ETSI TR 102 633 V1.1.1 (2008-10)

Corporate Networks (NGCN); Next Generation Corporate Networks (NGCN) - General

DTR/ECMA-00352

General Information

• Status: Published
• Publication Date: 30-Oct-2008
• Technical Committee: Ecma - Standardizing information and communications systems
• Current Stage: 12 - Completion
• Due Date: 12-Nov-2008
• Completion Date: 31-Oct-2008

Standards Content (Sample)

Technical Report
Corporate Networks (NGCN);
Next Generation Corporate Networks (NGCN) - General

2 ETSI TR 102 633 V1.1.1 (2008-10)

Reference
DTR/ECMA-00352
Keywords
IP, SIP
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3 ETSI TR 102 633 V1.1.1 (2008-10)

Contents
Intellectual Property Rights . 5
Foreword . 5
Introduction . 5
1 Scope . 6
2 References . 6
2.1 Normative references . 6
2.2 Informative references . 7
3 Definitions . . 7
3.1 Corporate telecommunication Network (CN) (EG 201 017) . 7
3.2 Domain . 7
3.3 Enterprise network . 7
3.4 Home server . 8
3.5 Transport service provider (TSP) . 8
3.6 Medium . 8
3.7 Next Generation CN (NGCN) . 8
3.8 Next Generation Network (NGN) . 8
3.9 Private network traffic . 8
3.10 Public network traffic . 8
3.11 Roaming . 9
3.12 Roaming hub . 9
3.13 Session service provider (SSP) . 9
3.14 SIP intermediary . 9
4 Abbreviations . 9
5 Background . 10
6 General concepts . 11
6.1 Basic communication architecture . 11
6.2 Session level architecture . 13
6.2.1 Signalling using SIP. 13
6.2.2 Media path . 14
6.2.3 Example . 15
6.3 Domains . 15
6.4 Mobility . 16
6.4.1 Roaming of enterprise users outside their home domain . 16
6.4.2 Accommodating guest users on an NGCN . 17
6.5 The hosting concept. 17
6.5.1 Dedicated NGCN . 19
6.5.2 Enterprise hosted by a single public network infrastructure . 19
6.5.3 Enterprise hosted by multiple public network infrastructures . 20
6.5.4 Enterprise hosted by a combination of enterprise infrastructure and a public network infrastructure . 20
6.6 Private network traffic and public network traffic . 21
6.7 Other technical considerations . 22
7 Scenarios for session-based communications . 22
7.1 Session-based intra-domain communications. 22
7.2 Session-based inter-domain communications within a single enterprise network. 24
7.2.1 Communication between domains supported by the same infrastructure . 24
7.2.2 Communications between domains of an NGCN via a TSP . 25
7.2.3 Communications between domains of an NGCN using private network traffic through a hosting
NGN. 25
7.2.4 Communications between domains of an NGCN using public network traffic through a public SSP
such as an NGN . 26
7.2.5 Communications between a dedicated NGCN domain and a domain hosted by an NGN . 26
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7.2.6 Communications between a dedicated NGCN domain and a domain hosted by an NGN as private
network traffic via an intermediate NGN domain . 27
7.3 Session-based communication between two enterprises . 27
7.3.1 Extension of enterprise network to include partner networks . 27
7.3.2 Direct peering . 27
7.3.3 Indirect peering . 28
7.3.4 Third party assistance . 29
7.3.5 Direct peering between two enterprises hosted by the same hosting enterprise infrastructure . 30
7.4 Session-based communication with users of public networks . 30
7.4.1 Communication between an NGCN user and an NGN public network user . 31
7.4.2 Communication between an NGCN user and an NGN public network user with break-in or break-
out function in the NGN . 31
7.4.3 Communication between an NGCN user and a public network user of a remote NGN . 32
7.4.4 Communication between an NGCN user and a public network user of a remote NGN with break-in
or break-out function in the local NGN . 32
7.4.5 Communication between an NGCN user and a public network user of a remote NGN with break-in
or break-out function in the remote NGN . 32
7.4.6 Communication between an NGCN user and a PSTN/ISDN user via an NGN . 33
7.4.7 Communication between an enterprise user hosted by a public network infrastructure and a public
network user of that same infrastructure . 33
7.5 Session-based roaming . 34
7.5.1 An NGCN user at a visited sub-domain of the NGCN . 34
7.5.2 An NGCN user at a visited sub-domain of the NGCN using private network traffic through an NGN . 34
7.5.3 An NGCN user at a visited NGN . 35
7.5.4 An NGCN user at a visited NGN using indirect roaming . 35
7.5.5 An NGN-hosted enterprise user at a visited NGN . 35
7.5.6 An NGN-hosted enterprise user at a visited NGCN . 36
8 NGN considerations . 36
8.1 Summary of scenarios involving NGN . 36
8.1.1 Scenarios involving NGN as a TSP . 36
8.1.2 Scenarios involving NGN as a SSP . 37
8.1.3 Roaming scenarios involving NGN as a SSP . 37
8.2 Interfacing NGCN to NGN . 37
8.2.1 Subscription-based business trunking . 37
8.2.2 Peering-based business trunking . 38
8.2.3 Roaming . 38
9 Application considerations . 38
10 Current standards and standardization efforts . 39
10.1 IETF Real-time Applications and Infrastructure (RAI) area . 39
10.2 SIP Forum . 40
10.3 ETSI TISPAN . 40
10.4 3GPP . 40
10.5 ITU-T Study Group 13 . 40
History . 41

