North American Network Operators Group
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Re: Shim6 vs PI addressing

  • From: Iljitsch van Beijnum
  • Date: Wed Mar 01 13:10:30 2006 
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.