Code Components extracted from this document must 68 include Simplified BSD License text as described in Section 4.e of 69 the Trust Legal Provisions and are provided without warranty as 70 described in the Simplified BSD License. 72 Table of Contents 74 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 75 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 76 3. Callback Scenarios . . . . . . . . . . . . . . . . . . . . . 4 77 3.1. Routing Asymmetry . . . . . . . . . . . . . . . . . . . . 5 78 3.2. Multi-Stage Routing . . . . . . . . . . . . . . . . . . . 6 79 3.3. Call Forwarding . . . . . . . . . . . . . . . . . . . . . 6 80 3.4. Network-based Service URN Resolution . . . . . . . . . . 8 81 3.5. PSTN Interworking . . . . . . . . . . . . . . . . . . . . 9 82 4. SIP PSAP Callback Indicator . . . . . . . . . . . . . . . . . 10 83 4.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 10 84 4.2. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 10 85 4.3. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 10 86 4.3.1. General . . . . . . . . . . . . . . . . . . . . . . . 10 87 4.3.2. ABNF . . . . . . . . . . . . . . . . . . . . . . . . 10 88 5. Security Considerations . . . . . . . . . . . . . . . . . . . 10 89 5.1. Security Threat . . . . . . . . . . . . . . . . . . . . . 10 90 5.2. Security Requirements . . . . . . . . . . . . . . . . . . 11 91 5.3. Security Solution . . . . . . . . . . . . . . . . . . . . 11 92 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 93 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 94 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 95 8.1. Normative References . . . . . . . . . . . . . . . . . . 13 96 8.2. Informative References . . . . . . . . . . . . . . . . . 13 98 1. Introduction 100 Summoning police, the fire department or an ambulance in emergencies 101 is one of the fundamental and most-valued functions of the telephone. 102 As telephone functionality moves from circuit-switched telephony to 103 Internet telephony, its users rightfully expect that this core 104 functionality will continue to work at least as well as it has for 105 the legacy technology. New devices and services are being made 106 available that could be used to make a request for help, which are 107 not traditional telephones, and users are increasingly expecting them 108 to be used to place emergency calls. 110 An overview of the protocol interactions for emergency calling using 111 the IETF emergency services architecture are described in [RFC6443] 112 and [RFC6881] specifies the technical details. As part of the 113 emergency call setup procedure two important identifiers are conveyed 114 to the PSAP call taker's user agent, namely the Address-Of-Record 115 (AOR), and, if available, the Globally Routable User Agent (UA) URIs 116 (GRUU). RFC 3261 [RFC3261] defines the AOR as: 118 "An address-of-record (AOR) is a SIP or SIPS URI that points to a 119 domain with a location service that can map the URI to another URI 120 where the user might be available. Typically, the location 121 service is populated through registrations. An AOR is frequently 122 thought of as the "public address" of the user." 124 In SIP systems a single user can have a number of user agents 125 (handsets, softphones, voicemail accounts, etc.) which are all 126 referenced by the same AOR. There are a number of cases in which it 127 is desirable to have an identifier which addresses a single user 128 agent rather than the group of user agents indicated by an AOR. The 129 GRUU is such a unique user-agent identifier, which is still globally 130 routable. RFC 5627 [RFC5627] specifies how to obtain and use GRUUs. 131 [RFC6881] also makes use of the GRUU for emergency calls. 133 Regulatory requirements demand that the emergency call setup 134 procedure itself provides enough information to allow the call taker 135 to initiate a callback to the emergency caller. This is desirable in 136 those cases where the call got dropped prematurely or when further 137 communication need arises. The AOR and the GRUU serve this purpose. 139 The communication attempt by the PSAP call taker back to the 140 emergency caller is called 'PSAP callback'. 