s than other methods. SIP: Protocol Operation Hop-by-Hop Treatment Remember that when proxies are in the path between two UAs, different transport protocols may also be between them (Figure 5-22). A UA using a reliable transport protocol towards a proxy can- not assume that same transport will be used end to end until the remote UA is reached. SIP provides a mechanism to ensure that the INVITE will eventually be delivered; namely, it makes proxies responsible for getting an INVITE to the next hop in its path. Note that stateless proxies cannot assume this responsibility because they don't maintain the state information needed for retransmission when an INVITE gets lost. Therefore, next hop with respect to transport refers to the next stateful proxy (or the destination UA). Transmitting an INVITE Because both a UA and a proxy have the same responsibility of ensuring that the INVITE reaches the next hop, the mechanisms used between a UA and a proxy, between two proxies, and between a proxy and a UA are exactly the same. In this section, we will explain the behavior of a UA sending an INVITE to a proxy, but that proxy will use exactly the same mechanisms towards the next proxy in the path. A SIP UA sending an INVITE to a proxy over a reliable transport proto- col does not have to perform any special task, but if an unreliable transport protocol such as UDP is used, it must be prepared to retransmit, sometimes repeatedly, until a previrional response is received. (Figure 5-23). Proxy servers receiving an INVITE always generate a 100 Trying provi- sional response. A UA receiving an INVITE can generate any provisional response, such as 180 Ringing. Laura Proxy Proxy Proxy Figure 5-22 Different types of transports are between Laura and Bob. Chapter 5 Proxy Figure 5-23 INVITES retransmit until a provisional (1) INVITE response arrives. (2) 100 Trying (3) INVITE (4) INVITE (5) 100 Trying Transmitting Responses to an INVITE We have seen that provisional responses are used to prevent hop-by-hop INVITE retransmissions. Howev