tx_pkt_history_by_pkt_num
pkt = tx_pkt_history_by_pkt_num(h, ack->ack_ranges[0].end);
pkt = tx_pkt_history_by_pkt_num(h, pkt_num);
pkt = tx_pkt_history_by_pkt_num(h, pn);
pkt = tx_pkt_history_by_pkt_num(h, ack->ack_ranges[0].end);