The SIP Architecture in Visual Studio .NET

Encode Data Matrix 2d barcode in Visual Studio .NET The SIP Architecture

The SIP Architecture
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SIP is a signaling protocol that handles the setup, modification, and teardown of multimedia sessions SIP, in combination with other protocols, is used to describe the session characteristics to potential session participants Although strictly speaking, SIP is written such that the media to be used in a given session could use any transport protocol, the media will
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Figure 5-1 Participation in SIPit
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The first seven events were known as bake-offs The Pillsbury company claimed, however, that the term bake-off is proprietary to that company Therefore, the term was dropped for subsequent interoperability-testing events
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The Session Initiation Protocol (SIP)
5
commonly be exchanged using the Real-Time Transport Protocol (RTP) as the transport protocol It is likely that SIP messages will pass through some of the same physical facilities as the media to be exchanged SIP signaling should be considered separately from the media itself, however Figure 5-2 shows the logical separation between signaling and session data This separation is important, because the signaling may pass via one or more proxy or redirect servers while the media stream itself takes a more direct path This approach can be considered somewhat analogous to the separation of signaling and media already described for H323
SIP Network Entities
SIP defines two basic classes of network entities: clients and servers Strictly speaking, a client (also known as a user agent client) is an application program that sends SIP requests A server is an entity that responds to those requests Thus, SIP is a client-server protocol VoIP calls using SIP are originated by a client and terminated at a server A client may be found within a user s device, which could be a PC with a headset attachment or a SIP phone, for example Clients may also be found within the same platform as a server For example, SIP enables the use of proxies, which act as both clients and servers Four different types of servers exist: proxy server, redirect server, useragent server, and registrar A proxy server acts in a similar way to a proxy server used for web access from a corporate local area network (LAN) Clients send requests to the proxy and the proxy either handles those requests itself or forwards them on to other servers, perhaps after performing some translation To those other servers, it appears as though the message is coming from the proxy rather than some entity hidden behind it
Figure 5-2 The separation of signaling and media
SIP User
SIP Signaling IP Network SIP User RTP Media Stream
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The Session Initiation Protocol (SIP)
The Session Initiation Protocol (SIP)
Given that a proxy both receives requests and sends requests, it incorporates both server and client functionality Figure 5-3 shows an example of the operation of a proxy server It does not take much imagination to realize how this type of functionality can be used for call forwarding, time-of-day routing, or follow-me services For example, if the message from the caller to Collins in Figure 5-3 is an invitation to participate in a call, the net effect is that the call is forwarded to Collins at home Of course, it is necessary that the proxy be aware that Collins happens to be at home instead of at work A redirect server is a server that accepts SIP requests, maps the destination address to zero or more new addresses, and returns the new address to the originator of the request Thereafter, the originator of the request can send requests directly to the address(es) returned by the redirect server A redirect server does not initiate any SIP requests of its own Figure 5-4 shows an example of the operation of a redirect server This can be another means of providing the call-forwarding/follow-me service that can be provided by a proxy server The difference is that, in the case of a redirect server, the originating client does the actual forwarding of the call The redirect server simply provides the information necessary to enable the originating client to correctly route the call, after which the redirect server is no longer involved A user-agent server accepts SIP requests and contacts the user A response from the user to the user-agent server results in a SIP response on behalf of the user In reality, a SIP device (such as a SIP-enabled telephone) will function as both a user-agent client and a user-agent server Acting as a user-agent client, it is able to initiate SIP requests Acting as a user-agent
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