Honest Cogent opinions without rhetoric.

Todd Underwood todd-nanog at renesys.com
Wed Mar 8 12:42:37 UTC 2006

On Wed, Mar 08, 2006 at 01:56:00AM -0500, alex at pilosoft.com wrote:

> (from Marty)
> >  From a global perspective[1], the top 12 (I stopped at Cogent since you
> > are asking about them) service providers whose customers and peering
> > partners reach the largest number of networks are listed below. You can
> > make some fairly interesting assumptions on your own:
> <snip>
> This gotta be the most meaningless metric ever. What does "reach"
> mean?  

now, that's a bit harsh.  'incompletely specified in this email',
perhaps, but 'meaningless'?  come on.  :-)

the marketing description for those rankings is: "Identify service
providers who are responsible for meeting Internet transit needs
(directly or indirectly) of significant numbers of large customer
networks within a given market, including both customer and peering

in actual, technical detail, what that means is that we take global
routing data (routeviews-style), determine the relationship of every
edge (customer, provider, peer), and weight the 'downstream cone' (a
caida term that is useful here) of each provider by the scaled prefix
space and the degree to which that provider actually provided transit
to that prefix space over the course of time.  it's critical to
integrate across time and across lots of peers in this process.
otherwise, the position of your peers and leaks come to dominate the

this metric (one of several that we've begun calculating daily) isn't
perfect, but we've found that for large SPs it matches expectations of
people familiar with traffic flows.  

> Note that many Cogent customers, while using Cogent for outbound, prefer
> not to announce any routes to Cogent for political reasons (or prepend or 
> depref their routes). So, that metric won't be exactly helpful.

this is, in fact, a useful point.  detecting and compensating for
asymmetric routing is difficult in a metric such as this, although
it's probably not impossible.


todd underwood                                 +1 603 643 9300 x101
renesys corporation                            chief of operations & security 
todd at renesys.com                               http://www.renesys.com/blog

More information about the NANOG mailing list