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Intellectual Property Rights
IPRs essential or potentially essential to the present document may have been declared to ETSI. The information
pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be found
in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in
respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the ETSI Web
server (http://webapp.etsi.org/IPR/home.asp).
Pursuant to the ETSI IPR Policy, no investigation, including IPR searches, has been carried out by ETSI. No guarantee
can be given as to the existence of other IPRs not referenced in ETSI SR 000 314 (or the updates on the ETSI Web
server) which are, or may be, or may become, essential to the present document.
Foreword
This Technical Report (TR) has been produced by ECMA on behalf of its members and those of the European
Telecommunications Standards Institute (ETSI).
Introduction
The present document is the first of a series of Ecma publications that explore IP-based enterprise communication
involving Corporate telecommunication Networks (CNs) (also known as enterprise networks) and in particular Next
Generation Corporate Networks (NGCN). The series particularly focuses on inter-domain communication, including
communication between parts of the same enterprise, between enterprises and between enterprises and carriers. The
present provides general information on the subject, defines some architectural concepts, identifies various
communication scenarios, and provides a framework in support of other publications that provide greater detail on
particular topics.
The present document is based upon the practical experience of Ecma member companies and the results of their active
and continuous participation in the work of ISO/IEC JTC1, ITU-T, ETSI, IETF and other international and national
standardization bodies. It represents a pragmatic and widely based consensus. In particular, Ecma acknowledges
valuable input from experts in ETSI TISPAN.
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1 Scope
The present document is part of a series of publications that provides an overview of IP-based enterprise
communication involving Corporate telecommunication Networks (CNs) (also known as enterprise networks) and in
particular Next Generation Corporate Networks (NGCN). The series particularly focuses on session level
communication based on the Session Initiation Protocol (SIP) [i.6], with an emphasis on inter-domain communication.
This includes communication between parts of the same enterprise (on dedicated infrastructures and/or hosted), between
enterprises and between enterprises and public networks. Key technical issues are investigated, current standardization
work and gaps in this area are identified and a number of requirements are stated.
The present document provides general information on the subject, defines some architectural concepts, identifies
various communication scenarios, and provides a framework in support of other publications that provide greater detail
on particular topics. At the time of publication of the present document, one further document in the series has been
published, on the subject of identification and routing [i.3].
The scope of the present document is limited to communications with a real-time element, including voice, video,
real-time text and instant messaging.
Further details on mobility in an NGCN environment are to be found in ECMA TR/92 [i.2].
2 References
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific.
• For a specific reference, subsequent revisions do not apply.
• Non-specific reference may be made only to a complete document or a part thereof and only in the following
cases:
- if it is accepted that it will be possible to use all future changes of the referenced document for the
purposes of the referring document;
- for informative references.
Referenced documents which are not found to be publicly available in the expected location might be found at
http://docbox.etsi.org/Reference.
For online referenced documents, information sufficient to identify and locate the source shall be provided. Preferably,
the primary source of the referenced document should be cited, in order to ensure traceability. Furthermore, the
reference should, as far as possible, remain valid for the expected life of the document. The reference shall include the
method of access to the referenced document and the full network address, with the same punctuation and use of upper
case and lower case letters.
NOTE: While any hyperlinks included in this clause were valid at the time of publication ETSI cannot guarantee
