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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: January 14, 2014 Nokia Siemens Networks 6 C. Holmberg 7 Ericsson 8 M. Patel 9 InterDigital Communications 10 July 13, 2013 12 Public Safety Answering Point (PSAP) Callback 13 draft-ietf-ecrit-psap-callback-10.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 January 14, 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 . . . . . . . . . . . . . . . . . . . . . 7 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 . . . . . . . . . . . . . . . . . . . . . . . . . 11 86 4.3.1. General . . . . . . . . . . . . . . . . . . . . . . . 11 87 4.3.2. ABNF . . . . . . . . . . . . . . . . . . . . . . . . 11 88 5. Security Considerations . . . . . . . . . . . . . . . . . . . 11 89 5.1. Security Threat . . . . . . . . . . . . . . . . . . . . . 11 90 5.2. Security Requirements . . . . . . . . . . . . . . . . . . 11 91 5.3. Security Solution . . . . . . . . . . . . . . . . . . . . 12 92 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 93 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13 94 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 95 8.1. Normative References . . . . . . . . . . . . . . . . . . 13 96 8.2. Informative References . . . . . . . . . . . . . . . . . 14 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 189 This section illustrates a number of scenarios where the currently 190 specified solution, as specified in [RFC6881], for preferential 191 treatment of callbacks fails. As explained in Section 1 a SIP entity 192 examines an incoming PSAP callback by comparing the domain of the 193 PSAP with the destination domain of the emergency call. 195 NOTE: All FQDNs used in the subsections below are used for 196 illustrative purposes. They are examples to demonstrate the 197 limitations of the technical solution outlined in RFC 6881. 199 3.1. Routing Asymmetry 201 In some deployment environments it is common to have incoming and 202 outgoing SIP messaging routed through different SIP entities. Figure 203 1 shows this graphically whereby a VoIP provider uses different SIP 204 proxies for inbound and for outbound call handling. Unless the two 205 devices are synchronized as to state the callback hitting the inbound 206 proxy would get treated like any other call since the emergency call 207 established state information at the outbound proxy only. 209 ,-------. 210 ,' `. 211 ,-------. / Emergency \ 212 ,' `. | Services | 213 / VoIP \ I | Network | 214 | Provider | n | | 215 | | t | | 216 | | e | | 217 | +-------+ | r | | 218 +--+---|Inbound|<--+-----m | | 219 | | |Proxy | | e | +------+ | 220 | | +-------+ | d | |PSAP | | 221 | | | i | +--+---+ | 222 +----+ | | | a-+ | | | 223 | UA |<---+ | | t | | | | 224 | |----+ | | e | | | | 225 +----+ | | | | | | | 226 | | | P | | | | 227 | | | r | | | | 228 | | +--------+ | o | | | | 229 +--+-->|Outbound|--+---->v | | +--+---+ | 230 | |Proxy | | i | | +-+ESRP | | 231 | +--------+ | d | | | +------+ | 232 | | e || | | 233 | | r |+-+ | 234 \ / | | 235 `. ,' \ / 236 '-------' `. ,' 237 '-------' 239 Figure 1: Example for Routing Asymmetry. 241 3.2. Multi-Stage Routing 243 Consider the following emergency call routing scenario shown in 244 Figure 2 where routing towards the PSAP occurs in several stages. In 245 this scenario we consider a SIP UA that uses LoST to learn the next 246 hop destination closer to the PSAP. This call is then sent to the 247 user's VoIP provider. The user's VoIP provider receives the 248 emergency call and creates state based on the destination domain, 249 namely state.org. It then routes it to the indicated ESRP. When the 250 ESRP receives it it needs to decide what the next hop is to get it 251 closer to the PSAP. In our example the next hop is the PSAP with the 252 URI psap@town.com. 254 When a callback is sent from psap@town.com towards the emergency 255 caller the call will get normal treatment by the VoIP providers 256 inbound proxy since the domain of the PSAP does not match the stored 257 state information. 