OpenVMS Cluster Systems - OpenVMS Systems - HP

Cluster Troubleshooting
C.9 Port Communications
C.9.2 LAN Communications
For clusters that include Ethernet or FDDI interconnects, a multicast scheme
is used to locate computers on the LAN. Approximately every 3 seconds, the
port emulator driver (PEDRIVER) sends a HELLO datagram message through
each LAN adapter to a cluster-specific multicast address that is derived from the
cluster group number. The driver also enables the reception of these messages
from other computers. When the driver receives a HELLO datagram message
from a computer with which it does not currently share an open virtual circuit,
it attempts to create a circuit. HELLO datagram messages received from a
computer with a currently open virtual circuit indicate that the remote computer
is operational.
A standard, three-message exchange handshake is used to create a virtual circuit.
The handshake messages contain information about the transmitting computer
and its record of the cluster password. These parameters are verified at the
receiving computer, which continues the handshake only if its verification is
successful. Thus, each computer authenticates the other. After the final message,
the virtual circuit is opened for use by both computers.
C.9.3 System Communications Services (SCS) Connections
System services such as the disk class driver, connection manager, and the
MSCP and TMSCP servers communicate between computers with a protocol
called System Communications Services (SCS). SCS is responsible primarily for
forming and breaking intersystem process connections and for controlling flow of
message traffic over those connections. SCS is implemented in the port driver (for
example, PADRIVER, PBDRIVER, PEDRIVER, PIDRIVER), and in a loadable
piece of the operating system called SCSLOA.EXE (loaded automatically during
system initialization).
When a virtual circuit has been opened, a computer periodically probes a remote
computer for system services that the remote computer may be offering. The
SCS directory service, which makes known services that a computer is offering,
is always present both on computers and HSC subsystems. As system services
discover their counterparts on other computers and HSC subsystems, they
establish SCS connections to each other. These connections are full duplex and
are associated with a particular virtual circuit. Multiple connections are typically
associated with a virtual circuit.
C.10 Diagnosing Port Failures
This section describes the hierarchy of communication paths and describes where
failures can occur.
C.10.1 Hierarchy of Communication Paths
Taken together, SCS, the port drivers, and the port itself support a hierarchy of
communication paths. Starting with the most fundamental level, these are as
follows:
• The physical wires. The Ethernet is a single coaxial cable. FDDI typically
has a pair of fiber-optic cables for redundancy. The CI has two pairs of
transmitting and receiving cables (path A transmit and receive and path B
transmit and receive). For the CI, the operating system software normally
sends traffic in automatic path-select mode. The port chooses the free path or,
if both are free, an arbitrary path (implemented in the cables and star coupler
and managed by the port).
C–18 Cluster Troubleshooting