Don't enter persistent congestion without an RTT sample

RFC 9002 §7.6.2: [..] a prior RTT sample existed when these two
packets were sent.

quinn-proto/src/connection/mod.rs

@@ -1159,6 +1159,10 @@ impl Connection {
             };
             let rtt = instant_saturating_sub(now, self.spaces[space].largest_acked_packet_sent);
             self.path.rtt.update(ack_delay, rtt);
+            if self.path.first_packet_after_rtt_sample.is_none() {
+                self.path.first_packet_after_rtt_sample =
+                    Some((space, self.spaces[space].next_packet_number));
+            }
         }
 
         // Must be called before crypto/pto_count are clobbered
@@ -1338,7 +1342,12 @@ impl Connection {
                         Some(start) if info.time_sent - start > congestion_period => {
                             in_persistent_congestion = true;
                         }
-                        None => {
+                        // Persistent congestion must start after the first RTT sample
+                        None if self
+                            .path
+                            .first_packet_after_rtt_sample
+                            .map_or(false, |x| x < (pn_space, packet)) =>
+                        {
                             persistent_congestion_start = Some(info.time_sent);
                         }
                         _ => {}

quinn-proto/src/connection/paths.rs

@@ -1,7 +1,7 @@
 use std::{cmp, net::SocketAddr, time::Duration, time::Instant};
 
 use super::pacing::Pacer;
-use crate::{congestion, INITIAL_MAX_UDP_PAYLOAD_SIZE, TIMER_GRANULARITY};
+use crate::{congestion, packet::SpaceId, INITIAL_MAX_UDP_PAYLOAD_SIZE, TIMER_GRANULARITY};
 
 /// Description of a particular network path
 pub struct PathData {
@@ -25,6 +25,10 @@ pub struct PathData {
     /// Total size of all UDP datagrams received on this path
     pub total_recvd: u64,
     pub max_udp_payload_size: u16,
+    /// Packet number of the first packet sent after an RTT sample was collected on this path
+    ///
+    /// Used in persistent congestion determination.
+    pub first_packet_after_rtt_sample: Option<(SpaceId, u64)>,
 }
 
 impl PathData {
@@ -52,6 +56,7 @@ impl PathData {
             total_sent: 0,
             total_recvd: 0,
             max_udp_payload_size: INITIAL_MAX_UDP_PAYLOAD_SIZE,
+            first_packet_after_rtt_sample: None,
         }
     }
 
@@ -75,6 +80,7 @@ impl PathData {
             total_sent: 0,
             total_recvd: 0,
             max_udp_payload_size: prev.max_udp_payload_size,
+            first_packet_after_rtt_sample: prev.first_packet_after_rtt_sample,
         }
     }