their long term validity.
2.1 Normative references
The following referenced documents are indispensable for the application of the present document. For dated
references, only the edition cited applies. For non-specific references, the latest edition of the referenced document
(including any amendments) applies.
Not applicable.
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2.2 Informative references
The following referenced documents are not essential to the use of the present document but they assist the user with
regard to a particular subject area. For non-specific references, the latest version of the referenced document (including
any amendments) applies.
[i.1] ECMA-269: "Services for Computer Supported Telecommunications Applications (CSTA) Phase
III".
[i.2] ECMA TR/92: "Corporate Telecommunication Networks - Mobility for Enterprise
Communication".
[i.3] ECMA TR/96: "Next Generation Corporate Networks (NGCN) - Identification and Routing".
[i.4] ITU-T Recommendation H.248: "Gateway control protocol".
[i.5] ITU-T Recommendation H.323: "Packet-based multimedia communications systems".
[i.6] IETF RFC 3261: "SIP: Session Initiation Protocol".
[i.7] IETF RFC 3550: "RTP: A Transport Protocol for Real-Time Applications".
[i.8] IETF RFC 4566: "SDP: Session Description Protocol".
[i.9] SIP Forum sf-adopted-twg-IP-PBX-SP-Interop-sibley-sipconnect: "IP-PBX / Service Provider
Interoperability - SIPConnect 1.0 Technical Recommendation".
[i.10] ETSI EG 201 017: "Corporate Telecommunication Networks (CN); Standardization plan".
[i.11] ETSI TR 180 000: "Telecommunications and Internet converged Services and Protocols for
Advanced Networking (TISPAN); NGN Terminology".
[i.12] IEEE 802.1x: "Port Based Network Access Control".
3 Definitions
For the purposes of the present document the following definitions apply:
3.1 Corporate telecommunication Network (CN) (EG 201 017)
Telecommunication network serving a corporation, i.e. a single organization, an extended enterprise, or an industry
application group as defined by the International Chamber of Commerce (ICC).
NOTE: Sets of equipment [Customer Premises Equipment (CPE) and/or Customer Premises Networks (CPN)]
are typically located at geographically dispersed locations and are interconnected to provide networking
services to a defined group of users. A CN can employ connection-oriented and connectionless
technology.
3.2 Domain
Session level capabilities within a single administrative area.
NOTE: A domain may or may not correspond to a DNS domain.
3.3 Enterprise network
A CN comprising session level capabilities and optionally application layer capabilities hosted on one or more
infrastructures.
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NOTE: Infrastructures can include the enterprise's own infrastructure (dedicated NGCN), the infrastructure of one
or more hosting NGNs, the infrastructure of one or more hosting NGCNs or any combination of these.
3.4 Home server
For a given user, as identified by a SIP address of record, the SIP intermediary that contains registrar and proxy
functionality in support of that user.
NOTE: It is therefore the SIP intermediary with which the user's UAs register.
3.5 Transport service provider (TSP)
A business or organization separate from an enterprise that provides services for transporting data based on the use of IP
at the network layer, thereby allowing the enterprise to communicate with entities outside the enterprise or with
geographically dispersed parts of the enterprise.
NOTE 1: Communication can but need not be via the public Internet.
NOTE 2: A TSP should not intervene above the transport layer. This does not preclude a business or organization
that acts as a TSP also acting as the provider of higher level services, e.g. as an SSP.
3.6 Medium
A given type of payload transported between session users (e.g. audio, video, text), separate from any signalling used
for session establishment.
3.7 Next Generation CN (NGCN)
That part of an enterprise network that is not based on public network infrastructure, that is designed to take advantage
of emerging IP-based communications solutions and that can have its own applications and service provisioning.
NOTE: An NGCN can be an entire enterprise network if none of that network is based on public network
infrastructure.
3.8 Next Generation Network (NGN)
The definition in [i.11] applies.
NOTE: This defines an NGN as follows: "A Next Generation Network is a packet-based network able to provide
services including Telecommunication Services and able to make use of multiple broadband,
