diff --git a/crates/irtt-client/src/lib.rs b/crates/irtt-client/src/lib.rs
index 34a1196..77df58d 100644
--- a/crates/irtt-client/src/lib.rs
+++ b/crates/irtt-client/src/lib.rs
@@ -1,4 +1,837 @@
-//! Client crate skeleton.
-//!
-//! Networking and session runtime are intentionally out of scope for the
-//! current milestone.
+#![forbid(unsafe_code)]
+
+use std::{
+    io,
+    net::{SocketAddr, ToSocketAddrs, UdpSocket},
+    time::{Duration, Instant, SystemTime},
+};
+
+use irtt_proto::{
+    close::CloseRequest, decode_open_reply, encode_close_request, encode_open_request, flags,
+    Clock, OpenRequest, Params, ReceivedStats, ServerFill, StampAt, PROTOCOL_VERSION,
+};
+use socket2::{Domain, Protocol, Socket, Type};
+use thiserror::Error;
+
+const DEFAULT_PORT: u16 = 2112;
+const DEFAULT_DURATION: Duration = Duration::from_secs(3);
+const DEFAULT_INTERVAL: Duration = Duration::from_secs(1);
+const DEFAULT_OPEN_TIMEOUTS: [Duration; 4] = [
+    Duration::from_secs(1),
+    Duration::from_secs(2),
+    Duration::from_secs(4),
+    Duration::from_secs(8),
+];
+const MIN_OPEN_TIMEOUT: Duration = Duration::from_millis(200);
+const MAX_OPEN_PACKET_SIZE: usize = 512;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ClientConfig {
+    pub server_addr: String,
+    pub duration: Option<Duration>,
+    pub interval: Duration,
+    pub length: u32,
+    pub received_stats: ReceivedStats,
+    pub stamp_at: StampAt,
+    pub clock: Clock,
+    pub dscp: u8,
+    pub hmac_key: Option<Vec<u8>>,
+    pub server_fill: Option<String>,
+    pub open_timeouts: Vec<Duration>,
+    pub run_mode: RunMode,
+    pub negotiation_policy: NegotiationPolicy,
+    pub socket_config: SocketConfig,
+}
+
+impl Default for ClientConfig {
+    fn default() -> Self {
+        Self {
+            server_addr: format!("127.0.0.1:{DEFAULT_PORT}"),
+            duration: Some(DEFAULT_DURATION),
+            interval: DEFAULT_INTERVAL,
+            length: 0,
+            received_stats: ReceivedStats::Both,
+            stamp_at: StampAt::Both,
+            clock: Clock::Both,
+            dscp: 0,
+            hmac_key: None,
+            server_fill: None,
+            open_timeouts: DEFAULT_OPEN_TIMEOUTS.to_vec(),
+            run_mode: RunMode::Normal,
+            negotiation_policy: NegotiationPolicy::Strict,
+            socket_config: SocketConfig::default(),
+        }
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Default)]
+pub struct SocketConfig {
+    pub bind_addr: Option<SocketAddr>,
+    pub ttl: Option<u32>,
+    pub ipv4_only: bool,
+    pub ipv6_only: bool,
+    pub recv_timeout: Option<Duration>,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum NegotiationPolicy {
+    Strict,
+    Loose,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum RunMode {
+    Normal,
+    NoTest,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct NegotiatedParams {
+    pub params: Params,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum OpenOutcome {
+    Started {
+        remote: SocketAddr,
+        token: u64,
+        negotiated: NegotiatedParams,
+        event: ClientEvent,
+    },
+    NoTestCompleted {
+        remote: SocketAddr,
+        event: ClientEvent,
+    },
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum ClientEvent {
+    SessionStarted {
+        remote: SocketAddr,
+        token: u64,
+        negotiated: NegotiatedParams,
+        at: ClientTimestamp,
+    },
+    NoTestCompleted {
+        remote: SocketAddr,
+        at: ClientTimestamp,
+    },
+    SessionClosed {
+        remote: SocketAddr,
+        token: u64,
+        at: ClientTimestamp,
+    },
+    Warning {
+        message: String,
+    },
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct ClientTimestamp {
