What are "re-transmissions" in SIP?

Re-transmissions in SIP (Session Initiation Protocol) refer specifically to the practice of resending SIP messages to ensure they are received, particularly in scenarios where the original message may not have been acknowledged or received by the intended recipient. This is crucial in SIP because it operates over UDP (User Datagram Protocol), which does not guarantee delivery of packets. If an endpoint does not receive a response within a certain timeframe, it will resend the initial request (like an INVITE or REGISTER) to ensure it reaches the destination and that the session can be established or maintained.

This process helps maintain the reliability of communications in SIP, allowing systems to effectively recover from potential packet loss that could occur in the network. The need for re-transmissions highlights the importance of managing state and ensuring successful exchanges between SIP endpoints to facilitate voice calls, video conferences, and other interactive media sessions.