[86416] in North American Network Operators' Group
Re: classful routes redux
daemon@ATHENA.MIT.EDU (Todd Vierling)
Thu Nov 3 15:30:49 2005
Date: Thu, 3 Nov 2005 15:29:35 -0500 (EST)
From: Todd Vierling <tv@duh.org>
To: "Stephen J. Wilcox" <steve@telecomplete.co.uk>
Cc: Fred Baker <fred@cisco.com>, Bill Woodcock <woody@pch.net>,
bmanning@vacation.karoshi.com,
Richard A Steenbergen <ras@e-gerbil.net>, nanog@merit.edu
In-Reply-To: <Pine.LNX.4.44.0511030051560.25860-100000@server2.tcw.telecomplete.net>
Errors-To: owner-nanog@merit.edu
On Thu, 3 Nov 2005, Stephen J. Wilcox wrote:
> well, /56 /48 /32 seem to have resonance but are not special in any way
Well, they are somewhat special. All of them are on eight-bit boundaries.
The importance of this comes in when deciding how to lay out a routing table
in a gate array or memory-based table.
A routing table capable of handling a flat 2^128 addressing space goes
beyond the realm of known physics -- and flat 2^64 comes close, at least for
a while (consider semiconductor atomic weights, and the fact that 1 mole is
approximately 2^79 atoms). That's quite a stretch, but should give a hint
as to why flat addressing does not work for every model.
Routing tables become much simpler when you have N-level (tree-like) tables,
a concept also used in MMUs. A tree done one bit at a time, while the most
compact form in many cases, is not very efficient at lookups. If you divide
the bitspace into sized chunks, the lookup time can be a better tradeoff
between speed and tree size.
Specifically, 8-bit dividing lines make this even easier. Much logic
programming (FPGA or similar) depends on power-of-two data sizes with a
minimum of 4 or 8 bits. This has led to well established 4-bit and 8-bit
data movement patterns that have been better optimized over time. If using
a store-and-forward mechanism with a more generic data processor (such as a
CPU), 8-bit dividing lines are all the more important for speed.
Or in summary of all of the above, "8-bit building blocks in routing tables
make writing the physical routing code much easier, and in many cases makes
the forwarding operation much faster."
--
-- Todd Vierling <tv@duh.org> <tv@pobox.com> <todd@vierling.name>
