A Taxonomy of Session Initiation Protocol (SIP) Back-to-Back User Agents
draft-ietf-straw-b2bua-taxonomy-02

STRAW Working Group                                            H. Kaplan
Internet-Draft                                                V. Pascual
Intended status: Informational                               Acme Packet
Expires: November 11, 2013                                  May 10, 2013

A Taxonomy of Session Initiation Protocol (SIP) Back-to-Back User Agents
                   draft-ietf-straw-b2bua-taxonomy-02

Abstract

   In many SIP deployments, SIP entities exist in the SIP signaling path
   between the originating UAC and final terminating UAS, which go
   beyond the definition of a Proxy, performing functions not defined in
   standards-track RFCs.  The only term for such devices provided in
   [RFC3261] is for a Back-to-Back User Agent (B2BUA), which is defined
   as the logical concatenation of a User Agent Server (UAS) and User
   Agent Client (UAC).

   There are numerous types of SIP Back-to-Back User Agents (B2BUAs),
   performing different roles in different ways.  For Example IP-PBXs,
   SBCs and Application Servers.  This document identifies several
   common B2BUA roles, in order to provide taxonomy other documents can
   use and reference.

Status of This Memo

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Copyright Notice

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

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Table of Contents

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

1.  Terminology

   B2BUA: a SIP Back-to-Back User Agent, which is the logical
   combination of a User Agent Server (UAS) and User Agent Client (UAC).

   UAS: a SIP User Agent Server.

   UAC: a SIP User Agent Client.

2.  Introduction

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   In current SIP deployments, there are numerous forms of B2BUAs,
   operating at various layers of the protocol stack, and for various
   purposes, and with widely varying behavior from a SIP protocol
   perspective.  Some act as pure SIP Proxies and only change to the
   role of B2BUA in order to generate BYEs to terminate dead sessions.
   Some are full User Agent stacks with only high-level event and
   application logic binding the UAS and UAC sides.  Some B2BUAs operate
   only in the SIP signaling plane, while others participate in the
   media plane as well.

   As more and more SIP domains get deployed and interconnect the
   probability of a single SIP session crossing multiple B2BUA's at both
   the signaling and media planes increases significantly.

   This document provides a taxonomy of several common B2BUA roles, so
   that other documents may refer to them using their given names
   without re-defining them in each document.

3.  B2BUA Role Types

   Within the context of this document, the terms refer to a B2BUA role,
   not a particular system type.  A given system type may change its
   role in the middle of a SIP session, for example when a Stateful
   Proxy tears-down a session by generating BYEs; or for example when an
   SBC performs transcoding or REFER termination.

   Furthermore, this document defines 'B2BUA' following the definition
   provided in [RFC3261], which is the logical concatenation of a UAS
   and UAC.  A typical centralized conference server, for example, is
   not a B2BUA because it is the target UAS of multiple UACs, whereby
   the UACs individually and independently initiate separate SIP
   sessions to the central conference server.  Likewise, a third-party
   call control transcoder as described in section 3.1 of [RFC5369] is
   not a B2BUA; whereas an inline transcoder based on [RFC5370] is a
   B2BUA.

3.1.  Signaling-plane B2BUA Roles

   A Signaling-plane B2BUA is one that operates only on the SIP message
   protocol layer, and only with SIP messages and headers but not the
   media itself in any way.  This implies it does not modify SDP bodies,
   although it may save them and/or operate on other MIME body types.
   This category is further subdivided into specific roles as described
   in this section.

3.1.1.  Proxy-B2BUA

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   A Proxy-B2BUA is one that appears, from a SIP protocol perspective,
   to be a SIP Proxy based on [RFC3261] and its extensions, except that
   it maintains sufficient dialog state to generate in-dialog SIP
   messages on its own and does so in specific cases.  The most common
   example of this is a SIP Proxy which can generate BYE requests to
   tear-down a dead session.

   A Proxy-B2BUA does not modify the received header fields such as the
   To, From, or Contact, and only modifies the Via and Record-Route
   header fields following the rules in [RFC3261] and its extensions.
   If a Proxy-B2BUA can generate in-dialog messages, then it will also
   need to modify the CSeq header, after it's generated its own.  A
   Proxy-B2BUA neither modifies nor inspects MIME bodies (including
   SDP), does not have any awareness of media, will forward any Method
   type, etc.

3.1.2.  Signaling-only

   A Signaling-only B2BUA is one that operates at the SIP layer but in
   ways beyond those of [RFC3261] Proxies, even for normally forwarded
   requests.  Such a B2BUA may or may not replace the Contact URI,
   modify or remove all Via and Record-Route headers, modify the To and
   From header fields, modify or inspect specific MIME bodies, etc.  No
   SIP header field is guaranteed to be copied from the received request
   on the UAS side to the generated request on the UAC side.

