Internet Engineering Task Force (IETF)
Request for Comments: 6443
Category: Informational
ISSN: 2070-1721
B. Rosen
H. Schulzrinne
Columbia U.
J. Polk
Cisco Systems
A. Newton
December 2011

Framework for Emergency Calling Using Internet Multimedia


The IETF has standardized various aspects of placing emergency calls. This document describes how all of those component parts are used to support emergency calls from citizens and visitors to authorities.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.

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). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see 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

Copyright Notice

Copyright © 2011 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 ( 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.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................6
   3. Overview of How Emergency Calls Are Placed ......................8
   4. Which Devices and Services Should Support Emergency Calls? .....12
   5. Identifying an Emergency Call ..................................12
   6. Location and Its Role in an Emergency Call .....................14
      6.1. Types of Location Information .............................16
      6.2. Location Determination ....................................17
           6.2.1. User-Entered Location Information ..................17
           6.2.2. Access Network "Wire Database" Location
                  Information ........................................18
           6.2.3. End System Measured Location Information ...........19
           6.2.4. Network Measured Location Information ..............19
      6.3. Who Adds Location, Endpoint, or Proxy? ....................20
      6.4. Location and References to Location .......................20
      6.5. End System Location Configuration .........................21
      6.6. When Location Should Be Configured ........................22
      6.7. Conveying Location ........................................23
      6.8. Location Updates ..........................................24
      6.9. Multiple Locations ........................................24
      6.10. Location Validation ......................................25
      6.11. Default Location .........................................26
      6.12. Location Format Conversion ...............................26
   7. LIS and LoST Discovery .........................................26
   8. Routing the Call to the PSAP ...................................27
   9. Signaling of Emergency Calls ...................................29
      9.1. Use of TLS ................................................29
      9.2. SIP Signaling Requirements for User Agents ................30
      9.3. SIP Signaling Requirements for Proxy Servers ..............30
   10. Call Backs ....................................................30
   11. Mid-Call Behavior .............................................31
   12. Call Termination ..............................................31
   13. Disabling of Features .........................................32
   14. Media .........................................................32
   15. Testing .......................................................32
   16. Security Considerations .......................................33
   17. Acknowledgments ...............................................33
   18. Informative References ........................................34

1. Introduction

Requesting help in an emergency using a communications device such as a telephone (landline or mobile) is an accepted practice in many parts of the world. As communications devices increasingly utilize the Internet to interconnect and communicate, users will expect to use such devices to request help. This document describes establishment of a communications session by a user to a "Public Safety Answering Point" (PSAP), that is, a call center established by response agencies to accept emergency calls. Such citizen-/ visitor-to-authority calls can be distinguished from those that are created by responders (authority-to-authority) using public communications infrastructure often involving some kind of priority access as defined in Emergency Telecommunications Service (ETS) in IP Telephony [RFC4190]. They can also be distinguished from emergency warning systems that are authority-to-citizen.

Supporting emergency calling requires cooperation by a number of elements, their vendors, and service providers. This document discusses how end devices and applications create emergency calls, how access networks supply location for some of these devices, how service providers assist the establishment and routing, and how PSAPs receive calls from the Internet.

The emergency response community will have to upgrade their facilities to support a wider range of communications services, but cannot be expected to handle wide variations in device and service capability. New devices and services are being made available that could be used to make a request for help that are not traditional telephones, and users are increasingly expecting to use them to place emergency calls. However, many of the technical advantages of Internet multimedia require re-thinking the traditional emergency calling architecture. This challenge also offers an opportunity to improve the operation of emergency calling technology, while potentially lowering its cost and complexity.

It is beyond the scope of this document to enumerate and discuss all the differences between traditional (Public Switched Telephone Network) and IP-based telephony, but calling on the Internet is characterized by:

  • interleaving over the same infrastructure of a wider variety of services;
  • separation of the access provider from the application provider;
  • media other than voice (for example, video and text in several forms);
  • potential mobility of all end systems, including endpoints nominally thought of as fixed systems and not just those using radio access technology. For example, consider a wired phone connected to a router using a mobile data network such as Evolution Data Optimized (EV-DO) as an uplink.

This document focuses on how devices using the Internet can place emergency calls and how PSAPs can handle Internet multimedia emergency calls natively, rather than describing how circuit-switched PSAPs can handle Voice over IP (VoIP) calls. In many cases, PSAPs making the transition from circuit-switched interfaces to packet- switched interfaces may be able to use some of the mechanisms described here, in combination with gateways that translate packet- switched calls into legacy interfaces, e.g., to continue to be able to use existing call taker equipment. There are many legacy telephone networks that will persist long after most systems have been upgraded to IP origination and termination of emergency calls. Many of these legacy systems route calls based on telephone numbers. Gateways and conversions between existing systems and newer systems defined by this document will be required. Since existing systems are governed primarily by local government regulations and national standards, the gateway and conversion details will be governed by national standards and thus are out of scope for this document.

Existing emergency call systems are organized locally or nationally; there are currently few international standards. However, the Internet crosses national boundaries, and thus Internet standards are required. To further complicate matters, VoIP endpoints can be connected through tunneling mechanisms such as virtual private networks (VPNs). Tunnels can obscure the identity of the actual access network that knows the location. This significantly complicates emergency calling, because the location of the caller and the first element that routes emergency calls can be on different continents, with different conventions and processes for handling of emergency calls.

The IETF has historically not created national variants of its standards. Thus, this document attempts to take into account best practices that have evolved for circuit-switched PSAPs, but it makes no assumptions on particular operating practices currently in use, numbering schemes, or organizational structures.

This document discusses the use of the Session Initiation Protocol (SIP) [RFC3261] by PSAPs and calling parties. While other inter- domain call signaling protocols may be used for emergency calling, SIP is ubiquitous and possesses the proper support of this use case. Only protocols such as H.323, XMPP/Jingle, ISUP, and SIP are suitable for inter-domain communications, ruling out Media Gateway Controller protocols such as the Media Gateway Control Protocol (MGCP) or H.248/ Megaco. The latter protocols can be used by the enterprise or carrier placing the call, but any such call would reach the PSAP through a media gateway controller, similar to how inter-domain VoIP calls would be placed. Other signaling protocols may also use protocol translation to communicate with a SIP-enabled PSAP. Peer- to-peer SIP (p2psip) is not considered in this document.

Existing emergency services rely exclusively on voice and conventional text telephony ("TTY") media streams. However, more choices of media offer additional ways to communicate and evaluate the situation as well as to assist callers and call takers in making and handling emergency calls, respectively. For example, instant messaging and video could improve the ability to communicate and evaluate the situation and to provide appropriate instruction prior to arrival of emergency crews. Thus, the architecture described here supports the creation of sessions of any media type, negotiated between the caller and PSAP using existing SIP mechanisms [RFC3264].

This document focuses on the case in which all three steps in the emergency calling process -- location configuration, call routing, and call placement -- can be and are performed by the calling endpoint, with the endpoint's Access Service Provider supporting the process by providing location information. In this case, calls may be routed via an application-layer Communications Service Provider (e.g., a Voice Service Provider) but need not be. The underlying protocols can also be used to support other models in which parts of the process are delegated to the Communications Service Provider. This document does not address in detail either these models or interoperability issues between them and the model described here.

Since this document is a framework document, it does not include normative behavior. [PHONEBCP] describes the best current practice for this subject and contains normative language for devices as well as access and calling network elements.

Supporting emergency calling does not require any specialized SIP header fields, request methods, status codes, message bodies, or event packages, but it does require that existing mechanisms be used in certain specific ways, as described below. User agents (UAs) unaware of the recommendations in this document may be able to place emergency calls, but functionality may be impaired. For example, if the UA does not implement the location mechanisms described, an emergency call may not be routed to the correct PSAP, and if the caller is unable to supply his exact location, dispatch of emergency responders may be delayed. Suggested behavior for both endpoints and servers is provided.

From the point of view of the PSAP, three essential elements characterize an emergency call:

  • The call is routed to the most appropriate PSAP, based primarily on the location of the caller.
  • The PSAP must be able to automatically obtain the location of the caller with sufficient accuracy to dispatch a responder to help the caller.
  • The PSAP must be able to re-establish a session to the caller if for any reason the original session is disrupted.

