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Figure 2-27: SIP transaction example 
A typical SIP message may look like this [20]. In the INVITE line we find the SIP 
version. The SIP message attaches a Via header which indicates the IP address of the 
sender. Under the Via header is the From and To headers which indicates the sender and 
receiver addresses. In this example the SIP addresses are email addresses. Other possible 
forms of SIP addresses could be telephone numbers or first/middle/last name, assuming 
that it is unique. SIP identifiers are called SIP Uniform Resource Identifier (URI) [21]. 
The Call-ID line uniquely identifies the call. The Content-Type header defines the format 
used to describe the content contained in the SIP message. The last of the headers, the 
Content-Length, shows the length in bytes of the content in the message. After the 
empty line, which indicated the end of the header fields, comes the content. In this case 
the IP address to who sent the INVITE message and how the sender wants to receive 
the audio which is to be sent. 
 

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2.6 Background information: : Session Initiation Protocol 
Page 23 of 93 
INVITE sip:[e-mail] SIP/2.0 
Via: SIP/2.0/UDP 167.180.112.24 
From: sip:[e-mail] 
To: sip:[e-mail] 
Call-ID: [e-mail] 
Content-type: application/audio 
Content-Length: 885 
C=IN IP4 167.180.112.24 
M=audio 38060 RTP/AVP 0 
As mentioned in [22], a session which is to be established is described by a session 
description. This description contains enough information about the session for the 
remote user to join the session. The information includes IP addresses and port 
numbers. Session descriptions are created using standard format, and the most common 
one is the Session Description Protocol (SDP).The SDP protocol consists of two parts, a 
session-level information part and a media-level information part. SIP is session 
description independent, and therefore SIP doesn’t need to use the SDP protocol, but 
can use whatever description format it wants.