You can enable or disable the TCP/IP window scale factor by using the TCP Window Scale Factor (TWSF) option as follows:

NW TCPIP OPT TWSF = <value>

where <value> is within the range -1 (negative one) to 14. Enter 0 (zero) or a positive value only if you want to enable window scaling.

The TCP Window Scale Factor option is an extension to the TCP protocol that improves performance over large bandwidth paths by allowing larger blocks of data to be sent and received. It is based on RFC 1323.

The TCP header uses a 16-bit window field to report the size of the receive window to the sender. Therefore, the largest window that can be used is 216 or 65,535 bytes, and the largest amount of data that can be sent or received is limited to 65,535 bytes.

The window scale extension expands the definition of the TCP window to 32 bits and then uses a scale factor to carry this 32-bit value in the 16-bit window field of the TCP header. The scale factor sets the number of bits that the TCP window is to be adjusted (left-shifted). This TCP option allows the window to increase to a maximum of 230 or 1 Gbyte.

The scale factor is carried in the TCP Window Scale option. This option is sent only in a SYN segment, so the window scale is fixed in each direction when a connection is opened. Both sides must send Window Scale options in their SYN segments to enable window scaling in either direction.

A scale factor of –1 means no scaling is performed and leaves the TCPIP window scale option unset in a SYN and SYN ACK frame.

A scale factor of 0 indicates that the MCP TCP window will not be scaled, but window scaling will be performed to the remote system if the remote system sets the option in its SYN or SYN ACK frame.

The default for all systems is 5.