2. Terminology

   This document uses terms from [RFC3261], [RFC5222], and [RFC5012].
   In addition, the following terms are used:
   Access network:  The access network supplies IP packet service to an
      endpoint.  Examples of access networks include digital subscriber
      lines (DSLs), cable modems, IEEE 802.11, WiMaX, enterprise local
      area networks, and cellular data networks.
   Confidence:  Confidence is an estimate indicating how sure the
      measuring system is that the actual location of the endpoint is
      within the bounds defined by the uncertainty value, expressed as a
      percentage.  For example, a value of 90% indicates that the actual
      location is within the uncertainty nine times out of ten.
   Dispatch location:  The dispatch location is the location used for
      dispatching responders to the person in need of assistance.  The
      dispatch location must be sufficiently precise to easily locate
      the caller; typically, it needs to be more accurate than the
      routing location.
   Location configuration:  During location configuration, an endpoint
      learns its physical location.
   Location Configuration Protocol (LCP):  A protocol used by an
      endpoint to learn its location.
   Location conveyance:  Location conveyance delivers location
      information to another element.
   Location determination:  Location determination finds where an
      endpoint is physically located.  For example, the endpoint may
      contain a Global Navigation Satellite System (GNSS) receiver used
      to measure its own location or the location may be determined by a
      network administrator using a wiremap database.
   Location Information Server (LIS):  A Location Information Server
      stores location information for retrieval by an authorized entity.
   Mobile device:  A mobile device is a user agent that may change its
      physical location and possibly its network attachment point during
      an emergency call.
   National Emergency Number Association (NENA):  The National Emergency
      Number Association is an organization of professionals to "foster
      the technological advancement, availability and implementation of
      a universal emergency telephone number system in North America".
      It develops emergency calling specifications and procedures.
   Nomadic device (user):  A nomadic user agent is connected to the
      network temporarily, for relatively short durations, but does not
      move significantly during the emergency call.  Examples include a
      laptop using an IEEE 802.11 hotspot or a desk IP phone that is
      moved occasionally from one cubicle to another.
   Physical location:  A physical location describes where a person or
      device is located in physical space, described by a coordinate
      system.  It is distinguished from the network location, described
      by a network address.
   Public Safety Answering Point (PSAP):  A PSAP is a call center that
      answers emergency calls.
   Routing location:  The routing location of a device is used for
      routing an emergency call and may not be as precise as the
      dispatch location.
   Stationary device:  An stationary device is not mobile and is
      connected to the network at a fixed, long-term-stable physical
      location.  Examples include home PCs or pay phones.
   Uncertainty:  Uncertainty is an estimate, expressed in a unit of
      length, indicating the diameter of a circle that contains the
      endpoint with the probability indicated by the confidence value.

3. Overview of How Emergency Calls Are Placed

An emergency call can be distinguished (Section 5) from any other call by a unique service URN [RFC5031] that is placed in the call setup signaling when a home or visited emergency dial string is detected. Because emergency services are local to specific geographic regions, a caller obtains his location (Section 6) prior to making emergency calls. To get this location, either a form of measuring, for example, GNSS (Section 6.2.3) is deployed or the endpoint is configured (Section 6.5) with its location from the access network's Location Information Server (LIS) using a Location Configuration Protocol (LCP). The location is conveyed (Section 6.7) in the SIP signaling with the call. The call is routed (Section 8) based on location using the Location-to-Service Translation (LoST) protocol [RFC5222], which maps a location to a set of PSAP URIs. Each URI resolves to a PSAP or an Emergency Services Routing Proxy (ESRP) that serves as an incoming proxy for a group of PSAPs. The call arrives at the PSAP with the location included in the INVITE request.

The following is a quick overview for a typical Ethernet-connected telephone using SIP signaling. It illustrates one set of choices for various options presented later in this document.

  • The phone "boots" and connects to its access network.
  • The phone gets location via a Location Configuration Protocol (LCP), for example, from the DHCP server in civic [RFC4776] and/or geo [RFC6225] forms, a HTTP-Enabled Location Delivery (HELD) server [RFC5985] or the first-level switch's Link-Layer Discovery Protocol (LLDP) server [LLDP].
  • The phone obtains the local emergency dial string(s) from the LoST [RFC5222] server for its current location. It also receives and caches the PSAP URI obtained from the LoST server.
  • Some time later, the user places an emergency call. The phone recognizes an emergency call from the dial strings and uses the "urn:service:sos" [RFC5031] URN to mark an emergency call.
  • It refreshes its location via DHCP and updates the PSAP's URI by querying the LoST mapping server with its location.
  • It puts its location in the SIP INVITE request in a Geolocation header [RFC6442] and forwards the call using its normal outbound call processing, which commonly involves an outbound proxy.
  • The proxy recognizes the call as an emergency call and routes the call using normal SIP routing mechanisms to the URI specified.
  • The call routing commonly traverses an incoming proxy server (ESRP) in the emergency services network. That proxy then routes the call to the PSAP.
  • The call is established with the PSAP and mutually agreed upon media streams are created.
  • The location of the caller is displayed to the call taker.
          Configuration Servers
    . . . . . . . . . . . . . . . . .
    .                               .
    .   +--------+    +----------+  .
    . +--------+ |  +----------+ |  .
    . | LIS    | |  | SIP      | |  .
    . |        |-+  | Registrar|-+  .
    . +--------+    +----------+    .
    .   ^               ^           .
    . . | . . . . . . . | . . . . . .
        |               |
        |[M1][M4]       |[M2]
        |               |         +--------+
        |+--------------+       +--------+ |
        ||                      | LoST   | |
        ||+-------------------->| Servers|-+
        |||        [M3][M5]     +--------+       +-------+
        |||                                      | PSAP2 |
        |||                                      +-------+
        |||  [M6]  +-------+ [M7]+------+ [M8]+-------+
      Alice ------>| Proxy |---->| ESRP |---->| PSAP1 |-----> Call Taker
                   +-------+     +------+     +-------+
                                                 | PSAP3 |

Figure 1: Emergency Call Component Topology

The typical message flow for this example using Alice as the caller:

  [M1] Alice -> LIS:  LCP Request(s) (ask for location)
       LIS -> Alice:  LCP Reply(s) (replies with location)
  [M2] Alice -> Registrar: SIP REGISTER
       Registrar -> Alice: SIP 200 OK (REGISTER)
  [M3] Alice -> LoST Server: Initial LoST Query (contains location)
       Lost Server -> Alice: Initial LoST Response (contains
                         PSAP-URI and dial string)

Some time later, Alice dials or otherwise initiates an emergency call:

  [M4] Alice -> LIS:  LCP Request (updates location)
       LIS -> Alice:  LCP Reply (replies with location)
  [M5] Alice -> LoST Server: Update LoST Query (contains location)
       Lost Server -> Alice: LoST Response (contains PSAP-URI)
  [M6] Alice -> Outgoing Proxy: SIP INVITE (contains service URN,
                                       Location and PSAP URI)
  [M7] Outgoing Proxy -> ESRP: SIP INVITE (contains service URN,
                                       Location and PSAP URI)
  [M8] ESRP -> PSAP: SIP INVITE (contains service URN,
                                       Location and PSAP URI)

The 200 OK response is propagated back from the PSAP to Alice and the ACK response is propagated from Alice to the PSAP.

Figure 2: Message Flow

Figure 1 shows emergency call component topology and the text above shows call establishment. These include the following components:

  • Alice - the user of a UA that places the emergency call.
  • Configuration servers - Servers providing Alice's UA its IP address and other configuration information, perhaps including location-by-value or location-by-reference. Configuration servers also may include a SIP registrar for Alice's UA. Most SIP UAs will register, so it will be a common scenario for UAs that make emergency calls to be registered with such a server in the originating calling network. In most cases, a UA would have to register in order for the PSAP to be able to call it back after an emergency call has been completed. All the configuration messages are labeled M1 through M3, but could easily require more than three messages to complete.
  • LoST server - Processes the LoST request for location plus a service URN to a PSAP-URI, either for an initial request from a UA or an in-call routing by the proxy server in the originating network, or possibly by an ESRP.
  • ESRP - Emergency Services Routing Proxy, a SIP proxy server that is the incoming call proxy in the emergency services domain. The ESRP makes further routing decisions (e.g., based on PSAP state and the location of the caller) to choose the actual PSAP that handles the call. In some jurisdictions, this may involve another LoST query.
  • PSAP - Emergency calls are answered at a Public Safety Answering Point, a call center.

Generally, Alice's UA either has location configured manually, has an integral location measurement mechanism, or runs an LCP [M1] to obtain location from the access (broadband) network. Then, Alice's UA will most likely register [M2] with a SIP registrar. This allows her to be contacted by other SIP entities. Next, her UA will perform an initial LoST query [M3] to learn a URI for use if the LoST query fails during an emergency call or to use to test the emergency call mechanism. The LoST response contains the dial string for emergency calls appropriate for the location provided.

At some time after her device has booted, Alice initiates an emergency call. She may do this by dialing an emergency dial string valid for her current ("local") location or for her "home" location.

The UA recognizes either dial string. The UA attempts to refresh its location [M4], and with that location, to refresh the LoST mapping [M5], in order to get the most accurate information to use for routing the call. If the location request or the LoST request fails, or takes too long, the UA uses values it has cached.

The UA creates a SIP INVITE [M6] request that includes the location. [RFC6442] defines a SIP Geolocation header that contains either a location-by-reference URI or a [RFC3986] "cid:" URL indicating where in the message body the location-by-value is.

The INVITE message is routed to the ESRP [M7], which is the first inbound proxy for the emergency services domain. This message is then routed by the ESRP towards the most appropriate PSAP for Alice's location [M8], as determined by the location and other information.

A proxy in the PSAP chooses an available call taker and extends the call to its UA.

The 200 OK response to the INVITE request traverses the path in reverse, from call taker UA to PSAP proxy to ESRP to originating network proxy to Alice's UA. The ACK request completes the call setup and the emergency call is established, allowing the PSAP call taker to talk to Alice about Alice's emergency.

4. Which Devices and Services Should Support Emergency Calls?

Current PSAPs support voice calls and real-time text calls placed through PSTN facilities or systems connected to the PSTN. However, future PSAPs will support Internet connectivity and a wider range of media types and provide higher functionality. In general, if a user could reasonably expect to be able to place a call for help with the device, then the device or service should support emergency calling. Certainly, any device or service that looks like and works like a telephone (wired or mobile) should support emergency calling, but increasingly, users have expectations that other devices and services should work.

Devices that create media sessions and exchange audio, video, and/or text and that have the capability to establish sessions to a wide variety of addresses and communicate over private IP networks or the Internet should support emergency calls.

Traditionally, enterprise support of emergency calling is provided by the telephony service provider to the enterprise. In some more recent systems, the enterprise Private Branch Exchange (PBX) assists emergency calling by providing more fine-grained location in larger enterprises. In the future, the enterprise may provide the connection to emergency services itself, not relying on the telephony service provider.

5. Identifying an Emergency Call

Using the PSTN, emergency help can often be summoned by dialing a nationally designated, widely known number, regardless of where the telephone was purchased. The appropriate number is determined by the infrastructure to which the telephone is connected. However, this number differs between localities, even though it is often the same for a country or region, as it is in many countries in the European Union. In some countries, there is only one uniform digit sequence that is used for all types of emergencies. In others, there are several sequences that are specific to the type of responder needed, e.g., one for police, another for fire. For end systems, on the other hand, it is desirable to have a universal identifier, independent of location, to allow the automated inclusion of location information and to allow the device and other entities in the call path to perform appropriate processing within the signaling protocol in an emergency call setup.

Since no such universal identifier existed, the overall emergency calling architecture described here defines common emergency call URNs [RFC5031]. When all emergency services use a single number, the URN is "urn:service:sos". Users are not expected to "dial" an emergency URN. Rather, appropriate emergency dial strings are translated to corresponding service URNs, carried in the Request-URI of the INVITE request. Such translation is best done by the endpoint, because, among other reasons, emergency calls convey location in the signaling but non-emergency calls normally do not. If the device recognizes the emergency call, it can include location, if known. A signaling intermediary (proxy server) can also recognize emergency dial strings if the endpoint fails to do so.

For devices that are mobile or nomadic, an issue arises of whether the home or visited dial strings should be used. Many users would prefer that their home dialing sequences work no matter where they are. However, local laws and regulations may require that the visited dialing sequence(s) work. Therefore, the visited dial string must work. Devices may have a way to be configured or learn home dial strings.

LoST [RFC5222] provides the mechanism for obtaining the dialing sequences for a given location. LoST servers must return dial strings for emergency services. If the endpoint does not support the translation of dial strings to service URNs, the dialing sequence from the endpoint to its proxy is represented as a dial string [RFC4967] and the outgoing proxy must recognize the dial string and translate it to the equivalent service URN. To determine the local emergency dial string, the proxy needs the location of the endpoint. This may be difficult in situations where the user can roam or be nomadic. Endpoint recognition of emergency dial strings is therefore preferred. If a service provider is unable to guarantee that it can correctly determine local emergency dial strings, wherever its subscribers may be, then it is required that the endpoint do the recognition.

Note: The emergency call practitioners consider it undesirable to have a single-button emergency call user interface element. These mechanisms tend to result in a very high rate of false or accidental emergency calls. In order to minimize this issue, practitioners recommend that devices should only initiate emergency calls based on entry of specific emergency call dial strings. Speed dial mechanisms may effectively create single-button emergency call invocation and practitioners recommend they not be permitted.

6. Location and Its Role in an Emergency Call

Location is central to the operation of emergency services. Location is used for two purposes in emergency call handling: routing of the call and dispatch of responders. It is frequently the case that the callers reporting an emergency are unable to provide a unique, valid location themselves. For this reason, location provided by the endpoint or the access network is needed. For practical reasons, each PSAP generally handles only calls for a certain geographic area, with overload arrangements between PSAPs to handle each others' calls. Other calls that reach it by accident must be manually re-routed (transferred) to the more appropriate PSAP, increasing call handling delay and the chance for errors. The area covered by each PSAP differs by jurisdiction, where some countries have only a small number of PSAPs, while others decentralize PSAP responsibilities to the level of counties or municipalities.

In most cases, PSAPs cover at least a city or town, but there are some areas where PSAP coverage areas follow old telephone rate center boundaries and may straddle more than one city. Irregular boundaries are common, often due to historical reasons. Routing must be done based on actual PSAP service boundaries -- the closest PSAP, or the PSAP that serves the nominal city name provided in the location, may not be the correct PSAP.

Accuracy of routing location is a complex subject. Calls must be routed quickly, but accurately, and location determination is often a time/accuracy trade-off, especially with mobile devices or self- measuring mechanisms. If a more accurate routing location is not available, it is considered acceptable to base a routing decision on an accuracy equal to the area of one sector of a mobile cell site.

Routing to the most appropriate PSAP is always based on the location of the caller, despite the fact that some emergency calls are placed on behalf of someone else, and the location of the incident is sometimes not the location of the caller. In some cases, there are other factors that enter into the choice of the PSAP that gets the call, such as time of day, caller media requests, language preference, and call load. However, location of the caller is the primary input to the routing decision.

Many mechanisms used to locate a caller have a relatively long "cold start" time. To get a location accurate enough for dispatch may take as much as 30 seconds. This is too long to wait for emergencies. Accordingly, it is common, especially in mobile systems, to use a coarse location, for example, the cell site and sector serving the call, for call-routing purposes, and then to update the location when a more precise value is known prior to dispatch. In this document, we use "routing location" and "dispatch location" when the distinction matters.

Accuracy of dispatch location is sometimes determined by local regulation, and is constrained by available technology. The actual requirement is more stringent than available technology can deliver: It is required that a device making an emergency call close to the "demising" or separation wall between two apartments in a high-rise apartment building report location with sufficient accuracy to determine on what side of the wall it is. This implies perhaps a 3 cm accuracy requirement. As of the date of this memo, assisted GNSS uncertainty in mobile phones with 95% confidence cannot be relied upon to be less than hundreds of meters. As technology advances, the accuracy requirements for location will need to be tightened. Wired systems using wire-tracing mechanisms can provide location to a wall jack in specific room on a floor in a building, and may even specify a cubicle or even smaller resolution. As this discussion illustrates, emergency call systems demand the most stringent location accuracy available.

