Representing Trunk Groups in tel/sip Uniform Resource Identifiers (URIs)
RFC 4904

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Network Working Group                                         V. Gurbani
Request for Comments:  4904            Bell Laboratories, Alcatel-Lucent
Category: Standards Track                                    C. Jennings
                                                           Cisco Systems
                                                               June 2007

                  Representing Trunk Groups in tel/sip
                  Uniform Resource Identifiers (URIs)

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.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document describes a standardized mechanism to convey trunk
   group parameters in sip and tel Uniform Resource Identifiers (URIs).
   An extension to the tel URI is defined for this purpose.

Gurbani & Jennings          Standards Track                     [Page 1]
RFC 4904              Trunk Groups in tel/sip URIs             June 2007

Table of Contents

   1.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Requirements and Rationale . . . . . . . . . . . . . . . . . .  5
     4.1.  sip URI or tel URI?  . . . . . . . . . . . . . . . . . . .  5
     4.2.  Trunk Group Namespace: Global or Local?  . . . . . . . . .  5
     4.3.  Originating Trunk Group and Terminating Trunk Group  . . .  6
     4.4.  Intermediary Processing of Trunk Groups  . . . . . . . . .  6
   5.  Trunk Group Identifier: ABNF and Examples  . . . . . . . . . .  6
   6.  Normative Behavior of SIP Entities Using Trunk Groups  . . . .  8
     6.1.  User Agent Client Behavior . . . . . . . . . . . . . . . .  9
     6.2.  User Agent Server Behavior . . . . . . . . . . . . . . . . 10
     6.3.  Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . 10
   7.  Example Call Flows . . . . . . . . . . . . . . . . . . . . . . 11
     7.1.  Reference Architecture . . . . . . . . . . . . . . . . . . 11
     7.2.  Basic Call Flow  . . . . . . . . . . . . . . . . . . . . . 12
     7.3.  Inter-Domain Call Flow . . . . . . . . . . . . . . . . . . 14
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   9.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 16
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 16
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     11.2. Informative References . . . . . . . . . . . . . . . . . . 17

Gurbani & Jennings          Standards Track                     [Page 2]
RFC 4904              Trunk Groups in tel/sip URIs             June 2007

1.  Background

   Call routing in the Public Switched Telephone Network (PSTN) is
   accomplished by routing calls over specific circuits (commonly
   referred to as "trunks") between Time Division Multiplexed (TDM)
   circuit switches.  In switches, a group of trunks that connect to the
   same target switch or network is called a "trunk group".
   Consequently, trunk groups have labels, which are used as the main
   indication for the previous and next TDM switch participating in
   routing the call.

   Formally, we define a trunk and trunk group and related terminology
   as follows (definition of "trunk" and "trunk group" is from [5]).

      Trunk:  In a network, a communication path connecting two
      switching systems used in the establishment of an end-to-end
      connection.  In selected applications, it may have both its
      terminations in the same switching system.

      Trunk Group:  A set of trunks, traffic engineered as a unit, for
      the establishment of connections within or between switching
      systems in which all of the paths are interchangeable.  A single
      trunk group can be shared across multiple switches for redundancy
      purposes.

      Digital Signal 0 (DS0):  Digital Signal X is a term for a series
      of standard digital transmission rates based on DS0, a
      transmission rate of 64 kbps (the bandwidth normally used for one
      telephone voice channel).  The European E-carrier system of
      transmission also operates using the DS series as a base multiple.

   Since the introduction of ubiquitous digital trunking, which resulted
   in the allocation of DS0s between end offices in minimum groups of 24
   (in North America), it has become common to refer to bundles of DS0s
   as a trunk.  Strictly speaking, however, a trunk is a single DS0