feat(server): add tokio udp runtime

crates/irtt-server/src/core.rs

@@ -25,7 +25,7 @@ use crate::{
 /// reply's timestamps come from — both of which are private injected seams, so
 /// the whole engine stays deterministically testable.
 ///
-/// The Tokio runtime slice will wrap it directly:
+/// The Tokio runtime wraps it directly:
 ///
 /// ```text
 /// recv_from -> core.handle_datagram(peer, packet) -> Some(reply) -> send_to
@@ -39,12 +39,13 @@ use crate::{
 ///
 /// # Scope
 ///
-/// This slice implements open handling, session creation and negotiation,
+/// The core implements open handling, session creation and negotiation,
 /// normal echo processing, per-session rate limiting, idle expiry,
 /// maximum-duration close and client-initiated close. DSCP application and the
-/// full server fill policy are separate slices, as is the Tokio runtime — so
-/// expiry is evaluated when a datagram arrives rather than on a timer, and
-/// echo payloads are zero-filled.
+/// full server fill policy are separate slices, and echo payloads are
+/// zero-filled. Incoming authenticated requests enforce logical expiry exactly;
+/// the runtime also calls a private maintenance hook to reclaim idle sessions
+/// when no traffic arrives.
 #[derive(Debug)]
 pub struct ServerCore {
     config: ServerConfig,
@@ -198,17 +199,23 @@ impl ServerCore {
         }
     }
 
+    /// Reclaims sessions whose idle deadline has passed.
+    ///
+    /// The Tokio runtime calls this for physical background reclamation. Every
+    /// authenticated structurally valid datagram still performs the same sweep
+    /// before dispatch, so the timer cadence never weakens logical expiry.
+    pub(crate) fn maintain(&mut self) {
+        let now_ns = self.clock.sample().mono_ns;
+        self.release_expired_sessions(now_ns);
+    }
+
     /// Drops every session whose idle deadline has passed, as of `now_ns`.
     ///
     /// Expiry is *logical* — a session is expired the instant its deadline
-    /// passes — but reclamation here is *packet-driven*, because this slice has
-    /// no runtime and so no timer. An expired session may therefore stay
-    /// resident until the next authenticated, structurally valid request causes
-    /// this sweep. Nothing observable turns on the difference: total state is
-    /// already bounded by [`max_sessions`](ServerConfig::max_sessions), the
-    /// sweep runs before a new open is admitted so a stale session cannot deny
-    /// capacity to a live one, and while no traffic arrives at all there is
-    /// no peer to observe anything. The Tokio slice may add scheduled sweeps.
+    /// passes. Authenticated, structurally valid requests run this sweep before
+    /// dispatch, while the runtime's maintenance timer also invokes it for
+    /// physical reclamation when no traffic arrives. The table remains bounded,
+    /// and a stale session cannot deny capacity to a live one.
     ///
     /// The sweep is global and runs before the request is dispatched, so a
     /// request touching one session can release another. That is deliberate:

crates/irtt-server/src/lib.rs

@@ -10,10 +10,11 @@
 //! receive state, rate and lifetime policy, resource decisions and reply
 //! construction. It performs no I/O; the clock its timestamps and deadlines
 //! come from is a private injected seam, not a runtime abstraction, so the
-//! engine stays testable. Socket and runtime orchestration — the UDP listener,
-//! address handling, timers, scheduled sweeps and shutdown — will live around
-//! it and is intentionally Tokio-native. There is no blocking or
-//! alternate-runtime counterpart and no transport abstraction.
+//! engine stays testable. [`Server`] owns one Tokio UDP listener and one core,
+//! and provides sequential receive, reply, scheduled expiry maintenance and
+//! caller-controlled shutdown. The crate is intentionally Tokio-native; there
+//! is no blocking or alternate-runtime counterpart and no transport
+//! abstraction.
 //!
 //! # Rejection is silence
 //!
@@ -40,10 +41,10 @@
 //! # Current scope
 //!
 //! Open handling and negotiation, session creation, normal echo processing,
-//! per-session rate limiting, idle expiry and the maximum-duration
-//! server-initiated close. DSCP application, the full server fill policy and
-//! the Tokio runtime are separate slices: echo payloads are zero-filled, and
-//! expiry is evaluated when a datagram arrives rather than on a timer.
+//! per-session rate limiting, idle expiry, the maximum-duration
+//! server-initiated close and Tokio UDP orchestration. DSCP application, the
+//! full server fill policy, wildcard-bind destination-address metadata and the
+//! CLI server applet remain separate slices. Echo payloads are zero-filled.
 #![forbid(unsafe_code)]
 
