TCP configuration

The configuration of TCP layer parameters is controlled with the TCPCONFIG parameter. The most significant parameters within this statement block are the parameters controlling the size of send and receive buffers. These parameters can have a significant impact on network performance, particularly when doing bulk data transfer.

These parameters are:

TCPMAXRCVBUFRSIZE: An individual application running on z/OS can request to increase the default receive buffer size. This parameter limits the size that can be requested by any application. If there are a large number of TCP applications running on the z/OS host, this value could be significant.
By limiting the receive buffer size, TCPMAXRCVBUFRSIZE can avoid an inbound flood of data arriving over multiple concurrent applications. In other words, the maximum buffer size is only significant as the number of applications using it become larger. The maximum is 512 KB.
TCPRCVBUFRSIZE: This is the default receive buffer size given to a TCP application on the z/OS host.
TCPSENDBFRSIZE: This is the default for the size of the buffers used to hold outbound data prior to transmission. It can be increased up to 256 KB.

Reminder: A TCP buffer size correlates directly to a window size. When a session is established, each side sends out its receive buffer size. Each host uses the remote host's receive window size as an indication of the maximum amount of data that can be transmitted without an acknowledgement.