Finisar Surveyor User Manual

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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 

 

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 

 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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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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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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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.