The word alignment issue was very strong and the router people had considerably
more influence than the host folks. I tried to propose variable length
addressing using four bit nibbles in August 1974 and I got no traction at all.
Steve
Sent from my iPhone
On Feb 14, 2012, at 6:31 PM, Bob Braden <braden(_at_)isi(_dot_)edu> wrote:
On 2/13/2012 7:53 PM, Noel Chiappa wrote:
> From: Brian E
Carpenter<brian(_dot_)e(_dot_)carpenter(_at_)gmail(_dot_)com>
> The design error was made in the late 1970s, when Louis Pouzin's
advice
> that catenet addresses should be variable length, with a format
prefix,
> was not taken during the design of IPv4.
Ironically, TCP/IP had variable length addresses put in _twice_, and they
were
removed both times! (You can't make this stuff up! :-)
Noel,
You probably remember this, but...
Within the ARPA-funded Internet research program that designed IP and TCP,
Jon Postel and
Danny Cohen argued strenuously for variable length addresses. (This must have
been
around 1979. I cannot name most of the other 10 people in the room, but I have
a clear mental picture of Jon, in the back of the room, fuming over this
issue. Jon believed
intensely in protocol extensibility.)
However, Vint Cerf, the ARPA program manager, rules against variable length
addresses and
decreed the fixed length 32 bit word-aligned addresses of RFC 791. His
argument was that
TCP/IP had to be simple to implement if it were to succeed (and survive the
juggernaut
of the ISO OSI protocol suite).
System programmers of that day were familiar with word-aligned data
structures with fixed offsets, and variable length addresses seemed to be
(and in fact
would be) harder to program and would make packet dumps harder to interpret.
It was a political as much as a technical judgment, and Vint may have been
right ... we
can never know. We do know that IP eventually succeeded and OSI failed, but it
was a near thing for awhile. Of course, there were other factors in the
success
of IP, such as Berkeley Unix.
It is to be noted that when it came time to define IPv6 some 20 years later,
the IETF
stuck with fixed length internet addresses.
Bob Braden
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