I wrote a proposal back in 1999 or so exploring IPv4-in-IPv4 as an alternative to IPv6. You would have used the outer IP header to traverse the internet backbo

I wrote a proposal back in 1999 or so exploring IPv4-in-IPv4 as an alternative to IPv6. You would have used the outer IP header to traverse the internet backbone, and the inner IP header locally. It completely avoided the need to change any inter-domain infrastructure and, with some DNS-based gatewaying where the gateway to the Interent added/removed the outer IP header, could even be incrementally deployed to unmodified end systems (the idea was you would eventually modify end-systems/applications to add the outer header, as this simplifies applications and avoids the DNS hack). But in the end, IPv6 is cleaner, and it seemed better not to distract too much from progress there. If I'd known it would have taken another quarter century to make much progress, I might have made a different decision. There were quite a few "extend IPv4" solutions over the years.