Internet Engineering Task Force (IETF) B. Rosen
Request for Comments: 6881 NeuStar
BCP: 181 J. Polk
Category: Best Current Practice Cisco Systems
ISSN: 2070-1721 March 2013
Best Current Practice for Communications Services in
Support of Emergency Calling
Abstract
The IETF and other standards organizations have efforts targeted at
standardizing various aspects of placing emergency calls on IP
networks. This memo describes best current practice on how devices,
networks, and services using IETF protocols should use such standards
to make emergency calls.
Status of This Memo
This memo documents an Internet Best Current Practice.
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
BCPs 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/rfc6881.
Copyright Notice
Copyright (c) 2013 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.
Rosen & Polk Best Current Practice [Page 1]RFC 6881 Emergency Call Phone BCP March 2013
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................3
3. Overview of How Emergency Calls Are Placed ......................3
4. Which Devices and Services Should Support Emergency Calls? ......4
5. Identifying an Emergency Call ...................................4
6. Location and Its Role in an Emergency Call ......................5
6.1. Types of Location Information ..............................6
6.2. Location Determination .....................................6
6.2.1. User-Entered Location Information ...................6
6.2.2. Access Network "Wire Database" Location
Information .........................................6
6.2.3. End System Measured Location Information ............7
6.2.4. Network Measured Location Information ...............7
6.3. Who Adds Location? The Endpoint, or the Proxy? ............8
6.4. Location and References to Location ........................8
6.5. End System Location Configuration ..........................8
6.6. When Location Should Be Configured ........................10
6.7. Conveying Location ........................................11
6.8. Location Updates ..........................................11
6.9. Multiple Locations ........................................12
6.10. Location Validation ......................................12
6.11. Default Location .........................................13
6.12. Other Location Considerations ............................13
7. LIS and LoST Discovery .........................................13
8. Routing the Call to the PSAP ...................................14
9. Signaling of Emergency Calls ...................................15
9.1. Use of TLS ................................................15
9.2. SIP Signaling Requirements for User Agents ................16
9.3. SIP Signaling Requirements for Proxy Servers ..............17
10. Callbacks .....................................................18
11. Mid-Call Behavior .............................................19
12. Call Termination ..............................................19
13. Disabling of Features .........................................19
14. Media .........................................................20
15. Testing .......................................................21
16. Security Considerations .......................................22
17. IANA Considerations ...........................................22
17.1. Test Service URN .........................................22
17.2. 'test' Subregistry .......................................23
18. Acknowledgements ..............................................23
19. References ....................................................23
19.1. Normative References .....................................23
19.2. Informative References ...................................27
Rosen & Polk Best Current Practice [Page 2]RFC 6881 Emergency Call Phone BCP March 2013
1. Introduction
This document describes how access networks, Session Initiation
Protocol [RFC3261] user agents, proxy servers, and Public Safety
Answering Points (PSAPs) support emergency calling, as outlined in
[RFC6443], which is designed to complement the present document in
section headings, numbering, and content. Understanding [RFC6443] is
necessary to understand this document. This Best Current Practice
(BCP) succinctly describes the requirements of end devices and
applications (requirements prefaced by "ED-"), access networks
(including enterprise access networks) (requirements prefaced by
"AN-"), service providers (requirements prefaced by "SP-"), and PSAPs
to achieve globally interoperable emergency calling on the Internet.
This document also defines requirements for "intermediate" devices
that exist between end devices or applications and the access
network. For example, a home router is an intermediate device.
Reporting location on an emergency call (see Section 6) may depend on
the ability of such intermediate devices to meet the requirements
prefaced by "INT-".
The access network requirements apply to those networks that may be
used to place emergency calls using IETF protocols. Local
regulations may impact the need to support this document's access
network requirements.
Other organizations, such as the National Emergency Number
Association (NENA), define the PSAP interface. NENA's documents
reference this document.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
This document uses terms from [RFC3261], [RFC5012], and [RFC6443].
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 must obtain his location (Section 6)
prior to making emergency calls. To get this location, either a form
of measuring (e.g., GPS) ([RFC6443] Section 6.2.3) device location in
Rosen & Polk Best Current Practice [Page 3]
RFC 6881 Emergency Call Phone BCP March 2013
the endpoint is deployed or the endpoint is configured (Section 6.5)
with its location from the access network's Location Information
Server (LIS). 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 a group of PSAPs. The call arrives at the PSAP with the
location included in the SIP INVITE request.
4. Which Devices and Services Should Support Emergency Calls?
ED-1: A device or application that implements SIP calling SHOULD
support emergency calling. Some jurisdictions have regulations
governing which devices need to support emergency calling, and
developers are encouraged to ensure that devices they develop meet
relevant regulatory requirements. Unfortunately, the natural
variation in those regulations also makes it impossible to
accurately describe the cases when developers do or do not have to
support emergency calling.
SP-1: If a device or application expects to be able to place a call
for help, the service provider that supports it MUST facilitate
emergency calling. Some jurisdictions have regulations governing
this.
ED-2: Devices that create media sessions and exchange real-time
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. Some jurisdictions have regulations governing this.
