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2.6.1
 
Architecture 
SIP is based on already existing protocols, especially HTTP, but it also borrowing design 
principals from Simple Network Management Protocol (SNMP) [15]. This is an 
important strength since those two protocols are well-known and two of the most 
successful protocols made, and this makes SIP an attractive protocol. SIP is also to be 
used with other protocols such as the Real-time Transport Protocol (RTP) [16], RTSP, 
and Media Gateway Control Protocol (MeGaCo) [17]to build a complete multimedia 

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2.6 Background information: : Session Initiation Protocol 
Page 22 of 93 
architecture. All these streaming protocols will be introduced in chapter 2.7. Since SIP is 
a text-based protocol, it makes it easier to extend, debug, and to build services. Other 
benefits are rapid programmability, and scripting capabilities [18]. 
 
SIP does the following things: 
ƒ
Provides mechanisms for establishing calls between a caller and a callee 
over an Internet Protocol (IP) [19] network. 
ƒ
Provides a mechanism for the caller to determine the Current IP address 
of the callee. 
ƒ
Provides a mechanism for call management such as adding new media 
streams during the call, changing the encoding during the call, inviting 
new participants during the call, call transfer, and all holding. 
SIP is based on a request/response transaction model, just like HTTP. Each transaction 
consists of a request that invokes a particular method or function on a server, and at least 
one response. Both types of messages consist of a start line, one or more header fields, 
an empty line indicating the end of the header field, and an optional message body. The 
most important SIP message is the INVITE message. This SIP message is used to 
establish a session between to participants. 
An example of a SIP transaction is given in the figure below.