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SIP – Why NAT and/or PAT is Insufficient?

There are a number of fundamental reasons why simple NAT and PAT are insufficient to resolve NAT traversal issues for VoIP traffic in general and for SIP signaling specifically. A small subset of the reasons:

• The SIP Standard requires a number of header fields to be specified, some of which need to contain the IP Address where responses need to be sent to. Since NAT and PAT operate only on the IP headers, and not on the UDP Payload, the WAN-to-LAN device does not address this issue at all.
• SIP does not only perform the tasks of setting up, adjusting calls, and tearing down calls, but also incorporates the media codec negotiation phase inside an embedded protocol called SDP (Session Description Protocol). Again, the WAN-to-LAN device does not address this additional content at NAT or PAT level .

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