|Document revision date: 15 July 2002|
This section pertains to PEDRIVER running on OpenVMS VAX Version 6.0 or OpenVMS AXP Version 1.5, or later.
The retransmission mechanism is an adaptation of the algorithms developed for the Internet TCP protocol by Van Jacobson and improves on the old mechanism by making both the window size and the retransmission timeout interval adapt to network conditions.
This section pertains to PEDRIVER running on VMS Version 5.5 or earlier.
PEDRIVER periodically multicasts a HELLO datagram over each network adapter attached to the node. The HELLO datagram serves two purposes:
HELLO datagram congestion and loss of HELLO datagrams can prevent connections from forming or cause connections to be lost. Table G-1 describes conditions causing HELLO datagram congestion and how PEDRIVER helps avoid the problems. The result is a substantial decrease in the probability of HELLO datagram synchronization and thus a decrease in HELLO datagram congestion.
|Conditions that cause congestion||How PEDRIVER avoids congestion|
|If all nodes receiving a HELLO datagram from a new node responded immediately, the receiving network adapter on the new node could be overrun with HELLO datagrams and be forced to drop some, resulting in connections not being formed. This is especially likely in large clusters.||
To avoid this problem on nodes running:
|If a large number of nodes in a network became synchronized and transmitted their HELLO datagrams at or near the same time, receiving nodes could drop some datagrams and time out channels.||
On nodes running VMS Version 5.5--2 or earlier, PEDRIVER multicasts
HELLO datagrams over each adapter every 3 seconds, making HELLO
datagram congestion more likely.
On nodes running OpenVMS VAX Version 6.0 or later, or OpenVMS AXP Version 1.5 or later, PEDRIVER prevents this form of HELLO datagram congestion by distributing its HELLO datagram multicasts randomly over time. A HELLO datagram is still multicast over each adapter approximately every 3 seconds but not over all adapters at once. Instead, if a node has multiple network adapters, PEDRIVER attempts to distribute its HELLO datagram multicasts so that it sends a HELLO datagram over some of its adapters during each second of the 3-second interval.
In addition, rather than multicasting precisely every 3 seconds, PEDRIVER varies the time between HELLO datagram multicasts between approximately 1.6 to 3 seconds, changing the average from 3 seconds to approximately 2.3 seconds.
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