In Internet emergency calling, where the endpoint is located is determined using a variety of measurement or wire-tracing methods. Endpoints may be configured with their own location by the access network. In some circumstances, a proxy server may insert location into the signaling on behalf of the endpoint. The location is mapped to the URI to send the call to, and the location is conveyed to the PSAP (and other elements) in the signaling. The terms "determination", "configuration", "mapping", and "conveyance" are used for specific aspects of location handling in IETF protocols. Likewise, we employ Location Configuration Protocols, Location Mapping Protocols, and Location Conveyance Protocols for these functions.

This document provides guidance for generic network configurations with respect to location. It is recognized that unique issues may exist in some network deployments. The IETF will continue to investigate these unique situations and provide further guidance, if warranted, in future documents.

6.1. Types of Location Information

Location can be specified in several ways:

   Civic:  Civic location information describes the location of a person
      or object by a street address that corresponds to a building or
      other structure.  Civic location may include more fine-grained
      location information such as floor, room, and cubicle.  Civic
      information comes in two forms:

"Jurisdictional" refers to a civic location using actual

political subdivisions, especially for the community name.

"Postal" refers to a civic location for mail delivery. The name of the post office sometimes does not correspond to the community name and a postal address may contain post office boxes or street addresses that do not correspond to an actual building. Postal addresses are generally unsuitable for emergency call dispatch because the post office conventions (for community name, for example) do not match those known by the responders. The fact that they are unique can sometimes be exploited to provide a mapping between a postal address and a civic address suitable to which to dispatch a responder. In IETF location protocols, there is an element (Postal Community Name) that can be included in a location to provide the post office name as well as the actual jurisdictional community name. There is also an element for a postal code. There is no other accommodation for postal addresses in these protocols.

   Geospatial (geo):  Geospatial addresses contain longitude, latitude,
      and altitude information based on an understood datum and earth
      shape model (datum).  While there have been many datums developed
      over time, most modern systems are using or moving towards the
      WGS84 [WGS84] datum.
   Cell tower/sector:  Cell tower/sector is often used for identifying
      the location of a mobile handset, especially for routing of
      emergency calls.  Cell tower and sectors identify the cell tower
      and the antenna sector that a mobile device is currently using.
      Traditionally, the tower location is represented as a point chosen
      to be within a certain PSAP service boundary that agrees to take
      calls originating from that tower/sector, and routing decisions
      are made on that point.  Cell tower/sector information could also
      be represented as an irregularly shaped polygon of geospatial
      coordinates reflecting the likely geospatial location of the
      mobile device.  Whatever representation is used must route
      correctly in the LoST database, where "correct" is determined by
      local PSAP management.

In IETF protocols, both civic and geospatial forms are supported. The civic forms include both postal and jurisdictional fields. A cell tower/sector can be represented as a geo point or polygon or civic location. Other forms of location representation must be mapped into either a geo or civic value for use in emergency calls.

For emergency call purposes, conversion of location information from civic to geo or vice versa prior to conveyance is not desirable. The location should be sent in the form it was determined. Conversion between geo and civic requires a database. Where PSAPs need to convert from whatever form they received to another for responder purposes, they have a suitable database. However, if a conversion is done before the PSAP's, and the database used is not exactly the same one the PSAP uses, the double conversion has a high probability of introducing an error.

6.2. Location Determination

As noted above, location information can be entered by the user or installer of a device ("manual configuration"), measured by the end system, be delivered to the end system by some protocol or measured by a third party, and be inserted into the call signaling.

In some cases, an entity may have multiple sources of location information, possibly some that are partially contradictory. This is particularly likely if the location information is determined both by the end system and a third party. Although self-measured location (e.g., GNSS) is attractive, location information provided by the access network could be much more accurate, and more reliable in some environments such as high-rise buildings in dense urban areas.

The closer an entity is to the source of location, the more likely it is able to determine which location is most appropriate for a particular purpose when there is more than one location determination for a given endpoint. In emergency calling, the PSAP is the least likely to be able to appropriately choose which location to use when multiple conflicting locations are presented to it. While all available locations can be sent towards the PSAP, the order of the locations should be the sender's best attempt to guide the recipient of which one(s) to use.

6.2.1. User-Entered Location Information

Location information can be maintained by the end user or the installer of an endpoint in the endpoint itself, or in a database.

Location information routinely provided by end users is almost always less reliable than measured or wire database information, as users may mistype location information or may enter civic address information that does not correspond to a recognized (i.e., valid, see Section 6.10) address. Users can forget to change the data when the location of a device changes.

However, there are always a small number of cases where the automated mechanisms used by the access network to determine location fail to accurately reflect the actual location of the endpoint. For example, the user may deploy his own WAN behind an access network, effectively removing an endpoint some distance from the access network's notion of its location. To handle these exceptional cases, there must be some mechanism provided to manually provision a location for an endpoint by the user or by the access network on behalf of a user. The use of the mechanism introduces the possibility of users falsely declaring themselves to be somewhere they are not. However, this is generally not a problem in practice. Commonly, if an emergency caller insists that he is at a location different from what any automatic location determination system reports he is, responders will always be sent to the user's self-declared location.

6.2.2. Access Network "Wire Database" Location Information

Location information can be maintained by the access network, relating some form of identifier for the end subscriber or device to a location database ("wire database"). In enterprise LANs, wiremap databases map Ethernet switch ports to building locations. In DSL installations, the local telephone carrier maintains a mapping of wire-pairs to subscriber addresses.

Accuracy of location historically has been to a street-address level. However, this is not sufficient for larger structures. The Presence Information Data Format (PIDF) Location Object [RFC4119], extended by [RFC5139] and [RFC5491], permits interior building, floor, and room and even finer specification of location within a street address. When possible, interior location should be supported.

The threshold for when interior location is needed is approximately 650 square meters. This value is derived from US fire brigade recommendations of spacing of alarm pull stations. However, interior space layout, construction materials, and other factors should be considered.

Even for IEEE 802.11 wireless access points, wire databases may provide sufficient location resolution. The location of the access point as determined by the wiremap may be supplied as the location for each of the clients of the access point. However, this may not be true for larger-scale systems such as IEEE 802.16 (WiMAX) and IEEE 802.22 that typically have larger cells than those of IEEE 802.11. The civic location of an IEEE 802.16 base station may be of little use to emergency personnel, since the endpoint could be several kilometers away from the base station.

Wire databases are likely to be the most promising solution for residential users where a service provider knows the customer's service address. The service provider can then perform address validation (see Section 6.10), similar to the current system in some jurisdictions.

6.2.3. End System Measured Location Information

Global Positioning System (GPS) and similar Global Navigation Satellite Systems (e.g., GLONAS and Galileo) receivers may be embedded directly in the end device. GNSS produces relatively high precision location fixes in open-sky conditions, but the technology still faces several challenges in terms of performance (time-to-fix and time-to-first-fix), as well as obtaining successful location fixes within shielded structures, or underground. It also requires all devices to be equipped with the appropriate GNSS capability. Many mobile devices require using some kind of "assist", that may be operated by the access network (A-GPS) or by a government (WAAS). A device may be able to use multiple sources of assist data.

The GNSS satellites are active continuously; thus, location will always be available as long as the device can "see" enough satellites. However, mobile devices may not be able to afford the power levels required to keep the measuring system active. In such circumstances, when location is needed, the device has to start up the measurement mechanism. Typically, this takes tens of seconds, far too long to wait to be able to route an emergency call. For this reason, devices that have end system measured location mechanisms but need a cold start period lasting more than a couple seconds need another way to get a routing location. Typically, this would be a location associated with a radio link (cell tower/sector).

6.2.4. Network Measured Location Information

The access network may locate end devices. Techniques include various forms of triangulation. Elements in the network infrastructure triangulate end systems based on signal strength, angle of arrival or time of arrival. Common mechanisms deployed include the following:

  • Time Difference Of Arrival - TDOA
  • Uplink Time Difference Of Arrival - U-TDOA
  • Angle of Arrival - AOA
  • Radio Frequency (RF) fingerprinting
  • Advanced Forward Link Trilateration - AFLT
  • Enhanced Forward Link Trilateration - EFLT

Sometimes multiple mechanisms are combined, for example A-GPS with AFLT.

