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Network Working Group                                         H. Kaplan
Internet Draft                                              Acme Packet
Intended status: Informational                         October 22, 2012
Expires: April 22, 2013

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

Status of this Memo

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

Table of Contents

   1. Terminology...................................................2
   2. Introduction..................................................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. Media-plane B2BUA Roles..................................5
         3.2.1 Media-relay 5
         3.2.2 Media-aware 5
         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......................................6
      4.3. Session Border Controllers...............................7
      4.4. Transcoders..............................................7
      4.5. Conference Servers.......................................7
      4.6. P-CSCF and IBCF Functions................................7
      4.7. S-CSCF Function..........................................8
   5. Security Considerations.......................................8
   6. IANA Considerations...........................................8
   7. Acknowledgments...............................................8
   8. References....................................................8
      8.1. Informative References...................................8
   Author's Address..................................................9

1. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119.  The
   terminology in this document conforms to RFC 2828, "Internet
   Security Glossary".

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

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   UAS: a SIP User Agent Server.

   UAC: a SIP User Agent Client.

2. Introduction

   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).

   In practice, 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

   As more and more SIP domains get deployed and interconnect, the odds
   of a SIP session crossing such media-plane B2BUAs increases, as well
   as the number of such B2BUAs any given SIP session may go through.
   In other words, any given SIP session may cross any number of B2BUAs
   both in the SIP signaling plane as well as media-plane.

   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 one-armed

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   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

   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.
   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.

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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

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

   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 RTP or RTCP, but not at a codec or

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   higher layer.  An example of such a role is an 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 MSRP agent.  This is the role
   performed by transcoders, conference servers, etc.

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, find-me-follow-me services, privacy services, call-
   center ACD services, call screening, and VCC services.  Some only
   operate in the signaling plane in either Poxy-B2BUA or Signaling-
   only B2BUA roles; others operate as full Media-termination B2BUAs,
   such as when providing IVR, rich-ringtone or integrated voicemail

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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

   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 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 form 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,

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   etc.) typically form a logical Media-relay or Media-aware B2BUA

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

   There are no security implications for this document, as it is only
   a taxonomy.

6.   IANA Considerations

   This document makes no request of IANA.

7.   Acknowledgments

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

8.   References

8.1. 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.

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

   [RFC5366] Camarillo, G., Johnston, A., "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, et al, "Requirements from Session
   Initiation Protocol (SIP) Session Border Control (SBC) Deployments",
   RFC 5853, April 2010.

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Author's Address

   Hadriel Kaplan
   Acme Packet
   Email: hkaplan@acmepacket.com

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