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Low level protocol

There isn't a project I've come along that didn't invent a proprietary protocol to transport data over e.g. a serial line or similar. I have yet to find the optimal protocol, i guess that's why developers invent new ones, but I have some hints for people that wish to invent their own.

If you wish to find some official directions have a look at the OSI model, or the SLIP protocol.

This article will contain a lot of ideas to play with and some recommendations.

Problem
Generalizing we have a client symbol set including e.g. all the bytes we want to transfer plus maybe a few extras for e.g. start and end of message. These client symbols need to be transferred over a communication channel with the ability to transfer another set of symbols, e.g. bytes only but no meta-symbols, or just a subset of byte values like printable ASCII.
An example would be a serial line (RS232C or similar) being able to transfer 8 bit bytes. Or maybe 7 bit bytes excluding NUL that is consumed by the device driver and XON, XOFF that are used for software handshake. It may also be e.g. a TCP/IP socket or a parallel port. From a  software point of view we see a stream of bytes though theoretically our bytes could be 16 bit or 9-bit or whatever.
Over the channel two or more conputer systems need to exchange data in some kind of messages, e.g. commands + parameters or responses containing data.
In some cases not all byte codes may be transmitted (e.g. nulls or most significant bit is consumed by underlying channel). In other cases not all codes need be transferred.
I assume the channel is bidirectional but implementing a unidirectional protocols can be done in a similar way only with less opportunity to recover from errors in transmission.
Usually the communication is a bottleneck, either bulk transfer speed or responsiveness. Thus we want to make the protocol efficient in using little overhead for communications. Also the protocol will be used on embedded systems possibly with limited computing power so it shouldn't require too much number crunching power.
Well defined responsibilities
In order to make a good protocol design i is important to look at the requirements one at a time and separate the responsibilities. It is easy to want a simple protocol and want to start coding immediately, but in general you will risk implementing the same function in many places and code will be unmanageable.
I recommend to layer the communication structure. Separate at least the message packaging from the data in the messages. The layer that sends and receives the messages should not be considered with the data in the messages and if there is some acknowledge of messages  in the package send/receive layer that should be handled in the same layer and not have to inspect the packages.
I have chosen to divide the problem in to the following areas:

Message separation
Somehow the messages need to be extracted from the stream of bytes. There are two basic ways to do this:
  • Encoded length of message
One or more bytes in beginning of message is defined as length specifier. Thi sway the receiver knows how many bytes to read.
len=3 byte 0 byte 1 Byte 2
Encoded length requires an error free transmission to be useful. A single bit error may cause length to be significantly misinterpreted and can consume several following messages and be very hard to re-sync. Imagine having a 32-bit message length and the most significant bit is flipped. One obvious solution to this is of course to have some error detection codes in the (fixed length) header containing the length, like a header CRC used in IP protocol.
If sync is lost for some reason it is difficult to re-sync the communication stream. There is no way to identify when the next message starts if count is the only indicator of message length. Possibly a timeout can serve as framing to identify next message. Or vast numbers of NULs causing many zeros in the current message plus a number of zero length messages could do. Anyway we're approaching an inter-message framing protocol.
A special case of the latter is when the message type implicitly defines the length of the message, e.g. when parameters to command 'A' are always 4 bytes.
CmdA 4 bytes data CmdB 6 bytes data
This is a simple implementation but does not separate messages from contents and each message must handle errors in transmissions. Better separate the two and handle trasnmission errors in general and command/parameter errors in a different level.
  • Inter-message framing
Here we use a special code meaning that we are  no tin a message. Detection of this framing code indicates end of message but it i sbeneficial to allow more than one framing codes in a row to e.g. indicate both start and stop of the mesasge (filter out nois eon line).
framing message1 framing message2 framing
Inter-message framing has the drawback that the symbol used for framing cannot be used in the message but has to be replaced by some other code. SLIP protocol has a minimalistic implementation of this.
Using both methods is usually not good since if there is a mismatch one method must take priority over the other making the unprioritized just a waste of bandwidth.
In general the inter-message framing approch is preferred. A variant is to use a start marker and a different end marker:
START message END
The benefit of this is that any symbols outside START/STOP may be transmitted with a different protocol, e.g. as text. This way you can have normal ASCII communication with e.g. a terminal or a printer but symbols between START and END are considered valid for a special protocol. Optimally the terminal/printer should be insensitive to the symbols between the markers but with a terminal a human will usually be good at sorting it out. Or you have a software driver that intercepts the messages and passes on the text.
One kind of framing may be a duration of idle communication. This wastes no byte codes but may cost performance. Most channels are buffered, often in hardware, and timeouts may need to be long for idle time to be detected. Maybe an embedded system can handle the tiing but a PC with an operating system cannot unless it is a hard real time OS.
Actually with RS232C communications there are some options to send symbols outside the normal 8-bit byte range:
  • Idle time
  • Break character
  • Parity used as 9th bit
In general these are difficult to use in the PC world so this article assumes we have only a set of 8-bit bytes at hand. If working with embedded to embedded communications feel free to experiment with these.

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