4.13 Reliability of Provisional Responses

When only the core protocol is used, SIP provisional responses are not transmitted reliably. Only requests and final responses are considered important and, thus, transmitted reliably. However, some applications need to ensure that provisional responses are delivered to the user agent client. For example, a telephony application may find it important to let the caller know whether or not the callee is being alerted. Since SIP transmits this information in a 180 (Ringing) provisional response this telephony application needs to use an extension for reliable provisional responses.

However, before describing such an extension, let us study how requests and final responses are transmitted. SIP is transport-protocol agnostic and, thus, can run over reliable transport protocols, such as TCP (Transport Control Protocol), and over unreliable transport protocols, such as UDP (User Datagram Protocol). The reader can find in the IEEE Network article [112] an evaluation of transport protocols for SIP that analyzes the pros and cons of UDP, TCP, and SCTP [308] to be used underneath SIP.