cwnd can now be less than 2

[larseggert]

@martinthomson raised this on the QUIC slack:

      W_max = cwnd;                 // save window size before reduction
      ssthresh = cwnd * beta_cubic; // new slow-start threshold
      ssthresh = max(ssthresh, 2);  // threshold is at least 2 MSS
      cwnd = cwnd * beta_cubic;     // window reduction

cwnd can now be less than 2.

[goelvidhi]

Ah.. for this code, a simple fix would be:

W_max = cwnd;                 // save window size before reduction
      ssthresh = cwnd * beta_cubic; // new slow-start threshold
      ssthresh = max(ssthresh, 2);  // threshold is at least 2 MSS
      cwnd = ssthresh;                    // window reduction

I can work on this one, if no one else is.

[sangtaeha]

Yes, Linux also implements it this way.