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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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Figure 20-2 An example of S<strong>IP</strong> request/response flows<br />

20.2.3 S<strong>IP</strong> protocol architecture<br />

S<strong>IP</strong> architects used the Internet model during the development of the st<strong>and</strong>ard.<br />

As such, the architecture of S<strong>IP</strong> is similar to the architecture of HTTP, <strong>and</strong> both<br />

systems use a request-response model for communication. The calling device<br />

sends a request to the called device. The called device then chooses to accept<br />

or reject the request <strong>and</strong> returns the response to the initiator.<br />

Another feature of HTTP leveraged by S<strong>IP</strong> is the HTTP-based message formats.<br />

S<strong>IP</strong> reuses most of the header fields, encoding rules, <strong>and</strong> error codes of HTTP,<br />

providing a readable, text-based format for displaying information.<br />

A S<strong>IP</strong> address is referred to as a S<strong>IP</strong> Uniform Resource Locator (S<strong>IP</strong>-URL). It<br />

has the format sip:username@hostname.domain. This address is used to<br />

identify an individual device or group of devices. It enables a user to initiate a call<br />

by selecting a link in a st<strong>and</strong>ard browser.<br />

734 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong><br />

S<strong>IP</strong> Basic Call Flow<br />

S<strong>IP</strong> Device Server<br />

IN V IT E<br />

100 Trying<br />

180 Ringing<br />

PRACK (Optional)<br />

ACK<br />

BYE<br />

20 0 O K (A n sw er to op tio na l P R A C K )<br />

20 0 O K (A n sw e r to IN V IT E )<br />

RTP - 2 way voice path<br />

ACK

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