Jnc: How we really messed up

2026-09-05T19:55:54Z

How we really messed up

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==Colossal fail==

I have a personal memory of how we all lost the variable-length addresses of IPv3. I was not actually in the meeting where this happened (at MIT-LCS, 15-16 June 1978), but just down the hall in my office (I hadn't yet been promoted into the group); Dave Reed told me what happened a few days later, and the story has stuck with me (see below for why).

So, during that TCP meeting, Bill Plummer of BBN (who wrote the [[TENEX]] TCP) was observed talking to Vint Cerf; Dave overheard him complaining that he had a limited number of registers available at interrupt time (since he needed registers for [[PDP-10]] byte pointers - needed to deal with variable-length fields). I don't know whether this tipped the scales on the fixed-length addresses used in [[IPv4]], but it cannot have helped. Certainly, the use of variable-length addresses made the work involved in getting a basic experimental system up and running considerably harder.

I have always felt that the fact that I wasn't invited to attend prevented me from making what would have been my greatest contribution to internetting. I have long understood that interfaces are the things which have the greatest lifetime in any technological system. (The example I use is the screw light-bulb base: forget the technology on either side; the ''basic science'' behind the technology on either side has changed out of all recognition - fission reactors and [[Light Emitting Diode|LED lights]]. Those weren't even ''known'' when the base was invented; but we're still using that base.) The packet formats are a key part of interfaces in networking; look at [[NAT]] - the ''syntax'' of the 'address' fields is the same; the ''semantics'' has changed somewhat. So the adoption of the fixed-length addresses really screwed us; retention of the variable-length addresses would have been a massive boon ''forever''.

I understand ''why'' the variable-length addresses were removed; however, I feel that had I been in the room, I could/would have suggested an intermediate path, one that gave the advantages of both. I would have suggested retaining the IPv3 packet formats (perhaps rearranged a bit, to improve alignments), but saying that for the moment, the ''only'' acceptable values for the address lengths would be '4'. In the short run, this would have made something that was as easy to implement as IPv4. In the long run, it would have allowed a slow, ''interoperable'', upgrade to longer addresses.

Yes, sites that weren't upgraded to support them would not have been able to communicate with sites that had been given longer addresses - but that would have been reasonably workable. (Look at the transition from 32-bit to 96-bit [[ARPANET]] headers. Sites that hadn't upgraded to support 96-bit leaders could not communicate with sites that had been given such addresses - but it worked.) [[User:Jnc|Jnc]] ([[User talk:Jnc|talk]]) 19:55, 5 September 2026 (UTC)

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Jnc: How we really messed up