Build and send a selective acknowledgement (SACK) chunk.

async def _send_sack(self):
        """
        Build and send a selective acknowledgement (SACK) chunk.
        """
        gaps = []
        gap_next = None
        for tsn in sorted(self._sack_misordered):
            pos = (tsn - self._last_received_tsn) % SCTP_TSN_MODULO
            if tsn == gap_next:
                gaps[-1][1] = pos
            else:
                gaps.append([pos, pos])
            gap_next = tsn_plus_one(tsn)

        sack = SackChunk()
        sack.cumulative_tsn = self._last_received_tsn
        sack.advertised_rwnd = max(0, self._advertised_rwnd)
        sack.duplicates = self._sack_duplicates[:]
        sack.gaps = [tuple(x) for x in gaps]

        await self._send_chunk(sack)

        self._sack_duplicates.clear()
        self._sack_needed = False