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Re: Upstreams blocking /24s? (was Re: How Not to Multihome)

  • From: Keegan . Holley
  • Date: Tue Oct 09 19:09:57 2007

[email protected] wrote on 10/08/2007 10:28:37 PM:

> Hi,
> On Oct 8, 2007, at 6:28 PM, Justin M. Streiner wrote:
> > On Mon, 8 Oct 2007, Jon Lewis wrote:
> >>>  adopted /24 as the cutoff point.  If you make the cutoff point  
> >>> smaller,
> >>>  what is the new point... /26?  /32?
> Presumably the fear is there being no limitation, that is, /32.
> >> Anything longer than /24 is unlikely to propagate far on the  
> >> internet.
> Pedantically speaking, there ain't no such thing as "the internet".

There ain't no such thing as ain't but somehow that term has been proliferated as well. (less pedantic)

> There are a series of interconnected private IP based networks, each  
> with their own policy about what they'll transmit and accept in terms  
> of routing updates.  What one ISP accepts and propagates is not  
> necessarily what the next ISP accepts and propagates.  

Unfortunately that also goes for the customers of that ISP.  So if one of the Tier I's decides not to accept my public /29 then the millions of
singlehomed subscribers go with it.  The idea of random AS's accepting and blocking a prefix scares the hell out of me. It's right under the idea of some director calling me into his office because some customer can't get to AOL subscribers and their NOC told us to beat it when we called and asked for the filters to be updated.

>What I'm  
> trying to understand is whether there is a sufficient critical mass  
> to define a consensus maximal prefix among those interconnected  
> networks.
> >> You can all check your filters to see.  I just checked mine, and  
> >> neither Level3 nor Time Warner has tried to send me anything  
> >> longer than /24 in recent history.  If they did, it'd show up as  
> >> hits on a distribute-list deny rule.
> >
> > I realize that - I was posing a rhetorical question to the previous  
> > poster :)
> The argument, as I understand it (and those who argue this direction  
> feel free to correct me if I misstate), is that as the IPv4 free pool  
> exhausts, there will be a natural pressure to increase address  
> utilization efficiency.  This will likely mean longer prefixes will  
> begin to be put (back) into use, either from assignments and  
> allocations that were "rediscovered" or from unused portions of  
> shorter prefixes.  Customers will approach ISPs to get these long  
> prefixes routed, shopping through ISPs until they find one that will  
> accept their money and propagate the long prefix.

Not if their engineering staff possess the gift of clue.. (See above)

> Now, of course announcing a route doesn't mean anyone will accept it,  
> but as I understand the theory, larger ISPs will agree to accept and  
> propagate longer prefixes from other larger ISPs if those other ISPs  
> will be willing to accept and propagate transmitted long prefixes  
> ("scratch my back and I'll scratch yours"), particularly if this  
> encourages the smaller ISPs to 'look for other employment  
> opportunities' when they can't afford the router upgrades.
> Personally, I fully expect the first part to happen.  Where I'm  
> having trouble is the second part (the accepting longer prefixes  
> part).  However, a few prominent members of the Internet operations  
> community whom I respect have argued strongly that this is going to  
> happen.  I thought I'd ask around to see what other folk think...

The DOD aside even if some of the larger ISP's are bribed into accepting the smaller blocks.  There are still some unanswered questions.  First there is no way to force every AS to accept the routes, so some medium sized transit as will respond with "not until ARIN makes us" and the long networks will have to reachibility to the subscribers of that AS.  Also, where do you stop? /26 /30?  The biggest argument against the short prefixes is stability.  Just imagine the route churn if I start advertising a /30 for some metro E link to China and then it starts flapping.  If this isn't enough picture 20 such links or 2000. Fiber cut anyone? Or if this is too unrealistic how about a random /27 owned by some colo customer who router is flapping constantly.  IMHO this is one instance where Pandora's box should remain closed.

> If people feel uncomfortable publicly stating their filter policy is,

Does anyone know how to write over my router in RPSL?

> I'd be happy to summarize responses sent to me directly, keeping  
> individual responses confidential.
> Regards,
> -drc