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2.6.2
 
Call invitation example 
Here we give an example [20] in use of the SIP protocol. In this example Alice is at her 
PC and wants to communicate with Bob who is on his PC. To make the example on how 
to set up a call using SIP more understandable, let’s say Alice knows Bob’s IP address. 
The SIP session starts with an INVITE message from Alice. This message is sent over 
User Datagram Protocol (UDP) [23] to the well-known port 5060. This message includes 
an identifier for Bob, Alice’s current IP address, an indication that Alice wants to receive 
an audio encoded in Pulse Code Modulation (PCM) format and encapsulated in RTP, 
and an indication that she wants to receive the audio on port 38060. Bob responds this 
INVITE message with a 200 OK, which includes Bob’s IP address, his desired encoding 
and packetization for reception, and the port number which the audio should be sent. 

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2.6 Background information: : Session Initiation Protocol 
Page 24 of 93 
Figure 2-28: SIP call establishment [20] 
In the example above, where Alice sends an INVITE message to Bob, she knows Bobs 
IP address. This is not particularly realistic. What Alice has to do when she doesn’t know 
Bobs IP address, is to send the INVITE message to a SIP proxy server which is a SIP 
router. The SIP proxy server will inspect the request URI and respond on Alice’s request 
with the IP address to the device Bob currently is using. The SIP proxy server knows all 
of Bobs IP addresses because of the SIP registrar. Whenever a user launches a SIP 
application on a device, the application sends an SIP register message to the registrar, and 
informs the registrar of its current IP address. The registrar can therefore keep track of 
Bobs IP address, and whenever Bob is switching to a different device, the device is 
sending a register message. If a user is staying on the same device for a long time, the 
device will send a refresh message to let the registrar know that the user is still on the 
same device and that the IP address is still valid. 
SIP is an out-of-bound protocol which means that the SIP messages are sent and 
received in sockets that are different from those used to end and receive the media data. 
As we can see from the figure, SIP also requires all messages to be acknowledged so it 
can be run over Transmission Control Protocol (TCP) [24]or UDP. 

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2.7 Background information: : Streaming Protocols 
Page 25 of 93 
Additionally, in [14]SIP is structured as a layered protocol. This means that the protocols 
behavior is described in terms of a set of fairly independent processing stages with only a 
loose coupling between each stage. The lowest layer is the syntax and encoding. The next 
layer is the transport layer which defines how a client sends and receives messages and 
how servers receives requests and respond to these. The third layer is the transaction 
layer which handles application-layer retransmissions, matching of responses to requests, 
and application-layer timeouts. The next layer is the transaction user layer.