Internet Engineering Task Force (IETF) J. Winterbottom
Request for Comments: 7840 Winterb Consulting Services
Updates: 5985, 6881 H. Tschofenig
Category: Standards Track
ISSN: 2070-1721 L. Liess
May 2016 A Routing Request Extension for
the HTTP-Enabled Location Delivery (HELD) Protocol
For cases where location servers have access to emergency routing
information, they are able to return routing information with the
location information if the location request includes a request for
the desired routing information. This document specifies an
extension to the HTTP-Enabled Location Delivery (HELD) protocol that
updates RFC 5985 to support this function. Allowing location and
routing information to be acquired in a single request response
exchange updates RFC 6881, as current location acquisition and route
determination procedures are separate operations.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
The general Emergency Context Resolution with Internet Technology
(ECRIT) calling models described in [RFC6443] and [RFC6881] require a
local Location-to-Service Translation (LoST) server or network of
forest guides in order to determine the address of the Public Safety
Answering Point (PSAP) in the best position to handle a call.
Networks of forest guides have not materialized and while PSAPs are
moving towards IP networks, LoST server deployment is not ubiquitous.
Some regions and countries have expressed reluctance to deploy LoST
servers making aspects of the current ECRIT architecture hard to
To address regulatory requirements, such as [M493], evolving
architectures in Europe couple location and routing information in
the access network while using a softswitch-centric approach to
emergency call processing. This document describes an extension to
the HELD protocol [RFC5985], so that a location information server
can provide emergency routing information in the absence of a LoST
server or network of forest guides.
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].
The terms "Location Information Server (LIS)", "Emergency Services
Routing Proxy (ESRP)", "Voice Service Provider (VSP)", and "Public
Safety Answering Point (PSAP)" are used as defined in [RFC6443].
The term "Access Network Provider" is used as defined in [RFC5687]
and encompasses both the Internet Access Provider (IAP) and Internet
Service Provider (ISP).
The term "forest guide" is used as defined in [RFC5582].
The Internet emergency calling architecture specified in [RFC6881]
describes two main models for emergency call processing. The first
is a device-centric model, where a device obtains location
information using a location configuration protocol, such as HELD
[RFC5985], and then proceeds to determine the address of the next hop
closer to the local PSAP using LoST [RFC5222]. Figure 1 shows this
model in a simplified form.
In the softswitch-centric model, when a VSP receives an emergency
call, it performs two tasks. The first task is to determine the
correct LIS to ask for location information; this is done using a
combination of reverse DNS lookup described in [RFC7216] to acquire
the serving domain name and then using [RFC5986] to determine the LIS
URI. Once the location is obtained from the LIS, the VSP determines
the LoST server associated with the domain serving the caller and
queries it for the correct PSAP address.
LoST server discovery is a domain-based activity, similar to the LIS
discovery technique. However, unlike the LIS that is a domain-bound
service, a LoST server is a geographically bound service. This means
that for a domain that spans multiple geographic regions, the LoST
server determined may not be able to provide a route to the necessary
PSAP. When this occurs, the contacted LoST server invokes the help
of other LoST servers, and this requires the deployment of forest
At the time of writing, several countries have expressed a reluctance
to deploy public LoST servers. In countries amenable to the use of
LoST and forest guides, no public forest guides have been deployed.
There appears to be little interest from the public sector in
establishing a global forest-guide network. These issues pose
threats to the ability of both the device-centric and the softswitch-
centric calling approaches to operate everywhere.
The device-centric and softswitch-centric calling models both involve
the notion of a LIS bound to the serving access network. In many
cases, the LIS already knows the destination PSAP URI for any given
location. In [RFC6881], for example, the LIS validates civic
locations using a location validation procedure based on the LoST
protocol [RFC5222]. The LoST validation request is similar to a LoST
routing request and provides the LIS with the same PSAP routing
information that a routing request would. In other cases, the LIS
knows the correct PSAP for a given location at provisioning time, or
the access network might always route to the same emergency provider.
Irrespective of the way in which the LIS learns the PSAP URI for a
location, the LIS will, in a great many cases, already have this
This document specifies an extension to the HELD protocol, so that
emergency routing information can be requested from the LIS at the
same time that location information is requested. This document
updates [RFC6881] by requiring devices and softswitches that
understand this specification to always request routing information
to avoid the risk of query failure where no LoST server or forest-
guide network is deployed.
3.1. LoST Reuse Considerations
The LoST protocol [RFC5222] defines a <mapping> element that
describes a service region and associated service URLs. Reusing this
element from LoST to provide the routing URIs was considered.
However, this would have meant that several of the mandatory
components in the <mapping> element would have had to contain
ambiguous or misleading values. Specifically, the "source" attribute
is required to contain a LoST application-unique string for the
authoritative server. However, in the situations described in this
specification, there may not be an authoritative LoST server, so any
value put into this attribute would be misleading. In addition to
this, routing information received in the manner described in this
specification should not be cached by the receiver, so detailing when
the routing information expires or was last updated is irrelevant.
The mechanism consists of adding an element to the HELD
locationRequest and an element to the locationResponse.
The request element indicates that the requestor wants the LIS to
provide routing information based on the location of the end device.
