Adapt runtime stream retransmit pacing

src/transport.rs

@@ -29,8 +29,8 @@ use tokio::net::UdpSocket;
 
 pub const DEFAULT_INITIAL_WINDOW: usize = 1024 * 1024;
 pub const DEFAULT_RETRANSMIT_AFTER: Duration = Duration::from_millis(200);
-const ADAPTIVE_RETRANSMIT_PAD: Duration = Duration::from_millis(10);
-const ADAPTIVE_RETRANSMIT_MIN: Duration = Duration::from_millis(10);
+pub const ADAPTIVE_RETRANSMIT_PAD: Duration = Duration::from_millis(10);
+pub const ADAPTIVE_RETRANSMIT_MIN: Duration = Duration::from_millis(10);
 pub const DEFAULT_KEEPALIVE_AFTER: Duration = Duration::from_secs(2);
 pub const DEFAULT_RETIRED_STREAM_TOMBSTONES: usize = 16 * 1024;
 pub const SERVICE_TARGET_PREFIX: &str = "@dosh-";
@@ -553,17 +553,7 @@ impl StreamMux {
     }
 
     pub fn effective_retransmit_after(&self) -> Duration {
-        let Some(srtt) = self.srtt else {
-            return self.config.retransmit_after;
-        };
-        let adaptive = srtt
-            .saturating_add(ADAPTIVE_RETRANSMIT_PAD)
-            .max(ADAPTIVE_RETRANSMIT_MIN);
-        if self.config.retransmit_after < ADAPTIVE_RETRANSMIT_MIN {
-            self.config.retransmit_after
-        } else {
-            adaptive.min(self.config.retransmit_after)
-        }
+        adaptive_retransmit_after(self.config.retransmit_after, self.srtt)
     }
 
     pub fn is_open(&self, stream_id: u64) -> bool {
@@ -709,13 +699,7 @@ impl StreamMux {
     }
 
     fn observe_rtt_sample(&mut self, sample: Duration) {
-        self.srtt = Some(match self.srtt {
-            None => sample,
-            Some(srtt) => {
-                let smoothed = ((srtt.as_micros() * 7) + sample.as_micros()) / 8;
-                Duration::from_micros(smoothed.min(u64::MAX as u128) as u64)
-            }
-        });
+        observe_retransmit_rtt(&mut self.srtt, sample);
     }
 
     fn add_credit(&mut self, stream_id: u64, bytes: usize) {
@@ -761,6 +745,30 @@ impl StreamMux {
     }
 }
 
+pub fn adaptive_retransmit_after(configured: Duration, srtt: Option<Duration>) -> Duration {
+    let Some(srtt) = srtt else {
+        return configured;
+    };
+    let adaptive = srtt
+        .saturating_add(ADAPTIVE_RETRANSMIT_PAD)
+        .max(ADAPTIVE_RETRANSMIT_MIN);
+    if configured < ADAPTIVE_RETRANSMIT_MIN {
+        configured
+    } else {
+        adaptive.min(configured)
+    }
+}
+
+pub fn observe_retransmit_rtt(srtt: &mut Option<Duration>, sample: Duration) {
+    *srtt = Some(match *srtt {
+        None => sample,
+        Some(srtt) => {
+            let smoothed = ((srtt.as_micros() * 7) + sample.as_micros()) / 8;
+            Duration::from_micros(smoothed.min(u64::MAX as u128) as u64)
+        }
+    });
+}
+
 pub struct DoshTransport {
     socket: Arc<UdpSocket>,
     role: SessionRole,