Internet Engineering Task Force
Internet Draft H. Schulzrinne
Columbia U.
draft-schulzrinne-sipping-sos-02.txt
May 28, 2002
Expires: July 2002
Universal Emergency Address for SIP-based Internet Telephony
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
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".
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
To view the list Internet-Draft Shadow Directories, see
http://www.ietf.org/shadow.html.
Abstract
This document defines a universal emergency SIP URI, sip:sos@domain,
that allows SIP user agents to contact the local emergency number.
1 Introduction
Using the PSTN, emergency help can often be summoned at a designated,
widely known number, regardless of where the telephone was purchased.
However, this number differs between localities, even though it is
often the same for a country or region (such as many countries in the
European Union). For SIP-based end systems, it is desirable to have a
universal identifier, independent of location, to simplify the user
experience and to allow the device to perform appropriate processing.
Here, we define a common user identifier, "sos", as the contact
mechanism for emergency assistance.
Schulzrinne [Page 1]
Internet Draft SOS May 28, 2002
1.1 Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [1] and
indicate requirement levels for compliant SIP implementations.
2 Requirements
A single, global (set of) identifiers for emergency services is
highly desirable, as it allows end system and network devices to be
built that recognize such services and can act appropriately. Such
actions may include restricting the functionality of the end system,
providing special features, overriding user service constraints or
routing session setup messages. The details of the emergency service
and the associated end system and network server policies can be
specific to jurisdictions and are beyond the scope of this document.
3 Emergency URI
It is RECOMMENDED that SIP-based [2] end systems and proxy servers |
support a uniform emergency call identifier, namely the user name |
"sos" at any domain, e.g., |
sip:sos@example.com |
The host part of the emergency URI SHOULD be the host portion of the |
address-of-record of the caller. |
The domain-of-record was chosen since a SIP user agent may |
not be able to determine the local domain it is visiting. |
This also allows each user to test this facility, as the |
user can ensure that such services are operational in his |
home domain. An outbound proxy in the visited domain can |
handle the call if it believes to be in a position to |
provide appropriate emergency services.
In addition, user agents and proxies SHOULD also recognize the
telephone numbers 911 and 112, expressed as a "tel" URI [3] such as
tel:911 and tel:112, for this purpose. Where feasible, user agents
SHOULD recognize additional, local emergency numbers. Outbound proxy
servers MUST be configurable to recognize additional local emergency
numbers.
Schulzrinne [Page 2]
Internet Draft SOS May 28, 2002
There are about 60 short numbers for emergency services in
the world; including them all is not practical, as that
would interfere with existing local two, three and four-
digit dialing plans.
In addition, we define subaddresses of sos for specific emergency |
services: |
sos.fire fire brigade |
sos.rescue ambulance (rescue) |
sos.marine marine guard |
sos.police police (law enforcement) |
sos.mountain mountain rescue |
In some areas, these emergency services use different
numbers.
4 Request Handling
A user agent SHOULD direct such a request to a outbound proxy server,
if configured, or send the request to the SIP multicast address if
there is no outbound proxy server.
Multicast offers additional robustness to visitors if
firewalls prevent contacting the home domain and if the
outbound proxy is configured by some non-DHCP means
inaccessible to a visiting user.
It is possible that there are several SIP proxies listening to the
same multicast address, each routing the request independently to
different emergency call centers. Proxies in such configurations MUST
take steps to prevent this from occuring, for example to route the
call based on the caller's identity or location. Determining and
conveying the location of the caller is beyond the scope of this
document.
The multicast mechanism differs slightly from standard SIP
processing; the use of an outbound proxy conforms to
standard procedures. Multicast allows systems to make
emergency calls with minimal configuration.
Using a proxy server that is local to the user agent is more likely
Schulzrinne [Page 3]
Internet Draft SOS May 28, 2002
to reach a geographically local server, although that is not
guaranteed if virtual private networks are being used.
The "sos" user name and user names starting with "sos." MUST NOT be
assigned to any regular user. It is RECOMMENDED that SIP MESSAGE
requests are directed to a TTY-for-the-deaf translator.
User agent servers and proxy servers MUST NOT require that the user
agent client be registered or authenticated in order to place an
emergency call.
For testing purposes, OPTIONS messages to the user "sos" and the
"sos.*" addresses (sos.fire, etc.) SHOULD return an indication
whether the address is defined, but cause no further action. It is
RECOMMENDED that user agents periodically automatically check for the
availability of the "sos"identifier and alert the user if the check
fails. The period of such automated checks SHOULD NOT be less than
once per day and MUST be randomly placed over the testing interval.
Any proxy, outbound or otherwise, that receives such a request MUST
forward (proxy) or redirect the request to the appropriate local
emergency number (e.g., 911 in the United States or 112 in Europe).
Typically, the proxy server routes the call to an appropriate PSTN
gateway, translating the request URI to the local emergency number.
Any SIP PSTN gateway shall translate this emergency identifier to the
locally supported emergency number.
If a proxy receives a "sos.*" request (such as sos.fire), the proxy
forwards it to the appropriate emergency service. If it does not
recognize the suffix (e.g., fire), it MUST forward the request to the
appropriate general emergency contact, handling it as if the address
was "sos".
It is beyond the scope of this document how the proxy determines the
appropriate public safety answering point or how it determines the
physical location of the SIP UA making the request.
5 Alternatives Considered |
The scheme proposed here follows the convention of RFC 2142 [4]. One |
drawback is that it may conflict with locally assigned addresses of |
the form "sos@somewhere". |
There are a number of possible alternatives, each with their own set |
of advantages and problems: |
tel:sos This solution avoids name conflicts, but is not really a |
Schulzrinne [Page 4]
Internet Draft SOS May 28, 2002
valid "tel" URI. It also only works if every outbound proxy |
knows how to route requests to a proxy that can reach |
emergency services. The SIP URI proposed here only requires |
a user's home domain to be appropriately configured. |
URI parameter: One could create a special URI, such as "aor- |
domain;user=sos". This avoids the name conflict problem. |
Special domain: A special domain, such as "fire@sos.int" could |
be used to identify emergency calls. This has similar |
properties as the "tel:sos" URI, except that it is indeed a |
valid URI. |
6 Security Considerations
The SIP specification [2] details a number of security
considerations. Security for emergency calls has conflicting goals,
namely to make it as easy and reliable as possible to reach emergency
services, while discouraging and possibly tracing prank calls. It
appears unlikely that classical authentication mechanisms can be
required by emergency call centers, but SIP proxy servers may be able
to add identifying information.
7 Bibliography
[1] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.
[2] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J.
Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: Session
initiation protocol," RFC 3261, Internet Engineering Task Force, May
2002.
[3] A. Vaha-Sipila, "URLs for telephone calls," RFC 2806, Internet
Engineering Task Force, Apr. 2000.
[4] D. Crocker, "Mailbox names for common services, roles and
functions," RFC 2142, Internet Engineering Task Force, May 1997.
8 Acknowledgements
Andrew Allen, James Polk and Brian Rosen contributed helpful
comments.
9 Authors' Addresses
Henning Schulzrinne
Dept. of Computer Science
Schulzrinne [Page 5]
Internet Draft SOS May 28, 2002
Columbia University
1214 Amsterdam Avenue
New York, NY 10027
USA
electronic mail: schulzrinne@cs.columbia.edu
Schulzrinne [Page 6]