Re: [quicwg/base-drafts] Limit CWND increase in slow start (#3232)

Junho Choi <notifications@github.com> Tue, 12 November 2019 23:48 UTC

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Date: Tue, 12 Nov 2019 15:48:02 -0800
From: Junho Choi <notifications@github.com>
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Subject: Re: [quicwg/base-drafts] Limit CWND increase in slow start (#3232)
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junhochoi commented on this pull request.



> @@ -1368,7 +1377,15 @@ acked_packet from sent_packets.
        return
      if (congestion_window < ssthresh):
        // Slow start.
-       congestion_window += acked_packet.size
+       if (pacing):
+         congestion_window += acked_packet.size
+       else:
+         // If pacing is not used, limit the increase to
+         // 2*max_datagram_size per ACK as described in
+         // {{?RFC3465}}
+         if (cwnd_increase < 2 * max_datagram_size):
+           cwnd_increase += bytes_acked
+           congestion_window += bytes_acked

I think this may allow increasing cwnd close to (3 x max_datagram_size).

e.g. max_datagram_size = 1000
ack 1 = 999 bytes
ack 2 = 999 bytes
ack 3 = 999 bytes

If we have 3 acks each acks 999 bytes, this code result in 999x3 bytes cwnd increase. 

To calculate remaining bytes correctly, I would propose the following:

```
  OnAckReceivedCC():
     cwnd_incr_max = 2 * max_datagram_size
...
  OnPacketAckedCC(acked_packet):
...
         // If pacing is not used, limit the increase to
         // 2*max_datagram_size per ACK as described in
         // {{?RFC3465}}
         cwnd_incr = cwnd_incr_max - max(cwnd_incr_max - acked_packet.size, 0)
         congestion_window += cwnd_incr
         cwnd_incr_max -= cnwd_incr
```

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