QoS-enabled transport technologies and in which service-related functions are independent from
underlying transport-related technologies. It offers unrestricted access by users to different service
providers. It supports generalized mobility which will allow consistent and ubiquitous provision of
services to users." It also goes on to list some fundamental aspects that characterize NGN.
3.9 Private network traffic
Signalling for session level communications that is handled according to rules specific to an enterprise network.
3.10 Public network traffic
Signalling for session level communications that is handled according to rules for public networks.
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3.11 Roaming
The use of session capabilities of a visited domain to allow a user to access session level services at his home domain.
NOTE 1: This usually requires a roaming agreement between the operators of the domains concerned.
NOTE 2: This definition of roaming reflects the concept of roaming as found in mobile telephone networks, for
example. It does not encompass certain other common uses of the term, e.g. concerning transport service
provision.
3.12 Roaming hub
A network or other entity with which an enterprise domain has a roaming agreement, allowing enterprise users to visit
other domains that have a roaming agreement with the roaming hub but not directly with the enterprise domain.
3.13 Session service provider (SSP)
A business or organization separate from an enterprise that provides communication capabilities at the session layer
using SIP and thereby allows the enterprise to communicate using SIP with entities outside the enterprise or with
geographically dispersed parts of the enterprise.
3.14 SIP intermediary
Any intermediate entity involved either actively or passively in SIP signalling between two UAs, including but not
limited to proxies, Back-to-Back User Agents (B2BUAs), Application Layer Gateways (ALGs) and Session Border
Controllers (SBCs).
4 Abbreviations
ALG Application Layer Gateway
B2BUA Back-to-Back User Agent
CN Corporate telecommunication Network
DNS Domain Name System
IP Internet Protocol
IPPBX IP Private Branch eXchange
IPSEC Internet Protocol Security
ISDN Integrated Services Digital Network
LAN Local Area Network
NAT Network Address Translator
NGCN Next Generation Corporate Network
NGN Next Generation Network
PNP Private Numbering Plan
PSTN Public Switched Telephone Network
QoS Quality of Service
RTP Real Time Protocol
SBC Session Border Controller
SDP Session Description Protocol
SIP Session Initiation Protocol
SRTP Secure Real Time Protocol
SSP Session Service Provider
TLS Transport Layer Security
TSP Transport Service Provider
UA User Agent
UAC User Agent Client
UAS User Agent Server
URI Universal Resource Identifier
VPN Virtual Private Network
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WAN Wide Area Network
5 Background
Many enterprises and other organizations require their own telecommunications capabilities to support their own
internal communications as well as supporting communications with the outside world. This avoids incurring
unnecessary charges and provides added value in terms of services and features available, integration with other
enterprise applications, etc. These capabilities are provided through enterprise telecommunication networks (or
corporate telecommunication networks, CN, or simply enterprise networks). Many administrations do not apply the
same licensing conditions or regulation to enterprise networks and their internal traffic as they do to public networks
and their public network traffic. Many public networks also offer optional services to corporate customers, such as
hosted (Centrex) services and the leasing and maintenance of customer premises equipment.
There has been a major evolution in enterprise telecommunications during the last few years. Prior to that, enterprise
network were based on 64 kbit/s circuit-switched technology, which had synergy with corresponding technology
deployed in public Integrated Services Digital Networks (ISDN) and traditional analogue services. Those enterprise
networks primarily delivered a voice or telephony service to their users, although in principle they were capable of other
services too, including video and various types of data service. For communication outside the enterprise, enterprise
networks were able to interwork with public ISDNs across standardized interfaces (at the T-Reference Point). The T
reference point formed the demarcation point between standards and regulations applicable to public networks and
