TCP receive window

The z/OS® Communications Server implementation calculates the receive window as two times the receive buffer size, minus any data in the receive buffer. The calculated receive window is then limited to the value of the TCPMAXRCVBUFRSIZE parameter from the TCPCONFIG profile statement, unless Dynamic Right Sizing (DRS) is active for the connection.

DRS is a stack performance optimization that allows the TCP receive buffer size to expand because of network conditions. DRS is enabled for connections that meet the following criteria:

Inbound streaming workload
High latency network
Application that is using a receive buffer size of at least 64 KB

If DRS is active for a connection, the advertised window follows a different set of rules:

The maximum receive buffer size is changed to 2 MB.
The window size is set to the estimated congestion window of the peer.

DRS is disabled for a connection when inbound data is being queued in the receive buffer faster than the application can read and process it. You can determine whether DRS is active for a connection by examining the TcpPrf field value for the connection on the Netstat ALL/-A report output.

Guideline: You must consider the send buffer size and the receive window size. For the z/OS Communications Server implementation, the amount of data on the network is limited by the smaller of the send buffer size and the receive window size.