Expand Configuration and Management Manual (G06.24+)

Subsystem Description
Expand Configuration and Management Manual—523347-008
18-73
Congestion Control Configuration
The congestion control feature uses end-to-end mechanisms for congestion control
and error-recovery. It does not provide any mechanisms for indicating congestion along
intermediate nodes.
Congestion Control Configuration
You select the congestion control feature by specifying the L4CONGCTRL_ON
modifier. This modifier enables the congestion control mechanism for sending packets
on a specific path. The L4CONGCTRL_OFF modifier enables you to disable the
congestion control feature. The default is L4CONGCTRL_ON for Expand-over-IP line-
handler processes and L4CONGCTRL_OFF for all other Expand line-handler types.
Congestion Control Considerations
If congestion control is enabled on a node, the out-of-sequence (OOS) timer on the
receiver end of the connection should be set to 3 (the default) or greater. This timer
value is set with the OSTIMEOUT modifier. A value of 3 is appropriate for networks
with both congestion-controlled and non-congestion-controlled connections. For
networks with congestion control enabled for all connections, a greater value (such as
10) should be used for the OSTIMEOUT modifier value.
If the congestion control feature is not enabled, the Expand line-handler process uses
a static flow control window to limit the maximum number of outstanding
unacknowledged requests. There is no selective retransmission of a lost packet; all
packets are retransmitted, starting from the packet for which an acknowledgment was
not received.
If congestion is experienced on the network and packets are lost or delayed, a large
number of packets may be retransmitted, increasing the network traffic and congestion.
Since congestion is already being experienced on the network, it is likely that the
retransmission will also result in lost packets, so the whole retransmission sequence
continues, drastically decreasing throughput.