This section introduces some basic SIP operations using a simple example. Let's examine this message sequence between two user agents in the next image. You can see several other flows associated with session establishment in the RFC3665.
The messages are labeled in sequence. In this example, userA uses an IP phone to call another IP phone over the network. To complete the call, two SIP proxies are used.
userA calls userB using its SIP identity, called the SIP URI. The URI is similar to an e-mail address such as
sip:userA@sip.com. A secure SIP URI can be used too, such as
sips:userA@sip.com. A call made using
sips: (secure SIP) will ...