Encoding payloads with Protobuf

The Protocol Buffers Encoding specifies how to encode or decode Protobuf messages to or from a stream of bytes.

However, this specification is silent on how Protobuf RPC methods frame their input and output messages. As a result, an RPC framework that uses Protobuf needs to fill in this blank and describe what the payloads of its requests and responses look like with Protobuf.

The IceRPC + Protobuf integration adopts the framing used by the gRPC protocol: a Protobuf input or output message is encoded as a Length-Prefixed-Message inside a request or response payload:

• the first byte (Compressed-Flag) can be 0 or 1 (not compressed or compressed)
• the next 4 bytes (Message-Length) hold the length of the Protobuf-encoded message as an unsigned integer (big-endian ordering)
• the following bytes are the Protobuf-encoded message

It's a simple and uniform framing. IceRPC + Protobuf does not currently provide support for gRPC-style compression (Compress-Flag set to 1) so Compress-Flag is always set to 0.