[89047] in North American Network Operators' Group
Re: Shim6 vs PI addressing
daemon@ATHENA.MIT.EDU (Iljitsch van Beijnum)
Wed Mar 1 13:09:47 2006
In-Reply-To: <20060301170517.27261.qmail@web31810.mail.mud.yahoo.com>
Cc: NANOG list <nanog@nanog.org>
From: Iljitsch van Beijnum <iljitsch@muada.com>
Date: Wed, 1 Mar 2006 19:08:59 +0100
To: David Barak <thegameiam@yahoo.com>
Errors-To: owner-nanog@merit.edu
On 1-mrt-2006, at 18:05, David Barak wrote:
> Is it easier to scale N routers, or scale 10000*N hosts?
Is it easier for the government to make a 5 year plan or for everyone
to spend time and energy finding the best deal for everything?
Every router has to search through its FIB tables for every packet it
forwards. That's something like 10 FIB lookups for every packet
flowing between two hosts. The hosts only have to search through
their TCBs for every packet. Number of TCBs in nearly all hosts is
smaller than the average FIB size (even if you consider that many
routers don't have a full table). 2 x relatively small is a lot less
than 10 x relatively large. Or, in other words: on the host you only
pay if you actually communicate. In routers, you pay more as there is
more routing information, whether the extra information is used or not.
> If we simply moved to an "everyone with an ASN
> gets a /32" model, we'd have about 30,000 /32s. It
> would be a really long time before we had as many
> routes in the table as we do today, let alone the
> umpteen-bazillion routes which scare everyone so
> badly.
1. We've already walked the edge of the cliff several times (CIDR had
to be implemented in a big hurry, later flap dampening and prefix
length filtering were needed)
2. We'll have to live with IPv6 a long time
3. Route processing and FIB lookups scale worse than linear
4. If the global routing table meltdown happens, it will be extremely
costly in a short time
5. Even if the meltdown doesn't happen a smaller routing table makes
everything cheaper and gives us more implementation options (5000
entry TCAM is nice, 500000 entries not so much as it basically uses
100 times as much power)
6. Moore can't go on forever, there are physical limitations
But the most important thing we should remember is that currently,
routing table growth is artificially limited by relatively strict
requirements for getting a /24 or larger. With IPv6 this goes away,
and we don't know how many people will want to multihome then.