What is delayACK processing in TCP/IP?

DelayACK processing is the process by which the TCP protocol will delay sending an ACK packet to acknowledge an incoming TCP segment to conserve bandwidth and CPU cycles. This is the default behavior for TCP.

For request/response workloads, delayACK processing results in significant CPU savings by piggybacking the ACK onto a response. For streaming workloads, TCP will ACK every second packet.

The DELAYACKS parameter on the TCPCONFIG statement delays transmission of acknowledgments when a packet is received with the PUSH bit on in the TCP header. The NODELAYACKS parameter on the TCPCONFIG statement specifies that an acknowledgment is returned immediately when a packet is received with the PUSH bit on in the TCP header.

You want to enable DELAYACKS if you are running a streaming workload where generally more than one packet will consistently arrive within 200 ms. You want to disable DELAYACKS if you are running an interactive workload where two packets may not consistently arrive within 200 ms. The difference between DELAYACKS and NODELAYACKS is 200ms.

To determine whether DELAYACKS or NODELAYACKS has been specified for this stack, issue NETSTAT CONFIG and check the value of the DelayAck field.

The DELAYACKS parameter on the TCPCONFIG statement can be overridden by specifying the NODELAYACKS parameter on

• The TCP/IP stack PORT or PORTRANGE profile statements for the port used by a TCP connection

• The TCP/IP stack BEGINROUTES or GATEWAY profile statements

• The Policy Agent RouteTable statement

• The OMPROUTE configuration statements

The DELAYACKS and NODELAYACKS parameters on the PORT statement affect only connections that use this port. To determine whether DELAYACKS or NODELAYACKS has been specified for a specific port, issue the NETSTAT PORTLIST command and check the Flags column entry for that port to see whether D (DelayAcks) is specified.

The DELAYACKS and NODELAYACKS parameters on the PORTRANGE statement affect only connections that use the ports in this portrange. To determine whether DELAYACKS or NODELAYACKS has been specified for a specific port range, issue the NETSTAT PORTLIST command and check the Flags column entry for the first value in that port range to see whether D (DelayAcks) is specified.

The DELAYACKS and NODELAYACKS parameters on the BEGINROUTES, GATEWAY, and RouteTable statements affect only the connections that use this route.

To determine whether DELAYACKS or NODELAYACKS has been specified for a route, issue NETSTAT ROUTE DETAIL and check the value of the DelayAcks field.