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rfc:rfc7090

Internet Engineering Task Force (IETF) H. Schulzrinne Request for Comments: 7090 Columbia University Category: Standards Track H. Tschofenig ISSN: 2070-1721

                                                           C. Holmberg
                                                              Ericsson
                                                              M. Patel
                                    Huawei Technologies (UK) Co., Ltd.
                                                            April 2014
           Public Safety Answering Point (PSAP) Callback

Abstract

 After an emergency call is completed (terminated either prematurely
 by the emergency caller or normally by the call taker), the call
 taker may feel the need for further communication.  For example, the
 call may have been dropped by accident without the call taker having
 sufficient information about the current state of an accident victim.
 A call taker may trigger a callback to the emergency caller using the
 contact information provided with the initial emergency call.  This
 callback could, under certain circumstances, be treated like any
 other call and, as a consequence, it may get blocked by authorization
 policies or may get forwarded to an answering machine.
 The IETF emergency services architecture specification already offers
 a solution approach for allowing Public Safety Answering Point (PSAP)
 callbacks to bypass authorization policies in order to reach the
 caller without unnecessary delays.  Unfortunately, the specified
 mechanism only supports limited scenarios.  This document discusses
 shortcomings of the current mechanisms and illustrates additional
 scenarios where better-than-normal call treatment behavior would be
 desirable.  We describe a solution based on a new header field value
 for the SIP Priority header field, called "psap-callback", to mark
 PSAP callbacks.

Schulzrinne, et al. Standards Track [Page 1] RFC 7090 PSAP Callback April 2014

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7090.

Copyright Notice

 Copyright (c) 2014 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Schulzrinne, et al. Standards Track [Page 2] RFC 7090 PSAP Callback April 2014

Table of Contents

 1. Introduction ....................................................3
 2. Terminology .....................................................5
 3. Callback Scenarios ..............................................5
    3.1. Routing Asymmetry ..........................................5
    3.2. Multi-Stage Routing ........................................7
    3.3. Call Forwarding ............................................8
    3.4. Network-Based Service URN Resolution ......................10
    3.5. PSTN Interworking .........................................11
 4. SIP PSAP Callback Indicator ....................................12
    4.1. General ...................................................12
    4.2. Usage .....................................................12
    4.3. Syntax ....................................................12
         4.3.1. General ............................................12
         4.3.2. ABNF ...............................................12
 5. Security Considerations ........................................12
    5.1. Security Threat ...........................................12
    5.2. Security Requirements .....................................13
    5.3. Security Solution .........................................13
 6. IANA Considerations ............................................15
 7. Acknowledgements ...............................................16
 8. References .....................................................16
    8.1. Normative References ......................................16
    8.2. Informative References ....................................17

1. Introduction

 Summoning police, the fire department, or an ambulance in emergencies
 is one of the fundamental and most valuable functions of the
 telephone.  As telephone functionality moves from circuit-switched
 telephony to Internet telephony, its users rightfully expect that
 this core functionality will continue to work at least as well as it
 has for the legacy technology.  New devices and services are being
 made available that could be used to make a request for help and that
 are not traditional telephones.  Users are increasingly expecting
 them to be used to place emergency calls.
 An overview of the protocol interactions for emergency calling using
 the IETF emergency services architecture is described in [RFC6443],
 and [RFC6881] specifies the technical details.  As part of the
 emergency call setup procedure, two important identifiers are
 conveyed to the PSAP call taker's user agent, namely the address-of-
 record (AOR), and if available, the Globally Routable User Agent (UA)
 URIs (GRUUs).  RFC 3261 [RFC3261] defines the AOR as:

