Envoy Application Programming Manual

Software Concepts
Envoy Application Programming Manual—427159-001
2-14
Point-to-Point Operation
Example 2-4 shows the following two important points:
•
The first WRITE by the transmitter does not complete until it is acknowledged when
an ACK1 is returned from the receiver. (If the receiver is another NonStop™
Himalaya server, the ACK1 is returned when it performs its next READ.)
•
The transmitter indicates that the transmission is complete by sending an EOT. An
EOT is sent when the transmitter calls the READ procedure following a call to the
WRITE procedure (EOT is also sent if CONTROL 13 is executed). The receipt of
the EOT results in the second READ on the receiver side completing with an error
163 (EOT received).
Example 2-4. WRITE-READ Interactions
Transmitter Receiver
Initial WRITE:
CALL WRITE CALL READ
| (bid) ENQ |
| <--- ACK0 |
| message1 ---> (READ completes)
| error = 0
| line state = READ
|
| CALL READ
(WRITE completes) <--- ACK1 |
error = 0 |
line state = WRITE |
|
Continuation WRITE: |
|
CALL WRITE |
| message2 ---> (READ completes)
| error = 0
| line state = READ
|
| CALL READ
(WRITE completes) <--- ACK0 |
error = 0 |
line state = WRITE |
|
CALL READ |
| EOT ---> (READ completes)
| error = 163
(READ completes) line state = CONTROL
error = 162
line state = CONTROL