   An example of such a B2BUA would be some forms of Application Servers
   and PBXs, such as a server which locally processes REFER requests and
   generates new INVITEs on behalf of the REFER's target.  Another
   example would be an [RFC3323] Privacy Service Proxy performing the
   'header' privacy function.

3.1.3.  SDP-Modifying Signaling-only

   An SDP-Modifying Signaling-only B2BUA is one that operates in the
   signaling plane only and is not in the media path, but does modify
   SDP bodies and is thus aware of and understands SDP syntax and
   semantics.  Some Application Servers and PBXs act in this role in
   some cases, for example to remove certain codec choices or merge two
   media endpoints into one SDP offer.

3.2.  Media-plane B2BUA Roles

   A Media-plane B2BUA is one that operates at both the SIP and media
   planes, not only on SIP messages but also SDP and potentially RTP/
   RTCP or other media.  Such a B2BUA may or may not replace the Contact
   URI, modify or remove all Via and Record-Route headers, modify the To
   and From header fields, etc.  No SIP header field is guaranteed to be

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   copied from the received request on the UAS side to the generated
   request on the UAC side, and SDP will also be modified.

   An example of such a B2BUA would be a Session Border Controller
   performing the functions defined in [RFC5853], a B2BUA transcoder as
   defined in [RFC5370], a rich-ringtone Application Server, or a
   recording system.  Another example would be an [RFC3323] Privacy
   Service Proxy performing the 'session' privacy function.

   Note that a Media-plane B2BUA need not be instantiated in a single
   physical system, but may be decomposed into separate signaling and
   media functions.

   The Media-plane B2BUA category is further subdivided into specific
   roles as described in this section.

3.2.1.  Media-relay

   A B2BUA which performs a media-relay role is one that terminates the
   media plane at the IP and UDP/TCP layers on its UAS and UAC sides,
   but neither modifies nor restricts which forms of UDP or TCP payload
   are carried within the UDP or TCP packets.  Such a role may only
   support UDP or only TCP or both, as well as other transport types or
   not.  Such a role may involve policing the IP packets to fit within a
   bandwidth limit, or converting from IPv4 to IPV6 or vice-versa.  This
   is typically similar to a NAT behavior, except a NAT operating in
   both directions on both the source and destination information; it is
   often found as the default behavior in SBCs.

3.2.2.  Media-aware

   A B2BUA which performs a media-aware role is similar to a media-
   relay except that it inspects and potentially modifies the payload
   carried in UDP or TCP, such as [RFC3550] RTP or RTCP, but not at a
   codec or higher layer.  An example of such a role is an [RFC3711]
   SRTP terminator, which does not need to care about the RTP payload
   but does care about the RTP header; or a device which monitors RTCP
   for QoS information; or a device which muxes/de-muxes RTP and RTCP
   packets on the same 5-tuple.

3.2.3.  Media-termination

   A B2BUA which performs a media-termination role is one that operates
   at the media payload layer, such as RTP/RTCP codec or MSRP message
   layer and higher.  Such a role may only terminate/generate specific
   RTP media, such as [RFC4733] DTMF packets, or it may convert between
   media codecs, or act as a Back-To-Back [RFC4975] MSRP agent.  This is
   the role performed by transcoders, conference servers, etc.

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4.  Mapping SIP Device Types to B2BUA Roles

   Although the B2BUA role types defined previously do not define a
   system type, as discussed in section 3, some discussion of what
   common system types perform which defined roles may be appropriate.
   This section provides such a 'mapping' for general cases, to aid in
   understanding of the roles.

4.1.  SIP PBXs and Softswitches

   SIP-enabled Private Branch eXchanges (SIP PBXs) and Softswitches are
   market category terms, and not specified in any standard.  In general
   they can perform every role described in this document at any given
   time, based on their architecture or local policy.  Some are based on
   architectures that make them the equivalent of a SIP UAS and UAC
   connected with a logical PRI loopback; others are built as a SIP
   Proxy core with optional modules to "do more".

4.2.  Application Servers

   Application Servers are a broad marketing term, and not specified in
   any standard in general, although 3GPP and other organizations
   specify some specific Application Server functions and behaviors.
   Common examples of Application Servers functions are message-waiting
   indication (MWI), find-me-follow-me services, privacy services, call-
   center Automatic Call Distributor (ACD) services, call screening, and
   Voice Call Continuity (VCC) services.  Some only operate in the
   signaling plane in either Proxy-B2BUA or Signaling- only B2BUA roles;
   others operate as full Media-termination B2BUAs, such as when
   providing Interactive Voice Recognition (IVR), rich-ringtone or
   integrated voicemail services.