Schulzrinne, et al. Standards Track [Page 3] RFC 7090 PSAP Callback April 2014

    An address-of-record (AOR) is a SIP or SIPS URI that points to a
    domain with a location service that can map the URI to another URI
    where the user might be available.  Typically, the location
    service is populated through registrations.  An AOR is frequently
    thought of as the "public address" of the user.
 In SIP systems, a single user can have a number of user agents
 (handsets, softphones, voicemail accounts, etc.) that are all
 referenced by the same AOR.  There are a number of cases in which it
 is desirable to have an identifier that addresses a single user agent
 rather than the group of user agents indicated by an AOR.  The GRUU
 is such a unique user-agent identifier, and it is also globally
 routable.  [RFC5627] specifies how to obtain and use GRUUs.
 [RFC6881] also makes use of the GRUU for emergency calls.
 Regulatory requirements demand that the emergency call setup
 procedure itself provides enough information to allow the call taker
 to initiate a callback to the emergency caller.  This is desirable in
 those cases where the call is dropped prematurely or when further
 communication needs arise.  The AOR and the GRUU serve this purpose.
 The communication attempt by the PSAP call taker back to the
 emergency caller is called a "PSAP callback".
 A PSAP callback may, however, be blocked by user-configured
 authorization policies or may be forwarded to an answering machine
 since SIP entities (SIP proxies as well as the SIP user equipment
 itself) cannot differentiate the PSAP callback from any other SIP
 call.  "Call barring", "do not disturb", or "call diversion" (also
 called call forwarding) are features that prevent delivery of a call.
 It is important to note that these features may be implemented by SIP
 intermediaries as well as by the user agent.
 Among the emergency services community, there is a desire to treat
 PSAP callbacks in such a way that the chances of reaching the
 emergency caller are increased.  At the same time, any solution must
 minimize the chance that other calls bypass call forwarding or other
 authorization policies.  Ideally, the PSAP callback has to relate to
 an earlier emergency call that was made "not too long ago".  An exact
 time interval is difficult to define in a global IETF standard due to
 the variety of national regulatory requirements, but [RFC6881]
 suggests 30 minutes.

Schulzrinne, et al. Standards Track [Page 4] RFC 7090 PSAP Callback April 2014

 Nevertheless, to meet the needs from the emergency services
 community, a basic mechanism for preferential treatment of PSAP
 callbacks was defined in Section 13 of [RFC6443].  The specification
 says:
    A UA may be able to determine a PSAP callback by examining the
    domain of incoming calls after placing an emergency call and
    comparing that to the domain of the answering PSAP from the
    emergency call.  Any call from the same domain and directed to the
    supplied Contact header or AOR after an emergency call should be
    accepted as a callback from the PSAP if it occurs within a
    reasonable time after an emergency call was placed.
 This approach mimics a stateful packet-filtering firewall and is
 indeed helpful in a number of cases.  It is also relatively simple to
 implement even though it requires call state to be maintained by the
 user agent as well as by SIP intermediaries.  Unfortunately, the
 solution does not work in all deployment scenarios.  In Section 3 we
 describe cases where the currently standardized approach is
 insufficient.

2. Terminology

 Emergency-services-related terminology is borrowed from [RFC5012].
 This includes terminology like emergency caller, user equipment, call
 taker, Emergency Service Routing Proxy (ESRP), and Public Safety
 Answering Point (PSAP).
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [RFC2119].

3. Callback Scenarios

 This section illustrates a number of scenarios where the currently
 specified solution, as described in [RFC6881], for preferential
 treatment of callbacks fails.  As explained in Section 1, a SIP
 entity examines an incoming PSAP callback by comparing the domain of
 the PSAP with the destination domain of the outbound emergency call
 placed earlier.