+    pub wall: SystemTime,
+    pub mono: Instant,
+}
+
+impl ClientTimestamp {
+    pub fn now() -> Self {
+        Self {
+            wall: SystemTime::now(),
+            mono: Instant::now(),
+        }
+    }
+}
+
+#[derive(Debug, Error)]
+pub enum ClientError {
+    #[error("failed to resolve server address {addr:?}")]
+    Resolve { addr: String },
+    #[error("socket error: {0}")]
+    Socket(#[from] io::Error),
+    #[error("protocol error: {0}")]
+    Protocol(#[from] irtt_proto::ProtoError),
+    #[error("all open requests timed out")]
+    OpenTimeout,
+    #[error("open timeout {timeout:?} is below the minimum {minimum:?}")]
+    OpenTimeoutTooSmall {
+        timeout: Duration,
+        minimum: Duration,
+    },
+    #[error("server rejected the open request")]
+    ServerRejected,
+    #[error("protocol version mismatch: requested {requested}, received {received}")]
+    ProtocolVersionMismatch { requested: i64, received: i64 },
+    #[error("server returned a zero connection token")]
+    ZeroToken,
+    #[error("strict negotiation rejected changed params: {reason}")]
+    NegotiationRejected { reason: String },
+    #[error("client is not open")]
+    NotOpen,
+    #[error("client session is already closed")]
+    AlreadyClosed,
+    #[error("operation is not implemented for this milestone")]
+    NotImplementedForMilestone,
+    #[error("duration is too large to encode as nanoseconds")]
+    DurationOverflow,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum ClientState {
+    Connected,
+    Open { token: u64 },
+    NoTestCompleted,
+    Closed,
+}
+
+#[derive(Debug)]
+pub struct Client {
+    config: ClientConfig,
+    socket: UdpSocket,
+    remote: SocketAddr,
+    requested: Params,
+    negotiated: Option<NegotiatedParams>,
+    state: ClientState,
+}
+
+impl Client {
+    pub fn connect(config: ClientConfig) -> Result<Self, ClientError> {
+        validate_open_timeouts(&config.open_timeouts)?;
+        let remote = resolve_remote(&config)?;
+        let socket = connect_udp_socket(&config.socket_config, remote)?;
+        let requested = params_from_config(&config)?;
+
+        Ok(Self {
+            config,
+            socket,
+            remote,
+            requested,
+            negotiated: None,
+            state: ClientState::Connected,
+        })
+    }
+
+    pub fn open(&mut self, now: ClientTimestamp) -> Result<OpenOutcome, ClientError> {
+        validate_open_timeouts(&self.config.open_timeouts)?;
+        let request = OpenRequest {
+            params: self.requested.clone(),
+            close: self.config.run_mode == RunMode::NoTest,
+        };
+        let packet = encode_open_request(&request, self.config.hmac_key.as_deref())?;
+        let mut buf = [0_u8; MAX_OPEN_PACKET_SIZE];
+
+        for timeout in &self.config.open_timeouts {
+            self.socket.set_read_timeout(Some(*timeout))?;
+            self.socket.send(&packet)?;
+
+            match self.socket.recv(&mut buf) {
+                Ok(size) => {
+                    let reply = decode_open_reply(&buf[..size], self.config.hmac_key.as_deref())?;
+                    return self.accept_open_reply(reply, now);
+                }
+                Err(err)
+                    if matches!(
+                        err.kind(),
+                        io::ErrorKind::WouldBlock | io::ErrorKind::TimedOut
+                    ) => {}
+                Err(err) => return Err(ClientError::Socket(err)),
+            }
+        }
+
+        Err(ClientError::OpenTimeout)
+    }
+
+    pub fn close(&mut self, now: ClientTimestamp) -> Result<Vec<ClientEvent>, ClientError> {
+        let token = match self.state {
+            ClientState::Open { token } => token,
