https://erlang.org/doc/programming_examples/bit_syntax.html

> say sendSeq was one bit but we still wanted it to take 3 bits:

> [0] ++ pad 3 sendSeq ++ poll ++ recvSeq

In Erlang you could do this to get something padded (or truncated) to the correct size:

  << 2:3 >> ;; corresponds to the 3 bits <<010>>

So you can do something like this to ensure the precise bit size of each field:

  << 0:1, SendSeq:3, Poll:3, RecvSeq:3>>