rs are not SIP entities, but they are an important part of any architecture that uses SIP. A location server stores and returns possible Chapter 4 Registrar at Figure 4-13 company.com Bob registers his current position to the registrar. I am "Bob Johnson." Today I will be reachable at: sip: Sob@123.160.1.112 131.160.1.112 locations for users. It can make use of information from registrars or from other databases. Most registrars upload location updates to a location server upon receipt. Figure 4-14 shows how this is done. In Figure 4-15, the proxy server at company.com consults a location server for a SIP URL where Bob might be reachable. The location server can provide the server with this information because the registrar previously uploaded it. However, SIP is not used between location servers and SIP servers. Some location servers use Lightweight Directory Access Protocol (LDAP) [RFC 1777] to communicate with SIP servers. Good Features of SIP We have seen what functionality can be expected from the core SIP specifi- cation and which entities it defines. Now let's look at what makes SIP dif- ferent from and better than many other protocols. SIP Is Part of the IETF Toolkit The IETF designed SIP with the Internet paradigm in mind. As a tool in the IETF toolkit, it performs its role and then takes advantage of other Inter- net mechanisms to perform additional tasks. This provides great flexibility because systems using SIP in conjunction with other Internet protocols can be upgraded in a modular way. The Session Initiation Protocol: SIP Location server Figure 4-14 Registrar uploading information to a location server. I am "Bob Johnson." Today I will be reachable at: sip: Bob@131.160.1.112 SIP server (registrar and proxy) at company.com I am "Bob Johnson." 131.160.1.112 Today I will be reachable at: sip: Bob@131.160.1.112 Location server Figure 4-15 Proxy server consulting a location server. (2) Where is reachable (3) Try sip :Bob@131.160.1.112 sip :Bob.Johnson@company.com? SIP server (registrar and proxy) at comp