[ih] IPv4 address size debate

[John Day]

Once one understands the bigger picture, one realizes that question 
of variable vs fixed is a non-sequitor. But one does have to get free 
of the constraints of a Ptolemaic approach to architecture.

At which point fixed is variable.

At 21:30 -0500 2009/11/08, Noel Chiappa wrote:
>     > From: Bob Braden <braden at ISI.EDU>
>
>     > I have often wondered who was right. In the short run, you were
>     > probably right about the threat of OSI. In the long run, would variable
>     > length addresses have avoided the IPv4/IPv6 mess? I can only speculate.
>
>You've pressed one of my long-standing hot buttons.... :-)
>
>The right answer, IMO, to the question of "who was right" is 'both/neither'.
>To be a bit less cryptic, what _I_ would have pushed for (had I been more
>involved at that point; I was part of the MIT crew, and vividly recall David
>Reed reporting on this debate, but I was not yet part of the TCP/IP group)
>would have been a _packet format_ which _allowed_ for variable length, but
>'temporarily' subsetted it so that for the immediate term, the only
>allowed/supported value for the addresz length was '4'.
>
>IMNSHO this choice would have been better than either of the two other
>options, as it would have had the advantages of both (easy initial
>implementation, as well as long-term flexibility), and the disadvantages of
>neither.
>
>(As a corollary to that observation, it should be obvious that I feel that
>both sides of the debate had some good points; a less obvious corollary is
>that both had such force that it would probably have been unwise to entirely
>blow off either - as I believe blowing off the variable-length crew has
>shown.)
>
>
>To explore a related question a bit, I must confess I'm somewhat puzzled why
>this wasn't done. Maybe it's just me, but it seems to me to be a relatively
>obvious idea, and also, to me at least, 'obviously' better than the other
>alternatives.
>
>Yes, phasing in support for lengths other than 4 would not have been trivial,
>but it would have been possible without being unutterably painful (witness
>phasing in support for first A/B/C, and then later, CIDR; although I concede
>these were somewhat easier than variable-length addresses would have been).
>
>It would, I guess, have meant the header would be a full-word longer (since
>extending it by a short, to hold two byte-length address length fields, would
>have left it non-long-word aligned), i.e. more overhead, but I can't think of
>any other flat-out downside.
>
>Was this possibility discussed at the time, or was the choice seen as only all
>or nothing (i.e. only '32-bits/fully-variable')?
>
>	Noel