Re: Drops in Core

[Job Snijders <job () instituut net>]

On Sun, Aug 16, 2015 at 08:00:55AM -0400, Patrick W. Gilmore wrote:
> On Aug 15, 2015, at 1:41 PM, Job Snijders <job () instituut net> wrote:
> > On Sat, Aug 15, 2015 at 11:01:56PM +0530, Glen Kent wrote:
>
> > > Is there a paper or a presentation that discusses the drops in the core?
> > >
> > > If i were to break the total path into three legs -- the first, middle
> > > and the last, then are you saying that the probability of packet loss
> > > is perhaps 1/3 in each leg (because the packet passes through
> > > different IXes).
> >
> > It is unlikely packets pass through an IXP more then once.
>
> “Unlikely”? That’s putting it mildly.
>
> Unless someone is selling transit over an IX, I do not see how it can
> happen. And I would characterize transit over IXes far more
> pessimistically than “unlikely”.

There is another scenario (which unfortunatly is not that uncommon)
where packets could traverse two IXPs, and no transit is sold over any
of those two IXs.

Imagine the following:

Network A purchases transit from network B & network C. Network B &
Network C peer with each other via an IXP. Network A announces a /16 to
network B but 2 x /17 to network C. Network D peers with B via an IX
(and not with C) and receives the /16 from B, but note that internally
network B has two more specifics covering the /16 received from C and
the /16 itself. Network B will export the /16 (received from customer)
but not the /17s (received over peering) to its peers.

Because of longest prefix matching, network B will route the packets
received from network D over an IXP, towards network C, again over an
IXP. 

This phenomenon is described extensively in the following
Internet-Draft:

    https://tools.ietf.org/html/draft-ietf-grow-filtering-threats-07

Kind regards,

Job