Cisco DS-C9216I-K9, Руководство по настройке

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Cisco MDS 9000 Fabric Manager Switch Configuration Guide
OL-7753-01
Chapter 24 Configuring IP Storage
Configuring FCIP
SACK
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 time. 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 computed using the max-bandwidth option, the
min-available-bandwidth option, and the dynamically-measured round-trip-time (RTT). The
interaction and the resulting TCP behavior is outlined below:
The configured round-trip-time option determines the window scaling factor of the TCP connection.
This option is only an approximation. The measured RTT value overrides the round-trip-time option 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.
•
If the average rate of the fc traffic over the preceding RTT is less than the min-available-bandwidth
* RTT, every FC burst is sent immediately at the min-available-bandwidth rate, provided no TCP
drops occur.
• If the average rate of the FC traffic is greater than min-available-bandwidth * RTT, but less than
max-bandwidth * RTT, then if the FC traffic is transmitted in burst sizes smaller than the configured
CWM value all the bursts are sent immediately by FCIP at the max-bandwidth rate.
•
If the average rate of the FC traffic is larger than the min-available-bandwidth * RTT and the burst
size is greater than the CWM value, some traffic will not be sent immediately.
The maximum-bandwidth option and the measured round-trip-time together determine the maximum
window size.
The min-available-bandwidth option and the measured round-trip-time together determine the
threshold below which TCP aggressively maintains a window size sufficient to transmit at
min-available-bandwidth . The software uses standard TCP rules to increase the window beyond the
one required to maintain the min-available-bandwidth in order to reach the max-bandwidth . The
defaults are max-bandwidth = 1G, min-available-bandwidth = 15 Mbps, and round-trip-time =1 ms.
Buffer Size
The send-buffer-size option defines the required additional buffering (beyond the normal send window
size) that TCP allows before flow controlling the switch’s egress path for the FCIP interface. The default
buffer size is 0 KB.
Quality of Service
The Quality of Service (QoS) feature specifies the differentiated services code point (DSCP) value to
mark all IP packets (type of service—STOS field in the IP header).
•
The control DSCP value applies to all FCIP frames in the control TCP connection.
• The data DSCP value applies to all FCIP frames in the data connection.
If the FCIP link has only one TCP connection, that data DSCP value is applied to all packets in that
connection.