142 A PSAP callback may, however, be blocked by user configured 143 authorization policies or may be forwarded to an answering machine 144 since SIP entities (SIP proxies as well as the SIP user equipment 145 itself) cannot differentiate the PSAP callback from any other SIP 146 call. "Call barring", "do not disturb", or "call diversion"(aka call 147 forwarding) are features that prevent delivery of a call. It is 148 important to note that these features may be implemented by SIP 149 intermediaries as well as by the user agent. 151 Among the emergency services community there is the desire to offer 152 PSAP callbacks a treatment such that chances are increased that it 153 reaches the emergency caller. At the same time a design must deal 154 with the negative side-effects of allowing certain calls to bypass 155 call forwarding or other authorization policies. Ideally, the PSAP 156 callback has to relate to an earlier emergency call that was made 157 "not too long ago". An exact time interval is difficult to define in 158 a global IETF standard due to the variety of national regulatory 159 requirements. 161 To nevertheless meet the needs from the emergency services community 162 a basic mechanism for preferential treatment of PSAP callbacks was 163 defined in Section 13 of [RFC6443]. The specification says: 165 "A UA may be able to determine a PSAP callback by examining the 166 domain of incoming calls after placing an emergency call and 167 comparing that to the domain of the answering PSAP from the 168 emergency call. Any call from the same domain and directed to the 169 supplied Contact header or AOR after an emergency call should be 170 accepted as a callback from the PSAP if it occurs within a 171 reasonable time after an emergency call was placed." 173 This approach mimics a stateful packet filtering firewall and is 174 indeed helpful in a number of cases. It is also relatively simple to 175 implement even though it requires call state to be maintained by the 176 user agent as well as by SIP intermediaries. Unfortunately, the 177 solution does not work in all deployment scenarios. In Section 3 we 178 describe cases where the currently standardized approach is 179 insufficient. 181 2. Terminology 183 Emergency services related terminology is borrowed from [RFC5012]. 184 This includes terminology like emergency caller, user equipment, call 185 taker, Emergency Service Routing Proxy (ESRP), and Public Safety 186 Answering Point (PSAP). 188 3. Callback Scenarios 190 This section illustrates a number of scenarios where the currently 191 specified solution, as specified in [RFC6881], for preferential 192 treatment of callbacks fails. As explained in Section 1 a SIP entity 193 examines an incoming PSAP callback by comparing the domain of the 194 PSAP with the destination domain of the outbound emergency call 195 placed earlier. 197 NOTE: All FQDNs used in the subsections below are used for 198 illustrative purposes. They are examples to demonstrate the 199 limitations of the technical solution outlined in RFC 6881. 201 3.1. Routing Asymmetry 203 In some deployment environments it is common to have incoming and 204 outgoing SIP messaging routed through different SIP entities. Figure 205 1 shows this graphically whereby a VoIP provider uses different SIP 206 proxies for inbound and for outbound call handling. Unless the two 207 devices are synchronized, the callback hitting the inbound proxy 208 would get treated like any other call since the emergency call 209 established state information at the outbound proxy only. 211 ,-------. 212 ,' `. 213 ,-------. / Emergency \ 214 ,' `. | Services | 215 / VoIP \ I | Network | 216 | Provider | n | | 217 | | t | | 218 | | e | | 219 | +-------+ | r | | 220 +--+---|Inbound|<--+-----m | | 221 | | |Proxy | | e | +------+ | 222 | | +-------+ | d | |PSAP | | 223 | | | i | +--+---+ | 224 +----+ | | | a-+ | | | 225 | UA |<---+ | | t | | | | 226 | |----+ | | e | | | | 227 +----+ | | | | | | | 228 | | | P | | | | 229 | | | r | | | | 230 | | +--------+ | o | | | | 231 +--+-->|Outbound|--+---->v | | +--+---+ | 232 | |Proxy | | i | | +-+ESRP | | 233 | +--------+ | d | | | +------+ | 234 | | e || | | 235 | | r |+-+ | 236 \ / | | 237 `. ,' \ / 238 '-------' `. ,' 239 '-------' 241 Figure 1: Example for Routing Asymmetry. 