ignaling and media messages. In this configuration, each user agent is configured with a default outbound proxy server, to which it sends all requests. This proxy server typically will authenticate the user agent and may pull up a profile of the user and apply outbound routing services. In an interdomain exchange, DNS SRV queries will be used to locate a proxy server in the other domain. This proxy, sometimes called an inbound proxy may apply inbound routing services on behalf of the called party. This proxy also has access to the cur- rent registration information for the user, and can route the request to the called party. In general, future SIP requests will be sent directly between the two user agents, unless one or both proxies inserts a Record-Route header field. A proxy server can be either stateless or stateful. A stateless proxy server processes each SIP request or response based solely on the message contents. Once the message has been parsed, processed, and forwarded or responded to, no information about the message is stored-no dialog information is stored. A stateless proxy never retransmits a message, and does not use any SIP timers. Note that the stateless loop detection using Via header fields described in RFC 2543 has been deprecated (removed) in RFC 3261 in favor of the use of a man- datory Max-Forwards header field in all requests. Location DNS server server Inbound Outbound proxy proxy server server Media (RTP) Calling user Called user agent agent Figure 3.3 SIP Trapezoid. SIP: Understanding the Session Initiation Protocol A stateful proxy server keeps track of requests and responses received in the past and uses that information in processing future requests and responses. For example, a stateful proxy server starts a timer when a request is forwarded. If no response to the request is received within the timer period, the proxy will retransmit the request, relieving the user agent of this task. Also, a stateful proxy can require user agent authentication, as described in Section 3.8. The most