+            ClientState::Closed => return Err(ClientError::AlreadyClosed),
+            ClientState::Connected | ClientState::NoTestCompleted => {
+                return Err(ClientError::NotOpen)
+            }
+        };
+
+        let packet =
+            encode_close_request(&CloseRequest { token }, self.config.hmac_key.as_deref())?;
+        self.socket.send(&packet)?;
+        self.state = ClientState::Closed;
+
+        Ok(vec![ClientEvent::SessionClosed {
+            remote: self.remote,
+            token,
+            at: now,
+        }])
+    }
+
+    fn accept_open_reply(
+        &mut self,
+        reply: irtt_proto::OpenReply,
+        now: ClientTimestamp,
+    ) -> Result<OpenOutcome, ClientError> {
+        if reply.params.protocol_version != PROTOCOL_VERSION {
+            return Err(ClientError::ProtocolVersionMismatch {
+                requested: PROTOCOL_VERSION,
+                received: reply.params.protocol_version,
+            });
+        }
+
+        let reply_is_close = flags::has(reply.flags, flags::FLAG_CLOSE);
+        match (self.config.run_mode, reply_is_close) {
+            (RunMode::Normal, true) => return Err(ClientError::ServerRejected),
+            (RunMode::Normal, false) if reply.token == 0 => return Err(ClientError::ZeroToken),
+            (RunMode::NoTest, true) => {
+                self.state = ClientState::NoTestCompleted;
+                let event = ClientEvent::NoTestCompleted {
+                    remote: self.remote,
+                    at: now,
+                };
+                return Ok(OpenOutcome::NoTestCompleted {
+                    remote: self.remote,
+                    event,
+                });
+            }
+            (RunMode::NoTest, false) if reply.token == 0 => return Err(ClientError::ZeroToken),
+            (RunMode::NoTest, false) => {}
+            (RunMode::Normal, false) => {}
+        }
+
+        validate_negotiated_params(
+            &self.requested,
+            &reply.params,
+            self.config.negotiation_policy,
+        )?;
+        let negotiated = NegotiatedParams {
+            params: reply.params.clone(),
+        };
+        self.negotiated = Some(negotiated.clone());
+        self.state = ClientState::Open { token: reply.token };
+        let event = ClientEvent::SessionStarted {
+            remote: self.remote,
+            token: reply.token,
+            negotiated: negotiated.clone(),
+            at: now,
+        };
+
+        Ok(OpenOutcome::Started {
+            remote: self.remote,
+            token: reply.token,
+            negotiated,
+            event,
+        })
+    }
+}
+
+fn validate_open_timeouts(timeouts: &[Duration]) -> Result<(), ClientError> {
+    for timeout in timeouts {
+        if *timeout < MIN_OPEN_TIMEOUT {
+            return Err(ClientError::OpenTimeoutTooSmall {
+                timeout: *timeout,
+                minimum: MIN_OPEN_TIMEOUT,
+            });
+        }
+    }
+    Ok(())
+}
+
+fn resolve_remote(config: &ClientConfig) -> Result<SocketAddr, ClientError> {
+    let addr = with_default_port(&config.server_addr);
+    let mut addrs = addr
+        .to_socket_addrs()
+        .map_err(|_| ClientError::Resolve { addr: addr.clone() })?;
+    addrs
+        .find(|addr| {
+            (!config.socket_config.ipv4_only || addr.is_ipv4())
+                && (!config.socket_config.ipv6_only || addr.is_ipv6())
+        })
+        .ok_or(ClientError::Resolve { addr })
+}
+
+fn with_default_port(addr: &str) -> String {
+    if addr.parse::<SocketAddr>().is_ok() {
+        return addr.to_owned();
+    }
+    if addr
+        .rsplit_once(':')
+        .is_some_and(|(_, port)| port.parse::<u16>().is_ok())
+    {
