Related specifications
RFC 3581, An Extension to the Session Initiation Protocol (SIP) for Symmetric Response Routing, available at http://www.ietf.org/
Implementation notes
Public IP address and port are discovered using STUN when SIP registration is undertaken over UDP and STUN is enabled in the provisioned VoIP settings. This is used to check whether the public address seen by the SIP server differs from the address received in the network link setup.
Support is provided for the rport parameter in the Via header field. (RFC 3581) This enables a client to request that the server sends the response back to the request’s source IP address and port.
When SIP registration is undertaken over TCP (or over UDP when STUN is disabled in the provisioned VoIP settings), the Via header (from the first response to REGISTER) is checked for the received and rport parameters. These parameters advertise the public IP address and port as seen by the SIP server. If they are different from the local addresses received in the network link setup, there are NATs on the route. In this case the public IP address and port are obtained from the first response to REGISTER (when rport is used). The obtained address is then used in the next registration / re-registration in the Contact header.
Default port number 5060 is used for SIP signalling if not otherwise discovered.
For the symmetric SIP signalling, SIP requests and responses are received and sent from the same address and port.