5. Identifying an Emergency Call
ED-3: Endpoints SHOULD recognize dial strings of emergency calls.
If the service provider always knows the location of the device
(the correct dial string depends on which country a caller is in),
the service provider may recognize them; see SP-2.
SP-2: Proxy servers SHOULD recognize emergency dial strings if for
some reason the endpoint does not recognize them.
ED-4/SP-3: Emergency calls MUST be marked with a service URN in the
Request-URI of the INVITE.
ED-5/SP-4: Geographically local dial strings MUST be recognized.
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ED-6/SP-5: Devices MUST be able to be configured with the home
country from which the home dial string(s) can be determined.
ED-7/SP-6: Emergency dial strings SHOULD be determined from LoST
[RFC5222]. Dial strings MAY be configured directly into the
device.
AN-1: LoST servers MUST return dial strings for emergency services.
ED-8: Endpoints that do not recognize emergency dial strings SHOULD
send dial strings as per [RFC4967].
SP-7: If a proxy server recognizes dial strings on behalf of its
clients, it MUST recognize emergency dial strings represented by
[RFC4967] and SHOULD recognize the emergency dial strings
represented by a tel URI [RFC3966].
ED-9: Endpoints SHOULD be able to have home dial strings
provisioned.
SP-8: Service providers MAY provision home dial strings in devices.
ED-10: Devices SHOULD NOT have one-button emergency calling
initiation.
ED-11/SP-9: All sub-services for the 'sos' service specified in
[RFC5031] MUST be recognized.
6. Location and Its Role in an Emergency Call
Handling location for emergency calling usually involves several
steps to process, and multiple entities are involved. In Internet
emergency calling, where the endpoint is located is determined using
a variety of measurement or wire-tracing methods. Endpoints can be
configured with their own location by the access network. In some
circumstances, a proxy server can 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
entities) in the signaling. Likewise, we employ Location
Configuration Protocols (LCPs), the Location-to-Service Mapping
Protocol, and Location Conveyance Protocols for these functions. The
Location-to-Service Translation protocol [RFC5222] is the Location
Mapping Protocol defined by the IETF.
Rosen & Polk Best Current Practice [Page 5]RFC 6881 Emergency Call Phone BCP March 2013
6.1. Types of Location Information
There are several forms of location. All IETF location configuration
and location conveyance protocols support both civic and geospatial
(geo) forms. The civic forms include both postal and jurisdictional
fields. A cell tower/sector can be represented as a point (geo or
civic) or polygon. Endpoints, intermediate devices, and service
providers receiving other forms of location representation MUST map
them into either a geo or civic for use in emergency calls.
ED-12/INT-1/SP-10: Endpoints, intermediate devices, and service
providers MUST be prepared to handle location represented in
either civic or geo form.
ED-13/INT-2/SP-11/AN-2: Entities MUST NOT convert (civic to geo or
geo to civic) from the form of location that the determination
mechanism (see Section 6.2) supplied prior to receipt by the PSAP.
6.2. Location Determination
ED-14/INT-3/AN-3: Any location determination mechanism MAY be used,
provided the accuracy of the location meets local requirements.
6.2.1. User-Entered Location Information
ED-15/INT-4/AN-4: Devices, intermediate devices, and/or access
networks SHOULD support a manual method to override the location
the access network determines. When the override location is
supplied in civic form, it MUST be possible for the resultant
Presence Information Data Format Location Object (PIDF-LO)
received at the PSAP to contain any of the elements specified in
[RFC4119] and [RFC5139].
6.2.2. Access Network "Wire Database" Location Information
AN-5: Access networks supporting copper, fiber, or other hard-wired
IP packet services SHOULD support location configuration. If the
network does not support location configuration, it MUST require
every device or intermediate device that connects to the network
to support end system measured location.
AN-6/INT-5: Access networks and intermediate devices providing wire
database location information SHOULD provide interior location
data (building, floor, room, cubicle) where possible. It is
RECOMMENDED that interior location be provided when spaces exceed
approximately 650 square meters. See [RFC6443] Section 6.2.2 for
a discussion of how this value was determined.
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AN-7/INT-6: Access networks and intermediate devices (including
enterprise networks) that support intermediate range wireless
connections (typically 100 m or less of range) and that do not
support a more accurate location determination mechanism such as
triangulation MUST support location configuration where the
location of the access point is reflected as the location of the
clients of that access point.
AN-8/INT-7: Where the access network provides location
configuration, intermediate devices MUST either be transparent to
it or provide an interconnected client for the supported
configuration mechanism and a server for a configuration protocol
supported by end devices downstream of the intermediate device
such that the location provided by the access network is available
to clients as if the intermediate device was not in the path.
6.2.3. End System Measured Location Information
ED-16/INT-8: Devices MAY support end system measured location. See
[RFC6443] Section 6 for a discussion of accuracy of location.
ED-17/INT-9/AN-9: Devices that support endpoint measuring of
location MUST have at least a coarse location capability
(typically <1 km accuracy) for the routing of calls. The location
mechanism MAY be a service provided by the access network.
