Re: ISP failures and site multihoming [Re: Enforcing unreachabilityof site local addresses]

[Iljitsch van Beijnum <iljitsch@muada.com>]

On Thu, 20 Feb 2003, Pekka Savola wrote:

> > We are _very_ far from a situation where even the best ISP provides a
> > service level that is better then the one you get from multihoming even
> > if you consider failover delays.

> In some cases, this may be better.  In some others, not.

> It's not IMO necessary to get significantly better but "roughly equal".

Don't forget that failover is just one of the benefits of multihoming.
Two important others are protection against losing service when ISPs
go out of business and more optimal traffic flow.

> > And the single service provider thing doesn't scale anyway: the end
> > result would have to be a single global ISP.

> It does scale, pretty well actually.  I'm not talking about your average
> neighborhood ISP's with 100 customers, though.  Currently in DFZ, there
> are about 3500 (ONLY!) AS numbers which transit at least one other AS
> number.

I don't see your point. What you were saying is that using a single
reliable ISP would be better than multihoming. Now obviously an ISP can
only control the QoS parameters inside its own network so if you want to
do reliable and high quality VoIP you need to use the same ISP
end-to-end. In other words: just one ISP.

> > Has the end-to-end principle failed to teach us anything? Reliability
> > begins and ends in the end hosts. If each host is connected over two
> > service providers there are four possible paths the hosts can switch
> > between on a per-packet basis. Then the only problem becomes detecting
> > failure. The end hosts are in an excellent position to do this without
> > having to generate keepalive messages; a well designed protocol could
> > switch to an alternate path within a few round trip times when a path
> > failure occurs.

> Compare this to a solution where the site has two connections to the same
> ISP, and you're left with major ISP backbone failure or upstream failure
> (any relevant ISP's have only one upstream)?

So ISPs get to multihome but not end-users?

There are many ways in which an ISP network can fail, as the large scale
Worldcom and AT&T outages six months ago illustrate. More common is the
situation that an ISP network has trouble reaching a certain destination
because the only link to that destination has failed (which in itself
may not be their fault) or there is congestion somewhere. And don't
forget maintenance windows.

> A solution without multi-connecting, ie. only one L1 connection to one
> ISP, is naturally out of question.

Ok, so why is multihoming to a single ISP better than multihoming to
several ISPs?

> > Multi6 has been gravitating towards multi-address multihoming solutions
> > for a while now, but unfortunately it seems impossible to move foward.

> Multi-address solutions solve certain problems well, but leave some
> unsolved.

Like what?