Setting Up a Call to a Known IP Address
To understand the essence of SIP, it is best to take a look at a concrete example. In this example, Alice is at her PC and she wants to call Bob, who is also working at his PC. Alice’s and Bob’s PCs are both equipped with SIP-based software for making and receiving phone calls. In this initial example, we’ll assume that Alice knows the IP address of Bob’s PC. Figure 7.12 illustrates the SIP call-establishment process. In Figure 7.12, we see that an SIP session begins when Alice sends Bob an INVITE message, which resembles an HTTP request message. This INVITE message is sent over UDP to the well-known port 5060 for SIP. (SIP messages can also be sent over TCP.) The INVITE message includes an identifier for Bob (bob@193.64.210.89), an indication of Alice’s current IPaddress, an indication that Alice desires to receive audio, which is to be encoded in format AVP 0 (PCM encoded -law) and encapsulated in RTP, and an indication that she wants to receive