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Troubleshooting the NISCA Protocol
F.3 Using SDA to Monitor LAN Communications
Table F–5 (Cont.) SHOW PORT/VC Display
Field Description
$ ReRcv (rereceive) Displays the number of packets received redundantly by this system. A
remote system may retransmit packets even though the local node has
already successfully received them. This happens when the cumulative delay
of the packet and its acknowledgment is longer than the estimated roundtrip
time being used as a timeout value by the remote node. Therefore, the
remote node retransmits the packet even though it is unnnecessary.
Underestimation of the round-trip delay by the remote node is not directly
harmful, but the retransmission and subsequent congestion-control behavior
on the remote node have a detrimental effect on data throughput. Large
numbers indicate frequent bursts of congestion in the network or adapters
leading to excessive delays. If the value in the ReRcv field is greater than
approximately 0.01% to 0.05% of the total messages received, there may be a
problem with congestion or network delays.
% Topology Change Indicates the number of times PEDRIVER has performed a failover from
FDDI to Ethernet, which necessitated closing and reopening the virtual
circuit. In Example F–2, there have been no failovers. However, if the field
indicates a number of failovers, a problem may exist on the FDDI ring.
& NPAGEDYN (nonpaged dynamic pool) Displays the number of times the virtual circuit was closed because of a pool
allocation failure on the local node. If this value is nonzero, you probably
need to increase the value of the NPAGEDYN system parameter on the local
node.
’ Congestion Control Displays information about the virtual circuit to control the pipe quota
(the number of messages that can be sent to the remote node [put into the
‘‘pipe’’] before receving an acknowledgment and the retransmission timeout).
PEDRIVER varies the pipe quota and the timeout value to control the
amount of network congestion.
( Pipe Quota/Slo/Max Indicates the current thresholds governing the pipe quota.
• The leftmost number (31) is the current value of the pipe quota
(transmit window). After a timeout, the pipe quota is reset to
1 to decrease congestion and is allowed to increase quickly as
acknowledgments are received.
• The middle number (7) is the slow-growth threshold (the size at which
the rate of increase is slowed) to avoid congestion on the network again.
• The rightmost number (31) is the maximum value currently allowed for
the VC based on channel limitations.
Reference: See Appendix G for PEDRIVER congestion control and channel
selection information.
) Pipe Quota Reached Indicates the number of times the entire transmit window was full. If
this number is small as compared with the number of sequenced messages
transmitted, it indicates that the local node is not sending large bursts of
data to the remote node.
+> Xmt C/T (transmission count/target) Shows both the number of successful transmissions since the last time the
pipe quota was increased and the target value at which the pipe quota is
allowed to increase. In the example, the count is 0 because the pipe quota is
already at its maximum value (31), so successful transmissions are not being
counted.
+? RndTrp uS (round trip in
microseconds)
F–12 Troubleshooting the NISCA Protocol
Displays values that are used to calculate the retransmission timeout in
microseconds. The leftmost number (18540) is the average round-trip time,
and the rightmost number (7764) is the average variation in round-trip
time. In the example, the values indicate that the round trip is about 19
milliseconds plus or minus about 8 milliseconds.
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