* t_partialacks<0 means we are not in fast recovery.
* t_partialacks==0 means we are in fast recovery, but we have not received
any partial acks yet.
* t_partialacks>0 means we are in fast recovery, and we have received
partial acks.
This is used to implement 2 changes in RFC 3782:
* We keep the notion that we are in fast recovery separate from t_dupacks, so
it is not reset due to out-of-order acks. (This affects both the Reno and
NewReno cases.)
* We only reset the retransmit timer on the first partial ack -- preventing us
from possibly taking one RTO per segment once fast recovery is initiated.
As before, it is hard to measure any difference between Reno and NewReno in the
real-world cases that I've tested.
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