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Chapter 4
FCIP over IP/MPLS Core
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TCP Window Size
TCP uses a sliding window to control the flow of data from end to end. The TCP maximum window size
(MWS) is the maximum amount of data the sender allows to be outstanding without acknowledgment at
one time. The minimum MWS is 14 KB; the maximum is 32 MB.
The sender can use a larger window size to allow more outstanding data and to make sure that the pipe
remains full. However, sending too much data at once can overrun intermediate routers, switches, and
end devices. The TCP congestion control manages changes to the window size.
You cannot configure the TCP window size directly. This value is automatically calculated from the
product of the maximum bandwidth x RTT x 0.9375 + 4 KB. In SAN-OS 1.3 and later, the RTT can
dynamically adjust up to four times the configured value in the FCIP profile according to network
conditions. The TCP sender dynamically changes the maximum window size accordingly.
TCP Maximum Bandwidth
The TCP maximum bandwidth is the maximum amount of bandwidth an FCIP link consumes from the
point of view of the TCP sender. The maximum bandwidth settings for an FCIP link can be asymmetric.
Set the TCP maximum bandwidth to the maximum amount of bandwidth you want the FCIP link to
consume. Set it no higher than the bandwidth of the slowest link in the FCIP link path. For example, if
the FCIP link is mapped over a dedicated DS3 WAN link, set the maximum bandwidth to 45 Mbps.
The TCP maximum bandwidth value is used as the bandwidth value in the bandwidth-delay product
calculation of the TCP MWS.
Observe the following guidelines when selecting a value for TCP maximum bandwidth:
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TCP Minimum Available Bandwidth
The value should represent the minimum amount of bandwidth in the FCIP path that you expect to be
always available. This value determines the aggressiveness of FCIP—a higher value is more aggressive,
a lower value is less aggressive. A value that is too high can cause congestion and packet drops for any
traffic traversing the shared network links.
Bandwidth allocation strongly favors the FCIP traffic when mixed with conventional TCP traffic, which
recovers from drops more slowly. To cause FCIP to behave more fairly, use a lower value for the
min-available-bw parameter. FCIP starts at a lower rate and increments the send rate every RTT, just like
classic TCP slow-start.
OL-12518-01
TCP window size
TCP maximum bandwidth
TCP minimum available bandwidth
Round Trip Time (RTT)
Set the TCP maximum bandwidth value no higher than the maximum path bandwidth available to
the FCIP.
If deploying FCIP over a shared link with critical traffic, lower the maximum bandwidth to a level
that allows the other traffic to coexist with minimal retransmissions. Quality of service (QoS) should
be considered in these situations.
When using the Cisco MDS 9000 software compression, set the maximum bandwidth value as
though compression is disabled. The Cisco MDS 9000 uses a dynamic moving average feedback
mechanism to adjust the TCP window size according to the compression rate.
Data Center High Availability Clusters Design Guide
Using FCIP Tape Acceleration
4-9
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