 mod clock;
@@ -51,6 +52,7 @@ mod config;
 mod core;
 mod error;
 mod negotiate;
+mod runtime;
 mod session;
 mod token;
 
@@ -63,3 +65,4 @@ pub use config::{
 };
 pub use core::ServerCore;
 pub use error::ServerError;
+pub use runtime::{Server, ServerRuntimeError};

crates/irtt-server/src/runtime.rs

@@ -0,0 +1,278 @@
+//! Tokio UDP orchestration around [`ServerCore`](crate::ServerCore).
+
+use std::{future::Future, io, net::SocketAddr, time::Duration};
+
+use thiserror::Error;
+use tokio::{net::UdpSocket, time::MissedTickBehavior};
+
+use crate::{ServerConfig, ServerCore, ServerError};
+
+/// Fixed transport receive capacity.
+///
+/// Standard UDP payloads fit in this buffer. A datagram reported as filling it
+/// exactly is dropped conservatively instead of passing potentially truncated
+/// bytes to the protocol core. IPv6 jumbograms are not supported.
+const RECEIVE_BUFFER_LEN: usize = 65_536;
+const MAINTENANCE_INTERVAL: Duration = Duration::from_secs(1);
+
+/// A Tokio UDP listener backed by one deterministic server core.
+///
+/// Processing is sequential: one task receives a datagram, lets the core
+/// process it, and sends the optional reply from the same socket. No
+/// per-datagram tasks or locks are involved.
+///
+/// Bind an explicit local address when reply source-address identity matters.
+/// Wildcard binds rely on the kernel's source-address choice; destination
+/// packet metadata and per-packet source selection are not yet implemented for
+/// multi-homed hosts.
+#[derive(Debug)]
+pub struct Server {
+    socket: UdpSocket,
+    core: ServerCore,
+    recv_buffer: Vec<u8>,
+}
+
+impl Server {
+    /// Binds one UDP listener at `addr` and creates its independent session
+    /// namespace.
+    pub async fn bind(addr: SocketAddr, config: ServerConfig) -> Result<Self, ServerRuntimeError> {
+        let socket = UdpSocket::bind(addr)
+            .await
+            .map_err(|source| ServerRuntimeError::Bind { addr, source })?;
+        Ok(Self::from_socket(socket, config))
+    }
+
+    /// Wraps an already prepared Tokio UDP socket in a server.
+    #[must_use]
+    pub fn from_socket(socket: UdpSocket, config: ServerConfig) -> Self {
+        Self {
+            socket,
+            core: ServerCore::new(config),
+            recv_buffer: vec![0; RECEIVE_BUFFER_LEN],
+        }
+    }
+
+    /// Returns the local endpoint selected for the listener.
+    pub fn local_addr(&self) -> Result<SocketAddr, ServerRuntimeError> {
+        self.socket
+            .local_addr()
+            .map_err(|source| ServerRuntimeError::LocalAddr { source })
+    }
+
+    /// Serves datagrams until `shutdown` completes.
+    ///
+    /// Graceful shutdown sends no session-close packets and leaves no hidden
+    /// receive task behind. Per-packet send failures and short sends drop only
+    /// that reply; receive failures and internal core failures terminate the
+    /// loop with an error.
+    pub async fn run<F>(&mut self, shutdown: F) -> Result<(), ServerRuntimeError>
+    where
+        F: Future<Output = ()>,
+    {
+        tokio::pin!(shutdown);
+        let mut maintenance = tokio::time::interval(MAINTENANCE_INTERVAL);
+        maintenance.set_missed_tick_behavior(MissedTickBehavior::Skip);
+        maintenance.tick().await;
+
+        loop {
+            tokio::select! {
+                _ = &mut shutdown => return Ok(()),
+                _ = maintenance.tick() => self.core.maintain(),
+                received = self.socket.recv_from(&mut self.recv_buffer) => {
+                    let (len, peer) = match received {
+                        Ok(received) => received,
+                        Err(source) if source.kind() == io::ErrorKind::Interrupted => continue,