+        return addr.to_owned();
+    }
+    format!("{addr}:{DEFAULT_PORT}")
+}
+
+fn connect_udp_socket(config: &SocketConfig, remote: SocketAddr) -> Result<UdpSocket, ClientError> {
+    let domain = if remote.is_ipv4() {
+        Domain::IPV4
+    } else {
+        Domain::IPV6
+    };
+    let socket = Socket::new(domain, Type::DGRAM, Some(Protocol::UDP))?;
+
+    if config.ipv6_only && remote.is_ipv6() {
+        socket.set_only_v6(true)?;
+    }
+    if let Some(ttl) = config.ttl {
+        socket.set_ttl(ttl)?;
+    }
+
+    let bind_addr = config.bind_addr.unwrap_or_else(|| {
+        if remote.is_ipv4() {
+            SocketAddr::from(([0, 0, 0, 0], 0))
+        } else {
+            SocketAddr::from(([0_u16; 8], 0))
+        }
+    });
+    socket.bind(&bind_addr.into())?;
+    socket.connect(&remote.into())?;
+
+    let socket: UdpSocket = socket.into();
+    socket.set_read_timeout(config.recv_timeout)?;
+    Ok(socket)
+}
+
+fn params_from_config(config: &ClientConfig) -> Result<Params, ClientError> {
+    Ok(Params {
+        protocol_version: PROTOCOL_VERSION,
+        duration_ns: duration_to_ns(config.duration.unwrap_or_default())?,
+        interval_ns: duration_to_ns(config.interval)?,
+        length: i64::from(config.length),
+        received_stats: config.received_stats,
+        stamp_at: config.stamp_at,
+        clock: config.clock,
+        dscp: i64::from(config.dscp),
+        server_fill: config.server_fill.clone().map(|value| ServerFill { value }),
+    })
+}
+
+fn duration_to_ns(duration: Duration) -> Result<i64, ClientError> {
+    i64::try_from(duration.as_nanos()).map_err(|_| ClientError::DurationOverflow)
+}
+
+fn validate_negotiated_params(
+    requested: &Params,
+    returned: &Params,
+    policy: NegotiationPolicy,
+) -> Result<(), ClientError> {
+    if returned.protocol_version != PROTOCOL_VERSION {
+        return Err(ClientError::ProtocolVersionMismatch {
+            requested: PROTOCOL_VERSION,
+            received: returned.protocol_version,
+        });
+    }
+    if returned.duration_ns > requested.duration_ns {
+        return Err(ClientError::NegotiationRejected {
+            reason: "duration increased".to_owned(),
+        });
+    }
+    if returned.length > requested.length {
+        return Err(ClientError::NegotiationRejected {
+            reason: "length increased".to_owned(),
+        });
+    }
+    if returned.interval_ns <= 0 {
+        return Err(ClientError::NegotiationRejected {
+            reason: "interval must be positive".to_owned(),
+        });
+    }
+
+    if policy == NegotiationPolicy::Strict && returned != requested {
+        return Err(ClientError::NegotiationRejected {
+            reason: "returned params differ from requested params".to_owned(),
+        });
+    }
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use irtt_proto::{
+        compute_hmac_in_place, flags::FLAG_HMAC, flags::FLAG_OPEN, flags::FLAG_REPLY, verify_hmac,
+        HMAC_SIZE, MAGIC,
+    };
+    use std::{
+        net::UdpSocket,
+        sync::mpsc,
+        thread::{self, JoinHandle},
+    };
+
+    const TOKEN: u64 = 0x1234_5678_90ab_cdef;
+    const HMAC_OFFSET: usize = 4;
+
+    struct FakeServer {
+        addr: SocketAddr,
+        rx: mpsc::Receiver<Vec<u8>>,
+        done: JoinHandle<()>,
+    }
+
+    impl FakeServer {
+        fn join(self) {
+            self.done.join().unwrap();
+        }
+    }
+
+    fn default_test_config(addr: SocketAddr) -> ClientConfig {
+        ClientConfig {
+            server_addr: addr.to_string(),
+            open_timeouts: vec![Duration::from_millis(200), Duration::from_millis(200)],
+            ..ClientConfig::default()
+        }
+    }
+
+    fn start_fake_server<F>(handler: F) -> FakeServer
