What does rport without port number mean in SIP

When you have a SIPĀ Via: header like

Via: SIP/2.0/UDP;rport;branch=z9hG4bKabc123

theĀ rport parameter means respond to the port from which the request was received

This is extremely useful in case of NAT traversal because you don’t know upfront which outgoing NAT port your packets will be mapped to.