6.2.4. Network Measured Location Information
AN-10: Access networks MAY provide network measured location
determination. Wireless access networks that do not supply
network measured location MUST require every device or
intermediate device connected to the network to support end system
measured location. Uncertainty and confidence may be specified by
local regulation. Where not specified, uncertainty of less than
100 meters with 95% confidence is RECOMMENDED for dispatch
location.
AN-11: Access networks that provide network measured location MUST
have at least a coarse location (typically <1 km when not location
hiding) capability at all times for the routing of calls.
AN-12: Access networks with a range of <10 meters (e.g., personal
area networks such as Bluetooth) MUST provide a location to mobile
devices connected to them. The location provided SHOULD be that
reported by the upstream access network unless a more accurate
mechanism is available.
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6.3. Who Adds Location? The Endpoint, or the Proxy?
ED-18/INT-10: Endpoints SHOULD attempt to configure their own
location using the Location Configuration Protocols (LCPs) listed
in ED-21.
SP-12: Proxies MAY provide location on behalf of devices if:
o The proxy has a relationship with all access networks the device
could connect to, and the relationship allows it to obtain
location.
o The proxy has an identifier, such as an IP address, that can be
used by the access network to determine the location of the
endpoint, even in the presence of NAT and VPN tunnels that may
obscure the identifier between the access network and the service
provider.
ED-19/INT-11/SP-13: Where proxies provide location on behalf of
endpoints, the service provider MUST ensure that either the end
device is provided with the local dial strings for its current
location (where the end device recognizes dial strings) or the
service provider proxy MUST detect the appropriate local dial
strings at the time of the call.
6.4. Location and References to Location
ED-20/INT-12: Devices SHOULD be able to accept and forward location-
by-value or location-by-reference. An end device that receives
location-by-reference (and does not also get the corresponding
value) MUST be able to perform a dereference operation to obtain a
value.
6.5. End System Location Configuration
Obtaining location from the access network may be preferable even if
the device can measure its own location, especially indoors where
most measurement mechanisms are not accurate enough. The
requirements listed in this section do not apply to devices that can
accurately measure their own location.
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ED-21/INT-13: Devices MUST support both the Dynamic Host
Configuration Protocol (DHCP) location options [RFC4776] [RFC6225]
and HTTP-Enabled Location Delivery (HELD) [RFC5985]. When devices
deploy a specific access network interface for which location
configuration mechanisms such as Link Layer Discovery Protocol -
Media Endpoint Discovery (LLDP-MED) [LLDP-MED] or 802.11v are
specified, the device SHOULD support the additional respective
access network specific location configuration mechanism.
AN-13/INT-14: The access network MUST support either DHCP location
options or HELD. The access network SHOULD support other location
configuration technologies that are specific to the type of access
network.
AN-14/INT-15: Where a router is employed between a LAN and WAN in a
small (less than approximately 650 square meters) area, the router
MUST be transparent to the location provided by the WAN to the
LAN. This may mean the router must obtain location as a client
from the WAN and supply an LCP server to the LAN with the location
it obtains. Where the area is larger, the LAN MUST have a
location configuration mechanism satisfying the requirements of
this document.
ED-22/INT-16: Endpoints SHOULD try all LCPs supported by the device
in any order or in parallel. The first one that succeeds in
supplying location MUST be used.
AN-15/INT-17: Access networks that support more than one LCP MUST
reply with the same location information (within the limits of the
data format for the specific LCP) for all LCPs it supports.
ED-23/INT-18/SP-14: When HELD is the LCP, the request MUST specify a
value of "emergencyRouting" for the "responseTime" parameter and
use the resulting location for routing. If a value for dispatch
location will be sent, another request with the "responseTime"
parameter set to "emergencyDispatch" must be completed, with the
result sent for dispatch purposes.
ED-24: Where the operating system supporting application programs
that need location for emergency calls does not allow access to
Layer 2 and Layer 3 functions necessary for a client application
to use DHCP location options and/or other location technologies
that are specific to the type of access network, the operating
system MUST provide a published API conforming to ED-12 through
ED-23 and ED-25 through ED-32. It is RECOMMENDED that all
operating systems provide such an API.
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6.6. When Location Should Be Configured
If an endpoint is manually configured, the requirements in this
section are not applicable.
ED-25/INT-19: Endpoints SHOULD obtain location immediately after
obtaining local network configuration information.
ED-26/INT-20: If the device is configured to use DHCP for
bootstrapping and does not use its own measurement to determine
location, it MUST include both options for location acquisition
(civic and geodetic), the option for LIS discovery, and the option
for LoST discovery as defined in [RFC4776], [RFC6225], [RFC5986],
and [RFC5223], respectively.
ED-27/INT-21: If the device sends a DHCPINFORM message, it MUST
include both options for location acquisition (civic and
geodetic), the option for LIS discovery, and the option for LoST
discovery as defined in [RFC4776], [RFC6225], [RFC5986], and
[RFC5223], respectively.
ED-28/INT-22: To minimize the effects of VPNs that do not allow
packets to be sent via the native hardware interface rather than
via the VPN tunnel, location configuration SHOULD be attempted
before such tunnels are established.
ED-29/INT-23: Software that uses LCPs SHOULD locate and use the
actual hardware network interface rather than a VPN tunnel
interface to direct LCP requests to the LIS in the actual access
network.