+                        Err(source) => return Err(ServerRuntimeError::Receive { source }),
+                    };
+                    if len == RECEIVE_BUFFER_LEN {
+                        continue;
+                    }
+
+                    if let Some(reply) = self.core.handle_datagram(peer, &self.recv_buffer[..len])? {
+                        let sent = self.socket.send_to(&reply, peer).await;
+                        if !matches!(sent, Ok(len) if len == reply.len()) {
+                            continue;
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+
+/// Failure to create or run a Tokio UDP server.
+#[derive(Debug, Error)]
+pub enum ServerRuntimeError {
+    /// Binding the requested listener failed.
+    #[error("could not bind UDP listener at {addr}: {source}")]
+    Bind {
+        addr: SocketAddr,
+        #[source]
+        source: io::Error,
+    },
+    /// Querying the listener's selected local endpoint failed.
+    #[error("could not query UDP listener address: {source}")]
+    LocalAddr {
+        #[source]
+        source: io::Error,
+    },
+    /// Receiving from the listener failed irrecoverably.
+    #[error("UDP receive failed: {source}")]
+    Receive {
+        #[source]
+        source: io::Error,
+    },
+    /// The deterministic protocol/session core failed internally.
+    #[error(transparent)]
+    Core(#[from] ServerError),
+}
+
+#[cfg(test)]
+mod tests {
+    use std::{net::Ipv4Addr, time::Duration};
+
+    use irtt_proto::{
+        decode_echo_reply, decode_open_reply, encode_request, Clock, Params, ReceivedStats,
+        RequestToEncode, StampAt,
+    };
+    use tokio::{net::UdpSocket, sync::oneshot, time::timeout};
+
+    use super::*;
+
+    #[tokio::test(flavor = "current_thread")]
+    async fn bind_reports_local_address_and_shutdown_stops_run() {
+        let requested = SocketAddr::from((Ipv4Addr::LOCALHOST, 0));
+        let mut server = Server::bind(requested, ServerConfig::default())
+            .await
+            .unwrap();
+        let bound = server.local_addr().unwrap();
+        assert_eq!(bound.ip(), requested.ip());
+        assert_ne!(bound.port(), 0);
+
+        let (shutdown_tx, shutdown_rx) = oneshot::channel();
+        shutdown_tx.send(()).unwrap();
+        server
+            .run(async {
+                let _ = shutdown_rx.await;
+            })
+            .await
+            .unwrap();
+    }
+
+    #[tokio::test(flavor = "current_thread")]
+    async fn ipv4_open_echo_close_crosses_a_real_udp_socket() {
+        exercise_udp_path(SocketAddr::from((Ipv4Addr::LOCALHOST, 0))).await;
+    }
+
+    #[tokio::test(flavor = "current_thread")]
+    async fn ipv6_open_echo_close_when_loopback_is_available() {
+        let addr = "[::1]:0".parse().unwrap();
+        let server = match Server::bind(addr, unthrottled()).await {
+            Ok(server) => server,
+            Err(ServerRuntimeError::Bind { source, .. })
+                if matches!(
+                    source.kind(),
+                    io::ErrorKind::AddrNotAvailable | io::ErrorKind::Unsupported
+                ) =>
+            {
+                return;
+            }
+            Err(error) => panic!("unexpected IPv6 loopback bind failure: {error}"),
+        };
+        exercise_bound_server(server).await;
+    }
+
+    async fn exercise_udp_path(addr: SocketAddr) {
+        let server = Server::bind(addr, unthrottled()).await.unwrap();
+        exercise_bound_server(server).await;
+    }
+
+    async fn exercise_bound_server(mut server: Server) {
+        let server_addr = server.local_addr().unwrap();
+        let client_bind = match server_addr {
+            SocketAddr::V4(_) => "127.0.0.1:0",
