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interconnection richness effects Re: Was [Re: Sprint peering policy]

  • From: Joseph T. Klein
  • Date: Sat Jun 29 15:44:12 2002

Preaching to the ministers here:

I would like to see more data. I don't think a network with large
aggregates (some who can not peer with tier 1s due to current
policies) has much impact on the global routing structure.

The primary problem is the noise of smaller announcements popping
on and off magnified by multihoming punching holes in large aggregates.

Small announcement show more churn because they are more granular.
They expand the route table thus slowing convergence.

Scientific investigation and public sharing of data can help
networks build policies based on service criteria.

Many large networks are reluctant to share internal data that could
help in the broader analysis of stability issues.

Where is the threshold? Can I turn it into a policy?

How much does topology effect stability? Hierarchal design tends
to mitigate instability when it can be localized to a small segment
of your network infrastructure.

Flat designs tend to ring like a bell when instability is introduced.
I think we held the world record for flapping at NAP.NET in 95-96.
That was a flat design executed during a time when the Cisco architecture
and software could not keep up with the growth and churn rate. The
inclusion of algorithms that enhanced oscillation ringing (and since has
been fixed in IOS) did not help.

--On Saturday, 29 June 2002 09:48 -0700 David Meyer <> wrote:


I think this is the key point. Its common sense that peering
with the downstreams will improve user quality of service by
both reducing latency and taking unnecessary points of failure
out of the network.
Is it really common sense? If so, is the common sense correct?
In fact, there is a lot of recent work that suggests that there
can be a very poor (and as it turns out poorly understood)
interaction between richness of interconnection and BGP dynamics;
this is due, at least in part, to amplification and coupling
effects that appear in some large systems. So many argue that
that given the current set of protocols (i.e. BGP and its
implementations), increased topological richness beyond some
threshold can actually hurt robustness and reliability. And just
to be clear about this, this is not a statement about peering
policies themselves (I'm explicitly not commenting on that), but
rather about our current understanding of some of the dynamics
that exist in today's Internet.

I've been trying to capture some of this in the following
document (with the able help of Randy, Tim, and many others):

On the topic of interconnection richness and its (possibly
unanticipated) effects, Craig and Abha's early work on this is
maybe the canonical reference. For something a little more
recent, see "What is the Sound of One Route Flapping", Timothy
G. Griffin,  IPAM Workshop on Large-Scale Communication Networks:
Topology, Routing, Traffic, and Control, March, 2002.

In any event, I guess the bottom line here is that sometimes what
looks like common sense (or even what we have a tendency to call
"conventional wisdom") may just be wrong.


Joseph T. Klein                               

   "Why do you continue to use that old Usenet style signature?"
                                                               -- anon

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