Jon,
I see these often too. In my case, it turns out if you look at the
sequence numbers that instead of a segment being lost it is merely
being received out of order. One indicator of this would be a dup
ack going back to the other system (supposed to be sent whenever a
packet arrives out of order). I believe it is a bug in the Ethereal
TCP session tracking feature. I use a program called "TCPTrace"
that does not have this problem. Before, Ethereal was reporting
retransmissions approaching 10% but it turned out that less than
1% were actual retransmissions and that the rest were just packets
out of order.
If you want to see the number of retransmitted packets in an
Ethereal filter, you can just set a filter for
tcp.analysis.retransmission and go to "Statistics->Summary". That
will tell you the number of packets total and the number displayed
(hence filtered). Just a warning that it might be wrong and I would
definitely go manually verify it first off of the sequence numbers.
Brian
-----Original Message-----
From: ethereal-users-bounces@xxxxxxxxxxxx
[mailto:ethereal-users-bounces@xxxxxxxxxxxx]On Behalf Of jon.johnston
Sent: Tuesday, August 23, 2005 13:26
To: Ethereal user support
Subject: [Ethereal-users] Analysis - Previous Segment Lost
I realize that Ethernet inherently includes some level of loss. However,
I am seeing what appears to be a large number of "previous segment lost"
packets, and I am concerned about how to measure the number of
re-transmitted packets with regards to overall traffic.
I assume I could write a filter, or something that would specifically
show the number of re-transmitted packets....which I would think would
involve counting the TCP sequence numbers by session. Is that possible
in Ethereal?
Any ideas are certainly welcome, I am pretty much a noob at this level
of Ethereal. Thank you.
Jon Johnston
Creative Business Solutions
IBM, Microsoft, Novell/Suse and Sophos Consulting
952-544-1108
http://www.cbsol.com
Blog:http://bingo.cbsol.com
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