Framing

Messages are binary data packet encoded via protocol buffers (protobuf) according to specifications in .proto files. The binary data packets do not by them self indicate their size (number of bytes). Therefore we need to add a length to the data. The HDLC layers for serial communication will handle length, thus any further framing is not needed. For communication over ethernet via TCP a framing is introduced by prepending a packet by its length.

HDLC

The HDLC layers handles length of frames, so for HDLC no additional framing is needed.

Length bytes

Protocol buffer encoded message

MyDefence devices will enable HDLC keep-alive, to detect broken connections. This is transparent to the application.

TCP

TCP provides a data stream, so each binary protobuf encoded message is prepended with a 4 byte length field encoded as a little endian 32 bit unsigned integer.

4 bytes	Length bytes
Length	Protocol buffer encoded message

MyDefence devices will enable TCP keep-alive, to detect broken connections. This is transparent to the application.