243 3.2. Multi-Stage Routing 245 Consider the following emergency call routing scenario shown in 246 Figure 2 where routing towards the PSAP occurs in several stages. In 247 this scenario we consider a SIP UA that uses LoST to learn the next 248 hop destination closer to the PSAP. This call is then sent to the 249 user's VoIP provider. The user's VoIP provider receives the 250 emergency call and creates state based on the destination domain, 251 namely state.org. It then routes it to the indicated ESRP. When the 252 ESRP receives it it needs to decide what the next hop is to get it 253 closer to the PSAP. In our example the next hop is the PSAP with the 254 URI psap@town.com. 256 When a callback is sent from psap@town.com towards the emergency 257 caller the call will get normal treatment by the VoIP providers 258 inbound proxy since the domain of the PSAP does not match the stored 259 state information. 261 ,-------. 262 +----+ ,' `. 263 | UA |--- esrp1@foobar.com / Emergency \ 264 +----+ \ | Services | 265 \ ,-------. | Network | 266 ,' `. | | 267 / VoIP \ | +------+ | 268 ( Provider ) | |PSAP | | 269 \ / | +--+---+ | 270 `. ,' | | 271 '---+---' | | | 272 | |psap@town.com | 273 esrp@state.org | | | 274 | | | | 275 | | | | 276 | | +--+---+ | 277 +------------+---+ESRP | | 278 | +------+ | 279 | | 280 \ / 281 `. ,' 282 '-------' 284 Figure 2: Example for Multi-Stage Routing. 286 3.3. Call Forwarding 287 Imagine the following case where an emergency call enters an 288 emergency network (state.org) via an ESRP but then gets forwarded to 289 a different emergency services network (in our example to police- 290 town.org, fire-town.org or medic-town.org). The same considerations 291 apply when the police, fire and ambulance networks are part of the 292 state.org sub-domains (e.g., police.state.org). 294 Similar to the previous scenario the problem here is with the wrong 295 state information being established during the emergency call setup 296 procedure. A callback would originate in the police-town.org, fire- 297 town.org or medic-town.org domain whereas the emergency caller's SIP 298 UA or the VoIP outbound proxy has stored state.org. 300 ,-------. 301 ,' `. 302 / Emergency \ 303 | Services | 304 | Network | 305 | (state.org) | 306 | | 307 | | 308 | +------+ | 309 | |PSAP +--+ | 310 | +--+---+ | | 311 | | | | 312 | | | | 313 | | | | 314 | | | | 315 | | | | 316 | +--+---+ | | 317 ------------------+---+ESRP | | | 318 esrp-a@state.org | +------+ | | 319 | | | 320 | Call Fwd | | 321 | +-+-+---+ | 322 \ | | | / 323 `. | | | ,' 324 '-|-|-|-' ,-------. 325 Police | | | Fire ,' `. 326 +------------+ | +----+ / Emergency \ 327 ,-------. | | | | Services | 328 ,' `. | | | | Network | 329 / Emergency \ | Ambulance | | fire-town.org | 330 | Services | | | | | | 331 | Network | | +----+ | | +------+ | 332 |police-town.org| | ,-------. | +----+---+PSAP | | 333 | | | ,' `. | | +------+ | 334 | +------+ | | / Emergency \ | | | 335 | |PSAP +----+--+ | Services | | | , 336 | +------+ | | Network | | `~~~~~~~~~~~~~~~ 337 | | |medic-town.org | | 338 | , | | | 339 `~~~~~~~~~~~~~~~ | +------+ | | 340 | |PSAP +----+ + 341 | +------+ | 342 | | 343 | , 344 `~~~~~~~~~~~~~~~ 346 Figure 3: Example for Call Forwarding. 348 3.4. Network-based Service URN Resolution 350 The IETF emergency services architecture also considers cases where 351 the resolution from the Service URN to the PSAP URI does not only 352 happen at the SIP UA itself but at intermediate SIP entities, such as 353 the user's VoIP provider. 355 Figure 4 shows this message exchange of the outgoing emergency call 356 and the incoming PSAP graphically. While the state information 357 stored at the VoIP provider is correct the state allocated at the SIP 358 UA is not. 360 ,-------. 361 ,' `. 362 / Emergency \ 363 | Services | 364 | Network | 365 |police-town.org| 366 | | 367 | +------+ | Invite to police.example.com 368 | |PSAP +<---+------------------------+ 369 | | +----+------------------+ ^ 370 | +------+ |Invite from | | 371 | ,police.example.com| | 372 `~~~~~~~~~~~~~~~ v | 373 +--------+ ++-----+-+ 374 | | query |VoIP | 375 | LoST |<-----------------------|Service | 376 | Server | police.example.com |Provider| 377 | |----------------------->| | 378 +--------+ +--------+ 379 | ^ 380 Invite| | Invite 381 from| | to 382 police.example.com| | urn:service:sos 383 V | 384 +-------+ 385 | SIP | 386 | UA | 387 | Alice | 388 +-------+ 390 Figure 4: Example for Network-based Service URN Resolution. 