AN-16: Network administrators MUST take care in assigning IP
addresses such that VPN address assignments can be distinguished
from local devices (by subnet choice, for example), and LISs
SHOULD NOT attempt to provide location to addresses that arrive
via VPN connections unless they can accurately determine the
location for such addresses.
AN-17: Placement of NAT devices where an LCP uses an IP address for
an identifier SHOULD consider the effect of the NAT on the LCP.
The address used to query the LIS MUST be able to correctly
identify the record in the LIS representing the location of the
querying device.
ED-30/INT-24: For devices that are not expected to change location,
refreshing location on the order of once per day is RECOMMENDED.
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ED-31/INT-25: For devices that roam, refresh of location information
SHOULD be more frequent, with the frequency related to the
mobility of the device and the ability of the access network to
support the refresh operation. If the device detects a link state
change that might indicate having moved, for example, when it
changes access points, the device SHOULD refresh its location.
ED-32/INT-26/AN-18: It is RECOMMENDED that location determination
not take longer than 250 ms to obtain routing location, and
systems SHOULD be designed such that the typical response time is
under 100 ms. However, as much as 3 seconds to obtain routing
location MAY be tolerated if location accuracy can be
substantially improved over what can be obtained in 250 ms.
6.7. Conveying Location
ED-33/SP-15: Location sent between SIP entities MUST be conveyed
using the extension described in [RFC6442].
6.8. Location Updates
ED-34/AN-19: Where the absolute location or the accuracy of location
of the endpoint may change between the time the call is received
at the PSAP and the time dispatch is completed, location update
mechanisms MUST be implemented and used.
ED-35/AN-20: Mobile devices MUST be provided with a mechanism to get
repeated location updates to track the motion of the device during
the complete processing of the call.
ED-36/AN-21: The LIS SHOULD provide a location reference that
permits a subscription with appropriate filtering.
ED-37/AN-22: For calls sent with location-by-reference, with a SIP
or Session Initiation Protocol Secure (SIPS) scheme, the server
resolving the reference MUST support a SUBSCRIBE [RFC6665] to the
presence event [RFC3856]. For other location-by-reference schemes
that do not support subscription, the PSAP will have to repeatedly
dereference the URI to determine if the device moved.
ED-38: If location was sent by value and the endpoint gets an
updated location, it MUST send the updated location to the PSAP
via a SIP re-INVITE or UPDATE request. Such updates SHOULD be
limited to no more than one update every 10 seconds, a value
selected to keep the load on a large PSAP manageable, and yet
provide sufficient indication to the PSAP of motion.
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6.9. Multiple Locations
ED-39/SP-16: If the LIS has more than one location for an endpoint,
it MUST conform to the rules in Section 3 of [RFC5491].
ED-40: If an endpoint has more than one location available to it, it
MUST choose one location to route the call towards the PSAP. If
multiple locations are in a single Presence Information Data
Format (PIDF), the procedures in [RFC5491] MUST be followed. If
the endpoint has multiple PIDFs and has no reasonable basis to
choose from among them, a random choice is acceptable.
SP-17: If a proxy inserts location on behalf of an endpoint and it
has multiple locations available for the endpoint, it MUST choose
one location to use to route the call towards the PSAP. If
multiple locations are in a single PIDF, the procedures in
[RFC5491] MUST be followed. If the proxy has multiple PIDFs and
has no reasonable basis to choose from among them, a random choice
is acceptable.
SP-18: If a proxy is attempting to insert location but the endpoint
conveyed a location to it, the proxy MUST use the endpoint's
location for routing in the initial INVITE and MUST convey that
location towards the PSAP. It MAY also include what it believes
the location to be in a separate Geolocation header.
SP-19: All location objects received by a proxy MUST be delivered to
the PSAP.
ED-41/SP-20: Location objects MUST be created with information about
the method by which the location was determined, such as GPS,
manually entered, or based on access network topology included in
a PIDF-LO "method" element. In addition, the source of the
location information MUST be included in a PIDF-LO "provided-by"
element.
ED-42/SP-21: A location with a method of "derived" MUST NOT be used
unless no other location is available.
6.10. Location Validation
AN-23: A LIS SHOULD perform location validation of civic locations
via LoST before entering a location in its database.
ED-43: Endpoints SHOULD validate civic locations when they receive
them from their LCP. Validation SHOULD be performed in
conjunction with the LoST route query to minimize load on the LoST
server.
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6.11. Default Location
AN-24: When the access network cannot determine the actual location
of the caller, it MUST supply a default location. The default
SHOULD be chosen to be as close to the probable location of the
device as the network can determine. See [RFC6443].
SP-22: Proxies handling emergency calls MUST insert a default
location in the INVITE if the incoming INVITE does not contain a
location and the proxy does not have a method for obtaining a
better location.
AN-25/SP-23: Default locations MUST be marked with method=Default,
and the proxy MUST be identified in a PIDF-LO "provided-by"
element.
6.12. Other Location Considerations
ED-44: If the LCP does not return location in the form of a PIDF-LO
[RFC4119], the endpoint MUST map the location information it
receives from the configuration protocol to a PIDF-LO.
