Technical data

Fabric OS FCIP Administrator’s Guide 21

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Adaptive Rate Limiting

Adaptive Rate Limiting (ARL) is performed on FCIP circuits to change the rate in which the FCIP

tunnel transmits data through the IP network. ARL uses information from the TCP connections to

determine and adjust the rate limit for the FCIP circuit dynamically. This allows FCIP connections to

utilize the maximum available bandwidth while providing a minimum bandwidth guarantee. ARL is

configured on a per-circuit basis because each circuit may have available differing amounts of

bandwidth.

ARL applies a minimum and maximum traffic rate, and allows the traffic demand and WAN

connection quality to dynamically determine the rate. If traffic is flowing error-free over the WAN,

the rate grows towards the maximum rate. If TCP reports an increase in retransmissions, the rate

reduces towards the minimum. ARL never attempts to exceed the maximum configured value and

reserves at least the minimum configured value. The aggregate of the minimum configured values

cannot exceed the speed of the Ethernet interface, which is 1 Gbps for GbE ports or 10 Gbps for

10GbE ports.

The maximum configured committed rate can be no larger than five times the minimum committed

rate.

For Fabric OS v7.0.0 and later, you can configure minimum and maximum rates for each circuit of a

tunnel using the XGE ports on the FX8-24 blade. This provides a maximum guaranteed rate of 10

Gbps combined for all tunnels over a single 10 GbE port and a maximum rate of 10 Gbps for any

single circuit. This feature is backwards-compatible with 1GbE ports on either the 7800 Extension

Switch or FX8-24 Extension Blade. For connections between 10GbE ports, ARL is supported only if

Fabric OS v7.0.0 and later is running on both switches.

FSPF link cost calculation when ARL is used

Fabric Shortest Path First (FSPF) is a link state path selection protocol that directs traffic along the

shortest path between the source and destination based upon the link cost. When ARL is used, the

link cost is equal to the sum of maximum traffic rates of all established, currently active low metric

circuits in the tunnel. The following formulas are used:

• If the bandwidth is greater than or equal to 2 Gbps, the link cost is 500.

• If the bandwidth is less than 2 Gbps, but greater than or equal to 1 Gbps, the link cost is

1,000,000 divided by the bandwidth in Mbps.

• If the bandwidth is less than 1 Gbps, the link cost is 2000 minus the bandwidth in Mbps.

QoS SID/DID priorities over an FCIP trunk

QoS SID/DID traffic prioritization is a capability of the Fabric OS Adaptive Networking licensed

feature. This feature allows you to prioritize FC traffic flows between initiators and targets.

Each circuit has four internal priorities that manage traffic over an FCIP tunnel, as illustrated in

Figure 7. The priorities are as follows:

• F class - F class is the highest priority, and is assigned bandwidth as needed at the expense of

lower priorities, if necessary.

• QoS high - The default value is 50 percent of the available bandwidth.