6.3. Who Adds Location, Endpoint, or Proxy?

The IETF emergency call architecture prefers endpoints to learn their location and supply it on the call. Where devices do not support location, proxy servers may have to add location to emergency calls. Some calling networks have relationships with all access networks the device may be connected to, and that may allow the proxy to accurately determine the location of the endpoint. However, NATs and other middleboxes often make it impossible to determine a reference identifier the access network could provide to a LIS to determine the location of the device. System designers are discouraged from relying on proxies to add location. The technique may be useful in some limited circumstances as devices are upgraded to meet the requirements of this document, or where relationships between access networks and calling networks are feasible and can be relied upon to get accurate location.

Proxy insertion of location complicates dial-string recognition. As noted in Section 6, local dial strings depend on the location of the caller. If the device does not know its own location, it cannot use the LoST service to learn the local emergency dial strings. The calling network must provide another way for the device to learn the local dial string, and update it when the user moves to a location where the dial string(s) change, or do the dial-string determination itself.

6.4. Location and References to Location

Location information may be expressed as the actual civic or geospatial value but can be transmitted as by value, i.e., wholly contained within the signaling message, or by reference, i.e., as a URI pointing to the value residing on a remote node waiting to be dereferenced.

When location is transmitted by value, the location information is available to entities in the call path. On the other hand, location objects can be large and only represent a single snapshot of the device's location. Location references are small and can be used to represent a time-varying location, but the added complexity of the dereference step introduces a risk that location will not be available to parties that need it if the dereference transaction were to fail.

6.5. End System Location Configuration

Unless a user agent has access to provisioned or locally measured location information, it must obtain it from the access network. There are several Location Configuration Protocols (LCPs) that can be used for this purpose including DHCP, HELD, and LLDP:

DHCP can deliver civic [RFC4776] or geospatial [RFC6225] information. User agents need to support both formats. Note that a user agent can use DHCP, via the DHCP REQUEST or INFORM messages, even if it uses other means to acquire its IP address.

HELD [RFC5985] can deliver a civic or geo location object, by value or by reference, via a Layer 7 protocol. The query typically uses the IP address of the requester as an identifier and returns the location value or reference associated with that identifier. HELD is typically carried in HTTP.

Link-Layer Discovery Protocol [LLDP] with Media Endpoint Device (MED) extensions [LLDP-MED] can be used to deliver location information directly from the Layer 2 network infrastructure and also supports both civic and geo formats identical in format to DHCP methods.

Each LCP has limitations in the kinds of networks that can reasonably support it. For this reason, it is not possible to choose a single mandatory-to-deploy LCP. For endpoints with common network connections, such as an Ethernet jack or a WiFi connection, location determination could easily fail unless every network supported every protocol, or alternatively, every device supported every protocol. For this reason, a mandatory-to-implement list of LCPs is established in [PHONEBCP]. Every endpoint that could be used to place emergency calls must implement all of the protocols on the list. Every access network must deploy at least one of them. Since it is the variability of the networks that prevent a single protocol from being acceptable, it must be the endpoints that implement all of them, and to accommodate a wide range of devices, networks must deploy at least one of them.

Often, network operators and device designers believe that they have a simpler environment and some other network specific mechanism can be used to provide location. Unfortunately, it is very rare to actually be able to limit the range of devices that may be connected to a network. For example, existing mobile networks are being used to support routers and LANs behind the WAN connection of a wireless data network, with Ethernet connected phones connected to that. It is possible that the access network could support a protocol not on the list and require every handset in that network to use that protocol for emergency calls. However, the Ethernet-connected phone will not be able to acquire location, and the user of the phone is unlikely to be dissuaded from placing an emergency call on that phone. The widespread availability of gateways, routers, and other network-broadening devices means that indirectly connected endpoints are possible on nearly every network. Network operators and vendors are cautioned that shortcuts to meeting this requirement are seldom successful.

Location for non-mobile devices is normally expected to be acquired at network attachment time and retained by the device. It should be refreshed when the cached value expires. For example, if DHCP is the acquisition protocol, refresh of location may occur when the IP address lease is renewed. At the time of an emergency call, the location should be refreshed, with the retained location used if the location acquisition does not immediately return a value. Mobile devices may determine location at network attachment time and periodically thereafter as a backup in case location determination at the time of call does not work. Mobile device location may be refreshed when a Time-to-Live (TTL) expires or the device moves beyond some boundaries (as provided by [RFC5222]). Normally, mobile devices will acquire their location at call time for use in an emergency call routing. See Section 6.8 for a further discussion on location updates for dispatch location.

There are many examples of endpoints that are user agent applications running on a more general purpose device, such as a personal computer. On some systems, Layer 2 protocols like DHCP and LLDP may not be directly accessible to applications. It is desirable for an operating system to have an API that provides the location of the device for use by any application, especially those supporting emergency calls.

6.6. When Location Should Be Configured

Devices should get routing location immediately after obtaining local network configuration information. The presence of NAT and VPN tunnels (that assign new IP addresses to communications) can obscure identifiers used by LCPs to determine location, especially for HELD.

In some cases, such as residential NAT devices, the NAT is placed between the endpoint and the access network demarcation point and thus the IP address seen by the access network is the right identifier for location of the residence. However, in many enterprise environments, VPN tunnels can obscure the actual IP address. Some VPN mechanisms can be bypassed so that a query to the LCP can be designated to go through the direct IP path, using the correct IP address, and not through the tunnel. In other cases, no bypass is possible, but location can be configured before the VPN is established. Of course, LCPs that use Layer 2 mechanisms (DHCP location options and LLDP-MED) are usually immune from such problems because they do not use the IP address as the identifier for the device seeking location.

It is desirable that routing location information be periodically refreshed. A LIS supporting a million subscribers each refreshing once per day would need to support a query rate of 1,000,000 / (24 * 60 * 60) = 12 queries per second. For networks with mobile devices, much higher refresh rates could be expected.

It is desirable for routing location information to be requested immediately before placing an emergency call. However, if there is any significant delay in getting more recent location, the call should be placed with the most recent location information the device has. In mobile handsets, routing is often accomplished with the cell site and sector of the tower serving the call, because it can take many seconds to start up the location determination mechanism and obtain an accurate location.

There is a trade-off between the time it takes to get a routing location and the accuracy (technically, confidence and uncertainty) obtained. Routing an emergency call quickly is required. However, if location can be substantially improved by waiting a short time (e.g., for some sort of "quick (location) fix"), it is preferable to wait. Three seconds, the current nominal time for a quick fix, is a very long time add to post-dial delay. NENA recommends [NENAi3TRD] that IP-based systems complete calls in two seconds from last dial press to ring at the PSAP.

6.7. Conveying Location

When an emergency call is placed, the endpoint should include location in the call signaling. This is referred to as "conveyance" to distinguish it from "configuration". In SIP, the location information is conveyed following the procedures in [RFC6442]. Since the form of the location information obtained by the acquisition protocol may not be the same as the conveyance protocol uses (PIDF-LO [RFC4119]), mapping by the endpoint from the LCP form to PIDF may be required.

6.8. Location Updates

As discussed above, it may take some time for some measurement mechanisms to get a location accurate enough for dispatch, and a routing location with less accuracy may be provided to get the call established quickly. The PSAP needs the dispatch location before it sends the call to the responder. This requires an update of the location. In addition, the location of some mobile callers, e.g., in a vehicle or aircraft, can change significantly during the emergency call.

A PSAP has no way to request an update of a location provided by value. If the User Agent Client (UAC) gets new location information, it must signal the PSAP using a new INVITE or an UPDATE transaction with a new Geolocation header field to supply the new location.

With the wide variation in determination mechanisms, the PSAP does not know when accurate location may be available. The preferred mechanism is that the LIS notifies the PSAP when an accurate location is available rather than requiring a poll operation from the PSAP to the LIS. The SIP Presence subscription [RFC3856] provides a suitable mechanism.

When using a HELD dereference, the PSAP must specify the value "emergencyDispatch" for the ResponseTime parameter. Since, typically, the LIS is local relative to the PSAP, the LIS can be aware of the update requirements of the PSAP.