392 3.5. PSTN Interworking 394 In case an emergency call enters the PSTN, as shown in Figure 5, 395 there is no guarantee that the callback some time later leaves the 396 same PSTN/VoIP gateway or that the same end point identifier is used 397 in the forward as well as in the backward direction making it 398 difficult to reliably detect PSAP callbacks. 400 +-----------+ 401 | PSTN |-------------+ 402 | Calltaker | | 403 | Bob |<--------+ | 404 +-----------+ | v 405 ------------------- 406 //// \\\\ +------------+ 407 | | |PSTN / VoIP | 408 | PSTN |---->|Gateway | 409 \\\\ //// | | 410 ------------------- +----+-------+ 411 ^ | 412 | | 413 +-------------+ | +--------+ 414 | | | |VoIP | 415 | PSTN / VoIP | +->|Service | 416 | Gateway | |Provider| 417 | |<------Invite----| Y | 418 +-------------+ +--------+ 419 | ^ 420 | | 421 Invite Invite 422 | | 423 V | 424 +-------+ 425 | SIP | 426 | UA | 427 | Alice | 428 +-------+ 430 Figure 5: Example for PSTN Interworking. 432 Note: This scenario is considered outside the scope of this document. 433 The specified solution does not support this use case. 435 4. SIP PSAP Callback Indicator 437 4.1. General 439 This section defines a new header field value, called "psap- 440 callback", for the SIP Priority header field defined in [RFC3261]. 441 The value is used to inform SIP entities that the request is 442 associated with a PSAP callback SIP session. 444 4.2. Usage 446 SIP entities that receive the header field value within an initial 447 request for a SIP session can, depending on local policies, apply 448 PSAP callback specific procedures for the session or request. 450 The PSAP callback specific procedures may be applied by SIP-based 451 network entities and by the callee. The specific procedures taken 452 when receiving such a PSAP callback marked call, such as bypassing 453 services and barring procedures, are outside the scope of this 454 document. 456 4.3. Syntax 458 4.3.1. General 460 This section defines the ABNF for the new SIP Priority header field 461 value "psap-callback". 463 4.3.2. ABNF 465 priority-value /= "psap-callback" 467 Figure 6: ABNF 469 5. Security Considerations 471 5.1. Security Threat 473 The PSAP callback functionality described in this document allows 474 marked calls to bypass blacklists, ignore call forwarding procedures 475 and other similar features used to raise the attention of emergency 476 callers when attempting to contact them. In the case where the SIP 477 Priority header value, 'psap-callback', is supported by the SIP UA, 478 it would override user interface configurations, such as vibrate-only 479 mode, to alert the caller of the incoming call. 481 5.2. Security Requirements 483 The requirement is to ensure that the mechanisms described in this 484 document can not be used for malicious purposes, including 485 telemarketing. 487 Furthermore, if the newly defined extension is not recognized, not 488 verified adequately, or not obeyed by SIP intermediaries or SIP 489 endpoints then it must not lead to a failure of the call handling 490 procedure. Such call must be treated like a call that does not have 491 any marking attached. 493 5.3. Security Solution 495 Figure 7 shows the architecture that utilizes the identity of the 496 PSAP to decide whether a preferential treatment of callbacks should 497 be provided. To make this policy decision, the identity of the PSAP 498 is compared with a white list of valid PSAPs available to the SIP 499 entity. The identity assurance in SIP can come in different forms, 500 such as SIP Identity [RFC4474] or with P-Asserted-Identity [RFC3325]. 501 The former technique relies on a cryptographic assurance and the 502 latter on a chain of trust. Also the usage of TLS between 503 neighboring SIP entities may provide useful identity information. 505 +----------+ 506 | List of |+ 507 | valid || 508 | PSAPs || 509 +----------+| 510 +----------+ 511 * 512 * white list 513 * 514 V 515 Incoming +----------+ Normal 516 SIP Msg | SIP |+ Treatment 517 -------------->| Entity ||======================> 518 + Identity | ||(if not in white list) 519 Info +----------+| 520 +----------+ 521 || 522 || 523 || Preferential 524 || Treatment 525 ++========================> 526 (if successfully verified) 528 Figure 7: Identity-based Authorization 530 An important aspect from a security point of view is the relationship 531 between the emergency services network (containing PSAPs) and the 532 VoIP provider (assuming that the emergency call travels via the VoIP 533 provider and not directly between the SIP UA and the PSAP). 