g, remote tag, Call - ID, and CSeq) is received when T1 expires, the request is re-sent. If a provisional (informa- tional class 1xx) response is received, the UAC or stateful proxy server ignores T1 and starts a new longer timer T2. No retransmissions are sent until T2 expires. After a request is retransmitted, the timer period is doubled until T2 is reached. After that, the remaining retransmissions occur at T2 intervals. This capped exponential backoff process is continued until a maximum of 10 retrans- missions at increasing intervals are sent. A stateful proxy server that receives a retransmission of a request discards the retransmission and continues its retrans- mission schedule based on its own timers. Typically, it will resend the last provi- sional response. For an INVITE request, the retransmission scheme is slightly different. After a provisional (1xx) response is received, the INVITE is never retransmit- ted. However, a proxy may discard transaction state after 3 minutes. SIP Clients and Servers A stateful proxy must store a forwarded request or generated response mes- sage for 32 seconds. An example message flow involving two user agents, a state- ful proxy, two lost messages (shown by "X" in Figure 3.6), and three retransmissions is shown in Figure 3.6. In this example, the OPTIONS sent by the stateful proxy server to the UAS is lost. As a result, it is retransmitted when the proxy's T1 timer expires and no response is received. The 200 OK for- warded by the proxy is also lost. When timer T1 in the UAC expires without a final response (non 1xx response), the OPTIONS is retransmitted. When the proxy receives the retransmitted OPTIONS, it deduces that the 200 OK was lost and resends it. The proxy recognizes the 200 OK as a retransmission and does not forward it. Suggested default values for T1 and T2 are 500 ms and 4 seconds, respec- tively. Timer T1 is supposed to be an estimate of the round-trip time (RTT) in the network. Longer values are allowed but not shorter ones, because this will generate mo