HP Cisco MDS 9216 - Fabric Switch Configuration Manual page 958

Using the FCIP Wizard
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
Path MTUs
Path MTU (PMTU) is the minimum MTU on the IP network between the two endpoints of the FCIP link.
PMTU discovery is a mechanism by which TCP learns of the PMTU dynamically and adjusts the
maximum TCP segment accordingly (RFC 1191).
By default, PMTU discovery is enabled on all switches with a timeout of 3600 seconds. If TCP reduces
the size of the maximum segment because of PMTU change, the reset-timeout specifies the time after
which TCP tries the original MTU.
Selective Acknowledgments
TCP may experience poor performance when multiple packets are lost within one window. With the
limited information available from cumulative acknowledgments, a TCP sender can only learn about a
single lost packet per round trip. A selective acknowledgment (SACK) mechanism helps overcome the
limitations of multiple lost packets during a TCP transmission.
The receiving TCP sends back SACK advertisements to the sender. The sender can then retransmit only
the missing data segments. By default, SACK is enabled on Cisco MDS 9000 Family switches.
Window Management
The optimal TCP window size is automatically calculated using the maximum bandwidth parameter, the
minimum available bandwidth parameter, and the dynamically measured round trip time (RTT).
The configured round-trip-time parameter determines the window scaling factor of the TCP
Note
connection. This parameter is only an approximation. The measured RTT value overrides the round trip
time parameter for window management. If the configured round-trip-time is too small compared to
the measured RTT, then the link may not be fully utilized due to the window scaling factor being too
small.
The min-available-bandwidth parameter and the measured RTT together determine the threshold below
which TCP aggressively maintains a window size sufficient to transmit at minimum available bandwidth.
The max-bandwidth-mbps parameter and the measured RTT together determine the maximum window
size.
Set the maximum bandwidth to match the worst-case bandwidth available on the physical link, keeping
Note
in mind other traffic that might be going across this link (for example, other FCIP tunnels, WAN
limitations)–in other words, maximum bandwidth should be the total bandwidth minus all other traffic
going across that link.
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:
•
Cisco MDS 9000 Family CLI Configuration Guide
48-20
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.
Chapter 48 Configuring FCIP
OL-16184-01, Cisco MDS SAN-OS Release 3.x