ecrit B. Rosen
Internet-Draft NeuStar
Intended status: BCP J. Polk
Expires: January 14, 2011 Cisco Systems
July 13, 2010
Best Current Practice for Communications Services in support of
Emergency Calling
draft-ietf-ecrit-phonebcp-15
Abstract
The IETF and other standards organization 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 should use such standards to make emergency
calls.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 14, 2011.
Copyright Notice
Copyright (c) 2010 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . 5
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 . . . . . . . 6
6.2.4. Network-measured location information . . . . . . . . 7
6.3. Who adds location, endpoint or proxy . . . . . . . . . . . 7
6.4. Location and references to location . . . . . . . . . . . 8
6.5. End system location configuration . . . . . . . . . . . . 8
6.6. When location should be configured . . . . . . . . . . . . 9
6.7. Conveying location in SIP . . . . . . . . . . . . . . . . 10
6.8. Location updates . . . . . . . . . . . . . . . . . . . . . 10
6.9. Multiple locations . . . . . . . . . . . . . . . . . . . . 11
6.10. Location validation . . . . . . . . . . . . . . . . . . . 11
6.11. Default location . . . . . . . . . . . . . . . . . . . . . 12
6.12. Other location considerations . . . . . . . . . . . . . . 12
7. LIS and LoST Discovery . . . . . . . . . . . . . . . . . . . . 12
8. Routing the call to the PSAP . . . . . . . . . . . . . . . . . 13
9. Signaling of emergency calls . . . . . . . . . . . . . . . . . 14
9.1. Use of TLS . . . . . . . . . . . . . . . . . . . . . . . . 14
9.2. SIP signaling requirements for User Agents . . . . . . . . 14
9.3. SIP signaling requirements for proxy servers . . . . . . . 16
10. Call backs . . . . . . . . . . . . . . . . . . . . . . . . . . 16
11. Mid-call behavior . . . . . . . . . . . . . . . . . . . . . . 17
12. Call termination . . . . . . . . . . . . . . . . . . . . . . . 17
13. Disabling of features . . . . . . . . . . . . . . . . . . . . 17
14. Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
15. Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
16. Security Considerations . . . . . . . . . . . . . . . . . . . 20
17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
18. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
19. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
19.1. Normative References . . . . . . . . . . . . . . . . . . . 20
19.2. Informative References . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
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1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document uses terms from [RFC3261], [RFC5012] and
[I-D.ietf-ecrit-framework].
2. Introduction
This document describes how access networks, Session Initiation
Protocol [RFC3261] user agents, proxy servers and PSAPs support
emergency calling, as outlined in [I-D.ietf-ecrit-framework], which
is designed to complement the present document in section headings,
numbering and content. This 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
which 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-".
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], which is placed in the call
set-up 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) (Section 6.2.3) device location in 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 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), which serves a group of PSAPs. The
call arrives at the PSAP with the location included in the SIP INVITE
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request.
4. Which devices and services should support emergency calls
ED-1 A device or application SHOULD support emergency calling if a
user could reasonably expect to be able to place a call for help with
the device. Some jurisdictions have regulations governing this.
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 audio, video
and/or text, and 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, then
the service provider could recognize them.
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 Local dial strings MUST be recognized.
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 in the device.
AN-1 LoST servers MUST return dial strings for emergency services
ED-8 Endpoints which 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
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[RFC4967] and SHOULD recognize 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 elements are involved. In Internet
emergency calling, where the endpoint is located is "determined"
using a variety of measurement or wiretracing 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. Likewise, we employ Location
Configuration Protocols, 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.
6.1. Types of location information
There are several forms of location. In IETF location configuration
and location conveyance protocols, civic and geospatial (geo) forms
are both supported. The civic forms include both postal and
jurisdictional fields. A cell tower/sector can be represented as a
point (geo or civic) or polygon. Other forms of location
representation must be mapped 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 Elements MUST NOT convert (civic to geo or geo
to civic) from the form of location the determination mechanism
supplied.
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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. Where a civic form of location is provided, all
fields in the PIDF-LO [RFC4119] and [RFC5139] MUST be able to be
specified.
6.2.2. Access network "wire database" location information
AN-5 Access networks supporting copper, fiber or other hard wired IP
packet service SHOULD support location configuration. If the network
does not support location configuration, it MUST require every 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.
AN-7/INT-6 Access networks and intermediate devices (including
enterprise networks) which support intermediate range wireless
connections (typically 100m or less of range) and which 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.
Uncertainty of less than 100 m with 95% confidence SHOULD be
available for dispatch.
