Sniffer Pro's Expert System feature is well-known for being faulty and
misleading. 4.5 DID have quite a few problems "detecting"
retransmissions properly - particularly if their Sniffer was placed at a
point where the packet loss would have already occurred (so that the
trace didn't contain the original segment AND the retransmitted segment)
it wouldn't flag any retransmissions.
...Which is one of the dangers of relying on the tool to perform 100% of
your analysis, 100% of the time. The tool may not be working (and you
may have no idea). Did they do no analysis of their own to see if they
could determine if packets were being lost/retransmitted? Especially in
a case where there is finger-pointing going on, they should be
double-checking the results - my guess is that they wouldn't know how.
Later versions of Sniffer, up through 4.75SP2 made some improvements in
this area, though it still doesn't do a terribly good job.
I'd trust Ethereal on this one, though do some spot-checking to make
sure it's not lying to you. Ethereal's TCP Analysis feature is very
very good, but not perfect - it does occasionally (rarely) provide false
positives or mis-diagnoses.
Use this technique to prove to them - try to arrange to have captures
taken simultaneously on both sides of the link that you suspect is
faulty. Find a retransmission event, and figure out what happened - was
the packet actually "lost" on the link? If so, you should be able to
find the original copy of a retransmitted segment sent out to the SP's
link AS WELL as the retransmitted segment (and in such a trace even
Sniffer *should* find the retransmission), but when you look at the
trace on the other side of the link, that original segment should be
gone. Ask them where the packet went?
Ian
Chula Bandara wrote:
Hi i am using Etereal Version 0.10.0a and this was very useful to me
solving connectivity issues with our Service Providers. Few days back
i had a similar problem that i see TCP retransmission but one of my
Service Provider did not those retransmissions. My SP was using
Network Associates Sniffer Pro Version 4.50.04. and i got some of
captured files from the Sniffer. Surprisingly when i open those files
with Ethereal , it sees Retransmissions , Packet Losses , Duplicate
ACks etc..
could this be a bug in Ethereal or the Sniffer
thanks you in advance.
cbandara
