code: func (r *reply) send(bv uint64) error {
  	type resp struct {
  		ReqID, BV uint64
  		Data      rlp.RawValue
  	}
  	return p2p.Send(r.w, r.msgcode, resp{r.reqID, bv, r.data})
  }
docs: // send sends the reply with the calculated buffer value