SOS Uniform Resource
Identifier (URI) Parameter for Marking of Session Initiation Protocol
(SIP) Requests related to Emergency Services
Nortel
Maidenhead Office Park, Westacott Way
Maidenhead
Berkshire, UK
milanpa@nortel.com
RAI
ECRIT Working Group
Emergency
Session Initiation Protocol
This document defines a new Session Initiation Protocol (SIP) Uniform
Resource Identifier (URI) parameter intended for marking SIP
registration requests related to emergency services. The usage of this
new URI parameter complements the usage of the Service Uniform Resource
Name (URN) and is not intended to replace it.
One way to differentiate a SIP-based emergency call from an ordinary
call is by the presence of the Service URN as defined in RFC 5031 (and used in the IETF emergency services
architecture described in PhoneBCP). The 3GPP IP Multimedia Subsystem
(IMS) emergency services architecture, illustrated in 3GPP TS 23.167
, specifies that the User Equipment (UE)
performs emergency registration prior to or during the initiation of an
emergency call. The circumstances where such an emergency registration
is beneficial are listed below: - the UE is not registered with its
home network;- the UE is currently registered but roaming (to
ensure that the emergency call is handled in the visited network, as
required by some jurisdictions).
Emergency registration is possible only when the UE has sufficient
credentials to register with its home network and can detect that an
emergency session is initiated. Unfortunately, marking of the emergency
registration can not be fulfilled by the use of the Service URN.
In some countries, it is a regulatory requirement that devices be
able to place emegency calls in circumstances where other calls may not
be permitted. When a UAC issues an emergency marked REGISTER request it
informs the registrar that the contact address and the address-of-record
being registered are to be used for emergency calls, and roaming and
barring restrictions should not be applied for the registered
address-of-record.
This document proposes a way to mark a REGISTER request as an
emergency registration.
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 BCP
14, RFC 2119
Req: It shall be possible to distinguish emergency registration from
non-emergency registration.
This section provides an overview of the proposed new URI parameter
to be used for marking REGISTER requests related to emergency
services.
A new URI parameter "sos" is defined in this document. The "sos"
parameter is appended to a URI consistent with RFC 3261 . It is proposed that use of this URI parameter is
restricted to the Contact header included in the REGISTER request (and
the 2xx response to the REGISTER request) related to an emergency call
only. The "sos" URI parameter MUST NOT be considered as a replacement
for the Service URN for emergency calls originated by a UA.
When the UA sends a REGISTER request for emergency registration,
the "sos" URI parameter MUST be appended to the URI in the Contact
header. This serves as an indication to the registrar that the request
is for emergency registration.
Example:Contact: "Alice" <sip:alice@example.com;sos>
;q=0.7; expires=3600
In the event that more than one Contact header field is included in
the REGISTER request, only the contact addresses that include the
"sos" URI parameter shall be considered as emergency registered
contact addresses.
The "sos" URI parameter MUST be present in the Contact header in
the 200 (OK) response sent by the registrar upon successful
registration. The "sos" URI parameter is appended to the URI included
in the Contact header, thus indicating to the UA that it needs to
include this contact address in the Contact header of an INVITE for
emergency call initiation.
The backwards compatibility scenario considered in this document is
where a legacy registrar does not support the "sos" URI parameter. In
this case, if the registrar receives a REGISTER request that includes
the "sos" URI parameter in the Contact header, the registrar proceeds
with registration procedures and silently ignores the URI-parameter in
accordance with RFC 3261. This ensures the
user is registered and thus can successfully initiate an emergency
call.
The drawback of proceeding with registration is if the
address-of-record is for example barred or has roaming restrictions
applied, then these restrictions will not be lifted and thus
registration will be unsuccessful. This can limit the UAC's ability to
successfully place an emergency call.
If registration is successful, the 200 (OK) response from a legacy
registrar is unlikely to include the "sos" URI parameter in the
Contact header since this registration is treated as a non-emergency
registration.
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in RFC 5234 .The
"sos" URI parameter is a "uri-parameter", as defined by RFC
3261.uri-parameter =/
sos-paramsos-param = "sos"
This specification defines one new SIP URI parameter, as per the
registry created by RFC 3969 Parameter Name:
sosPredefined Values: noneReference: [RFCXXXX][NOTE
TO IANA: Please replace XXXX with the RFC number of this
specification.]
As an identifier, the "sos" parameter itself does not raise any
particular security issues. The semantic described by the "sos"
parameter are meant to be well-known so privacy considerations do not
apply to the URI parameter. The main possibility of attack involves use
of the "sos" parameter to bypass the normal procedures in order to
achieve fraudulent use of services or to bypass security procedures. The
usage of this parameter as described in this document is purely for the
purpose of the REGISTER request and hence in presence of user
authentication it is ensured that the respective user can be held
accountable.
It is RECOMMENDED to log events of misuse of the "sos" URI parameter,
for example by including it in a request or response not related to an
emergency call.
The author would like to thank Keith Drage, Milo Orsic, Deb Barclay,
John-Luc Bakker, Andrew Allen, Hiroshi Ishikawa, Sean Schneyer, Peter
Leis, Georg Mayer, Marvin Bienn, Ricky Kaura, Steve Norreys, Laura
Liess, AC Mahendran, Roozbeh Atarius, Ramachandran Subramanian and
Sandeep Sharma, Brian Rosen, Hannes Tschofenig, Christer Holmberg and
Henning Schulzrinne for the discussions and contributions that led to
this work.