+    where
+        F: FnOnce(UdpSocket, mpsc::Sender<Vec<u8>>) + Send + 'static,
+    {
+        let socket = UdpSocket::bind("127.0.0.1:0").unwrap();
+        let addr = socket.local_addr().unwrap();
+        let (tx, rx) = mpsc::channel();
+        let done = thread::spawn(move || handler(socket, tx));
+        FakeServer { addr, rx, done }
+    }
+
+    fn recv_request(socket: &UdpSocket, tx: &mpsc::Sender<Vec<u8>>) -> (Vec<u8>, SocketAddr) {
+        let mut buf = [0_u8; 512];
+        let (size, peer) = socket.recv_from(&mut buf).unwrap();
+        let packet = buf[..size].to_vec();
+        tx.send(packet.clone()).unwrap();
+        (packet, peer)
+    }
+
+    fn open_reply(flags: u8, token: u64, params: &Params, hmac_key: Option<&[u8]>) -> Vec<u8> {
+        let mut packet = Vec::new();
+        packet.extend_from_slice(&MAGIC);
+        packet.push(flags | hmac_key.map_or(0, |_| FLAG_HMAC));
+        if hmac_key.is_some() {
+            packet.extend_from_slice(&[0_u8; HMAC_SIZE]);
+        }
+        packet.extend_from_slice(&token.to_le_bytes());
+        packet.extend_from_slice(&params.encode());
+        if let Some(key) = hmac_key {
+            compute_hmac_in_place(key, &mut packet, HMAC_OFFSET).unwrap();
+        }
+        packet
+    }
+
+    fn open_success_server(params: Params) -> FakeServer {
+        start_fake_server(move |socket, tx| {
+            let (_, peer) = recv_request(&socket, &tx);
+            let reply = open_reply(FLAG_OPEN | FLAG_REPLY, TOKEN, &params, None);
+            socket.send_to(&reply, peer).unwrap();
+        })
+    }
+
+    fn assert_open_started(outcome: OpenOutcome) -> NegotiatedParams {
+        match outcome {
+            OpenOutcome::Started {
+                token, negotiated, ..
+            } => {
+                assert_eq!(token, TOKEN);
+                negotiated
+            }
+            OpenOutcome::NoTestCompleted { .. } => panic!("unexpected no-test outcome"),
+        }
+    }
+
+    #[test]
+    fn client_config_default() {
+        let config = ClientConfig::default();
+        assert_eq!(config.duration, Some(Duration::from_secs(3)));
+        assert_eq!(config.interval, Duration::from_secs(1));
+        assert_eq!(config.length, 0);
+        assert_eq!(config.received_stats, ReceivedStats::Both);
+        assert_eq!(config.stamp_at, StampAt::Both);
+        assert_eq!(config.clock, Clock::Both);
+        assert_eq!(config.dscp, 0);
+        assert_eq!(config.hmac_key, None);
+        assert_eq!(config.server_fill, None);
+        assert_eq!(config.open_timeouts, DEFAULT_OPEN_TIMEOUTS);
+        assert_eq!(config.run_mode, RunMode::Normal);
+        assert_eq!(config.negotiation_policy, NegotiationPolicy::Strict);
+    }
+
+    #[test]
+    fn address_resolution_connects_to_local_fake_server() {
+        let server = start_fake_server(|socket, tx| {
+            let _ = recv_request(&socket, &tx);
+        });
+        let client = Client::connect(default_test_config(server.addr)).unwrap();
+        client.socket.send(b"ping").unwrap();
+        assert_eq!(server.rx.recv().unwrap(), b"ping");
+        server.join();
+    }
+
+    #[test]
+    fn successful_open_handshake() {
+        let config = default_test_config(SocketAddr::from(([127, 0, 0, 1], 1)));
+        let params = params_from_config(&config).unwrap();
+        let server = open_success_server(params.clone());
+        let mut client = Client::connect(default_test_config(server.addr)).unwrap();
+
+        let negotiated = assert_open_started(client.open(ClientTimestamp::now()).unwrap());
+        assert_eq!(negotiated.params, params);
+        assert!(matches!(client.state, ClientState::Open { token: TOKEN }));
+        server.join();