standards and regulations applicable to enterprise networks and their PBXs.
With the advent of technologies for transmitting voice and other real-time media over the Internet Protocol (IP)
(e.g. based on Real Time Protocol (RTP) [i.7]) and corresponding new signalling protocols (e.g. H.323 [i.5], the Session
Initiation Protocol (SIP) [i.6]), there was potential for providing telephony and other real-time person-to-person services
in the public Internet. Moreover, such services also became possible in the IP-based "intranets" already deployed in
enterprises for data services such as corporate email, file transfer, corporate web services and access to the world wide
web. Enterprises saw advantages such as savings on infrastructure costs (e.g. one wire to the desk) and the introduction
of innovative services that exploited the convergence of real-time and data communication. The traditional PBX
(Private Branch Exchange) was replaced by or evolved to an "IP-PBX" or "soft switch" that supported IP connectivity
to the desktop and IP connectivity between nodes. Direct IP-based transmission of multimedia between endpoints meant
that switching capabilities were no longer required, except gateways for interworking with "legacy" circuit-switched
networks and media servers for conference bridging, announcements, etc. The "IP-PBX" or "soft switch" was just
required to handle signalling.
IP-based enterprise networks are continuing to evolve, to support additional services, improved security, improved
Quality of Service (QoS), etc. Moreover, SIP has become the dominant signalling protocol. An enterprise network that
fully embraces IP technology and uses SIP as the signalling protocol is referred to here as a Next Generation CN
(NGCN). An NGCN could still contain some components that are not based on IP (e.g. traditional PISN components) or
that use signalling other than SIP (e.g. H.323 [i.5], H.248 [i.4]), but it would also include SIP components and be able to
interface externally using SIP.
Until recently, NGCNs generally fell back to legacy circuit-switched techniques for standardized communication
outside the enterprise, e.g. using public ISDN or circuit-switching over leased lines. Gateways provided the necessary
interworking of signalling and media. This was sometimes the case also for communication between different parts of
the same enterprise.
We are now witnessing a period when NGCNs are extending IP-based communication externally by interfacing to
public IP-based networks. This permits IP-based communication between:
• enterprise users supported by different NGCNs (i.e. different enterprises);
• enterprise users supported by different parts of the same NGCN at geographically dispersed locations
(different sites);
• enterprise users supported by NGCN and users supported by hosted enterprise services provided on public
network infrastructure;
• enterprise users supported by NGCNs and individual users of public networks (fixed or mobile); and
• enterprise users supported by an NGCN and users of legacy networks via a gateway outside the NGCN.
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Enterprise users in this context includes users of terminals based on IP and SIP and also users of legacy terminals
connected via gateways and other legacy equipment such as PBXs.
With this the NGCN no longer needs gateways to external legacy networks (except where required by existing
investment or economic considerations) and can enjoy the benefits of end-to-end IP-based communication with
appropriately equipped communication partners.
The public Internet is one example of a public IP-based network that an NGCN can use for external communications.
This can be used for direct connection between two enterprises, between two parts of the same enterprise (e.g. between
the main office and a branch or home office), or between an enterprise and a public telecommunications network. In
addition, some public network providers are starting to offer public IP-based networks that offer improvements
compared with the public Internet (e.g. in terms of QoS, security, mobility, applications, etc.) and are even basing their
entire telephony capabilities (including emulation of the PSTN/ISDN) on IP-based technology. These value added
public IP-based networks are collectively known as Next Generation Networks (NGN).
At present there are no defined standards for direct connection between enterprises. For connection between an
enterprise and a public network, various public network providers are offering their own specifications, based on SIP,