+            SocketAddr::V6(_) => "[::1]:0",
+        };
+        let client = UdpSocket::bind(client_bind).await.unwrap();
+        let (shutdown_tx, shutdown_rx) = oneshot::channel();
+        let server_task = tokio::spawn(async move {
+            server
+                .run(async {
+                    let _ = shutdown_rx.await;
+                })
+                .await
+        });
+
+        let requested = Params {
+            protocol_version: 1,
+            duration_ns: 1_000_000_000,
+            interval_ns: 100_000_000,
+            received_stats: ReceivedStats::Both,
+            stamp_at: StampAt::Both,
+            clock: Clock::Both,
+            ..Params::default()
+        };
+        let open = encode_request(
+            RequestToEncode::Open {
+                params: &requested,
+                no_test: false,
+            },
+            None,
+        )
+        .unwrap();
+        client.send_to(&open, server_addr).await.unwrap();
+
+        let mut buffer = vec![0; RECEIVE_BUFFER_LEN];
+        let (len, reply_source) = timeout(Duration::from_secs(1), client.recv_from(&mut buffer))
+            .await
+            .expect("OPEN reply timeout")
+            .unwrap();
+        assert_eq!(reply_source, server_addr);
+        let open_reply = decode_open_reply(&buffer[..len], None).unwrap();
+        assert_ne!(open_reply.token, 0);
+
+        let echo = encode_request(
+            RequestToEncode::Echo {
+                token: open_reply.token,
+                sequence: 0,
+                params: &open_reply.params,
+                payload: &[],
+            },
+            None,
+        )
+        .unwrap();
+        client.send_to(&echo, server_addr).await.unwrap();
+        let (len, reply_source) = timeout(Duration::from_secs(1), client.recv_from(&mut buffer))
+            .await
+            .expect("ECHO reply timeout")
+            .unwrap();
+        assert_eq!(reply_source, server_addr);
+        let echo_reply = decode_echo_reply(&buffer[..len], &open_reply.params, None).unwrap();
+        assert_eq!(echo_reply.token, open_reply.token);
+        assert_eq!(echo_reply.sequence, 0);
+        assert_eq!(echo_reply.recv_count, Some(1));
+        assert_eq!(echo_reply.recv_window, Some(1));
+
+        let close = encode_request(
+            RequestToEncode::Close {
+                token: open_reply.token,
+            },
+            None,
+        )
+        .unwrap();
+        client.send_to(&close, server_addr).await.unwrap();
+        client.send_to(&echo, server_addr).await.unwrap();
+        assert!(
+            timeout(Duration::from_millis(100), client.recv_from(&mut buffer))
+                .await
+                .is_err()
+        );
+
+        shutdown_tx.send(()).unwrap();
+        server_task.await.unwrap().unwrap();
+    }
+
+    fn unthrottled() -> ServerConfig {
+        ServerConfig::default().with_min_send_interval(Duration::ZERO)
+    }
+}

crates/irtt-server/src/tests/lifecycle.rs

@@ -110,6 +110,26 @@ fn a_session_that_never_carried_an_echo_still_expires() {
     );
 }
 
+#[test]
+fn maintenance_reclaims_an_idle_session_without_traffic() {
+    let clock = ManualClock::at(0);
+    let mut core = core_with_sources(
+        idle_after(Duration::from_secs(2)),
+        ScriptedTokens::new([TOKEN_A]),
+        clock.clone(),
+    );
+
+    assert!(core
+        .handle_datagram(peer(), &open_request(&params_at(SECOND), None))
+        .unwrap()
+        .is_some());
+    assert_eq!(core.session_count(), 1);
+
+    clock.set(2 * SECOND);
+    core.maintain();
+    assert_eq!(core.session_count(), 0);
+}
+
 #[test]
 fn a_served_echo_moves_the_idle_deadline_along() {
     let (mut core, clock, token, negotiated) = manual_core(