shim6 @ NANOG (forwarded note from John Payne)

Iljitsch van Beijnum iljitsch at
Thu Mar 2 15:51:28 UTC 2006

On 2-mrt-2006, at 16:20, Mark Newton wrote:

>>> Now, some may take that as a sign the IETF needs to figure out how
>>> to handle 10^6 BGP prefixes...  I'm not sure we'll be there for a
>>> few years with IPv6, but sooner or later we will, and someone needs
>>> to figure out what the Internet is going to look like at that point.

>> It won't look good. ISPs will have to buy much more expensive
>> routers. At some point, people will start to filter out routes that
>> they feel they can live without and universal reachability will be a
>> thing of the past.

> But don't we filter out routes we feel we can live without *right now*
> without the world ending?

Did I say that the world would end? All of this is important stuff  
but fortunately not THAT important...

> I mean, who accepts prefixes longer than /24 these days anyway?
> We've all decided that we "can live without" any network smaller
> than 254 hosts and it hasn't made a lick of difference to
> universal reachability.

...because everyone who can't get a /24 will find another way to  
connect to the internet. Now suppose that today we all filter at /24  
but tomorrow we start...

> What's to stop someone who wants to carry around less prefixes from
> saying, "Bugg'rit, I'm not going to accept anything smaller than
> a /18"?

This will break connectivity to MANY parts of the internet. And yes,  
you can do this if you want. Your customers may not like the new  
policy, though. A more realistic scenario would be to go from /24 to / 
23, which will make your routing tables a lot smaller but not break  
_too_ much. If enough people start doing this, people who have a /24  
will have to renumber into a larger block or move to PA space. And  
two years later it's /22 and so on. At this point, the people at the  
short end of that stick may start to think that shim6 wasn't so bad  
after all.

Note that in IPv6 all of this will play out differently because you  
basically have /32s for most ISPs (some shorter ones as well of  
course, but no longer ones for ISPs) So filtering on prefix length  
(other than as a safety mechanism against accidental deaggregation)  
won't be useful. Today you can deaggregate a /16 into 256 /24s, but  
deaggregating a /32 into 65536 /48s is of course more problematic.

But maybe in IPv6 people will filter out stuff from other regional  
registries. Especially when they discover multihoming in Asia.

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