ED-45/AN-26: To prevent against spoofing of the DHCP server,
entities implementing DHCP for location configuration SHOULD use
DHCPv4 message authentication [RFC3118] or DHCPv6 message
authentication [RFC3315], although the difficulty in providing
appropriate credentials is significant.
ED-46: If S/MIME [RFC5751] is used, the INVITE message MUST provide
enough information unencrypted for intermediate proxies to route
the call based on the location information included. This would
include the Geolocation header and any bodies containing location
information. Use of S/MIME with emergency calls is NOT
RECOMMENDED for this reason.
ED-47/SP-24: Transport Layer Security (TLS) [RFC5746] MUST be used
to protect location (but see Section 9.1). All SIP
implementations of this specification MUST support TLS.
7. LIS and LoST Discovery
ED-48: Endpoints MUST support one or more mechanisms that allow them
to determine their public IP address, for example, Session
Traversal Utilities for NAT (STUN) [RFC5389].
ED-49: Endpoints MUST support LIS discovery as described in
[RFC5986] and LoST discovery as described in [RFC5223].
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ED-50: The device MUST have a configurable default LoST server
parameter.
ED-51: DHCP LoST discovery MUST be used, if available, in preference
to configured LoST servers. That is, the endpoint MUST send
queries to this LoST server first, using other LoST servers only
if these queries fail.
AN-27: Access networks that support DHCP MUST implement the LIS and
LoST discovery options in their DHCP servers and return suitable
server addresses as appropriate.
8. Routing the Call to the PSAP
ED-52: Endpoints that obtain their own location SHOULD perform LoST
mapping to the PSAP URI.
ED-53: Mapping SHOULD be performed at boot time and whenever a
location changes beyond the service boundary obtained from a prior
LoST mapping operation, or when the time-to-live value of that
response has expired. The value MUST be cached for possible later
use.
ED-54: The endpoint MUST attempt to update its location at the time
of an emergency call. If it cannot obtain a new location quickly
(see Section 6), it MUST use the cached value.
ED-55: The endpoint SHOULD attempt to update the LoST mapping at the
time of an emergency call. If it cannot obtain a new mapping
quickly, it MUST use the cached value. If the device cannot
update the LoST mapping and does not have a cached value, it MUST
signal an emergency call without a Route header containing a PSAP
URI.
SP-25: Networks MUST be designed so that at least one proxy in the
outbound path will recognize emergency calls with a Request URI of
the service URN in the "sos" tree. An endpoint places a service
URN in the Request URI to indicate that the endpoint understood
the call was an emergency call. A proxy that processes such a
call looks for the presence of a SIP Route header field with a URI
of a PSAP. The absence of such a Route header indicates that the
endpoint was unable to invoke LoST, and the proxy MUST perform the
LoST mapping and insert a Route header field with the URI
obtained.
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SP-26: To deal with old user agents that predate this specification
and with endpoints that do not have access to their own location
data, a proxy that recognizes a call as an emergency call that is
not marked as such (see Section 5) MUST also perform this mapping,
with the best location it has available for the endpoint. The
resulting PSAP URI would be placed in a Route header with the
service URN in the Request URI.
SP-27: Proxy servers performing mapping SHOULD use location obtained
from the access network for the mapping. If no location is
available, a default location (see Section 6.11) MUST be supplied.
SP-28: A proxy server that attempts mapping and fails to get a
mapping MUST provide a default mapping. A suitable default
mapping would be the mapping obtained previously for the default
location appropriate for the caller.
ED-56/SP-29: [RFC3261] and [RFC3263] procedures MUST be used to
route an emergency call towards the PSAP's URI.
9. Signaling of Emergency Calls
9.1. Use of TLS
ED-57/SP-30: TLS is the primary mechanism used to secure the
signaling for emergency calls. IPsec [RFC4301] MAY be used
instead of TLS for any hop. Either TLS or IPsec MUST be used when
attempting to signal an emergency call.
ED-58/SP-31: If TLS session establishment is not available or fails,
the call MUST be retried without TLS.
ED-59/SP-32: Following the procedures described in [RFC5626] is
RECOMMENDED to maintain persistent TLS connections between
entities when one of the entities is an endpoint. Persistent TLS
connection between proxies is RECOMMENDED using any suitable
mechanism.
ED-60/AN-28: TLS SHOULD be used when attempting to retrieve location
(configuration or dereferencing) with HELD. The use of the
mechanism described in [RFC5077] is RECOMMENDED to minimize the
time to establish TLS sessions without keeping server-side state.
IPsec MAY be used instead of TLS.
ED-61/AN-29: When TLS session establishment fails, the location
retrieval MUST be retried without TLS.
Rosen & Polk Best Current Practice [Page 15]RFC 6881 Emergency Call Phone BCP March 2013
9.2. SIP Signaling Requirements for User Agents
ED-62: The initial SIP signaling method is an INVITE request:
1. The Request URI SHOULD be the service URN in the "sos" tree.
If the device does not interpret local dial strings, the
Request-URI MUST be a dial string URI [RFC4967] with the dialed
digits.
2. The To header field SHOULD be a service URN in the "sos" tree.
If the device does not interpret local dial strings, the To:
MUST be a dial string URI with the dialed digits.