+    }
+
+    #[test]
+    fn open_retries_after_first_timeout() {
+        let server = start_fake_server(|socket, tx| {
+            let (first, _) = recv_request(&socket, &tx);
+            let (_, peer) = recv_request(&socket, &tx);
+            let params = Params::decode(&first[4..]).unwrap();
+            let reply = open_reply(FLAG_OPEN | FLAG_REPLY, TOKEN, &params, None);
+            assert_eq!(first[3] & FLAG_OPEN, FLAG_OPEN);
+            socket.send_to(&reply, peer).unwrap();
+        });
+        let mut config = default_test_config(server.addr);
+        config.open_timeouts = vec![Duration::from_millis(200), Duration::from_millis(500)];
+        let params = params_from_config(&config).unwrap();
+        let reply_params = params.clone();
+        drop(reply_params);
+        let mut client = Client::connect(config).unwrap();
+        let outcome = client.open(ClientTimestamp::now()).unwrap();
+        assert_open_started(outcome);
+        assert_eq!(server.rx.iter().take(2).count(), 2);
+        server.join();
+    }
+
+    #[test]
+    fn open_timeout_after_all_timeouts() {
+        let server = start_fake_server(|socket, tx| {
+            socket
+                .set_read_timeout(Some(Duration::from_millis(700)))
+                .unwrap();
+            while recv_request_timeout(&socket, &tx).is_some() {}
+        });
+        let mut config = default_test_config(server.addr);
+        config.open_timeouts = vec![Duration::from_millis(200), Duration::from_millis(200)];
+        let mut client = Client::connect(config).unwrap();
+        assert!(matches!(
+            client.open(ClientTimestamp::now()),
+            Err(ClientError::OpenTimeout)
+        ));
+        assert_eq!(server.rx.iter().take(2).count(), 2);
+        server.join();
+    }
+
+    fn recv_request_timeout(
+        socket: &UdpSocket,
+        tx: &mpsc::Sender<Vec<u8>>,
+    ) -> Option<(Vec<u8>, SocketAddr)> {
+        let mut buf = [0_u8; 512];
+        match socket.recv_from(&mut buf) {
+            Ok((size, peer)) => {
+                let packet = buf[..size].to_vec();
+                tx.send(packet.clone()).unwrap();
+                Some((packet, peer))
+            }
+            Err(_) => None,
+        }
+    }
+
+    #[test]
+    fn strict_negotiation_accepts_identical_params() {
+        let config = ClientConfig::default();
+        let params = params_from_config(&config).unwrap();
+        assert!(validate_negotiated_params(&params, &params, NegotiationPolicy::Strict).is_ok());
+    }
+
+    #[test]
+    fn strict_negotiation_rejects_changed_params() {
+        let config = ClientConfig::default();
+        let requested = params_from_config(&config).unwrap();
+        let mut returned = requested.clone();
+        returned.dscp = 1;
+        assert!(matches!(
+            validate_negotiated_params(&requested, &returned, NegotiationPolicy::Strict),
+            Err(ClientError::NegotiationRejected { .. })
+        ));
+    }
+
+    #[test]
+    fn loose_negotiation_accepts_server_restricted_params() {
+        let config = ClientConfig::default();
+        let requested = params_from_config(&config).unwrap();
+        let mut returned = requested.clone();
+        returned.duration_ns /= 2;
+        returned.length = 0;
+        assert!(
+            validate_negotiated_params(&requested, &returned, NegotiationPolicy::Loose).is_ok()
+        );
+    }
+
+    #[test]
+    fn protocol_version_mismatch_fails() {
+        let mut config = default_test_config(SocketAddr::from(([127, 0, 0, 1], 1)));
+        config.negotiation_policy = NegotiationPolicy::Loose;
+        let mut params = params_from_config(&config).unwrap();
+        params.protocol_version = 2;