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ED-17/INT-9/AN-9 Devices that support endpoint measuring of location
MUST have at least a coarse location capability (typically <1km
accuracy when not location hiding) for 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 that all devices connected to the
network have end-system measured location. Uncertainty and
confidence may be specified by local regulation. Where not
specified, uncertainty of less than 100 m 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 <1km when not location
hiding) capability at all times for routing of calls.
AN-12 Access networks with 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 of the
access point location unless a more accurate mechanism is provided.
6.3. Who adds location, endpoint or proxy
ED-18/INT-10 Endpoints SHOULD attempt to configure their own location
using the 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.
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6.4. Location and references to location
ED-20/INT-12 Devices SHOULD be able to accept and forward location by
value or 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
ED-21/INT-13 Devices MUST support both the DHCP location options
[RFC4776], [RFC3825] and HELD
[I-D.ietf-geopriv-http-location-delivery]. When devices deploy a
specific access network interface for which location configuration
mechanisms such as 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. If the access network supports more than one location
configuration protocol, all such protocols MUST return the same
location, within the constraints of the protocols deployed.
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
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which 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.
6.6. When location should be configured
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, 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], [RFC3825],
[I-D.ietf-geopriv-lis-discovery] and [RFC5223].
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], [RFC3825], [I-D.ietf-geopriv-lis-discovery] and
[RFC5223].
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 which 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 it can accurately determine the location for such addresses.
AN-17 Placement of NAT devices where an LCP uses 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 which 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 which 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 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 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 in SIP
ED-33/SP-15 Location sent between SIP elements MUST be conveyed using
[I-D.ietf-sip-location-conveyance].
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 which permits
a subscription with appropriate filtering.
ED-37/AN-22 For calls sent with location-by-reference, with a SIP or
SIPS scheme, the server resolving the reference MUST support a
SUBSCRIBE [RFC3265] 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 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 a UA 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 PIDF, the procedures in [RFC5491] MUST be
followed. If the UA 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 UA conveyed
a location to it, the proxy MUST use the UA's location for routing
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.
ED-43/SP-22 When there are multiple Geolocation header field values
provided, the "used-for-routing" parameter MUST be set to the
location that was chosen for routing.
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-44 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 [I-D.ietf-ecrit-framework]
SP-23 Proxies handling emergency calls MUST insert a default location
in the INVITE if the call does not contain a location and the proxy
does not have a method for obtaining a better location.
AN-25/SP-24 Default locations MUST be marked with method=Default and
the proxy MUST be identified in provided-by element of the PIDF-LO.
6.12. Other location considerations
ED-45 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-46/AN-26 To prevent against spoofing of the DHCP server, elements
implementing DHCP for location configuration SHOULD use [RFC3118]
although the difficulty in providing appropriate credentials is
significant.
ED-47 If S/MIME 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.
ED-48/SP-25 Either TLS or IPSEC [RFC3986] MUST be used to protect
location (but see Section 9.1).
7. LIS and LoST Discovery
ED-49 Endpoints MUST support one or more mechanisms that allow them
to determine their public IP address, for example, STUN [RFC5389].
ED-50 Endpoints MUST support LIS discovery as described in
[I-D.ietf-geopriv-lis-discovery], and the LoST discovery as described
in [RFC5223].
ED-51 The device MUST have a configurable default LoST server
parameter. If the device is provided by or managed by a service
provider, it is expected that the service provider will configure
this option.
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ED-52 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 which 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-53 Endpoints who obtain their own location SHOULD perform LoST
mapping to the PSAP URI.
ED-54 Mapping SHOULD be performed at boot time and whenever location
changes beyond the service boundary obtained from a prior LoST
mapping operation or the time-to-live value of that response has
expired. The value MUST be cached for possible later use.
ED-55 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-56 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-26 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.
Absence of such a Route header indicates the UAC was unable to invoke
LoST and the proxy MUST perform the LoST mapping and insert a Route
header field with the URI obtained.
SP-27 To deal with old user agents that predate this specification
and with UAs 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.
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SP-28 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-29 A proxy server which 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-57/SP-30 [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-58/SP-31 Either TLS or IPsec MUST be used when attempting to
signal an emergency call.
ED-59/SP-32 If TLS session establishment is not available, or fails,
the call MUST be retried without TLS.
ED-60/SP-33 [RFC5626] is RECOMMENDED to maintain persistent TLS
connections between elements when one of the element is an endpoint.
Persistent TLS connection between proxies is RECOMMENDED using any
suitable mechanism.
ED-61/AN-28 TLS MUST be used when attempting to retrieve location
(configuration or dereferencing) with HELD. The use of [RFC5077] is
RECOMMENDED to minimize the time to establish TLS sessions without
keeping server-side state.
ED-62/AN-29 When TLS session establishment fails, the location
retrieval MUST be retried without TLS.
