Hector Santos writes:
Unless I misunderstanding, the above is based on the idea that the
CLIENT has already sent a command and is currently waiting for a
server response.
The client will see the 421 at once, or when it sends its next command,
depending on how it's written. That's why 421 is legal for all
commands.
I don't know if any internet client protocol, maybe with the exception
of FTP, where it can get a unsolicated server response.
IMAP, for example. And there are many internet protocols that don't work
well with an RPC client like this:
SendCommand("HELO")
WaitForResponse() // Synchronous concept
SendCommand("MAIL FROM:")
WaitForResponse() // Synchronous concept
Read the SMTP PIPELINING RFC, look at the NNTP client code in Leafnode,
look at the client part of any fast NFS implementation, etc.
Fast clients may look like this:
when necessary and possible:
send a command
when data arrives from the server:
figure out which command it belongs to
update the client's state machine
--Arn