SIP Retransmissions

What is the problem with SIP retransmits?

Sometimes you get messages in the console like these:

retrans_pkt: Hanging up call XX77yy  - no reply to our critical packet. retrans_pkt: Cancelling retransmit of OPTIONs

The SIP protocol is based on requests and replies. Both sides send requests and wait for replies. Some of these requests are important. In a TCP/IP network many things can happen with IP packets. Firewalls, NAT devices, Session Border Controllers and SIP Proxys are in the signalling path and they will affect the call.