What was sae-urn protocol on port 4500?

Port 4500 is a documented home to a couple of standards:

• 4500 – ipsec-nat-t – IPSec NAT Traversal
• 4500 – sae-urn

IP-Sec NAT traversal is explained in a number of RFCs:

• rfc3947 – Negotiation of NAT-Traversal in the IKE
• rfc3948 – UDP Encapsulation of IPsec ESP Packets
• rfc7296 – Internet Key Exchange Protocol Version 2 (IKEv2)
• rfc8229 – TCP Encapsulation of IKE and IPsec Packets

But what is sae-urn?

The oldest reference i can find to “sae-urn” comes from the port service mapping of nmap from May 21, 1997:^🕗

sae-urn         4500/tcp            # sae-urn
sae-urn         4500/udp            # sae-urn

Someone tried asking on a BSD newsgroup in 2002 what sae-urn is – but in typical Stackoverflow fashion, people condescendently didn’t answer the question.

Whatever the sae-urn was, it seems to have been lost to the sands of time.