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Re: in Chicago - high packet loss?!?

  • From: sdb
  • Date: Tue Sep 06 17:10:46 2005

On Tue, 6 Sep 2005, Christopher L. Morrow wrote:

> On Tue, 6 Sep 2005, chip wrote:
> > On 9/6/05, Joe Maimon <> wrote:
> > >
> > >
> > > If the hop(s) following the one you see loss for shows no loss, then
> > > disregard the loss for that hop, obviously whatever it is, it does not
> > > affect transit, which is what you really want to know.
> > >
> > > Is that correct?
> > >
> > This is one of the most misunderstood concepts in properly reading output
> > from a traceroute (mtr, visualtraceroute, whatever). Basically you are
> > seeing loss of packets destined directly *TO* that router, not THRU it. Most
> no... not destined TO the router, destined THROUGH the router that happen
> to TTL=0 ON that router.

Very true.  Most backbone kit on a tier 1 network is designed to switch
packets in a distributed fashion, shifting packets between ports/cards
over a backplane of some sort.  On such kit, generating things such as a
TTL-exceeded packet is usually punted to a central processor (whose
primary task is to build route tables to hand off to the cards), which
deals with the task in a much slower and much lower priority way than
packets which transit the routing device.  You also don't want your
central processor to have to deal with too much of this sort of thing,
which is (at least one of the reasons) why it's often rate limited.

> which is also misunderstood by just about everyone :( but anyway... 'not
> affecting transit' for reasons sited by yourself and min and adam already,
> yes.



Stewart Bamford (Posting as an individual)
Level3 Snr IP Engineer
*** Views expressed are my own and not necessarily those of Level3 ***
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