OpenVMS Cluster Systems - OpenVMS Systems - HP

NISCA Transport Protocol Channel Selection and Congestion Control
G.1 NISCA Transmit Channel Selection
A channel that satisfies all of these capacity criteria is classified as a peer. A
channel that is deficient with respect to any capacity criteria is classified as
inferior. A channel that exceeds one or more of the current capacity criteria,
and meets the other capacity criteria is classified as superior.
Note that detection of a superior channel will immediately result in
recalculation of the capacity criteria for membership. This recalculation
will result in the superior channel’s capacity criteria becoming the ECS’s
capacity criteria, against which all tight channels will be evaluated.
Similarly, if the last peer channel becomes unavailable or lossy, the capacity
criteria for ECS membership will be recalculated. This will likely result in
previously inferior channels becoming classified as peers.
Channels whose capacity values have not been evaluated against the current
ECS membership capacity criteria will sometimes be classified as ungraded.
Since they cannot affect the current ECS membership criteria, lossy channels
are marked as ungraded as a computational expedient when a complete
recalculation of ECS membership is being performed.
3. Delay
Channels that meet the preceding ECS membership criteria will be used if
their average round-trip delays are closely matched to that of the fastest
such channel—that is, they are fast. A channel that does not meet the ECS
membership delay criteria is considered slow.
The delay of each channel currently in the ECS is measured using cluster
communications traffic sent using that channel. If a channel has not been
used to send a datagram for a few seconds, its delay will be measured using
a round-trip handshake. Thus, a lossy or slow channel will be measured at
intervals of a few seconds to determine whether its delay, or datagram loss
rate, has improved enough so that it meets the ECS membership criteria.
Using the terminology introduced in this section, the ECS members are the
current set of tight, peer, and fast channels.
G.1.1.2 Local and Remote LAN Adapter Load Distribution
Once the ECS member channels are selected, they are ordered using an
algorithm that attempts to arrange them so as to use all local adapters for
packet transmissions before returning to reuse a local adapter. Also, the ordering
algorithm attempts to do the same with all remote LAN adapters. Once the order
is established, it is used round robin for packet transmissions.
With these algorithms, PEDRIVER will make a best effort at utilizing multiple
LAN adapters on a server node that communicates continuously with a client
that also has multiple LAN adapters, as well as with a number of clients. In
a two-node cluster, PEDRIVER will actively attempt to use all available LAN
adapters that have usable LAN paths to the other node’s LAN adapters, and that
have comparable capacity values. Thus, additional adapters provide both higher
availability and alternative paths that can be used to avoid network congestion.
G.1.2 Preferred Channel (OpenVMS Version 7.2 and Earlier)
This section describes the transmit-channel selection algorithm used by OpenVMS
VAX and Alpha prior to OpenVMS Version 7.3.
All available channels with a node can be used to receive datagrams from that
node. PEDRIVER chooses a single channel on which to transmit datagrams, from
the set of available channels to a remote node.
G–2 NISCA Transport Protocol Channel Selection and Congestion Control