Ah, dual stacks, a time tested transition strategy. But there was some
Application Layer Gateway cruft (ALG) although not at the level of
sophistication and beauty of a NAT ...
From RFC 801:
Because all hosts can not be converted to TCP simultaneously, and
some will implement only IP/TCP, it will be necessary to provide
temporarily for communication between NCP-only hosts and TCP-only
hosts. To do this certain hosts which implement both NCP and IP/TCP
will be designated as relay hosts. These relay hosts will support
Telnet, FTP, and Mail services on both NCP and TCP. These relay
services will be provided beginning in November 1981, and will be
fully in place in January 1982.
Initially there will be many NCP-only hosts and a few TCP-only hosts,
and the load on the relay hosts will be relatively light. As time
goes by, and the conversion progresses, there will be more TCP
capable hosts, and fewer NCP-only hosts, plus new TCP-only hosts.
But, presumably most hosts that are now NCP-only will implement
IP/TCP in addition to their NCP and become "dual protocol" hosts.
So, while the load on the relay hosts will rise, it will not be a
substantial portion of the total traffic.