[Info-vax] VMS process communication

[Stephen Hoffman]

On 2022-09-13 20:59:40 +0000, Dave Froble said:

> That's what "handshakes" are for.
> 
> Hey, I want to send some data, 4000 bytes ...
> Ok, ready to receive data ...
> Send the data, wait for a response ...
> Ok, received 4000 bytes ...
> 
> Anything else would be considered a failure.
> 
> In addition, checksums and such could be included to insure the 4000 
> bytes sent is the same 4000 bytes received.

4K is ~one (modern) disk block, but yeah.

More common to send 4K with a checksum, or 4K encrypted with a 
checksum, and let the receiver figure it out, though. An 
application-level TCP-style handshake is closer to what is described is 
certainly workable, but the chatter greatly slows things. I'd probably 
look at the QUIC handshake, or at a message-passing or transaction 
framework, if I needed the extra chatter.  DECdtm, or Paxos protocol, 
or MQTT, and tracking the transaction state, etc. But more generally 
with the use of TCP or QUIC, you will get notification of either a 
successful send, partial send, or of a failure to send.

It's up to the app or the transaction manager to determine where the 
failure occurred, just as it's up to app-local error processing the 
trap shadow on an Alpha to determine where the error occurred, or other 
sorts of errors.

Put somewhat differently, it's usually best to design the connection 
for speed and simplicity and for the common cause, and to (potentially) 
make the error path and error recovery path somewhat more ugly.

PS: The TCP send operation has four potential outcomes: all bytes 
transferred, a partial transfer with a need to loop and send more, a 
local failure with no transfer, or a connection failure from the remote 
side with no data transferred. Handling this usually involves a state 
machine.

PPS: one of the more common errors here with TCP and such is assuming a 
1-to-1 mapping of writes to reads. TCP and QUIC don't (necessarily) 
work that way. One big write can become many smaller reads, for 
instance.

PPPS: Depending on the platform, QUIC can have a specific API: 
https://developer.apple.com/documentation/network/quic_options — with 
that, QUIC can be switched to datagram, for instance.  And AFAIK, 
OpenVMS TCP/IP Services has UDP, TCP, and SCTP, but lacks QUIC.

PPPPS: having a framework that deals with this makes this whole area 
far easier to deal with, and with fewer mistakes, and with fewer 
problems with encryption or authentication.

-- 
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