ts_rtt is 1 plus the RTT, so that 0 can mean invalid measurement.
However, the code failed to subtract the 1 back out before use. With
this change, TCP from Massachusetts to France now typically has 1s RTO
values, rather than 1.5s.
This bug was found and fixed by Bev Schwartz of BBN. This material is
based upon work supported by the Defense Advanced Research Projects
Agency and Space and Naval Warfare Systems Center, Pacific, under
Contract No. N66001-09-C-2073. Approved for Public Release,
Distribution Unlimited
c238210804