6.9. Multiple Locations

Getting multiple locations all purported to describe the location of the caller is confusing to all, and should be avoided. Handling multiple locations at the point where a PIDF is created is discussed in [RFC5491]. Conflicting location information is particularly harmful if different routes (PSAPs) result from LoST queries for the multiple locations. When they occur anyway, the general guidance is that the entity earliest in the chain generally has more knowledge than later elements to make an intelligent decision, especially about which location will be used for routing. It is permissible to send multiple locations towards the PSAP, but the element that chooses the route must select exactly one location to use with LoST.

Guidelines for dealing with multiple locations are also given in [RFC5222]. If a UA gets multiple locations, it must choose the one to use for routing, but it may send all of the locations it has in the signaling. If a proxy is inserting location and has multiple locations, it must choose exactly one to use for routing and send it as well as any other locations it has that correspond to this UA.

The UA or proxy should have the ability to understand how and from whom it learned its location, and should include this information in the location objects that are sent to the PSAP. That labeling provides the call taker with information to make decisions upon, as well as guidance for, what to ask the caller and what to tell the responders.

Endpoints or proxies may be tempted to send multiple versions of the same location. For example a database may be used to "geocode" or "reverse geocode", that is, convert from civic to geo or vice versa. It is very problematic to use derived locations in emergency calls. The PSAP and the responders have very accurate databases that they use to convert most commonly from a reported geo to a civic suitable for dispatching responders. If one database is used to convert from, say, civic to geo, and another converts from geo to civic, errors will often occur where the databases are slightly different. Errors of even a single house number are serious as it may lead first responders to the wrong building. Derived locations should be marked with a "derived" method token [RFC4119]. If an entity gets a location that has a measured or other original method, and another with a derived method, it must use the original value for the emergency call.

6.10. Location Validation

Validation, in this context, means that there is a mapping from the address to a PSAP and that the PSAP understands how to direct responders to the location. It is recommended that location be validated prior to a device placing an actual emergency call; some jurisdictions require that this be done.

Determining whether an address is valid can be difficult. There are, for example, many cases of two names for the same street, or two streets with the same name but different "suffixes" (Avenue, Street, Circle) in a city. In some countries, the current system provides validation. For example, in the United States of America, the Master Street Address Guide (MSAG) records all valid street addresses and is used to ensure that the service addresses in phone billing records correspond to valid emergency service street addresses. Validation is normally only a concern for civic addresses, although there could be some determination that a given geo is within at least one PSAP service boundary; that is, a "valid" geo is one where there is a mapping in the LoST server.

LoST [RFC5222] includes a location validation function. Validation is normally performed when a location is entered into a Location Information Server. It should be confirmed periodically, because the mapping database undergoes slow change and locations that previously validated may eventually fail validation. Endpoints may wish to validate locations they receive from the access network, and will need to validate manually entered locations. Proxies that insert location may wish to validate locations they receive from a LIS. When the test functions (Section 15) are invoked, the location used should be validated.

When validation fails, the location given should not be used for an emergency call, unless no other valid location is available. Bad location is better than no location. If validation is completed when location is first loaded into a LIS, any problems can be found and fixed before devices could get the bad location. Failure of validation arises because an error is made in determining the location, although occasionally the LoST database is not up to date or has faulty information. In either case, the problem must be identified and should be corrected before using the location.

6.11. Default Location

Occasionally, the access network cannot determine the actual location of the caller. In these cases, it must supply a default location. The default location should be as accurate as the network can determine. For example, in a cable network, a default location for each Cable Modem Termination System (CMTS), with a representative location for all cable modems served by that CMTS could be provided if the network is unable to resolve the subscriber to anything more precise than the CMTS. Default locations must be marked as such so that the PSAP knows that the location is not accurate.

6.12. Location Format Conversion

The endpoint is responsible for mapping any form of location it receives from an LCP into PIDF-LO form if the LCP did not directly return a PIDF-LO.

7. LIS and LoST Discovery

Endpoints must be able to discover a LIS, if the HELD protocol is used and a LoST server. DHCP options are defined for this purpose, namely [RFC5986] and [RFC5223].

Until such DHCP records are widely available, it may be necessary for the service provider to provision a LoST server address in the device. The endpoint can also do a DNS SRV query to find a LoST server. In any environment, more than one of these mechanisms may yield a LoST server, and they may be different. The recommended priority is DHCP first, provisioned value second, and DNS SRV query in the SIP domain third.

8. Routing the Call to the PSAP

Emergency calls are routed based on one or more of the following criteria expressed in the call setup request (INVITE):

   Location:  Since each PSAP serves a limited geographic region and
      transferring existing calls delays the emergency response, calls
      need to be routed to the most appropriate PSAP.  In this
      architecture, emergency call setup requests contain location
      information, expressed in civic or geospatial coordinates, that
      allows such routing.
   Type of emergency service:  In some jurisdictions, emergency calls
      for specific emergency services such as fire, police, ambulance,
      or mountain rescue are directed to just those emergency-specific
      PSAPs.  This mechanism is supported by marking emergency calls
      with the proper service identifier [RFC5031].  Even in single-
      number jurisdictions, not all services are dispatched by PSAPs and
      may need alternate URNs to route calls to the appropriate call
   Media capabilities of caller:  In some cases, emergency call centers
      for specific caller media preferences, such as typed text or
      video, are separate from PSAPs serving voice calls.  ESRPs are
      expected to be able to provide routing based on media.  Also, even
      if media capability does not affect the selection of the PSAP,
      there may be call takers within the PSAP that are specifically
      trained, e.g., in real-time text or sign language communications,
      where routing within the PSAP based on the media offer would be

Providing a URL to route emergency calls by location and by type of service is the primary function LoST [RFC5222] provides. LoST accepts a query with location (by-value) in either civic or geo form, plus a service identifier, and returns a URI (or set of URIs) to which to route the call. Normal SIP [RFC3261] routing functions are used to resolve the URI to a next-hop destination.

The endpoint can complete the LoST mapping from its location at boot time, and periodically thereafter. It should attempt to obtain a "fresh" location, and from that a current mapping when it places an emergency call. If accessing either its location acquisition or its mapping functions fail, it should use its cached value. The call would follow its normal outbound call processing.

Determining when the device leaves the area provided by the LoST service can tax small mobile devices. For this reason, the LoST server should return a simple (small number of points) polygon for geospatial location. This can be a simple enclosing rectangle of the PSAP service area when the reported point is not near an edge, or a smaller polygonal edge section when the reported location is near an edge. Civic location is uncommon for mobile devices, but reporting that the same mapping is good within a community name, or even a street, may be very helpful for WiFi connected devices that roam and obtain civic location from the access point to which they are connected.

Networks that support devices that do not implement LoST mapping themselves may need the outbound proxy do the mapping. If the endpoint recognized the call was an emergency call, provided the correct service URN and/or included location on the call in a Geolocation header, a proxy server could easily accomplish the mapping.

However, if the endpoint did not recognize the call was an emergency call, and thus did not include location, the proxy's task is more difficult. It is often difficult for the calling network to accurately determine the endpoint's location. The endpoint may have its own location, but would not normally include it on the call signaling unless it knew it was an emergency call. There is no mechanism provided in [RFC6442] for a proxy to request the endpoint supply its location, because that would open the endpoint to an attack by any proxy on the path to get it to reveal location. The proxy can attempt to redirect a call to the service URN, which, if the device recognizes the significance, would include location in the redirected call from the device. All network elements should detect emergency calls and supply default location and/or routing if it is not already present.

The LoST server would normally be provided by the local emergency authorities, although the access network or calling network might run its own server using data provided by the emergency authorities. Some enterprises may have local responders and call centers, and could operate their own LoST server, providing URIs to in-house "PSAPs". Local regulations might limit the ability of enterprises to direct emergency calls to in-house services.

The ESRP, which is a normal SIP proxy server in the signaling path of the call, may use a variety of PSAP state information, the location of the caller, and other criteria to route onward the call to the PSAP. In order for the ESRP to route on media choice, the initial INVITE request has to supply an SDP offer.

