Does z/OS TCPIP support selective acknowledgement (SACK)?

A TCP connection can experience poor performance when multiple packets are lost from one window of data. With the limited information available from cumulative acknowledgments, a TCP sender can learn about only a single lost packet per round-trip time. A Selective Acknowledgment (SACK) mechanism with a selective repeat retransmission policy can help to overcome these limitations. The receiving TCP sends back SACK packets to the sender to inform the sender of data that was received. The sending TCP can then retransmit only the missing data segments.

Starting with Version 2 Release 1 of z/OS Communications Server, the Selective Acknowledgement (SACK) mechanism is supported. To enable SACK, specify the SELECTIVEACK parameter on the TCPCONFIG statement of the TCPIP profile. The default value is NOSELECTIVEACK.

To determine whether selective acknowledgement is enabled, issue the Netstat CONFIG/-f command.

The SelectiveACK field in that command's output indicates whether Selective Acknowledgment (SACK) support is active in the TCP/IP stack. This field can have the following values:

• Yes Indicates that SACK options are exchanged with partners when transmitting data. The SELECTIVEACK parameter was specified on the TCPCONFIG profile statement.

• No Indicates that SACK options will not be exchanged. The NOSELECTIVEACK parameter was specified on the TCPCONFIG profile statement or is in effect by default.