3.1. Detecting Support of NAT-Traversal
The NAT-Traversal capability of the remote host is determined by an
exchange of vendor ID payloads. In the first two messages of Phase
1, the vendor id payload for this specification MUST be sent if
supported (and it MUST be received by both sides) for the NAT-
Traversal probe to continue. The content of the payload is the MD5
hash of