If the routing request is sent with no attribute, then URIs for
urn:service:sos are returned. If the requestor wants routing
information for a specific service, then they may include an optional
service URN. This service MUST exist in the IANA "Service URN
Labels" repository created by [RFC5031]. If a service is specified,
and the LIS does not understand the requested service, then URIs for
urn:service:sos are returned.
If the LIS understands the routing request and has routing
information for the location, then it includes the information in a
routingInformation element returned in the locationResponse. How the
LIS obtains this information is left to implementation.
Possibilities are described in Section 3.
A LIS that does not understand the routing request element ignores it
and returns the location information in the normal manner.
A LIS that does support the routing request element MUST support
returning URIs for urn:service:sos and any regionally defined sub-
services while following the URN traversal rules defined in
A LIS that does understand the routing request element but can't
obtain any routing information for the end-device's location MUST set
the defaultRoute attribute to "true" and return a default PSAP or
gateway URI along with the determined location information in the
A LIS that understands the routing request element but not the
specified service URN MUST follow the URN traversal rules defined in
A LIS that receives a request for emergency routing information that
it understands MUST return the correct emergency routing information
if it has or is able to acquire the routing information for the
location of the target device.
The routing information in the location response consists of a
service element identified by a service name. The service name is a
URN and might contain a general emergency service URN such as
urn:service:sos or a specific service URN depending on what was
requested and what the LIS is able to provide. A list of one or more
service destinations is provided for the service name. Each
destination is expressed as a URI, and each URI scheme should only
appear once in this list. The routing URIs are intended to be used
at the time they are received. To avoid any risks of using stale
routing URIs, the values MUST NOT be cached by the receiving entity.
5. Modification to Phone BCP
This section describes the normative updates to Phone BCP [RFC6881].
It is important for devices and intermediaries to take all steps
possible to ensure that emergency calls are routed to the correct
PSAP. An alternative to providing routing information via global
forest guides or local LoST servers is for local networks to
configure the PSAP address information in the network location
server. This specification updates Phone BCP [RFC6881] to provide
this option. The update requires devices and intermediaries using
the HELD protocol to always include the HELD routing extension. If
the LIS is configured with the routing information, it can provide
it; if it is not, then the device or intermediary tries LoST to
acquire the PSAP URI.
Section 6.5 of [RFC6881] defines "End System Location Configuration".
Requirement ED-23/INT-18/SP-14 is updated when HELD is used as the
Location Configuration Protocol (LCP) such that "the request MUST
include the requestRoutingInformation element." The remainder of the
requirement remains unchanged.
This document adds a new requirement to Section 7 of [RFC6881].
"ED-51a : Endpoints MUST support the HELD requestRoutingInformation
element and be able to interpret and use any routing information
returned in the locationResponse."
This document adds two new requirements to Section 8 of [RFC6881].
"ED-52a : Endpoints that acquire routing information in a HELD
locationResponse SHOULD use this routing information but MAY perform
a LoST findService request if they have a location value."
"ED-52b : Endpoints that acquire routing information in a HELD
locationResponse with a defaultRoute attribute of "true" MUST perform
a LoST findService request if they have a location value. If a route
is provided by the LoST server, then this route MUST be used,
otherwise the routing information provided in the HELD response
SHOULD be used."
This document amends SP-26 from Section 8 of [RFC6881] such that a
LoST mapping need not be requested if non-default routing information
is provided in the HELD locationResponse.
6. HELD Schema Extension
This section describes the schema extension to HELD.
<xs:attribute name="service" type="xs:anyUri"
Figure 3 illustrates a <locationRequest> example that contains IP
flow information in the request.
Figure 3: Example Location Request
Figure 5 illustrates the <locationResponse> message containing
default routing information and an HTTPS location URI.
<service defaultRoute="true" serviceUri="urn:service:sos">
Figure 5: Example Location Response with Default Routing Information8. Privacy Considerations
This document makes no changes that require privacy considerations
beyond those already described in [RFC5985]. It does, however,
extend those described in [RFC6155].
[RFC5985] describes the privacy considerations surrounding the HELD
location configuration protocol, and this document makes no specific
changes to these considerations.
[RFC6155] extends HELD beyond a simple LCP by enabling authorized
third parties to acquire location information and describing the
issues in Section 4. The HELD routing extension supports returning
URIs that represent specific services operating in the Target's
vicinity. This represents additional information about the Target;
as a consequence, it is recommended that this option only be used
when the LIS returns a location URI, not a location value.
9. Security Considerations
This document imposes no additional security considerations beyond
those already described in [RFC5985] and [RFC6155].
10. IANA Considerations
10.1. URN Sub-Namespace Registration for
Per this document, IANA has registered a new XML namespace, following
the guidelines in [RFC3688].
Registrant Contact: IETF ECRIT working group (email@example.com),
James Winterbottom (firstname.lastname@example.org).
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<title>HELD Routing Information Extensions</title>
<h1>Additional Element for HELD Routing Information</h1>
<p>See <a href="http://www.rfc-editor.org/rfc/rfc7840.txt">
10.2. XML Schema Registration
This section registers an XML schema as per the procedures in
Registrant Contact: IETF ECRIT working group (email@example.com),
James Winterbottom (firstname.lastname@example.org).
XML: The XML for this schema can be found as the entirety of
Section 6 of this document.
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