Re: [quicwg/base-drafts] Notify the congestion controller of losses first (#3540)

Jana Iyengar <notifications@github.com> Tue, 31 March 2020 06:30 UTC

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Date: Mon, 30 Mar 2020 23:30:25 -0700
From: Jana Iyengar <notifications@github.com>
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Subject: Re: [quicwg/base-drafts] Notify the congestion controller of losses first (#3540)
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@janaiyengar commented on this pull request.

This looks like a good fix. A few comments.

> +DetectLostPackets is called every time an ACK is received or the time threshold
+loss detection timer expires and operates on the sent_packets for that packet
+number space.

```suggestion
DetectLostPackets is called every time an ACK is received or the time threshold
loss detection timer expires. This function operates on the sent_packets for that
packet number space and returns a list of packets newly detected as lost.
```

> @@ -1265,7 +1244,8 @@ OnLossDetectionTimeout():
     GetEarliestTimeAndSpace(loss_time)
   if (earliest_loss_time != 0):
     // Time threshold loss Detection
-    DetectLostPackets(pn_space)
+    lost_packets = DetectLostPackets(pn_space)
+    OnPacketsLost(lost_packets)

```suggestion
    if (!lost_packets.empty()):
      OnPacketsLost(lost_packets)
```

> @@ -1517,7 +1494,7 @@ Invoked from DetectLostPackets when packets are deemed lost.
      CongestionEvent(largest_lost_packet.time_sent)

How about also changing this to the following, and removing the line before this (last() is a language-specific method, so I'm suggesting largest())?
```suggestion
     CongestionEvent(lost_packets.largest().time_sent)
```

> @@ -1108,14 +1108,16 @@ OnAckReceived(ack, pn_space):
     largest_acked_packet[pn_space] =
         max(largest_acked_packet[pn_space], ack.largest_acked)
 
+  // Determine which packets have been acknowledged and remove

```suggestion
  // DetectNewlyAckedPackets finds packets have been newly acknowledged and removes
```

> @@ -1108,14 +1108,16 @@ OnAckReceived(ack, pn_space):
     largest_acked_packet[pn_space] =
         max(largest_acked_packet[pn_space], ack.largest_acked)
 
+  // Determine which packets have been acknowledged and remove
+  // them from sent_packets.
+  newly_acked_packets = DetectNewlyAckedPackets(ack, pn_space)

```suggestion
  newly_acked_packets = DetectAndRemoveAckedPackets(ack, pn_space)
```
here and below

> @@ -1128,13 +1130,12 @@ OnAckReceived(ack, pn_space):
   if (ACK frame contains ECN information):
       ProcessECN(ack, pn_space)
 
-  for acked_packet in newly_acked_packets:
-    OnPacketAcked(acked_packet.packet_number, pn_space)
-
-  DetectLostPackets(pn_space)
+  lost_packets = DetectLostPackets(pn_space)

```suggestion
  lost_packets = DetectAndRemoveLostPackets(pn_space)
```
here and below

> @@ -1434,26 +1411,27 @@ increases bytes_in_flight.
 ## On Packet Acknowledgement
 
 Invoked from loss detection's OnPacketAcked and is supplied with the

```suggestion
Invoked from loss detection's OnAckReceived and is supplied with the
```

> @@ -1499,15 +1477,14 @@ Invoked when an ACK frame with an ECN section is received from the peer.
 Invoked from DetectLostPackets when packets are deemed lost.
 
 ~~~
-   InPersistentCongestion(largest_lost_packet):
+   InPersistentCongestion(lost_packets):
      pto = smoothed_rtt + max(4 * rttvar, kGranularity) +
        max_ack_delay
      congestion_period = pto * kPersistentCongestionThreshold
      // Determine if all packets in the time period before the
      // newest lost packet, including the edges, are marked

```suggestion
     // largest newly lost packet, including the edges, are marked
```

>  
 ~~~
-   InCongestionRecovery(sent_time):
+   InLossRecovery(sent_time):

```suggestion
   InCongestionRecovery(sent_time):
```
Revert this change. Was this deliberate? The term congestion recovery is used earlier in the spec and the term is defined.

> -     if (InCongestionRecovery(acked_packet.time_sent)):
-       // Do not increase congestion window in recovery period.
-       return
-     if (IsAppOrFlowControlLimited()):
-       // Do not increase congestion_window if application
-       // limited or flow control limited.
-       return
-     if (congestion_window < ssthresh):
-       // Slow start.
-       congestion_window += acked_packet.size
-     else:
+   OnPacketsAcked(acked_packets):
+     for (packet in acked_packets):
+       // Remove from bytes_in_flight.
+       bytes_in_flight -= packet.size
+       if (InLossRecovery(packet.time_sent)):

```suggestion
       if (InCongestionRecovery(packet.time_sent)):
```

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