mechanisms suitable for the characteristics that all three exhibit. The remaining SIP requests follow common rules. OPTIONS, BYE, and REG- ISTER are treated in the same way with respect to reliability. In the next chapter, we will see that this commonality enables the protocol to be extended with new methods; a proxy will apply the common reliability rules to any unknown method. Therefore, regarding reliability, no difference appears between a BYE and any new method. The common reliability rules also employ the hop-by-hop mechanism used for INVITEs. A UA makes sure the request is received by the next proxy, and then the next proxy ensures that the following proxy in the path receives it, and so on. When the final response comes from the remote end, the proxy will ensure that it is delivered to the UA that originated the request. For reliable transport, the UA sends the request to the proxy. When the final response arrives, the proxy will return it to the UA also using reliable transport (Figure 5-30). Any provisional response that arrives before the final response is also sent to the UA over the reliable transport protocol. For unreliable transport, the UA has to ascertain that the proxy receives the request. When the proxy receives a response from the remote end, it has SIP: Protocol Operation Proxy Figure 5-30 The reliable transport ensures that both (1) BYE requests and responses arrive to (2) 100 Trying their destination. (3) 200 OK to ensure that the UA also receives it. The UA retransmits the request until the proxy proffers a final response. The proxy retransmits its final response as long as it continues receiving request retransmissions. When retrans- missions stop, the proxy determines that the UA has received the final response (Figure 5-31). Detailed Example Putting together all these pieces, we can now explore an example of how SIP works in detail. As noted, the examples usually consist of message flows that let us schematize SIP transactions and message exchanges. It is usu- ally not necessary to ex