3.1. Routing Asymmetry

 In some deployment environments, it is common to have incoming and
 outgoing SIP messaging routed through different SIP entities.
 Figure 1 shows this graphically whereby a Voice over IP (VoIP)
 provider uses different SIP proxies for inbound and for outbound call
 handling.  Unless the two devices are synchronized, the callback

Schulzrinne, et al. Standards Track [Page 5] RFC 7090 PSAP Callback April 2014

 reaching the inbound proxy would get treated like any other call
 since the emergency call established state information at the
 outbound proxy only.
                                                 ,-------.
                                               ,'         `.
                    ,-------.                 /  Emergency  \
                  ,'         `.              |   Services    |
                 /  VoIP       \      I      |   Network     |
                |   Provider    |     n      |               |
                |               |     t      |               |
                |               |     e      |               |
                |   +-------+   |     r      |               |
             +--+---|Inbound|<--+-----m      |               |
             |  |   |Proxy  |   |     e      |   +------+    |
             |  |   +-------+   |     d      |   |PSAP  |    |
             |  |               |     i      |   +--+---+    |
   +----+    |  |               |     a-+    |      |        |
   | UA |<---+  |               |     t |    |      |        |
   |    |----+  |               |     e |    |      |        |
   +----+    |  |               |       |    |      |        |
             |  |               |     P |    |      |        |
             |  |               |     r |    |      |        |
             |  |   +--------+  |     o |    |      |        |
             +--+-->|Outbound|--+---->v |    |   +--+---+    |
                |   |Proxy   |  |     i |    | +-+ESRP  |    |
                |   +--------+  |     d |    | | +------+    |
                |               |     e |    | |             |
                |               |     r +----+-+             |
                 \             /             |               |
                  `.         ,'               \             /
                    '-------'                  `.         ,'
                                                 '-------'
                Figure 1: Example for Routing Asymmetry

Schulzrinne, et al. Standards Track [Page 6] RFC 7090 PSAP Callback April 2014

3.2. Multi-Stage Routing

 Consider the emergency call routing scenario shown in Figure 2 where
 routing towards the PSAP occurs in several stages.  In this scenario,
 we consider a SIP UA that uses the Location-to-Service Translation
 (LoST) Protocol [RFC5222] to learn the next-hop destination, namely
 esrp@example.net, to get the call closer to the PSAP.  This call is
 then sent to the proxy of the user's VoIP provider (example.org).
 The user's VoIP provider receives the emergency call and creates a
 state based on the destination domain, namely example.net.  It then
 routes the call to the indicated ESRP.  When the ESRP receives the
 call, it needs to decide what the next hop is to get to the final
 PSAP.  In our example, the next hop is the PSAP with the URI
 psap@example.com.
 When a callback is sent from psap@example.com towards the emergency
 caller, the call will get normal treatment by the proxy of the VoIP
 provider since the domain of the PSAP does not match the stored state
 information.
                                       ,-----------.
     +----+                          ,'             `.
     | UA |--- esrp@example.net    /     Emergency    \
     +----+   \                    |      Services    |
               \  ,-------.        |      Network     |
                ,'         `.      |                  |
               /   VoIP      \     |     +------+     |
              (   Provider    )    |     | PSAP |     |
               \ example.org /     |     +--+---+     |
                `.         ,'      |        |         |
                  '---+---'        |        |         |
                      |            | psap@example.com |
              esrp@example.net     |        |         |
                      |            |        |         |
                      |            |        |         |
                      |            |     +--+---+     |
                      +------------+-----+ ESRP |     |
                                   |     +------+     |
                                   |                  |
                                    \                /
                                     `.            ,'
                                       '----------'
               Figure 2: Example for Multi-Stage Routing

Schulzrinne, et al. Standards Track [Page 7] RFC 7090 PSAP Callback April 2014

3.3. Call Forwarding

 Imagine the following case where an emergency call enters an
 emergency network (state.example) via an ESRP, but then it gets
 forwarded to a different emergency services network (in our example,
 to example.net, example.org, or example.com).  The same
 considerations apply when the police, fire and, ambulance networks
 are part of the state.example subdomains (e.g.,
 police.state.example).
 Similar to the previous scenario, the wrong state information is
 being set up during the emergency call setup procedure.  A callback
 would originate in the example.net, example.org, or example.com
 domains whereas the emergency caller's SIP UA or the VoIP outbound
 proxy has stored state.example.

