[quicwg/base-drafts] Recovery: exit initial slow start on RTT increase (#2728)

[Christian Huitema <[e-mail]>]

The current recovery spec says *7.2. Slow Start*: 
_QUIC begins every connection in slow start and exits slow start upon loss or upon increase in the ECN-CE counter. QUIC re-enters slow start anytime the congestion window is less than ssthresh, which only occurs after persistent congestion is declared. While in slow start, QUIC increases the congestion window by the number of bytes acknowledged when each acknowledgment is processed._ 

That's fine, but it is often advisable to exit the initial slow start before observing the first loss. Very often, the default behavior results in widely overshooting the "fair" window, leading to not just one loss but rather a series of losses. For example, implementations could monitor the evolution of the RTT during initial slow start, and exit slow start as soon that the measured RTT exits RTTMin + some margin -- maybe the min 1/8th of RTT and 10 milliseconds, or 1ms 1/8th of RTTMin was too small. 

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