network monitoring , tcp slow response, excessive udp retran

Unanswered Question
Nov 21st, 2008

Hi all, I have been doing some monitoring on my lan as there are various complaints of slowness, my software is telling me there is a number of slow tcp responses from servers, and also excessive udp restransmissions, Does udp retransmit? I thought that was only tcp.


I have this problem too.
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Joseph W. Doherty Fri, 11/21/2008 - 04:37

UDP itself doesn't retransmit, but the application using it can. E.g. tftp.



One of the biggest peformance killers is a high packet drop rate. The two most common causes I've seen have been either a duplex error along the path, or a highly congested interface. (Busy shared Ethenet can be a problem too, but I'm assuming you're running in a switched environment.)

Since you did mention UDP, something you have to sometimes watch for, someone brings in a "special" high speed transfer application "much faster and better than TCP", which uses it own custom protocol. There are reasons why TCP works as it does, some of which concern sharing bandwidth. Some of these "special" applications can really impact everyone else on a shared path.

ropethic Fri, 11/21/2008 - 04:45

UDP is an unreliable,When a message is sent, it cannot be known if it will reach its destination; it could get lost along the way. There is no concept of acknowledgment, retransmission and timeout

Deals with checksums and relies on application to deal with end to end communication problems.

In terms of slow TCP response from servers, check the latency between sites. What is the average TCP window size? Getting window 0?

Could be server does not have enough memory resources to buffer data. CPU utilization too high?

Assymetric routing to / from server? Dropped packets on switch / router interface?

Could be a number of things causing slowness.

carl_townshend Fri, 11/21/2008 - 05:53

Hi there, thanks for the reply, what should the average window size be? and if it is zero, why is this bad ?

ropethic Fri, 11/21/2008 - 11:07

Window 0 indicates the receiver does not have any memory buffer to queue packet prior to send off to application. When this occurs, it is basically telling all senders to stop transmitting until window size increases.

Typically you would expect to see a window size of 64K for server and 32K - 64K for client machines.


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