Using TCP large windows to improve performance

When using TCP large windows is beneficial

Long-haul networks have more capacity than can be used effectively with the standard TCP window size. Enabling TCP large windows dramatically improves performance over high-bandwidth, long-haul connections by using the capacity of the pipe more effectively. Using TCP large windows on local area networks, however, has no benefit.

Examples of networks that can benefit from enabling TCP large windows include the following:

Two appliances communicating with each other over a satellite link
Clients connecting to an appliance using DSL but with a high-capacity hop between them and the appliance

Determining which hosts or subnets to enable TCP large windows for

When the TCP Large Windows feature is configured, the appliance can communicate with specified hosts and subnets using TCP large windows. You can specify the systems or subnets with which the appliance should communicate using large windows, and also to specify the window size that the appliance should use with each.

Exercise caution when deciding which hosts or subnets the appliance should communicate with using large windows. Using large windows consumes more memory than using the standard window size; therefore, the appliance limits the number of simultaneous connections using large windows. The bigger the window size, the fewer simultaneous large windows connections the appliance can support.

Network Appliance recommends that you specify only a few hosts or subnets with which the appliance should communicate using large windows.

Option location

Setup tab > Network > General > TCP Large Windows Enable and TCP Large Windows List.

Determining the window size to use

To use the capacity of the link effectively, the window size should be greater than or equal to the bandwidth-delay product. Calculate the bandwidth delay product by multiplying the capacity of the link (bandwidth) by the round-trip time (delay), as follows:

Setting the window size precisely to optimize the usage of the link can require some experimentation, however.

Guidelines for common connection types

If you are using a T1 or T3 connection, you can use the window sizes in the following table.

Connection type	Recommended window size
T1	11264 bytes
T3	345088 bytes