In this illustration we see a series of servers and clients arranged in a pyramid. At the top of the pyramid is a server labeled "SIP registrar upenn.edu." Below and to the left is a server labeled "SIP proxy umass.edu," and below to the right is a server labeled "SIP registrar eurcom.fr." Below these to the left is a computer labeled "SIP client 217.123.56.89," and below to the right is a computer labeled "SIP client 197.87.54.21."

A black arrow, labeled 1 and corresponding to a series of steps given in the text starting on page 710, extends upwards from SIP client 217 to SIP proxy umass.edu. An arrow labeled 2 extends from SIP proxy umass.edu to SIP registrar upenn.edu. An arrow labeled 3 extends from the same registrar back to the previous SIP proxy. An arrow labeled 4 extends horizontally rightward from SIP proxy umass.edu to SIP registrar eurcom.fr. An arrow labeled 5 extends downward from this SIP registrar to SIP client 197. An arrow labeled 6 extends back from this client to the previous registrar. An arrow labeled 7 extends leftward from SIP registrar eurcom.fr to SIP proxy umass.edu. An arrow labeled 8 extends downward from this SIP proxy to SIP client 217. And a blue arrow labeled 9 extends horizontally back and forth between the two SIP clients.