4.3.  Session Border Controllers

   Session Border Controllers (SBCs) are a market category term, and not
   specified in any standard.  Some of the common functions performed by
   SBCs are described in [RFC5853], but in general they can perform
   every role described in this document at any given time, based on
   local policy.  By default, most SBCs are either Media-relay or Media-
   aware B2BUAs, and replace the Contact URI, remove the Via and Record-
   Route headers, modify the Call-ID, To, From, and various other
   headers, and modify SDP.  Some SBCs remove all headers, all bodies,
   and reject all Method types unless explicitly allowed by local
   policy; other SBCs pass all such elements through unless explicitly
   forbidden by local policy.

4.4.  Transcoders

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   Transcoders perform the function of transcoding one audio or video
   media codec type to another, such as G.711 to G.729.  As such they
   perform the Media-termination role, although they may only terminate
   media in specific cases of codec mismatch between the two ends.
   Although [RFC5369] and [RFC5370] define two types of SIP Transcoders,
   in practice a popular variant of the [RFC5370] inline model is to
   behave as a SIP B2BUA without using the resource-list mechanism, but
   rather simply route a normal INVITE request through an inline
   transcoder.  SIP Transcoders architectures are based on everything
   from SIP media servers, to SBCs, to looped-back Time Division
   Multiplexing (TDM) gateways, and thus run the gamut from replacing
   only specific headers/bodies and SDP content needed to perform their
   function, to replacing almost all SIP headers and SDP content.  Some
   transcoders save and remove SDP offers in INVITEs from the UAC, and
   wait for an offer in the response from the UAS, similar to a 3PCC
   model; others just insert additional codecs in SDP offers and only
   transcode if the inserted codec(s) are selected in the answer.

4.5.  Conference Servers

   In general Conference Servers do not fall under the term 'B2BUA' as
   defined by this document, since typically they involve multiple SIP
   UACs initiating independent SIP sessions to the single conference
   server UAS.  However, a conference server supporting [RFC5366],
   whereby the received INVITE triggers the conference focus UAS to
   initiate multiple INVITEs as a UAC, would be in a Media-termination
   B2BUA role when performing that function.

4.6.  P-CSCF and IBCF Functions

   Proxy-CSCFs and IBCFs are functions defined by 3GPP [IMS] standards,
   and when coupled with the IMS media-plane gateways (IMS AGW, TrGW,
   etc.)  typically form a logical Media-relay or Media-aware B2BUA
   role.

4.7.  S-CSCF Function

   S-CSCF is a function defined by 3GPP [IMS] standards, and typically
   follows a Proxy-B2BUA role.

5.  Security Considerations

   The security considerations related to the functionality described in
   this document are addressed in the relevant references.

6.  IANA Considerations

   This document makes no request of IANA.

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7.  Acknowledgments

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).

8.  Informative References

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3323]  Peterson, J., "A Privacy Mechanism for the Session
              Initiation Protocol (SIP)", RFC 3323, November 2002.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC4733]  Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
              Digits, Telephony Tones, and Telephony Signals", RFC 4733,
              December 2006.

   [RFC4975]  Campbell, B., Mahy, R., and C. Jennings, "The Message
              Session Relay Protocol (MSRP)", RFC 4975, September 2007.

   [RFC5366]  Camarillo, G. and A. Johnston, "Conference Establishment
              Using Request-Contained Lists in the Session Initiation
              Protocol (SIP)", RFC 5366, October 2008.

   [RFC5369]  Camarillo, G., "Framework for Transcoding with the Session
              Initiation Protocol (SIP)", RFC 5369, October 2008.

   [RFC5370]  Camarillo, G., "The Session Initiation Protocol (SIP)
              Conference Bridge Transcoding Model", RFC 5370, October
              2008.

   [RFC5853]  Hautakorpi, J., Camarillo, G., Penfield, R., Hawrylyshen,
              A., and M. Bhatia, "Requirements from Session Initiation
              Protocol (SIP) Session Border Control (SBC) Deployments",
              RFC 5853, April 2010.

   [IMS]      3GPP, "IP Multimedia Subsystem (IMS); Stage 2, 3GPP TS
              23.228", 2013.

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Authors' Addresses

   Hadriel Kaplan
   Acme Packet

   Email: hkaplan@acmepacket.com

   Victor Pascual
   Acme Packet

   Email: victor.pascual.avila@gmail.com

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