[121682] in North American Network Operators' Group
Re: Using /126 for IPv6 router links
daemon@ATHENA.MIT.EDU (Steven Bellovin)
Sun Jan 24 18:41:39 2010
From: Steven Bellovin <smb@cs.columbia.edu>
In-Reply-To: <31219.1264375574@localhost>
Date: Sun, 24 Jan 2010 18:41:18 -0500
To: Valdis.Kletnieks@vt.edu
Cc: "nanog@nanog.org" <nanog@nanog.org>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
On Jan 24, 2010, at 6:26 PM, Valdis.Kletnieks@vt.edu wrote:
> On Sun, 24 Jan 2010 17:01:21 EST, Steven Bellovin said:
>=20
>> Actually, Scott Bradner and I share most of the credit (or blame) for
>> the change from 64 bits to 128.
>>=20
>> During the days of the IPng directorate, quite a number of different
>> alternatives were considered. At one point, there was a compromise
>> proposal known as the "Big 10" design, because it was propounded at =
the
>> Big Ten Conference Center near O'Hare. One feature of it was =
addresses
>> of length 64, 128, 192, or 256 bits, determined by the high-order two
>> bits. That deal fell apart for reasons I no longer remember;
>=20
> I don't remember the details of Big 10, but I do remember the general =
objection
> to variable-length addresses (cf. some of the OSI-influenced schemes) =
was the
> perceived difficulty of building an ASIC to do hardware handling of =
the
> address fields at line rate. Or was Big 10 itself the compromise to =
avoid
> dealing with variable-length NSAP-style addresses ("What do you mean, =
the
> address can be between 7 and 23 bytes long, depending on bits in bytes =
3, 12,
> and 17?" :)
Precisely. The two bits could feed into a simple decoder that would =
activate one of four address handlers; depending on your design, they =
could all run in parallel, with only the output of one of them used. =
There were only four choices, all a multiple of 8 bytes.
But my goal is not to revisit the design issues, but rather to clarify =
the historical record.
--Steve Bellovin, http://www.cs.columbia.edu/~smb
