tries only one location, but several proxies are in the path between UAs (Figure 4-11). Forking Proxies When a proxy server tries more than one location for the user, it is said to fork the invitation. Forking proxies can perform par- allel or sequential searches depending on their configuration. A parallel search consists of trying all of the possible locations at the same time, whereas a sequential search consists of trying each location individually. Group Addresses Proxy servers also create group addresses. Figure 4-12 shows a forking proxy receiving an invitation for SIP:sales@company.com and trying all persons in the sales department until it finds one who is available. SIP server at Figure 4-11 company.com Scenario with two proxy servers. (1) Invitation to a session for SIP: Bob.Johnson@company.com (2) Invitation to a session for SIP: Bob@university.com Workstation1234.university.com SIP server at university.com (3) Invitation to a session for SIP: Bob@workstation1234.university.com The Session Initiation Protocol: SIP Figure 4-12 The forking proxy tries different (2) Invitation to a session for salespersons until it SIP: sales-person-1@company.com reaches one who is (3) I am busy not busy. (1) Invitation to a session for SIP: sales@company.com SIP server at company.com Workstation1234.university.com (4) Invitation to a session for SIP: sales-person-2@company.com (5) OK, I am available During session establishment, it is not uncommon for both kinds of servers (proxies and redirects) to be involved. The general term SIP server refers to both kinds of servers without differentiating on the basis of behav- ior. Actually, the same SIP server can act as a redirect or as a proxy depend- ing on the situation. For instance, a SIP server can redirect all session invitations received for certain individuals and proxy the rest. Registrars Registrar refers to a SIP server accepting registrations. A registrar is usu- ally co-located with a redirect server or a proxy server (Figure 4-13). Location Servers Location serve