[tcpm] hystart++

[Randall Stewart <[e-mail]>]

Greetings all:

I have implemented hystart++ off of the draft-03 for FreeBSD 
(both newreno and cubic) and I have noted one thing that I think
is a bug in the spec:

Draft-03 currently says:



        o  if (currentRoundMinRTT >= (lastRoundMinRTT + RttThresh))

           +  cssBaselineMinRtt = currentRoundMinRTT

           +  exit slow start and enter CSS


<And>



     -  if (currentRoundMinRTT < cssBaselineMinRtt)

        o  cssBaselineMinRtt = infinity

        o  resume slow start including HyStart++



But notice that the threshold for entering CSS is (lastRoundMinRtt + RttThresh) and
the exiting of CSS back to Slow Start is (currentRoundMinRTT < cssBaseLineMinRTT), which can
under the right circumstances set up an oscillation. In the case I observed I saw lastRoundMinRtt set
to like 63ms and currentRoundMinRtt set at 115ms. And the next RTT in was 114ms.. so out of 
CSS into Slow Start, but the next measurement (114ms again) pushes you back into CSS since
the lastRoundMinRTT + RttThresh is around 63ms or so.

I would think what you really want here is to set cssBaseLineMinRTT to (lastRoundMinRtt + RttThresh).

I.e. on entry:


        o  if (currentRoundMinRTT >= (lastRoundMinRTT + RttThresh))

           +  cssBaselineMinRtt = (lastRoundMinRtt + RttThresh)

           +  exit slow start and enter CSS

R      

------
Randall Stewart
[e-mail]