[NTLUG:Discuss] What is the best method to communicate between a device and a server?

[Kipton Moravec]

I am designing a device or appliance (it does not matter what) that
needs to report results to a server, over the Internet and get new
instructions once a day or whenever it powers up. (Or had a power
failure). There could be thousands of these devices.

I have to assume the device will be behind a firewall so the device will
have to contact the Linux server and the server will not be able to
initiate the communications to the device.

The device will send a progress report each day, reporting a few
statistics (probably 100 - 200 bytes). The only other time it will
report is when things go wrong.

When it checks in, it will look for any new commands for itself,
including software updates. A command may be to provide additional data,
or a recommendation to check in at a specific time (for load balancing
on the server). It may be required to download a new program revision,
and reprogram itself.

I do need to keep track of time, so I am thinking of implementing NTP
also. It will have its own clock that is good to a couple of seconds per
month.

What application protocols are best for something like this?

I can see using FTP to upload and download files with the data.

I can see using SMTP and a POP server (email) for sending and receiving
messages.

Some have suggested SNMP which I am not familiar with.

Another way is to open a socket to socket custom connection.

Someone suggested opening up a secure telnet connection.

I am sure there are more ways to do this. But what I do not know is what
is the best way to do this. I want something that is robust, and easy.
(KISS) And preferably small. 

It is hard to predict what will be required in the future so flexibility
is good for both sides, the device and the server.

Any suggestions? And more importantly why one way would be better than
another?

Kip
-- 
Kipton Moravec KE5NGX
"Always do right; this will gratify some people and astonish the rest."
--Mark Twain