[sane-standard] more network protocol proposals

[Johannes Berg]

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After having worked through the network protocol here are a few more
suggestions:

* make the handle only one byte long (this will suffice for 255
   simultaneous scans, I don't think that could reasonably be called a
   limitation)
* make the procedure code only one byte long
* prepend the request procedure code to the answer
   (that is, an answer that would previously have been
    |status (1 word)| would now be
    |code (1b)|status (1w)|
   and it would be allowed for the server to send other packets before
   the answer.

These both assist in data reduction with the following:

* transfer scan data in the same connection as control data

This can be achieved by adding two new reserved procedure codes for
answers only, scan_data and scan_data_eof.
When scan_data is returned, the data record looks like this:
| scan_data code (1b) | handle (1b) | length (2b) | data... |
for scan_data_eof, only scan_data_eof code and handle are present.

I have deliberately made the length field only 2 bytes long because more
would hurt the case of small packets more, and less is obviously bad
too, a maximum of 64k data per packet seems reasonable. At 64k / packet
the overhead of this would be negligible, at 1k packets it is still less
than 4 percent. The server could of course locally cache data to fill
the packets to 64k (this shouldn't hurt).

I'd say this is worth it for the reasons that it very much simplifies
the code (no more extra sockets etc), makes saned easier to work with
through firewalls and allows for encrypting the transport much easier.

As an extra bonus, this allows the server to instruct callbacks on the
client side (these need to be defined as procedure codes, and also in
the SANE API).

johannes

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