and there are some standardization activities, but these are at risk of being mutually incompatible. The present
document analyses enterprise requirements for IP- and SIP-based interworking with other networks, with a view to
influencing standardization and regulation. It also discusses communication between different parts of an enterprise,
including parts supported by enterprise infrastructure and parts supported through hosting on other network
infrastructures, including that of an NGN.
6 General concepts
6.1 Basic communication architecture
Communication networks in general and NGCNs in particular can be viewed as providing capabilities at three levels, as
shown in Figure 1:
• transport;
• session;
• application.
Communication network, e.g., NGCN
Application level
Session level
Transport level
Figure 1: Basic communication architecture
Capabilities at the transport level provide basic IP connectivity between physical items of equipment, such as servers,
PCs, phones, PDAs, etc., including connectivity to the outside world such as the public Internet and NGNs. This can be
used to transport signalling, media and other data. IP connectivity can be based on IP version 4 or version 6 and can
operate over different infrastructures (e.g. fixed or wireless, LAN or WAN) with different security mechanisms
(e.g. IEEE 802.1x [i.12], WiFi security). Capabilities present at the transport level include but are not limited to routing,
switching, firewall, network address translation (NAT), quality of service (QoS) provision, security, etc. Connectivity
may be limited to enterprise employees or may be extended to guests.
The session level provides capabilities in support of user-to-user communication, generally with a real-time element.
Communications can use a variety of media, including audio, video, real-time text, messaging, fax, etc., or a
combination of these. Within an NGCN (and likewise within an NGN) SIP signalling is assumed for establishment of
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communications. Users are not necessarily human users - in some cases automata (applications) can take the role of
users.
For the purposes of the present document, the application level comprises applications that have some relevance to
communication, as provided by the session level. This includes:
• applications that enhance communication capabilities in some way, e.g. advanced conferencing, presence;
• applications that use communication capabilities, e.g. a business process application that uses communication
capabilities for communicating with customers, suppliers, partners, etc.
The present document focuses on the session level, but also includes some discussion on the application level. The
transport level is mentioned only to the extent of how it impacts the session level, e.g. providing connectivity between
session level entities, the impact of NATs and firewalls.
Each level has its own security and management considerations and capabilities. In addition, security and management
have to be co-ordinated across all three levels, NGCN-wide.
The broad architectural framework described above can be realized in a number of ways. One example utilizes the IP
Multimedia Subsystem (IMS) specified by 3GPP, in which case the IMS layer corresponds to the session level
described above. The IMS layer runs on top of a transport layer and provides support to a service/applications layer.
The IMS layer itself comprises a number of functional components and inter-relationships. The present document
makes no assumptions regarding the use of IMS or any other architecture (beyond the framework described above and
the session level architecture described in clause 6.2) in NGCNs.
The three level architecture is applicable also to communication between networks. Two networks can each provide
functionality at each of the three levels, and interworking can take place potentially at all three levels, as shown in
Figure 2. Interworking does not need to occur right up to the application layer. For example, if applications are local to
the two networks, interworking could be just at the transport and session levels. If there is no real-time communication
session involved (e.g. data communication such as email or web, which is outside the scope of the present document),
interworking would be only at the transport level.
Network 1 (e.g., NGCN) Network 2 (e.g., NGN)
Application level
interworking
Application level Application level
Session level
interworking
Session level Session level
Transport level
interworking
Transport level Transport level