Schulzrinne, et al. Standards Track [Page 8] RFC 7090 PSAP Callback April 2014

                                 ,-------.
                               ,'         `.
                              /  Emergency  \
                             |   Services    |
                             |   Network     |
                             |(state.example)|
                             |               |
                             |               |
                             |   +------+    |
                             |   |PSAP  +--+ |
                             |   +--+---+  | |
                             |      |      | |
                             |      |      | |
                             |      |      | |
                             |      |      | |
                             |      |      | |
                             |   +--+---+  | |
           ------------------+---+ESRP  |  | |
           esrp-a@state.org  |   +------+  | |
                             |             | |
                             |    Call Fwd | |
                             |     +-+-+---+ |
                              \    | | |    /
                               `.  | | |  ,'
                                 '-|-|-|-'           ,-------.
                          Police   | | | Fire      ,'         `.
                      +------------+ | +----+     /  Emergency  \
       ,-------.      |              |      |    |   Services    |
     ,'         `.    |              |      |    |   Network     |
    /  Emergency  \   |          Ambulance  |    |    (Fire)     |
   |   Services    |  |              |      |    |               |
   |   Network     |  |              +----+ |    |   +------+    |
   |   (Police)    |  |     ,-------.     | +----+---+PSAP  |    |
   |               |  |   ,'         `.   |      |   +------+    |
   |   +------+    |  |  /  Emergency  \  |      |               |
   |   |PSAP  +----+--+ |   Services    | |      |  example.com  ,
   |   +------+    |    |   Network     | |      `~~~~~~~~~~~~~~~
   |               |    |  (Ambulance)  | |
   |  example.net  ,    |               | |
   `~~~~~~~~~~~~~~~     |   +------+    | |
                        |   |PSAP  +----+ +
                        |   +------+    |
                        |               |
                        |  example.org  ,
                        `~~~~~~~~~~~~~~~
                 Figure 3: Example for Call Forwarding

Schulzrinne, et al. Standards Track [Page 9] RFC 7090 PSAP Callback April 2014

3.4. Network-Based Service URN Resolution

 The IETF emergency services architecture also considers cases where
 the resolution from the Service URN to the PSAP URI does not only
 happen at the SIP UA itself but at intermediate SIP entities, such as
 the user's VoIP provider.
 Figure 4 shows this message exchange of the outgoing emergency call
 and the incoming PSAP graphically.  While the state information
 stored at the VoIP provider is correct, the state allocated at the
 SIP UA is not.
      ,-------.
    ,'         `.
   /  Emergency  \
  |   Services    |
  |   Network     |
  |  example.com  |
  |               |
  |   +------+    |    INVITE to police@example.com
  |   |PSAP  +<---+------------------------+
  |   |      +----+--------------------+   ^
  |   +------+    |INVITE from         |   |
  |               ,police@example.com  |   |
  `~~~~~~~~~~~~~~~                     |   |
                                       v   |
  +--------+  Query with location   +--+---+-+
  |        |  + urn:service:sos     |  VoIP  |
  | LoST   |<-----------------------|Service |
  | Server |   police@example.com   |Provider|
  |        |----------------------->|        |
  +--------+                        +--------+
                                     |     ^
                               INVITE|     | INVITE
                                 from|     | to
                   police@example.com|     | urn:service:sos
                                     V     |
                                    +-------+
                                    | SIP   |
                                    | UA    |
                                    | Alice |
                                    +-------+
      Figure 4: Example for Network-Based Service URN Resolution