259 ,-------. 260 +----+ ,' `. 261 | UA |--- esrp1@foobar.com / Emergency \ 262 +----+ \ | Services | 263 \ ,-------. | Network | 264 ,' `. | | 265 / VoIP \ | +------+ | 266 ( Provider ) | |PSAP | | 267 \ / | +--+---+ | 268 `. ,' | | 269 '---+---' | | | 270 | |psap@town.com | 271 esrp@state.org | | | 272 | | | | 273 | | | | 274 | | +--+---+ | 275 +------------+---+ESRP | | 276 | +------+ | 277 | | 278 \ / 279 `. ,' 280 '-------' 282 Figure 2: Example for Multi-Stage Routing. 284 3.3. Call Forwarding 286 Imagine the following case where an emergency call enters an 287 emergency network (state.org) via an ESRP but then gets forwarded to 288 a different emergency services network (in our example to police- 289 town.org, fire-town.org or medic-town.org). The same considerations 290 apply when the police, fire and ambulance networks are part of the 291 state.org sub-domains (e.g., police.state.org). 293 Similarly to the previous scenario the problem here is with the wrong 294 state information being established during the emergency call setup 295 procedure. A callback would originate in the police-town.org, fire- 296 town.org or medic-town.org domain whereas the emergency caller's SIP 297 UA or the VoIP outbound proxy has stored state.org. 299 ,-------. 300 ,' `. 301 / Emergency \ 302 | Services | 303 | Network | 304 | (state.org) | 305 | | 306 | | 307 | +------+ | 308 | |PSAP +--+ | 309 | +--+---+ | | 310 | | | | 311 | | | | 312 | | | | 313 | | | | 314 | | | | 315 | +--+---+ | | 316 ------------------+---+ESRP | | | 317 esrp-a@state.org | +------+ | | 318 | | | 319 | Call Fwd | | 320 | +-+-+---+ | 321 \ | | | / 322 `. | | | ,' 323 '-|-|-|-' ,-------. 324 Police | | | Fire ,' `. 325 +------------+ | +----+ / Emergency \ 326 ,-------. | | | | Services | 327 ,' `. | | | | Network | 328 / Emergency \ | Ambulance | | fire-town.org | 329 | Services | | | | | | 330 | Network | | +----+ | | +------+ | 331 |police-town.org| | ,-------. | +----+---+PSAP | | 332 | | | ,' `. | | +------+ | 333 | +------+ | | / Emergency \ | | | 334 | |PSAP +----+--+ | Services | | | , 335 | +------+ | | Network | | `~~~~~~~~~~~~~~~ 336 | | |medic-town.org | | 337 | , | | | 338 `~~~~~~~~~~~~~~~ | +------+ | | 339 | |PSAP +----+ + 340 | +------+ | 341 | | 342 | , 343 `~~~~~~~~~~~~~~~ 345 Figure 3: Example for Call Forwarding. 347 3.4. Network-based Service URN Resolution 349 The IETF emergency services architecture also considers cases where 350 the resolution from the Service URN to the PSAP URI does not only 351 happen at the SIP UA itself but at intermediate SIP entities, such as 352 the user's VoIP provider. 354 Figure 4 shows this message exchange of the outgoing emergency call 355 and the incoming PSAP graphically. While the state information 356 stored at the VoIP provider is correct the state allocated at the SIP 357 UA is not. 359 ,-------. 360 ,' `. 361 / Emergency \ 362 | Services | 363 | Network | 364 |police-town.org| 365 | | 366 | +------+ | Invite to police.example.com 367 | |PSAP +<---+------------------------+ 368 | | +----+------------------+ ^ 369 | +------+ |Invite from | | 370 | ,police.example.com| | 371 `~~~~~~~~~~~~~~~ v | 372 +--------+ ++-----+-+ 373 | | query |VoIP | 374 | LoST |<-----------------------|Service | 375 | Server | police.example.com |Provider| 376 | |----------------------->| | 377 +--------+ +--------+ 378 | ^ 379 Invite| | Invite 380 from| | to 381 police.example.com| | urn:service:sos 382 V | 383 +-------+ 384 | SIP | 385 | UA | 386 | Alice | 387 +-------+ 389 Figure 4: Example for Network-based Service URN Resolution. 391 3.5. PSTN Interworking 393 In case an emergency call enters the PSTN, as shown in Figure 5, 394 there is no guarantee that the callback some time later does leave 395 the same PSTN/VoIP gateway or that the same end point identifier is 396 used in the forward as well as in the backward direction making it 397 difficult to reliably detect PSAP callbacks. 