3. The From header field SHOULD contain the address of record (AoR)
of the caller.
4. A Route header field SHOULD be present with a PSAP URI obtained
from LoST (see Section 8). If the device does not interpret
dial plans or was unable to obtain a route from a LoST server,
no such Route header field will be present.
5. A Contact header field MUST be globally routable, for example, a
Globally Routable User Agent URI (GRUU) [RFC5627], and be valid
for several minutes following the termination of the call,
provided that the User Agent Client (UAC) remains registered
with the same registrar, to permit an immediate callback to the
specific device that placed the emergency call. It is
acceptable if the UAC inserts a locally routable URI and a
subsequent back-to-back user agent (B2BUA) maps that to a
globally routable URI.
6. Other header fields MAY be included as per normal SIP behavior.
7. If a geolocation URI is included in the INVITE, a Supported
header field MUST be included with a 'geolocation-sip' or
'geolocation-http" option tag, as appropriate [RFC6442].
8. If a device understands the SIP location conveyance [RFC6442]
extension and has its location available, it MUST include
location as either location-by-value or location-by-reference,
or both, according to the rules within RFC 6442.
9. An SDP offer SHOULD be included in the INVITE. If voice is
supported, the offer SHOULD include the G.711 codec; see
Section 14. As PSAPs may support a wide range of media types
and codecs, sending an offerless INVITE may result in a lengthy
return offer but is permitted. Cautions in [RFC3261] on
offerless INVITEs should be considered before such use.
Rosen & Polk Best Current Practice [Page 16]RFC 6881 Emergency Call Phone BCP March 2013
10. If the device includes location-by-value, the user agent (UA)
MUST support multipart message bodies, since SDP will likely be
also in the INVITE.
9.3. SIP Signaling Requirements for Proxy Servers
SP-33: SIP proxy servers processing emergency calls:
1. If the proxy interprets dial plans on behalf of user agents, the
proxy MUST look for the local emergency dial string at the
location of the end device and MAY look for the home dial string.
If it finds it, the proxy MUST:
* Insert a Geolocation header field. Location-by-reference MUST
be used because proxies are not allowed to insert bodies.
* Insert the Geolocation-Routing header with appropriate
parameters.
* Map the location to a PSAP URI using LoST.
* Add a Route header with the PSAP URI.
* Replace the Request-URI, which was the dial string, with the
service URN appropriate for the emergency dial string.
* Route the call using normal SIP routing mechanisms.
2. If the proxy recognizes the service URN in the Request URI and
does not find a Route header, it MUST query a LoST server
immediately. If a location was provided (which should be the
case), the proxy uses that location to query LoST. The proxy may
have to dereference a location-by-reference to get a value. If a
location is not present and the proxy can query a LIS that has
the location of the UA, it MUST do so. If no location is present
and the proxy does not have access to a LIS that could provide
location, the proxy MUST supply a default location (see
Section 6.11). The location (in the signaling, obtained from a
LIS, or default) MUST be used in a query to LoST with the service
URN received with the call. The resulting URI MUST be placed in
a Route header added to the call.
3. The proxy MAY add a Geolocation header field. Such an additional
location SHOULD NOT be used for routing; the location provided by
the UA should be used.
Rosen & Polk Best Current Practice [Page 17]RFC 6881 Emergency Call Phone BCP March 2013
4. Either a P-Asserted-Identity [RFC3325] or an Identity header
field [RFC4474], or both, SHOULD be included to identify the
sender. For services that must support emergency calls from
unauthenticated devices, valid identity may not be available.
Proxies encountering a P-Asserted-Identity will need to pass the
header to the PSAP, which is in a different domain. [RFC3325]
requires a "spec(T)" to determine what happens if either the "id"
privacy service or a Privacy header is present and requests
privacy. In the absence of another spec(T), such proxies should
pass the header unmodified if and only if the connection between
the proxy and the PSAP is, as far as the proxy can determine,
protected by TLS with mutual authentication using keys reliably
known by the parties, encrypted with no less strength than AES,
and the local regulations governing the PSAP do not specify
otherwise.
5. Proxies SHOULD NOT return a 424 error. They should process the
INVITE as best they can.
6. Proxies SHOULD NOT obey a Geolocation-Routing value of "no" or a
missing value if they must query LoST to obtain a route.
Emergency calls are always routed by location.
10. Callbacks
ED-63/SP-34: Devices SHOULD have a globally routable URI in a
Contact header field that remains valid for several minutes past
the time the original call containing the URI completes, unless
the device registration expires and is not renewed.
SP-35: Callbacks to the Contact header URI received within
30 minutes of an emergency call must reach the device regardless
of call features (e.g., do not disturb) or services (e.g., call
forwarding) that would normally cause the call to be routed to
some other entity.
SP-36: Devices MUST have a persistent AoR URI either in a
P-Asserted-Identity header field or From protected by an Identity
header field suitable for returning a call sometime after the
original call. Such a callback would not necessarily reach the
device that originally placed the call.
Rosen & Polk Best Current Practice [Page 18]RFC 6881 Emergency Call Phone BCP March 2013
11. Mid-Call Behavior
ED-64/SP-37: During the course of an emergency call, PSAPs and
responders may need to transfer the call to some other entity.