Schulzrinne, et al. Standards Track [Page 10] RFC 7090 PSAP Callback April 2014

3.5. PSTN Interworking

 In case an emergency call enters the Public Switched Telephone
 Network (PSTN), as shown in Figure 5, there is no guarantee that the
 callback sometime later leaves the same PSTN/VoIP gateway or that the
 same endpoint identifier is used in the forward as well as in the
 backward direction making it difficult to reliably detect PSAP
 callbacks.
   +-----------+
   | PSTN      |-------------+
   | Calltaker |             |
   | Bob       |<--------+   |
   +-----------+         |   v
              -------------------
          ////                   \\\\      +------------+
         |                           |     |PSTN / VoIP |
         |             PSTN          |---->|Gateway     |
          \\\\                   ////      |            |
              -------------------          +----+-------+
                         ^                      |
                         |                      |
                   +-------------+              |  +--------+
                   |             |              |  |VoIP    |
                   | PSTN / VoIP |              +->|Service |
                   | Gateway     |                 |Provider|
                   |             |<------INVITE----|   Y    |
                   +-------------+                 +--------+
                                                    |     ^
                                                    |     |
                                                  INVITE INVITE
                                                    |     |
                                                    V     |
                                                   +-------+
                                                   | SIP   |
                                                   | UA    |
                                                   | Alice |
                                                   +-------+
                Figure 5: Example for PSTN Interworking
 Note: This scenario is considered outside the scope of this document.
 The specified solution does not support this use case.

Schulzrinne, et al. Standards Track [Page 11] RFC 7090 PSAP Callback April 2014

4. SIP PSAP Callback Indicator

4.1. General

 This section defines a new header field value, called "psap-
 callback", for the SIP Priority header field defined in [RFC3261].
 The value is used to inform SIP entities that the request is
 associated with a PSAP callback SIP session.

4.2. Usage

 SIP entities that receive the header field value within an initial
 request for a SIP session can, depending on local policies, apply
 PSAP callback-specific procedures for the session or request.
 The PSAP callback-specific procedures may be applied by SIP-based
 network entities and by the callee.  The specific actions taken when
 receiving a call marked as a PSAP callback marked call, such as
 bypassing services and barring procedures, are outside the scope of
 this document.

4.3. Syntax

4.3.1. General

 This section defines the ABNF [RFC5234] for the new SIP Priority
 header field value "psap-callback".

4.3.2. ABNF

     priority-value  =/  "psap-callback"
             Figure 6: ABNF

5. Security Considerations

5.1. Security Threat

 The PSAP callback functionality described in this document allows
 marked calls to bypass blacklists and ignore call-forwarding
 procedures and other similar features used to raise the attention of
 emergency callers when attempting to contact them.  In the case where
 the SIP Priority header value, "psap-callback", is supported by the
 SIP UA, it would override user-interface configurations, such as
 vibrate-only mode, to alert the caller of the incoming call.

Schulzrinne, et al. Standards Track [Page 12] RFC 7090 PSAP Callback April 2014

5.2. Security Requirements

 The security threat discussed in Section 5.1 leads to the requirement
 to ensure that the mechanisms described in this document cannot be
 used for malicious purposes, including telemarketing.
 Furthermore, if the newly defined extension is not recognized, not
 verified adequately, or not obeyed by SIP intermediaries or SIP
 endpoints, then it must not lead to a failure of the call handling
 procedure.  Such a call must be treated like a call that does not
 have any marking attached.
 The indicator described in Section 4 can be inserted by any SIP
 entity, including attackers.  So it is critical that the indicator
 only lead to preferential call treatment in cases where the recipient
 has some trust in the caller, as described in the next section.

5.3. Security Solution

 The approach for dealing with the implementation of the security
 requirements described in Section 5.2 can be differentiated between
 the behavior applied by the UA and by SIP proxies.  A UA that has
 made an emergency call MUST keep state information so that it can
 recognize and accept a callback from the PSAP if it occurs within a
 reasonable time after an emergency call was placed, as described in
 Section 13 of [RFC6443].  Only a timer started at the time when the
 original emergency call has ended is required; information about the
 calling party identity is not needed since the callback may use a
 different calling party identity, as described in Section 3.  Since
 these SIP UA considerations are described already in [RFC6443] as
 well as in [RFC6881] the rest of this section focuses on the behavior
 of SIP proxies.
 Figure 7 shows the architecture that utilizes the identity of the
 PSAP to decide whether a preferential treatment of callbacks should
 be provided.  To make this policy decision, the identity of the PSAP
 (i.e., calling party identity) is compared with a PSAPs white list.