399 +-----------+ 400 | PSTN |-------------+ 401 | Calltaker | | 402 | Bob |<--------+ | 403 +-----------+ | v 404 ------------------- 405 //// \\\\ +------------+ 406 | | |PSTN / VoIP | 407 | PSTN |---->|Gateway | 408 \\\\ //// | | 409 ------------------- +----+-------+ 410 ^ | 411 | | 412 +-------------+ | +--------+ 413 | | | |VoIP | 414 | PSTN / VoIP | +->|Service | 415 | Gateway | |Provider| 416 | |<------Invite----| Y | 417 +-------------+ +--------+ 418 | ^ 419 | | 420 Invite Invite 421 | | 422 V | 423 +-------+ 424 | SIP | 425 | UA | 426 | Alice | 427 +-------+ 429 Figure 5: Example for PSTN Interworking. 431 Note: This scenario is considered outside the scope of this document. 432 The specified solution does not support this use case. 434 4. SIP PSAP Callback Indicator 436 4.1. General 438 This section defines a new header field value, called "psap- 439 callback", for the SIP Priority header field defined in [RFC3261]. 440 The value is used to inform SIP entities that the request is 441 associated with a PSAP callback SIP session. 443 4.2. Usage 445 SIP entities that receive the header field value within an initial 446 request for a SIP session can, depending on local policies, apply 447 PSAP callback specific procedures for the session or request. 449 The PSAP callback specific procedures may be applied by SIP-based 450 network entities and by the callee. The specific procedures taken 451 when receiving such a PSAP callback marked call, such as bypassing 452 services and barring procedures, are outside the scope of this 453 document. 455 4.3. Syntax 457 4.3.1. General 459 This section defines the ABNF for the new SIP Priority header field 460 value "psap-callback". 462 4.3.2. ABNF 464 priority-value /= "psap-callback" 466 Figure 6: ABNF 468 5. Security Considerations 470 5.1. Security Threat 472 The PSAP callback functionality described in this document allows 473 marked calls to bypass blacklists, ignore call forwarding procedures 474 and other similar features used to raise the attention of emergency 475 callers when attempting to contact them. In the case where the SIP 476 Priority header value, 'psap-callback', is supported by the SIP UA, 477 it would override user interface configurations, such as vibrate-only 478 mode, to alert the caller of the incoming call. 480 5.2. Security Requirements 482 The requirement is to ensure that the mechanisms described in this 483 document can not be used for malicious purposes, including 484 telemarketing. 486 Furthermore, if the newly defined extension is not recognized, not 487 verified adequately, or not obeyed by SIP intermediaries or SIP 488 endpoints then it must not lead to a failure of the call handling 489 procedure. Such call must be treated like a call that does not have 490 any marking attached. 492 5.3. Security Solution 494 Figure 7 shows the architecture that utilizes the identity of the 495 PSAP to decide whether a preferential treatment of callbacks should 496 be provided. To make this policy decision, the identity of the PSAP 497 is compared with a white list of valid PSAPs available to the SIP 498 entity. The identity assurance in SIP can come in different forms, 499 such as SIP Identity [RFC4474] or with P-Asserted-Identity [RFC3325]. 500 The former technique relies on a cryptographic assurance and the 501 latter on a chain of trust. Also the usage of TLS between 502 neighboring SIP entities may provide useful identity information. 504 +----------+ 505 | List of |+ 506 | valid || 507 | PSAPs || 508 +----------+| 509 +----------+ 510 * 511 * white list 512 * 513 V 514 Incoming +----------+ Normal 515 SIP Msg | SIP |+ Treatment 516 -------------->| Entity ||======================> 517 + Identity | ||(if not in white list) 518 Info +----------+| 519 +----------+ 520 || 521 || 522 || Preferential 523 || Treatment 524 ++========================> 525 (if successfully verified) 527 Figure 7: Identity-based Authorization 529 An important aspect from a security point of view is the relationship 530 between the emergency services network (containing PSAPs) and the 531 VoIP provider (assuming that the emergency call travels via the VoIP 532 provider and not directly between the SIP UA and the PSAP). 