Expand Configuration and Management Manual (G06.24+)

Tuning
Expand Configuration and Management Manual—523347-008
20-10
Congestion Control
Although the data-per-packet percentage is highest when the PATHPACKETBYTES
modifier is set to 4095 bytes, there are other effects of using a 4095-byte packet size
that you must consider. One of these considerations is the effect of the packet size on
a multi-line path. Refer to Multi-Line Paths on page 20-13 for more information about
packet size and multi-line paths.
Congestion Control
The Expand subsystem’s congestion control feature improves throughput in networks
that are subject to varying delays. The congestion control feature allows a connection
to reach an equilibrium point slowly and then pick up speed as additional bandwidth is
found. An additional mechanism allows quick error-recovery without wasting bandwidth
and is more efficient than the out-of-sequence timer recovery provided by Expand line-
handler processes that do not use the congestion control feature.
In networks with low delay variance and few or no errors, the congestion control
feature provides up to 8 percent improvement in response time with no significant
difference in processor utilization. In networks with high delay variance, the benefits of
congestion control are more significant, as recovery from lost packets due to
congestion is handled much more efficiently. In networks with high error or packet-loss
rates, congestion control provides a more efficient error-recovery mechanism;
however, congestion control cannot solve problems caused by noisy lines or poorly
configured bridges and routers.
Expand-Over-IP Connections
Expand-over-IP line-handler processes use the User Datagram Protocol (UDP)
services provided by a NonStop TCP/IP process to transmit data across an Internet
Protocol (IP) network. Because data transfer with UDP is not guaranteed, the Expand
End-to-End protocol is used to achieve reliable communications for Expand-over-IP
connections. You can avoid congestion and improve error-recovery by enabling the
congestion control feature on Expand-over-IP connections.
Congestion Control Configuration
You can enable the congestion control feature for outgoing transmissions on a per-
connection basis using the L4CONGCTRL_ON modifier. HP recommends that you
enable the congestion control feature for all connections in your Expand network.
If your network contains some Expand line-handler processes that support the
congestion control feature and some that do not, HP recommends that you enable the
feature for all connections that are capable of congestion control and set the out-of-
sequence (OOS) timeout value (OSTIMEOUT modifier) to the default value of 3
seconds for all Expand line-handler processes.
If all nodes in the network are running D30 or later versions of the Expand subsystem,
then HP recommends that the congestion control feature be enabled for all Expand
line-handler processes and that the OOS timeout value be set to 10 seconds.