Outbound serialization

In some cases, z/OS® Communications Server tailors its processing for certain workloads to improve in-order packet delivery on multiprocessors. This performance optimization, which is called outbound serialization, can reduce CPU consumption and improve throughput. When outbound serialization is enabled for a TCP/IP connection, outbound packets for the connection leave z/OS Communications Server in the correct order. This optimization path reduces out-of-order packet processing on the receiver and minimizes unnecessary retransmissions of packets at the sender.

A connection is eligible for outbound serialization when it meets the following criteria:
  • The connection has received four or more consecutive duplicate acknowledgements (ACKs).
  • The initial send buffer size is at least 64 KB.
The stack activates outbound serialization for a connection in any of the following conditions:
  • The connection has been identified as operating in bulk-data fashion, such as an FTP data connection, and has been registered to the bulk-mode ancillary input queue (AIQ) of an OSA-Express® feature that is enabled for inbound workload queueing.
  • The connection is eligible for outbound serialization and IPSEC is also enabled for this connection.
  • The connection is eligible for outbound serialization, the round trip time for the connection equals or exceeds the specified or defaulted QUEUEDRTT parameter value on the TCPCONFIG profile statement. The stack determined that a significant number of retransmissions occurred for packets that were delivered out of order initially.