Cisco MDS 9000 series Configuration Manual page 39

Chapter 2 Configuring FCIP
S e n d d o c u m e n t a t i o n c o m m e n t s t o m d s f e e d b a c k - d o c @ c i s c o . c o m
Monitoring Congestion
By enabling the congestion window monitoring (CWM) parameter, you allow TCP to monitor
congestion after each idle period. The CWM parameter also determines the maximum burst size allowed
after an idle period. By default, this parameter is enabled and the default burst size is 50 KB.
The interaction of bandwidth parameters and CWM and the resulting TCP behavior is outlined as
follows:
•
•
•
The software uses standard TCP rules to increase the window beyond the one required to maintain the
min-available-bandwidth to reach the max-bandwidth.
The default burst size is 50 KB.
Note
We recommend that this feature remain enabled to realize optimal performance. Increasing the CWM
Tip
burst size can result in more packet drops in the IP network, impacting TCP performance. Only if the IP
network has sufficient buffering, try increasing the CWM burst size beyond the default to achieve lower
transmit latency.
To change the CWM defaults, follow these steps:
Command
Step 1
switch(config-profile)# no tcp cwm
switch(config-profile)# tcp cwm
switch(config-profile)# tcp cwm burstsize 30
switch(config-profile)# no tcp cwm burstsize
25
Estimating Maximum Jitter
Jitter is defined as a variation in the delay of received packets. At the sending side, packets are sent in a
continuous stream with the packets spaced evenly apart. Due to network congestion, improper queuing,
or configuration errors, this steady stream can become lumpy, or the delay between each packet can vary
instead of remaining constant.
OL-19525-01,Cisco MDS NX-OS Release 4.2(1)
If the average rate of the Fibre Channel traffic over the preceding RTT is less than the
min-available-bandwidth multiplied by the RTT, the entire burst is sent immediately at the
min-available-bandwidth rate, provided no TCP drops occur.
If the average rate of the Fibre Channel traffic is greater than min-available-bandwidth multiplied
by the RTT, but less than max-bandwidth multiplied by the RTT, then if the Fibre Channel traffic is
transmitted in burst sizes smaller than the configured CWM value the entire burst is sent
immediately by FCIP at the max-bandwidth rate.
If the average rate of the Fibre Channel traffic is larger than the min-available-bandwidth multiplied
by the RTT and the burst size is greater than the CWM value, then only a part of the burst is sent
immediately. The remainder is sent with the next RTT.
Purpose
Disables congestion monitoring.
Enables congestion monitoring and sets the
burst size to its default size.
Changes the burst size to 30 KB. The valid
range is from 10 to 100 KB.
Leaves the CWM feature in an enabled state but
changes the burst size to its factory default.
Cisco MDS 9000 Family NX-OS IP Services Configuration Guide
Configuring FCIP
2-15