The request for such a transfer is signaled by a REFER request
within the dialog with method=INVITE and a Refer-To header field
[RFC3515]. Devices MUST react to such a transfer request with the
appropriate INVITE.
12. Call Termination
ED-65: Normal [RFC3261] procedures for termination MUST be used for
termination of the call.
13. Disabling of Features
ED-66/SP-38: User agents and proxies MUST disable features that will
interrupt an ongoing emergency call, such as:
o Call waiting
o Call transfer
o Three-way call
o Hold
o Outbound call blocking
when an emergency call is established, but see ED-65 with respect to
call waiting. Also see ED-73 in Section 14.
ED-67/SP-39: The emergency dial strings SHOULD NOT be permitted in
call forward numbers or speed dial lists.
ED-68/SP-40: The user agent and proxies MUST disable call features
that would interfere with the ability of callbacks from the PSAP
to be completed, such as:
o Do not disturb
o Call forward (all kinds)
These features SHOULD be disabled for approximately 30 minutes
following termination of an emergency call.
Rosen & Polk Best Current Practice [Page 19]RFC 6881 Emergency Call Phone BCP March 2013
ED-69: Callbacks SHOULD be determined by retaining the domain of the
PSAP that answers an outgoing emergency call and instantiating a
timer that starts when the call is terminated. If a call is
received from the same domain and within the timer period, and it
is sent to the URI in a Contact header or the AoR used in the
emergency call, then it should be assumed to be a callback. The
suggested timer period is 5 minutes. The mechanism described in
[RFC4916] can be used by the PSAP to inform the endpoint of the
PSAP's domain. Recognizing a callback from the domain of the PSAP
will not always work, and further standardization will be required
to give the endpoint the ability to recognize a callback.
14. Media
ED-70: Endpoints MUST send and receive media streams on RTP
[RFC3550].
ED-71: Normal SIP offer/answer [RFC3264] negotiations MUST be used
to agree on the media streams to be used.
ED-72/SP-41: G.711 A-law (and mu-law if they are intended to be used
in North America) encoded voice as described in [RFC3551] MUST be
supported. If the endpoint cannot support G.711, a transcoder
MUST be used so that the offer received at the PSAP contains
G.711. It is desirable to include wideband codecs such as G.722
and Adaptive Multi-Rate - WideBand (AMR-WB) in the offer. PSAPs
SHOULD support narrowband codecs common on endpoints in their area
to avoid transcoding.
ED-73: Silence suppression (Voice Activity Detection methods) MUST
NOT be used on emergency calls. PSAP call takers sometimes get
information on what is happening in the background to determine
how to process the call.
ED-74: Endpoints supporting Instant Messaging (IM) MUST support
either [RFC3428] or [RFC4975].
ED-75: Endpoints supporting real-time text MUST comply with
[RFC4103]. The expectations for emergency service support for the
real-time text medium are described in [RFC5194] Section 7.1.
ED-76: Endpoints supporting video MUST support H.264 per [RFC6184].
Rosen & Polk Best Current Practice [Page 20]RFC 6881 Emergency Call Phone BCP March 2013
15. Testing
ED-77: INVITE requests to a service URN starting with "test."
indicate a request for an automated test, for example,
"urn:service:test.sos.fire". As in standard SIP, a 200 (OK)
response indicates that the address was recognized and a 404 (not
found) that it was not. A 486 (busy here) MUST be returned if the
test service is busy, and a 404 (not found) MUST be returned if
the PSAP does not support the test mechanism.
ED-78: In its response to the test, the PSAP MAY include a text body
(text/plain) indicating the identity of the PSAP, the requested
service, and the location reported with the call. For the latter,
the PSAP SHOULD return location-by-value even if the original
location delivered with the test was location-by-reference. If
the location-by-reference was supplied and the dereference
requires credentials, the PSAP SHOULD use credentials supplied by
the LIS for test purposes. This alerts the LIS that the
dereference is not for an actual emergency call, and therefore
location-hiding techniques, if they are being used, may be
employed for this dereference. Use of SIPS for the request would
assure that the response containing the location is kept private.
ED-79: A PSAP accepting a test call SHOULD accept a media loopback
[RFC6849] test and SHOULD support the "rtp-pkt-loopback" and
"rtp-media-loopback" options. The user agent would specify a
loopback attribute of "loopback-source", the PSAP being the
mirror. User agents should expect the PSAP to loop back no more
than 3 packets of each media type accepted (which limits the
duration of the test), after which the PSAP would normally send
BYE.
ED-80: User agents SHOULD perform a full call test, including media
loopback, after a disconnect and subsequent change in IP address
not due to a reboot. After an initial test, a full test SHOULD be
repeated approximately every 30 days with a random interval.
ED-81: User agents MUST NOT place a test call immediately after
booting. If the IP address changes after booting, the endpoint
should wait a random amount of time (in perhaps a 30-minute
period, sufficient for any avalanche-restart event to complete)
and then test.
ED-82: PSAPs MAY refuse repeated requests for test from the same
device in a short period of time. Any refusal is signaled with a
486 (busy here) or 488 (not acceptable here) response.