+        let server = open_success_server(params);
+        let mut client = Client::connect(default_test_config(server.addr)).unwrap();
+        assert!(matches!(
+            client.open(ClientTimestamp::now()),
+            Err(ClientError::ProtocolVersionMismatch { received: 2, .. })
+        ));
+        server.join();
+    }
+
+    #[test]
+    fn server_rejection_fails_in_normal_mode() {
+        let config = default_test_config(SocketAddr::from(([127, 0, 0, 1], 1)));
+        let params = params_from_config(&config).unwrap();
+        let server = start_fake_server(move |socket, tx| {
+            let (_, peer) = recv_request(&socket, &tx);
+            let reply = open_reply(FLAG_OPEN | FLAG_REPLY | flags::FLAG_CLOSE, 0, &params, None);
+            socket.send_to(&reply, peer).unwrap();
+        });
+        let mut client = Client::connect(default_test_config(server.addr)).unwrap();
+        assert!(matches!(
+            client.open(ClientTimestamp::now()),
+            Err(ClientError::ServerRejected)
+        ));
+        server.join();
+    }
+
+    #[test]
+    fn no_test_open_close_succeeds_on_open_reply_close() {
+        let mut config = default_test_config(SocketAddr::from(([127, 0, 0, 1], 1)));
+        config.run_mode = RunMode::NoTest;
+        let params = params_from_config(&config).unwrap();
+        let server = start_fake_server(move |socket, tx| {
+            let (request, peer) = recv_request(&socket, &tx);
+            assert_eq!(request[3] & flags::FLAG_CLOSE, flags::FLAG_CLOSE);
+            let reply = open_reply(FLAG_OPEN | FLAG_REPLY | flags::FLAG_CLOSE, 0, &params, None);
+            socket.send_to(&reply, peer).unwrap();
+        });
+        config.server_addr = server.addr.to_string();
+        let mut client = Client::connect(config).unwrap();
+        assert!(matches!(
+            client.open(ClientTimestamp::now()).unwrap(),
+            OpenOutcome::NoTestCompleted { .. }
+        ));
+        assert!(matches!(
+            client.close(ClientTimestamp::now()),
+            Err(ClientError::NotOpen)
+        ));
+        server.join();
+    }
+
+    #[test]
+    fn close_sends_one_close_packet_with_negotiated_token() {
+        let config = default_test_config(SocketAddr::from(([127, 0, 0, 1], 1)));
+        let params = params_from_config(&config).unwrap();
+        let server = start_fake_server(move |socket, tx| {
+            let (_, peer) = recv_request(&socket, &tx);
+            let reply = open_reply(FLAG_OPEN | FLAG_REPLY, TOKEN, &params, None);
+            socket.send_to(&reply, peer).unwrap();
+            let _ = recv_request(&socket, &tx);
+        });
+        let mut client = Client::connect(default_test_config(server.addr)).unwrap();
+        assert_open_started(client.open(ClientTimestamp::now()).unwrap());
+        let events = client.close(ClientTimestamp::now()).unwrap();
+        assert_eq!(events.len(), 1);
+        let packets: Vec<_> = server.rx.iter().take(2).collect();
+        let close = &packets[1];
+        assert_eq!(close[3], flags::FLAG_CLOSE);
+        assert_eq!(u64::from_le_bytes(close[4..12].try_into().unwrap()), TOKEN);
+        server.join();
+    }
+
+    #[test]
+    fn hmac_open_success() {
+        let key = b"secret".to_vec();
+        let mut config = default_test_config(SocketAddr::from(([127, 0, 0, 1], 1)));
+        config.hmac_key = Some(key.clone());
+        let params = params_from_config(&config).unwrap();
+        let server = start_fake_server(move |socket, tx| {
+            let (request, peer) = recv_request(&socket, &tx);
+            verify_hmac(&key, &request, HMAC_OFFSET).unwrap();
+            let reply = open_reply(FLAG_OPEN | FLAG_REPLY, TOKEN, &params, Some(&key));