9.2. SIP signaling requirements for User Agents
ED-63 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 cannot interpret local dial strings, the Request-URI
SHOULD 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 cannot interpret local dial strings, the To:
SHOULD be a dial string URI with the dialed digits.
3. The From header field SHOULD contain the AoR of the caller.
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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
GRUU [RFC5627], and be valid for several minutes following the
termination of the call, provided that the UAC remains
registered with the same registrar, to permit an immediate call-
back to the specific device which placed the emergency call. It
is acceptable if the UAC inserts a locally routable URI and a
subsequent B2BUA maps that to a globally routable URI.
6. Other header fields MAY be included as per normal SIP behavior.
7. A Supported header field MUST be included with the 'geolocation'
option tag [I-D.ietf-sip-location-conveyance], unless the device
does not understand the concept of SIP location.
8. If a device understands the SIP location conveyance
[I-D.ietf-sip-location-conveyance] extension and has its
location available, it MUST include location either by-value,
by-reference or both.
9. If a device understands the SIP Location Conveyance extension
and has its location unavailable or unknown to that device, it
MUST include a Supported header field with a "geolocation"
option tag, and MUST NOT include a Geolocation header field, and
not include a PIDF-LO message body.
10. If a device understands the SIP Location Conveyance extension
and supports LoST [RFC5222], the Geolocation "used-for-routing"
header parameter MUST be added to the corresponding URI in the
Geolocation header field. If the device is unable to obtain a
PSAP URI for any reason it MUST NOT include "used-for-routing"
on a Geolocation URI, so that downstream entities know that LoST
routing has not been completed.
11. A SDP offer SHOULD be included in the INVITE. If voice is
supported the offer MUST 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.
12. If the device includes location-by-value, the UA MUST support
multipart message bodies, since SDP will likely be also in the
INVITE.
13. A UAC SHOULD include a "inserted-by" header parameter with its
own hostname on all Geolocation header fields. This informs
downstream elements which device entered the location at this
URI (either cid-URL or location-by-reference URI).
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9.3. SIP signaling requirements for proxy servers
SP-34 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 must not insert bodies.
* Include the Geolocation "inserted-by" and "used-for-routing"
parameters with its own hostname (which should match the Via
it inserts) on the inserted-by.
* 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 a Route header, it MUST query a LoST server. If
multiple locations were provided, the proxy uses the location
that has the "used-for-routing" marker set. 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 which 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 which 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 "inserted-by" parameter in any Geolocation: header received
on the call MUST NOT be modified or deleted in transit.
4. The proxy SHOULD NOT modify any parameters in Geolocation header
fields received in the call. It 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.
5. Either a P-Asserted-Identity [RFC3325] or an Identity header
field [RFC4474], or both, SHOULD be included to identify the
sender. For services which must support emergency calls from
unauthenticated devices, valid identity may not be available.
10. Call backs
ED-64/SP-35 Devices device SHOULD have a globally routable URI in a
Contact: header field which remains valid for several minutes past
the time the original call containing the URI completes unless the
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device registration expires and is not renewed.
SP-36 Call backs to the Contact: header URI received within 30
minutes of an emergency call must reach the device regardless of call
features or services that would normally cause the call to be routed
to some other entity.
SP-37 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 some time after the original call.
Such a call back would not necessarily reach the device that
originally placed the call.
11. Mid-call behavior
ED-65/SP-38 During the course of an emergency call, devices and
proxies MUST complete a call transfer upon receipt of REFER request
within the dialog with method=INVITE and the Referred-by: header
field [RFC3515] in that request.
12. Call termination
ED-66 There can be a case where the session signaling path is lost,
and the user agent does not receive the BYE. If the call is hung up,
and the session timer (if implemented) expires, the call MAY be
declared lost. If in the interval, an incoming call is received from
the domain of the PSAP, the device MUST drop the old call and alert
for the (new) incoming call. Dropping of the old call MUST only
occur if the user is attempting to hang up; the domain of an incoming
call can only be determined from the From header, which is not
reliable, and could be spoofed. Dropping an active call by a new
call with a spoofed From header field would be a DoS attack.
13. Disabling of features
ED-67/SP-39 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. Also see ED-74 in Section 14.
ED-68/SP-40 The emergency dial strings SHOULD NOT be permitted in
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Call Forward numbers or speed dial lists.
ED-69/SP-41 The User Agent and Proxies MUST disable call features
which would interfere with the ability of call backs from the PSAP to
be completed such as:
o Do Not Disturb
o Call Forward (all kinds)
ED-70 Call backs SHOULD be determined by retaining the domain of the
PSAP which answers an outgoing emergency call and instantiating a
timer which starts when the call is terminated. If a call is
received from the same domain and within the timer period, sent to
the Contact: or AoR used in the emergency call, it should be assumed
to be a call back. The suggested timer period is 5 minutes.
