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
The interaction of bandwidth parameters and CWM and the resulting TCP behavior is outlined as
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.
We recommend that this feature remain enabled to realize optimal performance. Increasing the CWM
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
To change the CWM defaults, follow these steps:
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
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.
You can configure the maximum estimated jitter in microseconds by the packet sender. The estimated
variation should not include network queuing delay. By default, this parameter is enabled in Cisco MDS
switches when IPS modules or MPS-14/2 modules are present.
The default value is 1000 microseconds for FCIP interfaces.
Cisco MDS 9000 Family CLI Configuration Guide
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.
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.
OL-16184-01, Cisco MDS SAN-OS Release 3.x