OpenVMS Cluster Systems - OpenVMS Systems - HP

Troubleshooting the NISCA Protocol
F.7 Understanding NISCA Datagrams
Figure F–10 TR Header
LAN
Header
DX
Header
space, a TR/CC flag identifies the type of message being transmitted over the
channel.
Figure F–10 shows the portions of the TR header that are needed for network
troubleshooting, and Table F–11 describes these fields.
0
Datagram
Flags
1 2
4 6
... Message
Acknowledgment
Sequence
Number
...
10 14 18
Extended Message
Acknowledgment
Extended Sequence
Number
...
ZK−5925A−GE
Note: The TR header shown in Figure F–10 is used when both nodes are running
Version 1.4 or later of the NISCA protocol. If one or both nodes are running
Version 1.3 or an earlier version of the protocol, then both nodes will use the
message acknowledgment and sequence number fields in place of the extended
message acknowledgment and extended sequence number fields, respectively.
Table F–11 Fields in the TR Header
Field Description
Datagram flags (bits ) Provide additional information about the transport datagram.
Value
Abbreviated
Datagram
Type
Expanded
Datagram
Type Function
0 DATA Packet data Contains data to be delivered to
the upper levels of software.
1 SEQ Sequence flag Set to 1 if this is a sequenced
message and the sequence
number is valid.
2 Reserved Set to 0.
3 ACK Acknowledgment Acknowledges the field is valid.
4 RSVP Reply flag Set when an ACK datagram is
needed immediately.
5 REXMT Retransmission Set for all retransmissions of a
sequenced message.
6 Reserved Set to 0.
7 TR/CC flag Transport flag Set to 0; indicates a TR
datagram.
Message acknowledgment An increasing value that specifies the last sequenced message segment received by
the local node. All messages prior to this value are also acknowledged. This field
is used when one or both nodes are running Version 1.3 or earlier of the NISCA
protocol.
Extended message
acknowledgment
F–24 Troubleshooting the NISCA Protocol
An increasing value that specifies the last sequenced message segment received by
the local node. All messages prior to this value are also acknowledged. This field is
used when both nodes are running Version 1.4 or later of the NISCA protocol.
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