OpenVMS Cluster Systems - OpenVMS Systems - HP

Figure F–4 Lost ACKs Cause Retransmissions
Troubleshooting the NISCA Protocol
F.6 Retransmission Problems
NISCA
Local Node Remote Node
(timeout)
(lost)
Message n
ACK n
Retransmitted message n
ACK n
Note: n represents a number that identifies
each sequenced message.
ZK−5942A−GE
In Figure F–4, the remote node receives the message and transmits an
acknowledgment (ACK) to the sending node. However, because the ACK from the
receiving node is lost, the sending node retransmits the message.
F.6.2 Techniques for Troubleshooting
You can troubleshoot cluster retransmissions using a LAN protocol analyzer for
each LAN segment. If multiple segments are used for cluster communications,
then the LAN analyzers need to support a distributed enable and trigger
mechanism (see Section F.8). See also Section G.1 for more information about
how PEDRIVER chooses channels on which to transmit datagrams.
Reference: Techniques for isolating the retransmitted datagram using a
LAN analyzer are discussed in Section F.10.2. See also Appendix G for more
information about congestion control and PEDRIVER message retransmission.
F.7 Understanding NISCA Datagrams
Troubleshooting NISCA protocol communication problems requires an
understanding of the NISCA protocol packet that is exchanged across the
OpenVMS Cluster system.
F.7.1 Packet Format
The format of packets on the NISCA protocol is defined by the $NISCADEF
macro, which is located in [DRIVER.LIS] on VAX systems and in [LIB.LIS] for
Alpha systems on your CD listing disk.
Figure F–5 shows the general form of NISCA datagrams. A NISCA datagram
consists of the following headers, which are usually followed by user data:
• LAN headers, including an Ethernet or an FDDI header
• Datagram exchange (DX) header
Troubleshooting the NISCA Protocol F–19