+            socket.send_to(&reply, peer).unwrap();
+        });
+        config.server_addr = server.addr.to_string();
+        let mut client = Client::connect(config).unwrap();
+        assert_open_started(client.open(ClientTimestamp::now()).unwrap());
+        server.join();
+    }
+
+    #[test]
+    fn hmac_open_rejects_missing_hmac() {
+        let key = b"secret".to_vec();
+        let mut config = default_test_config(SocketAddr::from(([127, 0, 0, 1], 1)));
+        config.hmac_key = Some(key);
+        let params = params_from_config(&config).unwrap();
+        let server = start_fake_server(move |socket, tx| {
+            let (_, peer) = recv_request(&socket, &tx);
+            let reply = open_reply(FLAG_OPEN | FLAG_REPLY, TOKEN, &params, None);
+            socket.send_to(&reply, peer).unwrap();
+        });
+        config.server_addr = server.addr.to_string();
+        let mut client = Client::connect(config).unwrap();
+        assert!(matches!(
+            client.open(ClientTimestamp::now()),
+            Err(ClientError::Protocol(
+                irtt_proto::ProtoError::HmacPresenceMismatch
+            ))
+        ));
+        server.join();
+    }
+
+    #[test]
+    fn hmac_open_rejects_bad_hmac() {
+        let key = b"secret".to_vec();
+        let wrong_key = b"wrong".to_vec();
+        let mut config = default_test_config(SocketAddr::from(([127, 0, 0, 1], 1)));
+        config.hmac_key = Some(key);
+        let params = params_from_config(&config).unwrap();
+        let server = start_fake_server(move |socket, tx| {
+            let (_, peer) = recv_request(&socket, &tx);
+            let reply = open_reply(FLAG_OPEN | FLAG_REPLY, TOKEN, &params, Some(&wrong_key));
+            socket.send_to(&reply, peer).unwrap();
+        });
+        config.server_addr = server.addr.to_string();
+        let mut client = Client::connect(config).unwrap();
+        assert!(matches!(
+            client.open(ClientTimestamp::now()),
+            Err(ClientError::Protocol(irtt_proto::ProtoError::BadHmac))
+        ));
+        server.join();
+    }
+
+    #[test]
+    fn hmac_close_packet_includes_valid_hmac() {
+        let key = b"secret".to_vec();
+        let mut config = default_test_config(SocketAddr::from(([127, 0, 0, 1], 1)));
+        config.hmac_key = Some(key.clone());
+        let params = params_from_config(&config).unwrap();
+        let server_key = key.clone();
+        let server = start_fake_server(move |socket, tx| {
+            let (_, peer) = recv_request(&socket, &tx);
+            let reply = open_reply(FLAG_OPEN | FLAG_REPLY, TOKEN, &params, Some(&server_key));
+            socket.send_to(&reply, peer).unwrap();
+            let _ = recv_request(&socket, &tx);
+        });
+        config.server_addr = server.addr.to_string();
+        let mut client = Client::connect(config).unwrap();
+        assert_open_started(client.open(ClientTimestamp::now()).unwrap());
+        client.close(ClientTimestamp::now()).unwrap();
+        let packets: Vec<_> = server.rx.iter().take(2).collect();
+        let close = &packets[1];
+        assert_eq!(close[3], flags::FLAG_CLOSE | FLAG_HMAC);
+        verify_hmac(&key, close, HMAC_OFFSET).unwrap();
+        assert_eq!(
+            u64::from_le_bytes(close[4 + HMAC_SIZE..12 + HMAC_SIZE].try_into().unwrap()),
+            TOKEN
+        );
+        server.join();
+    }
+
+    #[test]
+    fn minimum_open_timeout_under_200ms_is_rejected() {
+        let config = ClientConfig {
+            open_timeouts: vec![Duration::from_millis(199)],
+            ..ClientConfig::default()
+        };
+        assert!(matches!(
+            Client::connect(config),
+            Err(ClientError::OpenTimeoutTooSmall { .. })
+        ));
+    }
+}