534 If there is some form of relationship between the emergency services 535 operator and the VoIP provider then the identification of a PSAP 536 callback is less problematic than in the case where the two entities 537 have not entered in some form of relationship that would allow the 538 VoIP provider to verify whether the marked callback message indeed 539 came from a legitimate source. 541 The establishment of a whitelist with PSAP identities maybe be 542 operationally complex. When there is a local relationship between 543 the VoIP provider and the PSAP then populating the whitelist is 544 fairly simple. For SIP UAs there is no need to maintain a list of 545 PSAPs. Instead SIP UAs are assumed to trust the correct processing 546 of their VoIP provider, i.e., the VoIP provider processes the PSAP 547 callback marking and, if it cannot be verified, the PSAP callback 548 marking is removed. If it is left untouched then the SIP UA should 549 assume that it has been verified successfully by the VoIP provider 550 and it should therefore be obeyed. 552 6. IANA Considerations 554 This document adds the "psap-callback" value to the SIP Priority 555 header IANA registry allocated by [RFC6878]. The semantic of the 556 newly defined "psap-callback" value is defined in Section 4. 558 7. Acknowledgements 560 We would like to thank the following persons for their feedback: Paul 561 Kyzivat, Martin Thomson, Robert Sparks, Keith Drage, Cullen Jennings 562 Brian Rosen, Martin Dolly, Bernard Aboba, Andrew Allen, Atle Monrad, 563 John-Luc Bakker, John Elwell, Geoff Thompson, Dan Romascanu, James 564 Polk, John Medland, Hadriel Kaplan, Kenneth Carlberg, Timothy Dwight, 565 Janet Gunn 567 Finally, we would like to thank the ECRIT working group chairs, Marc 568 Linsner and Roger Marshall, for their support. Roger Marshall was 569 the document shepherd for this document. 571 8. References 573 8.1. Normative References 575 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 576 A., Peterson, J., Sparks, R., Handley, M., and E. 577 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 578 June 2002. 580 [RFC5627] Rosenberg, J., "Obtaining and Using Globally Routable User 581 Agent URIs (GRUUs) in the Session Initiation Protocol 582 (SIP)", RFC 5627, October 2009. 584 [RFC6878] Roach, A., "IANA Registry for the Session Initiation 585 Protocol (SIP) "Priority" Header Field", RFC 6878, March 586 2013. 588 8.2. Informative References 590 [RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private 591 Extensions to the Session Initiation Protocol (SIP) for 592 Asserted Identity within Trusted Networks", RFC 3325, 593 November 2002. 595 [RFC4474] Peterson, J. and C. Jennings, "Enhancements for 596 Authenticated Identity Management in the Session 597 Initiation Protocol (SIP)", RFC 4474, August 2006. 599 [RFC5012] Schulzrinne, H. and R. Marshall, "Requirements for 600 Emergency Context Resolution with Internet Technologies", 601 RFC 5012, January 2008. 603 [RFC6443] Rosen, B., Schulzrinne, H., Polk, J., and A. Newton, 604 "Framework for Emergency Calling Using Internet 605 Multimedia", RFC 6443, December 2011. 607 [RFC6881] Rosen, B. and J. Polk, "Best Current Practice for 608 Communications Services in Support of Emergency Calling", 609 BCP 181, RFC 6881, March 2013. 611 Authors' Addresses 613 Henning Schulzrinne 614 Columbia University 615 Department of Computer Science 616 450 Computer Science Building 617 New York, NY 10027 618 US 620 Phone: +1 212 939 7004 621 EMail: hgs+ecrit@cs.columbia.edu 622 URI: http://www.cs.columbia.edu 624 Hannes Tschofenig 625 Nokia Siemens Networks 626 Linnoitustie 6 627 Espoo 02600 628 Finland 630 Phone: +358 (50) 4871445 631 EMail: Hannes.Tschofenig@gmx.net 632 URI: http://www.tschofenig.priv.at 633 Christer Holmberg 634 Ericsson 635 Hirsalantie 11 636 Jorvas 02420 637 Finland 639 EMail: christer.holmberg@ericsson.com 641 Milan Patel 642 InterDigital Communications 644 EMail: Milan.Patel@interdigital.com