Schulzrinne, et al. Standards Track [Page 13] RFC 7090 PSAP Callback April 2014

                     +----------+
                     | List of  |+
                     | valid    ||
                     | PSAPs    ||
                     +----------+|
                      +----------+
                          *
                          * white list
                          *
                          V
       Incoming      +----------+    Normal
       SIP Msg       | SIP      |+   Treatment
      -------------->| Entity   ||======================>
       + Identity    |          ||(if not in white list)
         Info        +----------+|
                     +----------+
                          ||
                          ||
                          || Preferential
                          || Treatment
                          ++========================>
                            (if successfully verified)
                Figure 7: Identity-Based Authorization
 The identity assurance in SIP can come in different forms, namely via
 the SIP Identity [RFC4474] or the P-Asserted-Identity [RFC3325]
 mechanisms.  The former technique relies on a cryptographic assurance
 and the latter on a chain of trust.  Also, the usage of Transport
 Layer Security (TLS) between neighboring SIP entities may provide
 useful identity information.  At the time of writing, these identity
 technologies are being revised in the Secure Telephone Identity
 Revisited (stir) working group [STIR] to offer better support for
 legacy technologies interworking and SIP intermediaries that modify
 the content of various SIP headers and the body.  Once the work on
 these specifications has been completed, they will offer a stronger
 calling party identity mechanism that limits or prevents identity
 spoofing.
 An important aspect from a security point of view is the relationship
 between the emergency services network (containing the PSAPs) and the
 VoIP provider, assuming that the emergency call travels via the VoIP
 provider and not directly between the SIP UA and the PSAP.
 The establishment of a white list with PSAP identities may be
 operationally complex and dependent on the relationship between the
 emergency services operator and the VoIP provider.  If there is a
 relationship between the VoIP provider and the PSAP operator, for

Schulzrinne, et al. Standards Track [Page 14] RFC 7090 PSAP Callback April 2014

 example, when they are both operating in the same geographical
 region, then populating the white list is fairly simple and
 consequently the identification of a PSAP callback is less
 problematic compared to the case where the two entities have never
 interacted with each other before.  In the end, the VoIP provider has
 to verify whether the marked callback message indeed came from a
 legitimate source.
 VoIP providers MUST only give PSAP callbacks preferential treatment
 when the calling party identity of the PSAP was successfully matched
 against entries in the white list.  If it cannot be verified (because
 there was no match), then the VoIP provider MUST remove the PSAP
 callback marking.  Thereby, the callback reverts to a normal call.
 As a second step, SIP UAs MUST maintain a timer that is started with
 the original emergency call and this timer expires within a
 reasonable amount of time, such as 30 minutes per [RFC6881].  Such a
 timer also ensures that VoIP providers cannot misuse the PSAP
 callback mechanism, for example, to ensure that their support calls
 reach their customers.
 Finally, a PSAP callback MUST use the same media as the original
 emergency call.  For example, when an initial emergency call
 established a real-time text communication session, then the PSAP
 callback must also attempt to establish a real-time communication
 interaction.  The reason for this is twofold.  First, the person
 seeking help may have disabilities that prevent them from using
 certain media and hence using the same media for the callback avoids
 unpleasant surprises and delays.  Second, the emergency caller may
 have intentionally chosen a certain media and does not prefer to
 communicate in a different way.  For example, it would be unfortunate
 if a hostage tries to seek help using instant messaging to avoid any
 noise when subsequently the ringtone triggered by a PSAP callback
 using a voice call gets the attention of the hostage-taker.  User-
 interface designs need to cater to such situations.

6. IANA Considerations

 This document adds the "psap-callback" value to the SIP "Priority
 Header Field Values" registry allocated by [RFC6878].  The semantic
 of the newly defined "psap-callback" value is defined in Section 4.

