A Taxonomy of Session Initiation Protocol (SIP) Back-to-Back User Agents
RFC 7092

Document Type RFC - Informational (December 2013; No errata)
Last updated 2013-12-20
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Internet Engineering Task Force (IETF)                         H. Kaplan
Request for Comments: 7092                                        Oracle
Category: Informational                                       V. Pascual
ISSN: 2070-1721                                                   Quobis
                                                           December 2013

A Taxonomy of Session Initiation Protocol (SIP) Back-to-Back User Agents

Abstract

   In many SIP deployments, SIP entities exist in the SIP signaling path
   between the originating and final terminating endpoints, which go
   beyond the definition of a SIP proxy, performing functions not
   defined in Standards Track RFCs.  The only term for such devices
   provided in RFC 3261 is for a Back-to-Back User Agent (B2BUA), which
   is defined as the logical concatenation of a SIP User Agent Server
   (UAS) and User Agent Client (UAC).

   There are numerous types of SIP B2BUAs performing different roles in
   different ways; for example, IP Private Branch Exchanges (IPBXs),
   Session Border Controllers (SBCs), and Application Servers (ASs).
   This document identifies several common B2BUA roles in order to
   provide taxonomy other documents can use and reference.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see 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
   http://www.rfc-editor.org/info/rfc7092.

Kaplan & Pascual              Informational                     [Page 1]
RFC 7092                   Taxonomy of B2BUAs              December 2013

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  B2BUA Role Types  . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Signaling Plane B2BUA Roles . . . . . . . . . . . . . . .   4
       3.1.1.  Proxy-B2BUA . . . . . . . . . . . . . . . . . . . . .   4
       3.1.2.  Signaling-only  . . . . . . . . . . . . . . . . . . .   4
       3.1.3.  SDP-Modifying Signaling-only  . . . . . . . . . . . .   5
     3.2.  Signaling/Media Plane B2BUA Roles . . . . . . . . . . . .   5
       3.2.1.  Media Relay . . . . . . . . . . . . . . . . . . . . .   5
       3.2.2.  Media Aware . . . . . . . . . . . . . . . . . . . . .   6
       3.2.3.  Media Termination . . . . . . . . . . . . . . . . . .   6
   4.  Mapping SIP Device Types to B2BUA Roles . . . . . . . . . . .   6
     4.1.  SIP PBXs and Softswitches . . . . . . . . . . . . . . . .   6
     4.2.  Application Servers . . . . . . . . . . . . . . . . . . .   7
     4.3.  Session Border Controllers  . . . . . . . . . . . . . . .   7
     4.4.  Transcoders . . . . . . . . . . . . . . . . . . . . . . .   7
     4.5.  Conference Servers  . . . . . . . . . . . . . . . . . . .   8
     4.6.  P-CSCF and IBCF Functions . . . . . . . . . . . . . . . .   8
     4.7.  S-CSCF Function . . . . . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  Informative References  . . . . . . . . . . . . . . . . . . .   9

Kaplan & Pascual              Informational                     [Page 2]
RFC 7092                   Taxonomy of B2BUAs              December 2013

1.  Introduction

   In current SIP deployments, there are numerous forms of Back-to-Back
   User Agents (B2BUAs), operating at various levels of transparency and
   for various purposes, with widely varying behavior from a SIP
   perspective.  Some act as pure SIP proxies and only change to the
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