OpenVMS Cluster Systems - OpenVMS Systems - HP

G
NISCA Transport Protocol Channel Selection
and Congestion Control
G.1 NISCA Transmit Channel Selection
This appendix describes PEDRIVER running on OpenVMS Version 7.3 (Alpha
and VAX) and PEDRIVER running on earliers versions of OpenVMS Alpha and
VAX.
G.1.1 Multiple-Channel Load Distribution on OpenVMS Version 7.3 (Alpha and
VAX) or Later
While all available channels with a node can be used to receive datagrams from
that node, not all channels are necessarily used to transmit datagrams to that
node. The NISCA protocol chooses a set of equally desirable channels to be used
for datagram transmission, from the set of all available channels to a remote
node. This set of transmit channels is called the equivalent channel set (ECS).
Datagram transmissions are distributed in round-robin fashion across all the
ECS members, thus maximizing internode cluster communications throughput.
G.1.1.1 Equivalent Channel Set Selection
When multiple node-to-node channels are available, the OpenVMS Cluster
software bases the choice of which set of channels to use on the following criteria,
which are evaluated in strict precedence order:
1. Packet loss history
Channels that have recently been losing LAN packets at a high rate are
termed lossy and will be excluded from consideration. Channels that have
an acceptable loss history are termed tight and will be further considered for
use.
2. Capacity
Next, capacity criteria for the current set of tight channels are evaluated. The
capacity criteria are:
a. Priority
Management priority values can be assigned both to individual channels
and to local LAN devices. A channel’s priority value is the sum of these
management-assigned priority values. Only tight channels with a priority
value equal to, or one less than, the highest priority value of any tight
channel will be further considered for use.
b. Packet size
Tight, equivalent-priority channels whose maximum usable packet size is
equivalent to that of the largest maximum usable packet size of any tight
equivalent-priority channel will be further considered for use.
NISCA Transport Protocol Channel Selection and Congestion Control G–1