Citrix Systems CITRIX NETSCALER 9.3 Configuring TCP Window Scaling, System Time Tue Feb 165044

Chapter 5 Advanced Configurations

IST-Asia/Colombo

System Time: Tue Feb 22

16:50:44 2011

Last Config Changed Time: Tue Feb 22

16:48:02 2011

Last Config Saved Time: Tue Feb 22

16:48:19 2011 Done

To view the system date and time by using the configuration utility

1.In the navigation pane, click System.

2.In the details pane, select the System Information tab.

3.Under System Information, view the system date and time.

Configuring TCP Window Scaling

The TCP window scaling option, which is defined in RFC 1323, increases the TCP receive window size beyond its maximum value of 65,535 bytes. This option is required for efficient transfer of data over long fat networks (LFNs).

A TCP window determines the amount of outstanding (unacknowledged by the recipient) data a sender can send on a particular connection before receiving any acknowledgment from the receiver. The main purpose of the window is flow control.

The window size field in the TCP header is 16 bits, which limits the ability of the sender to advertise a window size larger than 65535 ( 2^16 - 1). The TCP window scale extension expands the definition of the TCP window by applying a scale factor to the value in the 16 bit window size field of the TCP header. (Although RFC 1323 describes expanding the definition to up to 30 bits, NetScaler window scaling expands the definition of the TCP window to up to 24 bits.) The scale factor is carried in the new TCP window scale field. This field is sent only in a SYN packet (a segment with the SYN bit on)

To fit a larger window size value into the 16-bit field, the sender right shifts the value by the number of bit positions specified by the scale factor. The receiver left shifts the value by the same number of positions. Therefore, the actual window size is equivalent to:

(2^<scale factor>) * <received window size>.

Before configuring window scaling, make sure that:

wYou do not set a high value for the scale factor, because this could have adverse effects on the NetScaler and the network.

wYou have enabled selective acknowledgement (SACK).

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