On Jan 24, 2010, at 6:26 PM, valdis.kletni...@vt.edu wrote: > On Sun, 24 Jan 2010 17:01:21 EST, Steven Bellovin said: > >> Actually, Scott Bradner and I share most of the credit (or blame) for >> the change from 64 bits to 128. >> >> 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; > > 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