UA initiating the INVITE. In Figure 5-10, the proxy server ACKs non-successful responses in mes- sages (4) and (7). However, the UA ACKs the 200 OK response in message (11). This enables a proxy to try multiple locations without having to inform the originating UA about the unsuccessful attempts to locate the final user. Once a user responds positively to an INVITE, the originating UA must receive the remote session description in order to establish the session. SIP: Protocol Operation Stateful workstation 1234 Figure 5-10 Laura Stateful forking proxy. SIP proxy university.com 131.160.1.112 (1) INVITE (2) INVITE (3) 404 Not found (4) ACK (5) INVITE (6) 404 Not found (7) ACK (8) INVITE (9) 200 OK (10) 200 OK (11) ACK Conversation (12) BYE (13) 200 OK Stateless Proxy Stateless proxies don't keep any state. They receive a request, forward it to the next hop, and immediately delete all state related to that request. When a stateless proxy receives a response, it determines routing based solely on the Via header and it does not maintain state for it. (We'll discuss Via head- ers later in this chapter.) Distribution of Proxies An analysis of the IP traffic in a network invariably shows that the core is more stressed than the edges. This is true of SIP traffic as well. SIP servers in the core need to be able to handle many messages, whereas SIP servers in the periphery do not have to support equally as heavy loads. SIP is designed for stateless servers at the core. They perform Chapter 5 routing based on Request-URI or Via headers as fast and efficiently as we know how to do it. At the edges of the network, call stateful and stateful servers can be implemented to perform routing based on more complicated variables (such as the time of day or identity of the sender in the From field), or they can fork requests and provide services to the user. Distributing servers this way makes SIP a very scalable protocol that can be implemented in increasingly large networks such as the Internet. SIP keeps the core fast and simple and