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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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► The proxy server provides application layer routing of S<strong>IP</strong> requests <strong>and</strong><br />

responses. When the server receives a request, it forwards the request to a<br />

next hop server having more information about the location of the called<br />

device. These are also intermediary entities, which can act as both a client<br />

<strong>and</strong> a server in order to make requests on behalf of other clients. These<br />

requests are either h<strong>and</strong>led internally or by passing them on to other servers.<br />

► Registrar servers are used to record the S<strong>IP</strong> address <strong>and</strong> the associated <strong>IP</strong><br />

address of a device.<br />

► The application server provides call processing, personal <strong>and</strong> group services,<br />

<strong>and</strong> service management.<br />

► The network server provides policy-based call routing <strong>and</strong> translations.<br />

► The media server provides specialized media resources, interactive voice<br />

response, three-way conferencing, <strong>and</strong> other regulatory requirements.<br />

S<strong>IP</strong> network servers usually implement a combination of the different server<br />

types.<br />

20.2.1 S<strong>IP</strong> request <strong>and</strong> response<br />

There are seven types (methods) of S<strong>IP</strong> requests:<br />

INVITE Indicates a user or service is being invited to participate in<br />

a call session.<br />

ACK Confirms that the client has received a final response to<br />

an INVITE request.<br />

BYE Terminates a call, <strong>and</strong> can be sent by either the caller or<br />

the callee.<br />

CANCEL Cancels any pending searches but does not terminate a<br />

call that has already been accepted.<br />

OPTIONS Queries the capabilities of servers.<br />

PRACK Provisional acknowledgement.<br />

REGISTER Registers the address listed in the “To” header field with a<br />

S<strong>IP</strong> server. Registrations can require authentication.<br />

The following types of S<strong>IP</strong> responses are used.<br />

S<strong>IP</strong> 1xx Informational responses (for example, 180 Ringing)<br />

S<strong>IP</strong> 2xx Successful responses (for example, 200 OK)<br />

S<strong>IP</strong> 3xx Redirection responses (for example, 302 Temporarily<br />

Moved)<br />

732 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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