9. Signaling of Emergency Calls

9.1. Use of TLS

Best current practice for SIP user agents [RFC4504] including handling of audio, video, and real-time text [RFC4103] should be applied. As discussed above, location is carried in all emergency calls in the call signaling. Since emergency calls carry privacy- sensitive information, they are subject to the requirements for geospatial protocols [RFC3693]. In particular, signaling information should be carried in Transport Layer Security (TLS), i.e., in 'sips' mode with a ciphersuite that includes strong encryption, such as AES. There are exceptions in [RFC3693] for emergency calls. For example, local policy may dictate that location is sent with an emergency call even if the user's policy would otherwise prohibit that. Nevertheless, protection from eavesdropping of location by encryption should be provided.

It is unacceptable to have an emergency call fail to complete because a TLS connection was not created for any reason. Thus, the call should be attempted with TLS, but if the TLS session establishment fails, the call should be automatically retried without TLS. [RFC5630] recommends that to achieve this effect, the target specify a sip URI, but use TLS on the outbound connection. An element that receives a request over a TLS connection should attempt to create a TLS connection to the next hop.

In many cases, persistent TLS connections can be maintained between elements to minimize the time needed to establish them [RFC5626]. In other circumstances, use of session resumption [RFC5077] is recommended. IPsec [RFC4301] is an acceptable alternative to TLS when used with an equivalent crypto suite.

Location may be used for routing by multiple proxy servers on the path. Confidentiality mechanisms such as Secure/Multipurpose Internet Mail Extensions (S/MIME) encryption of SIP signaling [RFC3261] cannot be used because they obscure location. Only hop-by- hop mechanisms such as TLS should be used. Implementing location conveyance in SIP mandates inclusion of TLS support.

9.2. SIP Signaling Requirements for User Agents

SIP UAs that recognize local dial strings, insert location, and perform emergency call routing will create SIP INVITE messages with the service URN in the Request-URI, the LoST-determined URI for the PSAP in a Route header, and the location in a Geolocation header. The INVITE request must also have appropriate callback identifiers (in Contact and From headers). To enable media-sensitive routing, the call should include a Session Description Protocol (SDP) offer [RFC3264].

SIP caller preferences [RFC3841] can be used to signal how the PSAP should handle the call. For example, a language preference expressed in an Accept-Language header may be used as a hint to cause the PSAP to route the call to a call taker who speaks the requested language. SIP caller preferences may also be used to indicate a need to invoke a relay service for communication with people with disabilities in the call.

9.3. SIP Signaling Requirements for Proxy Servers

At least one SIP proxy server in the path of an emergency call must be able to assist UAs that are unable to provide any of the location- based routing steps and recognition of dial strings. A proxy can recognize the lack of location awareness by the lack of a Geolocation header. It can recognize the lack of dial-string recognition by the presence of the local emergency call dial string in the From header without the service URN being present. They should obtain the location of the endpoint if possible, and use a default location if they cannot, inserting it in a Geolocation header. They should query LoST with the location and put the resulting URI in a route, with the appropriate service URN in the Request-URI. In any event, they are also expected to provide information for the caller using SIP Identity or P-Asserted-Identity. It is often a regulatory matter whether calls normally marked as anonymous are passed as anonymous when they are emergency calls. Proxies must conform to the local regulation or practice.

10. Call Backs

The call taker must be able to reach the emergency caller if the original call is disconnected. In traditional emergency calls, wireline and wireless emergency calls include a callback identifier for this purpose. There are two kinds of call backs. When a call is dropped, or the call taker realizes that some important information is needed that it doesn't have, it must call back the device that placed the emergency call. The PSAP, or a responder, may need to call back the caller much later, and for that purpose, it wants a normal SIP address of record (AOR). In SIP systems, the caller must include a Contact header field in an emergency call containing a globally routable URI, possibly a Globally Routable User Agent URI (GRUU) [RFC5627]. This identifier would be used to initiate callbacks immediately by the call taker if, for example, the call is prematurely dropped. A concern arises with back-to-back user agents (B2BUAs) that manipulate Contact headers. Such B2BUAs should always include a Contact header that routes to the same device.

In addition, a callback identifier as an address of record (AoR) must be included either as the URI in the From header field [RFC3261] verified by SIP Identity [RFC4474] or as a network-asserted URI [RFC3325]. If the latter, the PSAP will need to establish a suitable spec(t) with the proxies that send it emergency calls. This identifier would be used to initiate a callback at a later time and may reach the caller, not necessarily on the same device (and at the same location) as the original emergency call as per normal SIP rules. It is often a regulatory matter whether calls normally marked as anonymous are passed as anonymous when they are emergency calls. Proxies must conform to the local regulation or practice.

11. Mid-Call Behavior

Some PSAPs often include dispatchers, responders, or specialists on a call. Some responders' dispatchers are not located in the primary PSAP, the call may have to be transferred to another PSAP. Most often, this will be an attended transfer, or a bridged transfer. Therefore, a PSAP may need to a REFER request [RFC3515] a call to a bridge for conferencing. Devices that normally involve the user in transfer operations should consider the effect of such interactions when a stressed user places an emergency call. Requiring user interface manipulation during such events may not be desirable. Relay services for communication with people with disabilities may be included in the call with the bridge. The UA should be prepared to have the call transferred (usually attended, but possibly blind) per [RFC5359].

12. Call Termination

It is undesirable for the caller to terminate an emergency call. A PSAP terminates a call using the normal SIP call termination procedures, i.e., with a BYE request.

13. Disabling of Features

Certain features that can be invoked while a normal call is active are not permitted when the call is an emergency call. Services such as call waiting, call transfer, three-way calling, and hold should be disabled.

Certain features such as call forwarding can interfere with calls from a PSAP and should be disabled. There is no way to reliably determine a PSAP call back. A UA may be able to determine a PSAP call back 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.

14. Media

PSAPs should always accept RTP media streams [RFC3550]. Traditionally, voice has been the only media stream accepted by PSAPs. In some countries, text, in the form of Baudot codes or similar tone encoded signaling within a voiceband is accepted ("TTY") for persons who have hearing disabilities. Using SIP signaling includes the capability to negotiate media. Normal SIP offer/answer [RFC3264] negotiations should be used to agree on the media streams to be used. PSAPs should accept real-time text [RFC4103]. All PSAPs should accept G.711 A-law (and mu-law in North America) encoded voice as described in [RFC3551]. Newer text forms are rapidly appearing, with instant messaging now very common, PSAPs should accept IM with at least "pager-mode" MESSAGE request [RFC3428] as well as Message Session Relay Protocol [RFC4975]. Video media in emergency calling is required to support Video Relay Service (sign language interpretation) as well as modern video phones.

It is desirable for media to be kept secure by the use of Secure RTP [RFC3711], using DTLS [RFC5764] for keying.

15. Testing

Since the emergency calling architecture consists of a number of pieces operated by independent entities, it is important to be able to test whether an emergency call is likely to succeed without actually occupying the human resources at a PSAP. Both signaling and media paths need to be tested since NATs and firewalls may allow the session setup request to reach the PSAP, while preventing the exchange of media.

[PHONEBCP] includes a description of an automated test procedure that validates routing, signaling, and media path continuity. This test should be used within some random interval after boot time, and whenever the device location changes enough that a new PSAP mapping is returned by the LoST server.

The PSAP needs to be able to control frequency and duration of the test, and since the process could be abused, it may temporarily or permanently suspend its operation.

There is a concern associated with testing during a so-called "avalanche-restart" event where, for example, a large power outage affects a large number of endpoints, that, when power is restored, all attempt to reboot and, possibly, test. Devices need to randomize their initiation of a boot time test to avoid the problem.

16. Security Considerations

Security considerations for emergency calling have been documented in [RFC5069] and [RFC6280].

This document suggests that security (TLS or IPsec) be used hop-by- hop on a SIP call to protect location information, identity, and other privacy-sensitive call data. It also suggests that if the attempt to create a security association fails, the call be retried without the security. It is more important to get an emergency call through than to protect the data; indeed, in many jurisdictions privacy is explicitly waived when making emergency calls. Placing a call without security may reveal user information, including location. The alternative, failing the call if security cannot be established, is considered unacceptable.

17. Acknowledgments

This document was created from "Emergency Services for Internet Telephony Systems" (Schulzrinne, 2004) together with sections from "Emergency Context Routing of Internet Technologies Architecture Considerations" (Polk, 2006).

