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682 SIGNALING FOR VoIPRedirect Server. This server returns one or more addresses upon receiving arequest from a client (a UA or a proxy). It cannot initiate requests; its only functionis to respond to a client and provide new addresses to redirect the call. Afterservicing a request a redirect server returns control of the call to the source of therequest. In H.323 networks the gatekeeper performs the function of the redirectserver.Registrar. This server receives a user’s current location by servicing a REGISTERrequest from a UAC and then records that information in a location service (seebelow). The registrar’s only function is to receive and record location information,not to provide it to clients; that role is played by the location service. The registrarfunction is typically implemented in a server that also hosts proxy or redirectfunctions. The interface between a registrar and a location service is outside thescope of the SIP specification.Location Service. This server provides a called party’s possible locations to arequesting redirect or proxy server. A location service may obtain location informationfrom a registrar or from other sources. The interface between a locationservice and registrars, proxy, or redirect servers is outside the scope of the SIPspecification.21.3.3 SIP SessionsSIP sessions typically involve four components: two user agents (one for the source,the other for the destination) and two proxy servers (one proxying for the sourceand one for the destination). The standard configuration of a SIP connection(SIP trapezoid) is shown in Fig. 21.3-1.Two forms of message exchanges are used for signaling communication betweenSIP entities:. Dialog. TransactionA dialog is a logical connection between two user agents and was called a call leg inthe original version of SIP. A dialog starts when a called UA accepts and starts processinga request for a connection from a calling UA. In each UA a dialogis identified by a dialog ID, consisting of three parameters: Call-ID, local tag, andremote tag. The Call-ID is globally unique; that is, both UAs involved in a dialoghave the same Call-ID. The local tag is assigned separately by each UA; theremote tag has the value of the local tag of the other UA (see next section formore details on tags).A transaction is the basic building block of SIP signaling. It consists of anexchange of messages between a client and a server, encompassing a request andits responses. A call setup typically requires multiple transactions, depending on