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diff --git a/docs/concepts.rst b/docs/concepts.rst index 0df5ad66..b0fd43aa 100644 --- a/docs/concepts.rst +++ b/docs/concepts.rst @@ -299,6 +299,25 @@ An example of this is available in *tests/test_encode_extensions.c* and .. _`extension fields`: https://developers.google.com/protocol-buffers/docs/proto#extensions +Message framing +=============== +Protocol Buffers does not specify a method of framing the messages for transmission. +This is something that must be provided by the library user, as there is no one-size-fits-all +solution. Typical needs for a framing format are to: + +1. Encode the message length. +2. Encode the message type. +3. Perform any synchronization and error checking that may be needed depending on application. + +For example UDP packets already fullfill all the requirements, and TCP streams typically only +need a way to identify the message length and type. Lower level interfaces such as serial ports +may need a more robust frame format, such as HDLC (high-level data link control). + +Nanopb provides a few helpers to facilitate implementing framing formats: + +1. Functions *pb_encode_delimited* and *pb_decode_delimited* prefix the message data with a varint-encoded length. +2. Union messages and oneofs are supported in order to implement top-level container messages. +3. Message IDs can be specified using the *(nanopb_msgopt).msgid* option and can then be accessed from the header. Return values and error handling ================================ |