docs: @Override
      public void writeTo(DataOutput out) throws IOException {
          out.writeByte(type);
          switch(type) {
              case DATA:
                  Bits.writeLong(seqno, out);
                  out.writeShort(conn_id);
                  out.writeBoolean(first);
                  break;
              case ACK:
                  Bits.writeLong(seqno, out);
                  out.writeShort(conn_id);
                  Bits.writeInt(timestamp, out);
                  break;
              case SEND_FIRST_SEQNO:
                  Bits.writeInt(timestamp, out);
                  break;
              case XMIT_REQ:
                  break;
              case CLOSE:
                  out.writeShort(conn_id);
                  break;
          }
      }