des of the same name. In strict routing, a proxy must use the first URI in the Route header field to rewrite the Request- URI, which is then forwarded. In loose routing, a proxy does not re-write the Request-URI, but either forwards the request to the first URI in the Route header field or it may forward the request to another loose routing element. In loose routing, the request must route through every server in the Route list (but may also route through other servers) before it may be routed based on the Request-URI. In strict routing, the request must only route through the set of servers in the Route header field with the Request-URI being rewritten at each hop. A proxy or UAC can tell if the next element in the route set supports loose routing by the presence of a lr parameter. An example is: Route: <sip:proxy@example.com; lr> Chapter 10 contains an example of the use of the Record-Route and Route header fields. Examples of Route header fields constructed from the example Record-Route header fields in Section 6.2.12 are: Route: <sip:firewall33.corporation.com lr>, <sip:proxy1.carrier.com;lr> Route: <sip: 139.23.1.44 lr> 6.2.29 RAck The RACK header field [10] is used within a response to a PRACK request to reliably acknowledge a provisional response that contained a RSeq header field. The RACK header field echoes the CSeq and the RSeq from the provisional SIP Header Fields response. The reliable sequence number is incremented for each response sent reliably. A call flow is shown in Figure 4.11. An example is: RACK: 8342523 1 INVITE 6.2.30 Session-Expires The Session-Expires header field [21] is used to specify the expiration time of the session. To extend the session, either UA can send a re-INVITE or UPDATE with a new Session-Expires header field. At the expiration of the interval in the Session-Expires, either UA may send a BYE and call- stateful proxies may destroy any state information. A proxy may shorten the expiration time by reducing the interval in the header field as it proxies the request. A UAS co