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User’s Guide
TCP Zero Window
Counter
TCP Zero Window is a counter of all TCP Zero Window events over a period of
time per segment. A count of all TCP Zero Window events displays in the
Overview
counters of Expert View. A threshold for this counter can be set in Expert Alarms.
Expert Symptom
TCP Zero Window events are automatically logged as expert symptoms. The
Symptom Summary field provides information about the time, location, and the
well-known ports (WKP) involved, including the port number and the IP address.
For example:
Stuck at 0 for [14 ms] between [206.250.228.69]/[TCP/IP
WKP:1988] and [206.250.228.11]/[SMTP]
The TCP window size is examined for every packet to check against a window size
of zero. If the window size remains zero for a threshold interval for one connection
in one direction, the event is logged. Events of this type indicate when a receiver’s
buffer is full which can indicate problems with the network.
Expert Diagnosis
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Problem Description:
A TCP/IP packet indicates zero window size for longer than the threshold interval. The
receiver is shutting down communication and will accept no more data from the other end.
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Probable Cause(s):
1. The receiver is overloaded.
2. The receiver has run out of buffer space.
3. The non-responsive receiver intends the sender to close the connection.
4. There may be a problem with the receiver’s TCP/IP stack.
5. There are too many connections to the receiver resulting in reduced buffer space.
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Recommended Action(s):
1. Upgrade the receiver’s CPU and/or Memory.
2. Reduce the number of connections to the receiver.
3. Increase the bandwidth of your network.