3.2.5.1 Receiving Message #1
On receipt of message #1, a NAT-T supporting host MUST check for the presence of the NAT-T vendor ID payloads that are specified in section 3.2.4.1. If NAT-T vendor ID payloads are present in the message, the host MUST set the Selected Revision for the corresponding MMSAD entry according to the following rules:
If both hosts support [RFC3947] and [DRAFT-NATT], the host MUST set the Selected Revision to [RFC3947]. For more information, see [DRAFT-NATT].
If both hosts share only one common revision, the host MUST set the Selected Revision to the common revision.
If the hosts do not share a common revision, the host MUST ignore the payload.
Then, the host MUST construct message #2 (as specified in [RFC2409] section 5) and add vendor ID payloads that advertise its NAT-T capabilities, setting the values of those payloads exactly as it would if it were constructing IKE message #1. For details, see section 3.2.4.