"Compatible on the wire" myth [Re: I-D Action:draft-azinger-scalable-addressing-00.txt]

[Brian E Carpenter]

On 2010-09-26 21:41, Michael Sinatra wrote:
....
> Obstacle 1: IPv6 is on-the-wire incompatible with IPv4.  This has been
> pointed out by many others (including Randy Bush in his various IPv6 vs.
> Operational Reality presentations), but the point still stands.  Could
> the IETF have created a protocol that was sufficiently compatible? 

Ever since I saw that slide of Randy's, it has been bothering me greatly.
Randy isn't known for propagating myths, but this is a myth.

An IPv4 host has no way to deal with an incoming packet that doesn't
conform to RFC 791. It has no way to deal with a version number other
than 4, and it has no way to deal with a destination address that
(unknown to it) is longer than 32 bits.

There is no such thing as an IP protocol with extended addressing
that is on-the-wire compatible with IPv4.

There are many ways we could have extended addressing to more than
32 bits with fewer changes than introduced by IPv6, but *none* of
them would be on-the-wire compatible with IPv4.

In retrospect, I wish we'd made fewer changes, but let's not turn
that into a myth that we could have been backwards compatible at
layer 3 without dual stack or NAT/PT.

    Brian