Internet DRAFT - draft-ietf-straw-b2bua-taxonomy
draft-ietf-straw-b2bua-taxonomy
STRAW Working Group H. Kaplan
Internet-Draft Oracle
Intended status: Informational V. Pascual
Expires: April 21, 2014 Quobis
October 18, 2013
A Taxonomy of Session Initiation Protocol (SIP) Back-to-Back User Agents
draft-ietf-straw-b2bua-taxonomy-03
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 [RFC3261] 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 Back-to-Back User Agents, performing
different roles in different ways. For Example IP-PBXs, Session
Border Controllers (SBC) [RFC5853] and Application Servers (AS).
This document identifies several common Back-to-Back User Agent
roles, in order to provide taxonomy other documents can use and
reference.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 21, 2014.
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. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. B2BUA Role Types . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Signaling-plane B2BUA Roles . . . . . . . . . . . . . . . 3
3.1.1. Proxy-B2BUA . . . . . . . . . . . . . . . . . . . . . 4
3.1.2. Signaling-only . . . . . . . . . . . . . . . . . . . 4
3.1.3. SDP-Modifying Signaling-only . . . . . . . . . . . . 4
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 . . . . . . . . . . . . . . . 6
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. Informative References . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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, 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 User Agent Server (UAS) and User
Agent Client (UAC) sides. Some B2BUAs operate only in the SIP
signaling plane, while others participate in the media plane as well.
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As more SIP domains get deployed and interconnected 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.
2. Terminology
The following terms are defined in [RFC3261], Section 6.
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.
3. B2BUA Role Types
Within the context of this document, the classification refers 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 (conference bridge) 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 that it does not modify
Session Description Protocol (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.
It should be noted that there is a large variety of modifications
made by "Signaling-plane B2BUA's"
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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.
If a Proxy-B2BUA can generate in-dialog messages, then it will also
need to modify the CSeq header, after it has 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. For example, such a B2BUA might 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 form of Application Server
and PBX, 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.
These B2BUAs don't do anything that changes the path that media takes
(in particular, they don't insert themselves on the media path), but
they may make SDP changes that affect what is sent on the media
plane.
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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 Real-
time Transport Protocol (RTP) / Real Time Control Protocol (RTCP)
[RFC3550] 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 modified.
An example of such a B2BUA would be a Session Border Controller (SBC)
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 transport (UDP/TCP) layers on its UAS and
UAC sides, but neither modifies nor restricts which forms of payload
are carried within the packets. Rather, the payload is transparently
copied from one side to the other. 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 (as it could be [RFC3550] RTP or RTCP) but it
does 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 multiplexes/de-
multiplexes RTP and RTCP packets on the same 5-tuple.
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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, [RFC5366] based conference
servers, etc.
4. Mapping SIP Device Types to B2BUA Roles
Although the B2BUA roles defined previously do not define system
types, 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
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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 conference
bridge model is to behave as a SIP B2BUA without using the resource-
list mechanism, but rather simply route a normal INVITE request
through a B2BUA with built-in 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-Call Session Control Function (P-CSCF) and Interconnection
Border Control Function (IBCF) are functions defined by 3GPP [IMS]
standards, and when coupled with the IMS media-plane gateways (IMS
Access Gateway IMS-AGW, Transition Gateway TrGW, etc.) typically form
a logical Media-relay or Media-aware B2BUA role.
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4.7. S-CSCF Function
Serving-Call Session Control Function (S-CSCF) is a function defined
by 3GPP [IMS] standards, and typically follows a Proxy-B2BUA role.
5. Security Considerations
Security risks are specific to each type of B2BUA, so little can be
said in general. Of course, adding extra systems in the
communication path creates extra points of attack, reduces or
eliminates the ability to perform end to end encryption, decreases
the privacy of SIP communications, and complicates trust models.
Thus, every B2BUA design requires particular attention to security
analysis.
A few general points can be made:
1. The B2BUA processing of SDP and media packets is an impediment to
deployment of end-to-end SRTP, and reduces the ability to deploy
new, stronger forms of SRTP key exchange.
2. The ability for B2BUAs to modify any SIP header field value and
body impacts the ability to deploy SIP Identity and message
integrity.
3. The management and configuration mechanisms of B2BUAs are a
tempting point of attack, and must be strongly defended.
Further 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.
7. 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.
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[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.
Authors' Addresses
Hadriel Kaplan
Oracle
Email: hadriel.kaplan@oracle.com
Victor Pascual
Quobis
Email: victor.pascual@quobis.com
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