rtt measurement in loss recovery state

when we use rtt for pacing. the rtt measurement is usually too conservative for loss recovery state.

"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.