Network Working Group K. Carlberg
Request for Comments: 5115 G11
Category: Standards Track P. O'Hanlon
Telephony Routing over IP (TRIP) Attribute for Resource Priority
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
This document defines a new attribute for the Telephony Routing over
IP (TRIP) protocol. The attribute associates protocols/services in
the PSTN offering authorized prioritization during call setup that
are reachable through a TRIP gateway. Current examples of
preferential service in the Public Switched Telephone Network (PSTN)
are Government Emergency Telecommunications Service (GETS) in the
U.S. and Government Telephone Preference Scheme (GTPS) in the U.K.
The proposed attribute for TRIP is based on the NameSpace.Value tuple
defined for the SIP Resource-Priority field.
An IP telephony gateway allows nodes on an IP-based network to
communicate with other entities on the circuit switched telephone
network. The Telephony Routing over IP (TRIP) protocol [rfc3219]
provides a way for nodes on the IP network to locate a suitable
gateway through the use of Location Servers. TRIP is a policy-
driven, inter-administrative domain protocol for advertising the
reachability, negotiating the capabilities, and specifying the
attributes of these gateways.
The TRIP protocol is modeled after BGP-4 [rfc4271] and uses path-
vector advertisements distributed in a hop-by-hop manner (resembling
a Bellman-Ford routing algorithm) via Location Servers. These
Location Servers are grouped in administrative clusters known as
Internet Telephony Administrative Domains (ITADs). A more extensive
framework discussion on TRIP can be found in [rfc2871].
Carlberg & O'Hanlon Standards Track [Page 1]RFC 5115 Resource Priority Attribute January 2008
This document defines a new attribute that has been registered with
IANA. The purpose of this new attribute, and the rationale behind
its specification, is explained in the following sections.
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].
2. Emergency Telecommunications Service
Emergency Telecommunications Service is a broad term that refers to
the services provided by systems used to support emergency
communications. One example of these systems is the U.S. Government
Emergency Telecommunications Service (GETS). This system currently
operates over the U.S. Public Switched Telephone Network (PSTN) as a
pay-per-use system by authorized personnel. It uses the T1.631-1993
ANSI standard [ANSI] to signal the presence of the National Security
/ Emergency Preparedness (NS/EP) codepoint in an ISDN User Part
(ISUP) Initial Address Message (IAM) for Signaling System No. 7
(SS7). A key aspect of GETS is that a signaling standard in the U.S.
PSTN is used to convey the activation/request of the GETS service.
From a protocol perspective, other examples of priority (and which
can be argued as emergency/disaster related) standards are the
H.460.4 ITU [itu460] standard on Call Priority designation for H.323
calls, and the I.255.3 [itu255] ITU standard on Multi-Level
Precedence and Preemption Service. The latter has been integrated
into private telephony systems like AUTOVON. In both cases,
signaling codepoints exist to distinguish telephony calls by
authenticated and prioritized end-user from the normal end-user. The
form of this distinction varies and is outside the scope of this
document. [rfc3689] and [rfc3690] provide additional information on
ETS and its requirements.
3. SIP Resource-Priority Effort
The initial discussions in the IEPREP working group list, along with
the presentation at the Adelaide IETF [ADEL00], led to strawman
requirements to augment SIP to have the ability to prioritize call
signaling. This effort was then advanced formally in the SIPPING