"strange case 2" in FACK.

OK. now it's clear. Most (if not all) the NS2 implementation of scoreboard ignores DSACK, which is generated by the Sacker.

If a DSACK is generated, one SACK block is created for highest_ack_+1 (which is actually never retransmitted.)

To fix this bug, check DSACK in Scoreboard before creating a block.