Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.csd.uwm.edu!cs.utexas.edu!rutgers!bellcore!ka9q.bellcore.com!karn
From: karn@ka9q.bellcore.com (Phil Karn)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Repeated ping responses?
Message-ID: <17428@bellcore.bellcore.com>
Date: 19 Aug 89 04:16:09 GMT
References: <3936@phri.UUCP>
Sender: news@bellcore.bellcore.com
Reply-To: karn@ka9q.bellcore.com (Phil Karn)
Organization: Secular Humanists for No-Code
Lines: 26

In article <3936@phri.UUCP> roy@phri.UUCP (Roy Smith) writes:
>
>	Note that ping 15 got answered twice.  What causes that?  It
>doesn't seem like a fluke -- I tried pinging 132.247.5.1 three times and
>each time got a doublet within the first 10 or 20 packets. [...]

A few days ago I was pinging my way around the NSFNet/Internet, measuring
round trip delays and comparing them to some expected values I had computed.
I too noticed that quite a lot of packet duplication was going on.

When pinging berkeley.edu from rotgut.bellcore.com, I consistently saw a 10%
packet duplication rate. Traceroute showed a perfectly reasonable path from
bellcore-net through JvNCNet, NSFNet and BARRNET to Berkeley.

This got me curious, so I started to isolate the problem. I could ping JvNC's
last router (the one adjacent to their NSF backbone switch) with no packet
duplication and virtually no loss. But when I pinged the NSS at Merit, just
one hop away from JvNC, I began to see duplicate packets.

I therefore concluded that the duplication was happening inside NSFNet. Does
anyone have any independent knowledge of this, or can anyone offer an
explanation of what mechanism could be responsible? It's hard to understand
how packets can get duplicated in the Internet unless there is a
retransmitting link-layer protocol somewhere.

Phil