Byte counting at Congestion avoidance

There is an issue in current congestion avoidance pseudocode
when `max_datagram_size * acked_packet.sent_bytes < congestion_window`
cwnd increment effectively becomes zero, especially because most
implementations use integer operation to calculate cwnd.

This happens when `acked_packet.sent_bytes` is a small value or
`congestion_window` is very big. It may lead to too conservative
cwnd growth during congestion avoidance.

To fix the issue, I propose to use a TCP ABC style cwnd growth
during congestion avoidance: https://www.rfc-editor.org/rfc/rfc3465.html#section-2.1

Basically it stores accumulated acked bytes in a new
variable `bytes_acked` and increase cwnd by `max_datagram_size` when
`bytes_acked` reaches to cwnd.

draft-ietf-quic-recovery.md

@@ -1483,6 +1483,10 @@ ssthresh:
   the mode is slow start and the window grows by the number of bytes
   acknowledged.
 
+bytes_acked:
+: Records number of bytes acked and used during congestion
+  avoidance, for incrementing the congestion window.
+
 
 ## Initialization
 
@@ -1496,6 +1500,7 @@ variables as follows:
    ssthresh = infinite
    for pn_space in [ Initial, Handshake, ApplicationData ]:
      ecn_ce_counters[pn_space] = 0
+   bytes_acked = 0
 ~~~
 
 
@@ -1541,9 +1546,10 @@ newly acked_packets from sent_packets.
       congestion_window += acked_packet.sent_bytes
     else:
       // Congestion avoidance.
-      congestion_window +=
-        max_datagram_size * acked_packet.sent_bytes
-        / congestion_window
+      bytes_acked += acked_packet.sent_bytes
+      if (bytes_acked >= congestion_window):
+        bytes_acked -= congestion_window
+        congestion_window += max_datagram_size
 ~~~
 
 
@@ -1562,6 +1568,7 @@ window.
        congestion_window *= kLossReductionFactor
        congestion_window = max(congestion_window, kMinimumWindow)
        ssthresh = congestion_window
+       bytes_acked *= kLossReductionFactor
        // A packet can be sent to speed up loss recovery.
        MaybeSendOnePacket()
 ~~~
@@ -1604,6 +1611,7 @@ Invoked when DetectAndRemoveLostPackets deems packets lost.
      // Collapse congestion window if persistent congestion
      if (InPersistentCongestion(lost_packets)):
        congestion_window = kMinimumWindow
+       bytes_acked = 0
 ~~~
 
 ## Upon dropping Initial or Handshake keys