Re: Minutes / Notes

[Pekka Savola <pekkas@netcore.fi>]

On Fri, 18 Jul 2003, Joe Abley wrote:
> > ISPs' networks don't crash every other day (if they do, change the 
> > ISPs;
> > we just can't design protocols to work around broken operational
> > practices: that's going to fail, no matter what).  Might happen a 
> > couple
> > of times during the year, at most, but I'm not convinced that's
> > necessarily a huge problem.
> 
> This isn't about ISPs' networks "crashing" -- it's about transient 
> changes in routing topology, which *do* happen with monotonous 
> regularity (circuits fail, or take errors and have to be shut down; 
> upstream providers fat-finger routing policy and stopping routes from  
> propagating, etc). 

Fat-fingering in critical places is a real problem -- more so than many 
other problems like circuit failures, but most of these are not visible to 
the edge network.

> Also remember that the effects of a re-homing event 
> can be felt several levels down; if your path to the DFZ is through 
> three intermediate ASes, then you're three-times as likely to feel the 
> impact of a re-homing event if the multihoming architecture presents an 
> impact to be felt. In some countries, *all* the ISPs are two or three 
> ASes away from the core.

Nope, it's not as simple as that.

Please remember the context here: we're talking about the connection
surviability of the end-user.  What happens several AS's upstream doesn't
typically matter *at all*.  Sure, some circuit goes down, and traffic gets
re-routed.  These almost always cause a quick re-routing, not e.g. falling
back to BGP hello timeouts. These are not things that are (typically)  
visible to the end-users, and do not trigger rehoming events.

The most important things, as it seems to me, are:
 - your own resiliency
 - the connection between you and your ISP(s)
 - your first-hop ISP's network characteristics, configuration, etc.

So, in this context, I'm not sure that in practice the connection 
survivability is a strict requirement.  IMHO, the quality of your 
first-hop ISP is the thing that matters the most.

-- 
Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings