A buffer serialized as its size first as a nat0, then the payload itself.
A value serialized by first having its size as a nat0, then the encoding of the value itself.
This also uses the “size-prefixed binary protocol”.
This uses the “size-prefixed binary protocol”. https://ocaml.janestreet.com/ocaml-core/v0.13/doc/async_unix/Async_unix/Writer/index.html#val-write_bin_prot