SIP UA in his laptop for making VoIP calls. When he is working in his office, he tends to use the SIP server at company.com. He knows the name of the domain, company.com, so he configures his SIP UA in order to send REGISTERs to that SIP server. Bob usually works in the university in the afternoon. When he gets there, he'll have to change the configuration of his SIP UA. He sends the REGISTERS to the proxy at university.com, and also configures this proxy as the outbound proxy. All requests from his UA will be sent first to this proxy because the university has a firewall that will only admit SIP mes- sages have traversed the proxy at university.com. This security proxy works much like the HTTP proxy used by many companies to access the Internet. Bob is accustomed to working in both the university.com and company.com domains, SO he knows the configuration data by heart. How- ever, in any other domain, he probably has no idea about how to configure his UA. This week Bob is visiting another university where he is collaborating with a colleague. When Bob connects his laptop to the Internet, he does not know his current domain name or his outbound proxy. He will most likely have to hunt down the network administrator of the university and ask for this information in person. Mechanisms are available for SIP server discovery that can make Bob's life easier when he is traveling around trying to use his SIP UA. With these mechanisms, Bob would not need to bother configuring his SIP UA. Just as an IP address is automatically assigned to his laptop when he connects to the Internet, the parameters needed to configure his SIP UA can be auto- matically delivered to his laptop. How It's Done This configuration process can be made automatic through a couple of ways. They are actually extensions to protocols other than SIP, but we consider them important enough for SIP applications to mention them. One option is to use Dynamic Host Configuration Protocol (DHCP) [RFC 2131] to retrieve the domain name of the SIP server. In the sam