From bf8a783ca6c638af568f182e70d2f0a6b3407bbd Mon Sep 17 00:00:00 2001
From: Nick Banks <nibanks@microsoft.com>
Date: Thu, 20 Jan 2022 15:16:29 -0600
Subject: [PATCH] Some Cubic Clean Up (#2305)

---
 src/core/cubic.c | 41 ++++++++++++++++++++++-------------------
 src/core/cubic.h |  4 ++--
 2 files changed, 24 insertions(+), 21 deletions(-)

diff --git a/src/core/cubic.c b/src/core/cubic.c
index f702fdde07..b8ece9e4ec 100644
--- a/src/core/cubic.c
+++ b/src/core/cubic.c
@@ -364,7 +364,7 @@ CubicCongestionControlOnDataAcknowledged(
 {
     QUIC_CONGESTION_CONTROL_CUBIC* Cubic = &Cc->Cubic;
 
-    uint64_t TimeNow = US_TO_MS(AckEvent->TimeNow);
+    const uint64_t TimeNowUs = AckEvent->TimeNow;
     QUIC_CONNECTION* Connection = QuicCongestionControlGetConnection(Cc);
     BOOLEAN PreviousCanSendState = CubicCongestionControlCanSend(Cc);
 
@@ -384,7 +384,7 @@ CubicCongestionControlOnDataAcknowledged(
                 Connection);
             Cubic->IsInRecovery = FALSE;
             Cubic->IsInPersistentCongestion = FALSE;
-            Cubic->TimeOfCongAvoidStart = CxPlatTimeMs64();
+            Cubic->TimeOfCongAvoidStart = CxPlatTimeUs64();
         }
         goto Exit;
     } else if (AckEvent->NumRetransmittableBytes == 0) {
@@ -398,12 +398,13 @@ CubicCongestionControlOnDataAcknowledged(
         //
 
         //
-        // TODO: CongestionWindow should only be allowed to grow up to SlowStartThreshold
-        // here, and then any spare bytes should be handled in the Congestion Avoidance path.
+        // TODO: CongestionWindow should only be allowed to grow up to
+        // SlowStartThreshold here, and then any spare bytes should be handled
+        // in the Congestion Avoidance path.
         //
         Cubic->CongestionWindow += AckEvent->NumRetransmittableBytes;
         if (Cubic->CongestionWindow >= Cubic->SlowStartThreshold) {
-            Cubic->TimeOfCongAvoidStart = CxPlatTimeMs64();
+            Cubic->TimeOfCongAvoidStart = TimeNowUs;
         }
 
     } else {
@@ -422,21 +423,18 @@ CubicCongestionControlOnDataAcknowledged(
         // growth during the gap.
         //
         if (Cubic->TimeOfLastAckValid) {
-            uint64_t TimeSinceLastAck = CxPlatTimeDiff64(Cubic->TimeOfLastAck, TimeNow);
-            if (TimeSinceLastAck > Cubic->SendIdleTimeoutMs &&
-                TimeSinceLastAck > US_TO_MS(Connection->Paths[0].SmoothedRtt + 4 * Connection->Paths[0].RttVariance)) {
+            const uint64_t TimeSinceLastAck = CxPlatTimeDiff64(Cubic->TimeOfLastAck, TimeNowUs);
+            if (TimeSinceLastAck > MS_TO_US((uint64_t)Cubic->SendIdleTimeoutMs) &&
+                TimeSinceLastAck > (Connection->Paths[0].SmoothedRtt + 4 * Connection->Paths[0].RttVariance)) {
                 Cubic->TimeOfCongAvoidStart += TimeSinceLastAck;
-                if (CxPlatTimeAtOrBefore64(TimeNow, Cubic->TimeOfCongAvoidStart)) {
-                    Cubic->TimeOfCongAvoidStart = TimeNow;
+                if (CxPlatTimeAtOrBefore64(TimeNowUs, Cubic->TimeOfCongAvoidStart)) {
+                    Cubic->TimeOfCongAvoidStart = TimeNowUs;
                 }
             }
         }
 
-        uint64_t TimeInCongAvoid =
-            CxPlatTimeDiff64(Cubic->TimeOfCongAvoidStart, CxPlatTimeMs64());
-        if (TimeInCongAvoid > UINT32_MAX) {
-            TimeInCongAvoid = UINT32_MAX;
-        }
+        const uint64_t TimeInCongAvoidUs =
+            CxPlatTimeDiff64(Cubic->TimeOfCongAvoidStart, TimeNowUs);
 
         //
         // Compute the cubic window:
@@ -455,9 +453,14 @@ CubicCongestionControlOnDataAcknowledged(
         //
 
         int64_t DeltaT =
-            (int64_t)TimeInCongAvoid -
-            (int64_t)Cubic->KCubic +
-            (int64_t)US_TO_MS(AckEvent->SmoothedRtt);
+            US_TO_MS(
+                (int64_t)TimeInCongAvoidUs -
+                (int64_t)MS_TO_US(Cubic->KCubic) +
+                (int64_t)AckEvent->SmoothedRtt
+            );
+        if (DeltaT > 2500000) {
+            DeltaT = 2500000;
+        }
 
         int64_t CubicWindow =
             ((((DeltaT * DeltaT) >> 10) * DeltaT *
@@ -528,7 +531,7 @@ CubicCongestionControlOnDataAcknowledged(
 
 Exit:
 
-    Cubic->TimeOfLastAck = TimeNow;
+    Cubic->TimeOfLastAck = TimeNowUs;
     Cubic->TimeOfLastAckValid = TRUE;
     return CubicCongestionControlUpdateBlockedState(Cc, PreviousCanSendState);
 }
diff --git a/src/core/cubic.h b/src/core/cubic.h
index 5df5b13a7d..b31199c211 100644
--- a/src/core/cubic.h
+++ b/src/core/cubic.h
@@ -74,8 +74,8 @@ typedef struct QUIC_CONGESTION_CONTROL_CUBIC {
     //
     uint8_t Exemptions;
 
-    uint64_t TimeOfLastAck; // millisec
-    uint64_t TimeOfCongAvoidStart; // millisec
+    uint64_t TimeOfLastAck; // microseconds
+    uint64_t TimeOfCongAvoidStart; // microseconds
     uint32_t KCubic; // millisec
     uint32_t PrevKCubic; // millisec
     uint32_t WindowMax; // bytes