feat(congestion control): Migrate the BBR implementation of quinn into smoltcp

Author: Jadon-Chan

Cargo.toml

@@ -93,6 +93,9 @@ defmt = ["dep:defmt", "heapless/defmt"]
 # Enable Reno TCP congestion control algorithm, and it is used as a default congestion controller.
 "socket-tcp-reno" = []
 
+# Enable BBR TCP congestion control algorithm, and it is used as a default congestion controller.
+"socket-tcp-bbr" = []
+
 "packetmeta-id" = []
 
 "async" = []

src/socket/tcp.rs

@@ -197,6 +197,10 @@ impl RttEstimator {
         Duration::from_millis(self.rto as _)
     }
 
+    pub(super) fn min_rtt(&self) -> Duration {
+        Duration::from_millis(self.srtt as _)
+    }
+
     fn sample(&mut self, new_rtt: u32) {
         if self.have_measurement {
             // RFC 6298 (2.3) When a subsequent RTT measurement R' is made, a host MUST set (...)
@@ -454,6 +458,9 @@ pub enum CongestionControl {
 
     #[cfg(feature = "socket-tcp-cubic")]
     Cubic,
+
+    #[cfg(feature = "socket-tcp-bbr")]
+    Bbr,
 }
 
 /// A Transmission Control Protocol socket.
@@ -657,6 +664,9 @@ impl<'a> Socket<'a> {
 
             #[cfg(feature = "socket-tcp-cubic")]
             CongestionControl::Cubic => AnyController::Cubic(cubic::Cubic::new()),
+
+            #[cfg(feature = "socket-tcp-bbr")]
+            CongestionControl::Bbr => AnyController::Bbr(bbr::Bbr::new()),
         }
     }
 
@@ -672,6 +682,9 @@ impl<'a> Socket<'a> {
 
             #[cfg(feature = "socket-tcp-cubic")]
             AnyController::Cubic(_) => CongestionControl::Cubic,
+
+            #[cfg(feature = "socket-tcp-bbr")]
+            AnyController::Bbr(_) => CongestionControl::Bbr,
         }
     }
 
@@ -2368,6 +2381,12 @@ impl<'a> Socket<'a> {
             .inner_mut()
             .pre_transmit(cx.now());
 
+        // Notify congestion controller about available data for app-limited tracking
+        let bytes_available = self.tx_buffer.len();
+        self.congestion_controller
+            .inner_mut()
+            .on_send_ready(cx.now(), bytes_available);
+
         // Check if any state needs to be changed because of a timer.
         if self.timed_out(cx.now()) {
             // If a timeout expires, we should abort the connection.

src/socket/tcp/congestion.rs

@@ -10,6 +10,9 @@ pub(super) mod cubic;
 #[cfg(feature = "socket-tcp-reno")]
 pub(super) mod reno;
 
+#[cfg(feature = "socket-tcp-bbr")]
+pub(super) mod bbr;
+
 #[allow(unused_variables)]
 pub(super) trait Controller {
     /// Returns the number of bytes that can be sent.
@@ -30,6 +33,12 @@ pub(super) trait Controller {
 
     /// Set the maximum segment size.
     fn set_mss(&mut self, mss: usize) {}
+
+    /// Called when the socket is about to send data.
+    /// `bytes_available` indicates how many bytes are waiting in the send buffer.
+    /// This allows the congestion controller to track whether the application
+    /// is app-limited (not enough data to send) or cwnd-limited.
+    fn on_send_ready(&mut self, now: Instant, bytes_available: usize) {}
 }
 
 #[derive(Debug)]
@@ -42,15 +51,20 @@ pub(super) enum AnyController {
 
     #[cfg(feature = "socket-tcp-cubic")]
     Cubic(cubic::Cubic),
+
+    #[cfg(feature = "socket-tcp-bbr")]
+    Bbr(bbr::Bbr),
 }
 
 impl AnyController {
     /// Create a new congestion controller.
     /// `AnyController::new()` selects the best congestion controller based on the features.
     ///
+    /// - If `socket-tcp-bbr` feature is enabled, it will use `Bbr`.
     /// - If `socket-tcp-cubic` feature is enabled, it will use `Cubic`.
     /// - If `socket-tcp-reno` feature is enabled, it will use `Reno`.
-    /// - If both `socket-tcp-cubic` and `socket-tcp-reno` features are enabled, it will use `Cubic`.
+    /// - Priority: BBR > Cubic > Reno > NoControl
+    ///    - `BBR` is optimized for high bandwidth-delay product networks.
     ///    - `Cubic` is more efficient regarding throughput.
     ///    - `Reno` is more conservative and is suitable for low-power devices.
     /// - If no congestion controller is available, it will use `NoControl`.
@@ -60,6 +74,11 @@ impl AnyController {
     #[allow(unreachable_code)]
     #[inline]
     pub fn new() -> Self {
+        #[cfg(feature = "socket-tcp-bbr")]
+        {
+            return AnyController::Bbr(bbr::Bbr::new());
+        }
+
         #[cfg(feature = "socket-tcp-cubic")]
         {
             return AnyController::Cubic(cubic::Cubic::new());
@@ -83,6 +102,9 @@ impl AnyController {
 
             #[cfg(feature = "socket-tcp-cubic")]
             AnyController::Cubic(c) => c,
+
+            #[cfg(feature = "socket-tcp-bbr")]
+            AnyController::Bbr(b) => b,
         }
     }
 
@@ -96,6 +118,9 @@ impl AnyController {
 
             #[cfg(feature = "socket-tcp-cubic")]
             AnyController::Cubic(c) => c,
+
+            #[cfg(feature = "socket-tcp-bbr")]
+            AnyController::Bbr(b) => b,
         }
     }
 }