How the “OK” message looks like?

Question

I have a device that sends data to a server.

            Data
[ Client ]  == >  [ Server ]

After the validation on the server I want to return a message:

            OK
[ Client ] < == [ Server ]

Is there a standard "OK" message to return? And an "ERROR" message? How does it looks like? (e.g. ":0011", ":110F")

Answer 1

You've got to design an application-level protocol. TCP is a byte stream, so even the definition of "Data" in your client->server piece needs some protocol so that the receiver can know what bytes make up the data (when to stop reading).

A couple of common types of protocols are...

• Length-delimited chunks. Every message starts with a 16 or 32-bit length prefix. Then that many bytes follow. The length needs to be in a defined byte order (see htons, ntohs, etc). Everyone who uses this protocol knows to read the length prefix then read that many bytes. Having defined that "chunk" on the network, you might put a header on the contents of the chunk. Maybe a message type (ACK, NAK, Data, etc) followed by some contents.
• ASCII newline delimited. Each message is a line of ASCII (or UTF8, etc) text. It ends at a newline. Newline endings for the lines play the same role as the length prefix for the chunks above. You then define what's in each line (like space or comma-delimited ASCII/UTF8/whatever fields). Somewhere in that you'd define what data looks like, ACK, etc.

I'm sure you could come up with other ideas, but that's the basic job: defining your application-level protocol on top of TCP's byte stream.