Shim6 vs PI addressing
stephen at sprunk.org
Fri Mar 3 18:50:03 UTC 2006
Thus spake "Iljitsch van Beijnum" <iljitsch at muada.com>
> On 1-mrt-2006, at 18:05, David Barak wrote:
>> Is it easier to scale N routers, or scale 10000*N hosts?
> 2 x relatively small is a lot less than 10 x relatively large. Or, in
> 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.
OTOH, hosts go a lot longer between upgrades and generally don't have
professional admins. It'll be a long, long time (if ever) until shim6 is
deployed widely enough for folks to literally bet their company on
>> 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
> 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)
At least this time we know what the likely problems are, and we can build in
safeguards that can be quickly implemented if we get too close to the edge.
Not that I agree we'll even get there...
> 2. We'll have to live with IPv6 a long time
Perhaps. I know the goal was for it to last 100 years, but what technology
has ever lasted that long without significant improvements that altered it
almost beyond recognition?
> 3. Route processing and FIB lookups scale worse than linear
With an mtrie+ FIB, routing lookups scale far, far better than linear. What
matters is the length of the prefix being matched, not how many there are.
TCAMs scale linearly, provided you can build them big enough (and costs
certainly aren't 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
Every time folks claim that, someone makes a breakthrough that continues the
curve. Surely we can't count on this forever, but so far money has
consistently trumped "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.
The requirements for getting a /24 are pretty darn lax, actually, and the
current proposals for PI space being debated within ARIN are significantly
The reality today is that v4 routing tables are well within our capabilities
and growing slowly. If we were on the verge of another serious problem,
like we where when the CIDR fire drill happened, ISPs could easily cut the
tables in half simply by filtering prefixes longer than RIR minima.
Stephen Sprunk "Stupid people surround themselves with smart
CCIE #3723 people. Smart people surround themselves with
K5SSS smart people who disagree with them." --Aaron Sorkin
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