Design Team members participating in this document creation include Martin Dolly, Stu Goldman, Ted Hardie, Marc Linsner, Roger Marshall, Shida Schubert, Tom Taylor, and Hannes Tschofenig. Further comments and input were provided by Richard Barnes, Barbara Stark, and James Winterbottom.

18. Informative References

   [LLDP]       IEEE, "IEEE802.1ab Station and Media Access Control",
                December 2004.
   [LLDP-MED]   ANSI/TIA, "Link Layer Discovery Protocol - Media
                Endpoint Discovery", TIA Standard, TIA-1057, April 2006.
   [NENAi3TRD]  NENA, "08-751 v1 - i3 Technical Requirements (Long Term
                Definition)", 2006.
   [PHONEBCP]   Rosen, B. and J. Polk, "Best Current Practice for
                Communications Services in support of Emergency
                Calling", Work in Progress, September 2011.
   [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.
   [RFC3264]    Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
                with Session Description Protocol (SDP)", RFC 3264,
                June 2002.
   [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.
   [RFC3428]    Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema,
                C., and D. Gurle, "Session Initiation Protocol (SIP)
                Extension for Instant Messaging", RFC 3428,
                December 2002.
   [RFC3515]    Sparks, R., "The Session Initiation Protocol (SIP) Refer
                Method", RFC 3515, April 2003.
   [RFC3550]    Schulzrinne, H., Casner, S., Frederick, R., and V.
                Jacobson, "RTP: A Transport Protocol for Real-Time
                Applications", STD 64, RFC 3550, July 2003.
   [RFC3551]    Schulzrinne, H. and S. Casner, "RTP Profile for Audio
                and Video Conferences with Minimal Control", STD 65,
                RFC 3551, July 2003.
   [RFC3693]    Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and
                J. Polk, "Geopriv Requirements", RFC 3693,
                February 2004.
   [RFC3711]    Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
                K. Norrman, "The Secure Real-time Transport Protocol
                (SRTP)", RFC 3711, March 2004.
   [RFC3841]    Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
                Preferences for the Session Initiation Protocol (SIP)",
                RFC 3841, August 2004.
   [RFC3856]    Rosenberg, J., "A Presence Event Package for the Session
                Initiation Protocol (SIP)", RFC 3856, August 2004.
   [RFC3986]    Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
                Resource Identifier (URI): Generic Syntax", STD 66,
                RFC 3986, January 2005.
   [RFC4103]    Hellstrom, G. and P. Jones, "RTP Payload for Text
                Conversation", RFC 4103, June 2005.
   [RFC4119]    Peterson, J., "A Presence-based GEOPRIV Location Object
                Format", RFC 4119, December 2005.
   [RFC4190]    Carlberg, K., Brown, I., and C. Beard, "Framework for
                Supporting Emergency Telecommunications Service (ETS) in
                IP Telephony", RFC 4190, November 2005.
   [RFC4301]    Kent, S. and K. Seo, "Security Architecture for the
                Internet Protocol", RFC 4301, December 2005.
   [RFC4474]    Peterson, J. and C. Jennings, "Enhancements for
                Authenticated Identity Management in the Session
                Initiation Protocol (SIP)", RFC 4474, August 2006.
   [RFC4504]    Sinnreich, H., Lass, S., and C. Stredicke, "SIP
                Telephony Device Requirements and Configuration",
                RFC 4504, May 2006.
   [RFC4776]    Schulzrinne, H., "Dynamic Host Configuration Protocol
                (DHCPv4 and DHCPv6) Option for Civic Addresses
                Configuration Information", RFC 4776, November 2006.
   [RFC4967]    Rosen, B., "Dial String Parameter for the Session
                Initiation Protocol Uniform Resource Identifier",
                RFC 4967, July 2007.
   [RFC4975]    Campbell, B., Mahy, R., and C. Jennings, "The Message
                Session Relay Protocol (MSRP)", RFC 4975,
                September 2007.
   [RFC5012]    Schulzrinne, H. and R. Marshall, "Requirements for
                Emergency Context Resolution with Internet
                Technologies", RFC 5012, January 2008.
   [RFC5031]    Schulzrinne, H., "A Uniform Resource Name (URN) for
                Emergency and Other Well-Known Services", RFC 5031,
                January 2008.
   [RFC5069]    Taylor, T., Tschofenig, H., Schulzrinne, H., and M.
                Shanmugam, "Security Threats and Requirements for
                Emergency Call Marking and Mapping", RFC 5069,
                January 2008.
   [RFC5077]    Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
                "Transport Layer Security (TLS) Session Resumption
                without Server-Side State", RFC 5077, January 2008.
   [RFC5139]    Thomson, M. and J. Winterbottom, "Revised Civic Location
                Format for Presence Information Data Format Location
                Object (PIDF-LO)", RFC 5139, February 2008.
   [RFC5222]    Hardie, T., Newton, A., Schulzrinne, H., and H.
                Tschofenig, "LoST: A Location-to-Service Translation
                Protocol", RFC 5222, August 2008.
   [RFC5223]    Schulzrinne, H., Polk, J., and H. Tschofenig,
                "Discovering Location-to-Service Translation (LoST)
                Servers Using the Dynamic Host Configuration Protocol
                (DHCP)", RFC 5223, August 2008.
   [RFC5359]    Johnston, A., Sparks, R., Cunningham, C., Donovan, S.,
                and K. Summers, "Session Initiation Protocol Service
                Examples", BCP 144, RFC 5359, October 2008.
   [RFC5491]    Winterbottom, J., Thomson, M., and H. Tschofenig,
                "GEOPRIV Presence Information Data Format Location
                Object (PIDF-LO) Usage Clarification, Considerations,
                and Recommendations", RFC 5491, March 2009.
   [RFC5626]    Jennings, C., Mahy, R., and F. Audet, "Managing Client-
                Initiated Connections in the Session Initiation Protocol
                (SIP)", RFC 5626, October 2009.
   [RFC5627]    Rosenberg, J., "Obtaining and Using Globally Routable
                User Agent URIs (GRUUs) in the Session Initiation
                Protocol (SIP)", RFC 5627, October 2009.
   [RFC5630]    Audet, F., "The Use of the SIPS URI Scheme in the
                Session Initiation Protocol (SIP)", RFC 5630,
                October 2009.
   [RFC5764]    McGrew, D. and E. Rescorla, "Datagram Transport Layer
                Security (DTLS) Extension to Establish Keys for the
                Secure Real-time Transport Protocol (SRTP)", RFC 5764,
                May 2010.
   [RFC5985]    Barnes, M., "HTTP-Enabled Location Delivery (HELD)",
                RFC 5985, September 2010.
   [RFC5986]    Thomson, M. and J. Winterbottom, "Discovering the Local
                Location Information Server (LIS)", RFC 5986,
                September 2010.
   [RFC6225]    Polk, J., Linsner, M., Thomson, M., and B. Aboba,
                "Dynamic Host Configuration Protocol Options for
                Coordinate-Based Location Configuration Information",
                RFC 6225, July 2011.
   [RFC6280]    Barnes, R., Lepinski, M., Cooper, A., Morris, J.,
                Tschofenig, H., and H. Schulzrinne, "An Architecture for
                Location and Location Privacy in Internet Applications",
                BCP 160, RFC 6280, July 2011.
   [RFC6442]    Polk, J., Rosen, B., and J. Peterson, "Location
                Conveyance for the Session Initiation Protocol",
                RFC 6442, December 2011.
   [WGS84]      NIMA, "NGA: DoD World Geodetic System 1984, Its
                Definition and Relationships with Local Geodetic
                Systems", Technical Report TR8350.2, Third Edition,
                July 1997.

Authors' Addresses

   Brian Rosen
   NeuStar, Inc.
   470 Conrad Dr
   Mars, PA  16046


   Henning Schulzrinne
   Columbia University
   Department of Computer Science
   450 Computer Science Building
   New York, NY  10027
   Phone: +1 212 939 7042
   James Polk
   Cisco Systems
   3913 Treemont Circle
   Colleyville, Texas  76034
   Phone: +1-817-271-3552
   Andrew Newton
   4900 Seminary Road
   Alexandria, VA  22311
   Phone: +1 703 845 0656