recovery: update packet.time_sent with pacing rate

Sent/Acked.time_sent is the timestamp when the packet
is transmitted. With the pacing rate calculation,
it might indicate the future and impact the delivery
rate esimation which uses the value. Now we update
Sent.time_sent with the timestamp after packet
scheduling based on the pacing rate.

When pacing is not used in the application, this doesn't
have an impact because the application will ignore
the pacing rate.

Author: junhochoi

quiche/src/lib.rs

@@ -5553,6 +5553,11 @@ pub struct Stats {
     pub pmtu: usize,
 
     /// The most recent data delivery rate estimate in bytes/s.
+    ///
+    /// Note that this value could be inaccurate if the application does not
+    /// respect pacing hints (see [`SendInfo.at`]).
+    ///
+    /// [`SendInfo.at`]: struct.SendInfo.html#structfield.at
     pub delivery_rate: u64,
 
     /// The maximum idle timeout.

quiche/src/recovery/mod.rs

@@ -293,8 +293,6 @@ impl Recovery {
             self.set_loss_detection_timer(handshake_status, now);
         }
 
-        self.sent[epoch].push_back(pkt);
-
         // HyStart++: Start of the round in a slow start.
         if self.hystart.enabled() &&
             epoch == packet::EPOCH_APPLICATION &&
@@ -316,6 +314,10 @@ impl Recovery {
 
         self.schedule_next_packet(epoch, now, sent_bytes);
 
+        pkt.time_sent = self.get_packet_send_time();
+
+        self.sent[epoch].push_back(pkt);
+
         self.bytes_sent += sent_bytes;
         trace!("{} {:?}", trace_id, self);
     }
@@ -483,7 +485,10 @@ impl Recovery {
         }
 
         if largest_newly_acked_pkt_num == largest_acked && has_ack_eliciting {
-            let latest_rtt = now - largest_newly_acked_sent_time;
+            // The packet's sent time could be in the future if pacing is used
+            // and the network has a very short RTT.
+            let latest_rtt =
+                now.saturating_duration_since(largest_newly_acked_sent_time);
 
             let ack_delay = if epoch == packet::EPOCH_APPLICATION {
                 Duration::from_micros(ack_delay)