Schulzrinne, et al. Standards Track [Page 15] RFC 7090 PSAP Callback April 2014

7. Acknowledgements

 We would like to thank the following persons for their feedback:
 Bernard Aboba, Andrew Allen, John-Luc Bakker, Kenneth Carlberg,
 Martin Dolly, Keith Drage, Timothy Dwight, John Elwell, Janet Gunn,
 Cullen Jennings, Hadriel Kaplan, Paul Kyzivat, John Medland, Atle
 Monrad, James Polk, Dan Romascanu, Brian Rosen, Robert Sparks, Geoff
 Thompson, and Martin Thomson.
 We would also like to thank the ECRIT working group chairs, Marc
 Linsner and Roger Marshall, for their support.  Roger Marshall was
 the document shepherd for this document.  Vijay Gurbani provided the
 general area review.
 During IESG review, the document received good feedback from Barry
 Leiba, Spencer Dawkins, Richard Barnes, Joel Jaeggli, Stephen
 Farrell, and Benoit Claise.

8. References

8.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
            A., Peterson, J., Sparks, R., Handley, M., and E.
            Schooler, "SIP: Session Initiation Protocol", RFC 3261,
            June 2002.
 [RFC5234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
            Syntax Specifications: ABNF", STD 68, RFC 5234, January
            2008.
 [RFC5627]  Rosenberg, J., "Obtaining and Using Globally Routable User
            Agent URIs (GRUUs) in the Session Initiation Protocol
            (SIP)", RFC 5627, October 2009.
 [RFC6878]  Roach, A., "IANA Registry for the Session Initiation
            Protocol (SIP) "Priority" Header Field", RFC 6878, March
            2013.

Schulzrinne, et al. Standards Track [Page 16] RFC 7090 PSAP Callback April 2014

8.2. Informative References

 [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
            Extensions to the Session Initiation Protocol (SIP) for
            Asserted Identity within Trusted Networks", RFC 3325,
            November 2002.
 [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
            Authenticated Identity Management in the Session
            Initiation Protocol (SIP)", RFC 4474, August 2006.
 [RFC5012]  Schulzrinne, H. and R. Marshall, "Requirements for
            Emergency Context Resolution with Internet Technologies",
            RFC 5012, January 2008.
 [RFC5222]  Hardie, T., Newton, A., Schulzrinne, H., and H.
            Tschofenig, "LoST: A Location-to-Service Translation
            Protocol", RFC 5222, August 2008.
 [RFC6443]  Rosen, B., Schulzrinne, H., Polk, J., and A. Newton,
            "Framework for Emergency Calling Using Internet
            Multimedia", RFC 6443, December 2011.
 [RFC6881]  Rosen, B. and J. Polk, "Best Current Practice for
            Communications Services in Support of Emergency Calling",
            BCP 181, RFC 6881, March 2013.
 [STIR]     IETF, "Secure Telephone Identity Revisited (stir) Working
            Group", http://datatracker.ietf.org/wg/stir/charter/,
            October 2013.

Schulzrinne, et al. Standards Track [Page 17] RFC 7090 PSAP Callback April 2014

Authors' Addresses

 Henning Schulzrinne
 Columbia University
 Department of Computer Science
 450 Computer Science Building
 New York, NY  10027
 US
 Phone: +1 212 939 7004
 EMail: hgs+ecrit@cs.columbia.edu
 URI:   http://www.cs.columbia.edu
 Hannes Tschofenig
 EMail: Hannes.Tschofenig@gmx.net
 URI:   http://www.tschofenig.priv.at
 Christer Holmberg
 Ericsson
 Hirsalantie 11
 Jorvas  02420
 Finland
 EMail: christer.holmberg@ericsson.com
 Milan Patel
 Huawei Technologies (UK) Co., Ltd.
 300 South Oak Way, Green Park
 Reading, Berkshire  RG2 6UF
 U.K.
 EMail: Milan.Patel@huawei.com

Schulzrinne, et al. Standards Track [Page 18]

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