535 If there is some form of relationship between the emergency services 536 operator and the VoIP provider then the identification of a PSAP 537 callback is less problematic than in the case where the two entities 538 have not entered in some form of relationship that would allow the 539 VoIP provider to verify whether the marked callback message indeed 540 came from a legitimate source. 542 The establishment of a whitelist with PSAP identities maybe be 543 operationally complex. When there is a local relationship between 544 the VoIP provider and the PSAP then populating the whitelist is 545 fairly simple. For SIP UAs there is no need to maintain a list of 546 PSAPs. Instead SIP UAs are assumed to trust the correct processing 547 of their VoIP provider, i.e., the VoIP provider processes the PSAP 548 callback marking and, if it cannot be verified, the PSAP callback 549 marking is removed. If it is left untouched then the SIP UA should 550 assume that it has been verified successfully by the VoIP provider 551 and it should therefore be obeyed. 553 6. IANA Considerations 555 This document adds the "psap-callback" value to the SIP Priority 556 header IANA registry allocated by [RFC6878]. The semantic of the 557 newly defined "psap-callback" value is defined in Section 4. 559 7. Acknowledgements 561 We would like to thank the following persons for their feedback: Paul 562 Kyzivat, Martin Thomson, Robert Sparks, Keith Drage, Cullen Jennings 563 Brian Rosen, Martin Dolly, Bernard Aboba, Andrew Allen, Atle Monrad, 564 John-Luc Bakker, John Elwell, Geoff Thompson, Dan Romascanu, James 565 Polk, John Medland, Hadriel Kaplan, Kenneth Carlberg, Timothy Dwight, 566 Janet Gunn 568 We would like to thank the ECRIT working group chairs, Marc Linsner 569 and Roger Marshall, for their support. Roger Marshall was the 570 document shepherd for this document. Vijay Gurbani provided the 571 general area review. 573 8. References 575 8.1. Normative References 577 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 578 A., Peterson, J., Sparks, R., Handley, M., and E. 579 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 580 June 2002. 582 [RFC5627] Rosenberg, J., "Obtaining and Using Globally Routable User 583 Agent URIs (GRUUs) in the Session Initiation Protocol 584 (SIP)", RFC 5627, October 2009. 586 [RFC6878] Roach, A., "IANA Registry for the Session Initiation 587 Protocol (SIP) "Priority" Header Field", RFC 6878, March 588 2013. 590 8.2. Informative References 592 [RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private 593 Extensions to the Session Initiation Protocol (SIP) for 594 Asserted Identity within Trusted Networks", RFC 3325, 595 November 2002. 597 [RFC4474] Peterson, J. and C. Jennings, "Enhancements for 598 Authenticated Identity Management in the Session 599 Initiation Protocol (SIP)", RFC 4474, August 2006. 601 [RFC5012] Schulzrinne, H. and R. Marshall, "Requirements for 602 Emergency Context Resolution with Internet Technologies", 603 RFC 5012, January 2008. 605 [RFC6443] Rosen, B., Schulzrinne, H., Polk, J., and A. Newton, 606 "Framework for Emergency Calling Using Internet 607 Multimedia", RFC 6443, December 2011. 609 [RFC6881] Rosen, B. and J. Polk, "Best Current Practice for 610 Communications Services in Support of Emergency Calling", 611 BCP 181, RFC 6881, March 2013. 613 Authors' Addresses 614 Henning Schulzrinne 615 Columbia University 616 Department of Computer Science 617 450 Computer Science Building 618 New York, NY 10027 619 US 621 Phone: +1 212 939 7004 622 EMail: hgs+ecrit@cs.columbia.edu 623 URI: http://www.cs.columbia.edu 625 Hannes Tschofenig 626 Nokia Solutions and Networks 627 Linnoitustie 6 628 Espoo 02600 629 Finland 631 Phone: +358 (50) 4871445 632 EMail: Hannes.Tschofenig@gmx.net 633 URI: http://www.tschofenig.priv.at 635 Christer Holmberg 636 Ericsson 637 Hirsalantie 11 638 Jorvas 02420 639 Finland 641 EMail: christer.holmberg@ericsson.com 643 Milan Patel 644 InterDigital Communications 646 EMail: Milan.Patel@interdigital.com