Rosen & Polk Best Current Practice [Page 21]RFC 6881 Emergency Call Phone BCP March 2013
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, etc. It also suggests that if the attempt to
create a security association fails the call be retried without the
security. It's 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. IANA Considerations
This document registers service URNs in the Service URN Labels
registry per [RFC5031] for testing.
17.1. Test Service URN
A new entry in the URN Service Label registry has been added. The
new service is "test", the reference is this document, and the
description is "self-test".
Rosen & Polk Best Current Practice [Page 22]
RFC 6881 Emergency Call Phone BCP March 2013
17.2. 'test' Subregistry
A new subregistry has been created: 'test' Sub-Services. The
registration process is Expert Review per [RFC5226]. The expert
review should consider that the entries in this registry nominally
track the entries in the 'sos' subregistry, although it is not
required that every entry in 'sos' have an entry in 'test', and it is
possible that entries in the 'test' subregistry may not necessarily
be in the 'sos' subregistry. For example, testing of non-emergency
URNs may be allowed. The reference is this document. The initial
content of the subregistry is:
Service Reference Description
------------------------------------------------------------------
test.sos RFC 6881 test for sos
test.sos.ambulance RFC 6881 test for sos.ambulance
test.sos.animal-control RFC 6881 test for sos.animal-control
test.sos.fire RFC 6881 test for sos.fire
test.sos.gas RFC 6881 test for sos.gas
test.sos.marine RFC 6881 test for sos.marine
test.sos.mountain RFC 6881 test for sos.mountain
test.sos.physician RFC 6881 test for sos.physician
test.sos.poison RFC 6881 test for sos.poison
test.sos.police RFC 6881 test for sos.police
18. Acknowledgements
Working group members participating in the creation and review of
this document include Hannes Tschofenig, Ted Hardie, Marc Linsner,
Roger Marshall, Stu Goldman, Shida Schubert, James Winterbottom,
Barbara Stark, Richard Barnes, and Peter Blatherwick.
19. References
19.1. Normative References
[LLDP-MED] ANSI/TIA, "Link Layer Discovery Protocol - Media Endpoint
Discovery", TIA Standard, TIA-1057, April 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP
Messages", RFC 3118, June 2001.
Rosen & Polk Best Current Practice [Page 23]RFC 6881 Emergency Call Phone BCP March 2013
[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.
[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263,
June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[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.
[RFC3856] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856, August 2004.
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers",
RFC 3966, December 2004.
[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.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
Rosen & Polk Best Current Practice [Page 24]RFC 6881 Emergency Call Phone BCP March 2013
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC4776] Schulzrinne, H., "Dynamic Host Configuration Protocol
(DHCPv4 and DHCPv6) Option for Civic Addresses
Configuration Information", RFC 4776, November 2006.
[RFC4916] Elwell, J., "Connected Identity in the Session Initiation
Protocol (SIP)", RFC 4916, June 2007.
[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.
[RFC5031] Schulzrinne, H., "A Uniform Resource Name (URN) for
Emergency and Other Well-Known Services", RFC 5031,
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.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
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.
Rosen & Polk Best Current Practice [Page 25]RFC 6881 Emergency Call Phone BCP March 2013
[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.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, February 2010.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 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.
[RFC6184] Wang, Y., Even, R., Kristensen, T., and R. Jesup, "RTP
Payload Format for H.264 Video", RFC 6184, May 2011.
[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.
[RFC6442] Polk, J., Rosen, B., and J. Peterson, "Location
Conveyance for the Session Initiation Protocol",
RFC 6442, December 2011.
[RFC6665] Roach, A., "SIP-Specific Event Notification", RFC 6665,
July 2012.
[RFC6849] Kaplan, H., Ed., Hedayat, K., Venna, N., Jones, P., and
N. Stratton, "An Extension to the Session Description
Protocol (SDP) and Real-time Transport Protocol (RTP) for
Media Loopback", RFC 6849, February 2013.
Rosen & Polk Best Current Practice [Page 26]RFC 6881 Emergency Call Phone BCP March 2013
19.2. 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.
[RFC5012] Schulzrinne, H. and R. Marshall, "Requirements for
Emergency Context Resolution with Internet Technologies",
RFC 5012, 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.
[RFC5194] van Wijk, A. and G. Gybels, "Framework for Real-Time Text
over IP Using the Session Initiation Protocol (SIP)",
RFC 5194, June 2008.
[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.
[RFC6443] Rosen, B., Schulzrinne, H., Polk, J., and A. Newton,
"Framework for Emergency Calling Using Internet
Multimedia", RFC 6443, December 2011.
Rosen & Polk Best Current Practice [Page 27]RFC 6881 Emergency Call Phone BCP March 2013
Authors' Addresses
Brian Rosen
NeuStar
470 Conrad Dr.
Mars, PA 16046
USA
Phone: +1 724 382 1051
EMail: br@brianrosen.net
James Polk
Cisco Systems
3913 Treemont Circle
Colleyville, TX 76034
USA
Phone: +1-817-271-3552
EMail: jmpolk@cisco.com
Rosen & Polk Best Current Practice [Page 28]
