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Eric,
Ack-ing the same packet every time is tcp's only
way of letting the sending party know that it has lost the tcp frame following
the acked one. The reciever ack's every packet that it recieves after the lost
frame with the sequence number of the frame it hasn't recieved. The server
should resend that frame. When the frame has been resent, tcp acks the last
sequence number + length of the last frame it has gotten in good order from the
sender. The fact that you see the reciever ack-ing 9-10 times has to do with the
nagle algorithm. This algorithm has an internal clock which goes off every 200
ms (standard setting). During that time it waits with ack-ing packets until it
has something to send as well. If the 200 ms period has passed, and there is
nothing to send, then it has to ack the packets it recieved. Your sender is
probably filling up the windowsize of your reciever during that waiting period
(fast sender, slow reciever), and tcp has lost a packet.
Retransmissions like this can slow things down
considerably. Try replacing the lan-cables first, then the patch panels (if
any), then switch switchports, then replace nic's. Test every
step.
Marco.
----- Original Message -----
Sent: Friday, September 05, 2003 2:47
AM
Subject: [Ethereal-users] Application
Keeps Acking Same Packet,then Suddenly Catches Up
Network
geeks:
I
have an application called ProLaw that runs off a NetWare server on the other
side of a PIX firewall.
Most
of my workstations perform fine. But two or three of them slow way down after
a few hours of use, to the point that queries which usually take 10-15 seconds
take 1.5 minutes! Meanwhile, other clients on the same switch keep working
fine.
Traces
show that during the slow periods, the client retransmits a lot of ACKs of the
same data. It will transmit the same ACK maybe 9 or 10 times in a row with
only about 0.000002 seconds between. Then the server finally retransmits a
packet of its own.
When
this happens, the client finally ACKs all of the previously received data at
once and catches up.
Somebody
please offer some insight on what could be happening
here!
--Eric
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