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NonStop S-Series Server Description Manual (G06.27+)

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Interprocessor Communication
HP NonStop S-Series Server Description Manual—520331-004
9-6
Message System Protocol
Message System Protocol
When the linker and listener processes exchange messages back and forth (as
described in the preceding topic), the message system adds its own protocol
information to those same messages. In Figure 9-3, that added information is shown
enclosed in shaded boxes in the messages; the unshaded information in the messages
is exactly the same as shown in Figure 9-2. Details about the movement of control and
data between processors, described in the preceding topic, are therefore omitted in
this description. Note, however, that message system icons have been added and the
flow of control information to and from the message systems is shown.
In comparison with Figure 9-2, a primary difference in the messages is that request
setup information has been added to the request messages and reply setup has been
added to the reply messages. As mentioned earlier, setup information tells the peer
message system what the byte count of the data is (if present) and provides ServerNet
addresses for the various buffers.
In addition to exchanging setup information, it also is the responsibility of the message
systems to send an acknowledgment back to the sending message system whenever
a request message or a reply message has been received. Such acknowledgments
can, for the sake of reducing message traffic, be included in (“piggy-backed” on) any
setup packet going in the same direction to the same processor.
If there happens to be no setup packet in which the acknowledge can be included, a
one-packet message needs to be sent back separately to the other processor’s
message system after the message has been received. That is the case shown as an
example in Figure 9-3, wherein the reply acknowledge is returned separately and alone
from the linker to the listener in both cases. The reply acknowledge could have been
returned in a request setup going in the same direction.