[RFC4916] may be used by the PSAP to inform the UA of the domain of
the PSAP. Recognizing a call back from the domain of the PSAP will
not always work, and further standardization will be required to give
the UA the ability to recognize a call back.
14. Media
ED-71 Endpoints MUST send and receive media streams on RTP [RFC3550].
ED-72 Normal SIP offer/answer [RFC3264] negotiations MUST be used to
agree on the media streams to be used.
ED-73 Endpoints supporting voice MUST support G.711 A law (and mu Law
if they are intended be used in North America) encoded voice as
described in [RFC3551]. It is desirable to include wideband codecs
such as AMR-WB in the offer.
ED-74 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-75 Endpoints supporting Instant Messaging (IM) MUST support both
[RFC3428] and [RFC4975].
ED-76 Endpoints supporting real-time text MUST use [RFC4103]. The
expectations for emergency service support for the real-time text
medium are described in [RFC5194], Section 7.1.
ED-77 Endpoints supporting video MUST support H.264 per
[I-D.ietf-avt-rtp-rfc3984bis].
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15. Testing
ED-78 INVITE requests to a service URN ending in ".test" indicates a
request for an automated test. For example,
"urn:service.sos.fire.test". 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-79 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 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 location hiding techniques, if they are
being used, may be employed for this dereference. Use of SIPS for
the request would assure the response containing the location is kept
private
ED-80 A PSAP accepting a test call SHOULD accept a media loopback
test [I-D.ietf-mmusic-media-loopback] and SHOULD support the "rtp-
pkt-loopback" and "rtp-start-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-81 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-82 User agents MUST NOT place a test call immediately after
booting. If the IP address changes after booting, the UA should wait
a random amount of time (in perhaps a 30 minute period, sufficient
for any avalanche restart to complete) and then test.
ED-83 PSAPs MAY refuse repeated requests for test from the same
device in a short period of time. Any refusal is signaled with a 486
or 488 response.
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16. Security Considerations
Security considerations for emergency calling have been documented in
[RFC5069], and [I-D.ietf-geopriv-arch].
17. IANA Considerations
This document has no actions for IANA.
18. Acknowledgements
Work group members participating in the creation and review of this
document include 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
[I-D.ietf-avt-rtp-rfc3984bis]
Wang, Y., Even, R., Kristensen, T., and R. Jesup, "RTP
Payload Format for H.264 Video",
draft-ietf-avt-rtp-rfc3984bis-11 (work in progress),
June 2010.
[I-D.ietf-geopriv-http-location-delivery]
Barnes, M., Winterbottom, J., Thomson, M., and B. Stark,
"HTTP Enabled Location Delivery (HELD)",
draft-ietf-geopriv-http-location-delivery-16 (work in
progress), August 2009.
[I-D.ietf-geopriv-lis-discovery]
Thomson, M. and J. Winterbottom, "Discovering the Local
Location Information Server (LIS)",
draft-ietf-geopriv-lis-discovery-15 (work in progress),
March 2010.
[I-D.ietf-mmusic-media-loopback]
Sivachelvan, C., Venna, N., Jones, P., Stratton, N.,
Roychowdhury, A., and K. Hedayat, "An Extension to the
Session Description Protocol (SDP) for Media Loopback",
draft-ietf-mmusic-media-loopback-13 (work in progress),
April 2010.
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[I-D.ietf-sip-location-conveyance]
Polk, J. and B. Rosen, "Location Conveyance for the
Session Initiation Protocol",
draft-ietf-sip-location-conveyance-13 (work in progress),
March 2009.
[LLDP-MED]
TIA, "ANSI/TIA-1057 Link Layer Discovery Protocol - Media
Endpoint Discovery".
[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.
[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.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 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.
[RFC3825] Polk, J., Schnizlein, J., and M. Linsner, "Dynamic Host
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Configuration Protocol Option for Coordinate-based
Location Configuration Information", RFC 3825, July 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.
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers",
RFC 3966, December 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.
[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.
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[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.
[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.
[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.
19.2. Informative References
[I-D.ietf-ecrit-framework]
Rosen, B., Schulzrinne, H., Polk, J., and A. Newton,
"Framework for Emergency Calling using Internet
Multimedia", draft-ietf-ecrit-framework-10 (work in
progress), July 2009.
[I-D.ietf-geopriv-arch]
Barnes, R., Lepinski, M., Cooper, A., Morris, J.,
Tschofenig, H., and H. Schulzrinne, "An Architecture for
Location and Location Privacy in Internet Applications",
draft-ietf-geopriv-arch-02 (work in progress), May 2010.
[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.
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[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.
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
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