??? ResSetup and ResWork are action symbols that represent responses from the
server to commands Setup and Work, respectively. A setup (work) response is
an action ResSetup(m,c,s) (ResWork(m,c,s)), where m is a message ID, c is a
number of granted credits, and s is a status value.
??? Cancel is used as an action symbol that represents a request from the client to
cancel a previous operation; that is, the command is Cancel. A cancel request
has the form Cancel(m), where m is a message ID. There is no response to a
cancel request.
Each action symbol has a number of parameters that corresponds to the number of
relevant fields in the messages. As said, not all of the fields of messages are relevant.
If we were to consider more features of the protocol, the number of parameters of
the action symbols would be bigger but the remaining arguments would be ignored
in the model programs of the above features.
14.2.1 Credits feature
The protocol is designed to prevent the client from overwhelming the server with too
many requests. The client can only use certain message identifiers to communicate
with the server. Each identifier can only be used once. The identifiers that the client
can use can be seen as a pool of numbers. The client can ask for additional credits
in the requests that it sends to the server in order to expand the pool. The server
may grant credits in its responses to the client.
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