Protobuf is a great encoding format. It’s become wildly popular as an encoding protocol, not only for gRPC messages, but for all kinds of information exchange.
When using Protobuf in a streaming situation — i.e. when writing to a file, or over a socket — the problem of delimiting one message from the next becomes apparent. Encoded Protobuf messages do not mark the end of each message, so it is not be possible to reliably distinguish one message from another when reading the stream on the other end.