NAT affects SIP because SIP is a session protocol. The translation of the address occurs in the network headers leaving the SIP headers in the session layer untouched. Thus, the server is unable to contact the user using the addresses provided in the SIP headers. The following headers, Contact, Via, Route, and Record-Route are not automatically translated by the NAT device. Session Description Protocol (SDP) is also affected. See the points where some layer-3 addresses appear in a SIP request in the following highlighted code:
U 189.101.207.211:11266 -> 208.109.122.193:5060 INVITE sip:[email protected] SIP/2.0. Via: SIP/2.0/UDP 192.168.1.160:11266;branch=z9hG4bK-d8754z-1f2cd509;rport. Max-Fowards: 70. Contact: <sip:flavio@192.168.1.160:11266>. To: "8580"<sip:[email protected]>. From: "flavio"<sip:[email protected]>;tag=99494a4b. Call-ID: NmYwNjAzMDE3MTE0YWM5MWIxNjNiMWNjZDY3NjI0MWQ.. CSeq: 1 INVITE. Allow: INVITE, ACK, CANCEL, OPTIONS, BYE,...