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

Tue Oct 9 02:28:37 UTC 2007

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 propogate far on the  
>> internet.

Pedantically speaking, there ain't no such thing as "the internet".   
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.  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.

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...

If people feel uncomfortable publicly stating their filter policy is,  
I'd be happy to summarize responses sent to me directly, keeping  
individual responses confidential.

Regards,
-drc