Figure 2: Overall architecture for inter-network operation
Similar principles can be extended to multiple networks in series, where intermediate networks might not be involved at
the higher levels. Figure 3 shows an example where networks 1 and 4 communicate up to the application level, with
network 2 involved only at the transport level (e.g. a transport service provider, TSP) and network 3 involved only up to
the session level (management and security omitted for clarity).
Network 1
(e.g., NGCN) Network 2 Network 3 Network 4
Application level Application level
Session level Session level Session level
Transport level Transport level Transport level Transport level

Figure 3: Example of operation through multiple networks
(management and security omitted for clarity)
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6.2 Session level architecture
During a session, two users (more in conferencing arrangements) exchange media information. To do this each user has
an endpoint device, which in the case of a human user can be a dedicated phone (fixed or wireless) or a PC or PDA
running a soft client or some higher level application. Other types of endpoint include application servers (e.g. media
applications, conferencing applications, presence applications) and gateways to non-IP-based or non-SIP-based
networks or equipment, etc. Establishment, clear down and maintenance of a session are achieved using signalling, the
protocol being SIP.
Signalling and media are transported separately between endpoints. Signalling typically passes through one or more
intermediaries, as shown in Figure 4, whereas media is often transported directly between endpoints (intermediaries
such as NATs and firewalls can be involved up to the transport layer). However, see also clause 6.2.2 for further
discussion of the media path.
Signalling
Intermediary
Endpoint Endpoint
Media path(s)
Signalling path
Figure 4: Session architecture
6.2.1 Signalling using SIP
SIP messages are used to exchange session descriptions using the embedded Session Description Protocol (SDP) [i.8].
Session descriptions indicate factors such as the media to be used, codecs, packet rates, security contexts and IP
addresses and port numbers for media reception.
Using SIP terminology, each endpoint contains a SIP user agent (UA). In principle, SIP can operate directly between
two UAs. However, the SIP standards do define other SIP entities that can assist. In particular the main SIP
standard [i.6] defines the concept of a location service at which UAs can register to assist in locating UAs representing a
particular user. Related to this are the concepts of a registrar (a SIP entity that registers UAs at the location service) and
a proxy (a SIP entity that queries the location service in order to assist in the forwarding of requests to appropriate
UAs).
NOTE: There is a redirect, which is similar to a proxy but redirects requests (by instructing an upstream entity to
remake the request to a different destination) rather than forwarding requests. For the purposes of the
present document a redirect is treated as a proxy unless otherwise stated.
The handling of a SIP request from one UA (the UA client, UAC) to another UA (the UA server, UAS) typically passes
through a proxy in the destination domain (as determined by the domain part of a SIP URI), which queries the location
service in order to locate UAs that might be able to handle the request. Also the UAC often sends requests to a local
proxy in the originating domain (known as an outbound proxy) in order to reduce the routing burden on the UAC. This
gives rise to the typical SIP trapezoid involving two UAs and two proxies (see Figure 5).
Domain 1 Domain 2
Proxy Proxy
UA UA
Figure 5: Typical SIP trapezoid
ETSI
14 ETSI TR 102 633 V1.1.1 (2008-10)

Sometimes a SIP request can result in the formation of an association between two UAs, in the context of which further
SIP requests in either direction can be sent. This is known as a dialog. The most important dialog in SIP is that initiated
by an INVITE request, because such a dialog has an associated session, comprising a collection of media between the
UAs. An INVITE-initiated dialog and its associated session can be regarded as a call. Proxies involved in routing the
dialog-initiating request may or may not remain involved in the dialog for routing subsequent requests.
Typically a proxy is collocated with a location serv
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