async-std-utp 0.1.0

use crate::error::SocketError;
use crate::packet::*;
use crate::time::*;
use crate::util::*;
use async_std::task;
use async_std::{
    io,
    net::{SocketAddr, ToSocketAddrs, UdpSocket},
};
use futures::future::BoxFuture;
use futures::ready;
use log::debug;
use std::cmp::{max, min};
use std::collections::VecDeque;
use std::io::{ErrorKind, Result};
use std::task::Poll;
use std::time::{Duration, Instant};

// For simplicity's sake, let us assume no packet will ever exceed the
// Ethernet maximum transfer unit of 1500 bytes.
pub(crate) const BUF_SIZE: usize = 1500;
const GAIN: f64 = 1.0;
const ALLOWED_INCREASE: u32 = 1;
const TARGET: f64 = 100_000.0; // 100 milliseconds
const MSS: u32 = 1400;
const MIN_CWND: u32 = 2;
const INIT_CWND: u32 = 2;
const INITIAL_CONGESTION_TIMEOUT: u64 = 1000; // one second
const MIN_CONGESTION_TIMEOUT: u64 = 500; // 500 ms
const MAX_CONGESTION_TIMEOUT: u64 = 60_000; // one minute
const BASE_HISTORY: usize = 10; // base delays history size
const MAX_SYN_RETRIES: u32 = 5; // maximum connection retries
const MAX_RETRANSMISSION_RETRIES: u32 = 5; // maximum retransmission retries
const WINDOW_SIZE: u32 = 1024 * 1024; // local receive window size

// Maximum time (in microseconds) to wait for incoming packets when the send window is full
const PRE_SEND_TIMEOUT: u32 = 500_000;

// Maximum age of base delay sample (60 seconds)
const MAX_BASE_DELAY_AGE: Delay = Delay(60_000_000);

#[derive(PartialEq, Eq, Debug, Copy, Clone)]
enum SocketState {
    New,
    Connected,
    SynSent,
    FinSent,
    ResetReceived,
    Closed,
}

#[derive(Debug)]
struct DelayDifferenceSample {
    received_at: Timestamp,
    difference: Delay,
}

/// Returns the first valid address in a `ToSocketAddrs` iterator.
async fn take_address<A: ToSocketAddrs>(addr: A) -> Result<SocketAddr> {
    addr.to_socket_addrs()
        .await
        .and_then(|mut it| it.next().ok_or_else(|| SocketError::InvalidAddress.into()))
}

/// A structure that represents a uTP (Micro Transport Protocol) connection between a local socket
/// and a remote socket.
///
/// The socket will be closed when the value is dropped (either explicitly or when it goes out of
/// scope).
///
/// The default maximum retransmission retries is 5, which translates to about 16 seconds. It can be
/// changed by assigning the desired maximum retransmission retries to a socket's
/// `max_retransmission_retries` field. Notice that the initial congestion timeout is 500 ms and
/// doubles with each timeout.
///
/// # Examples
///
/// ```no_run
/// # fn main() { async_std::task::block_on(async {
/// use async_std_utp::UtpSocket;
///
/// let mut socket = UtpSocket::bind("127.0.0.1:1234").await.expect("Error binding socket");
///
/// let mut buf = [0; 1000];
/// let (amt, _src) = socket.recv_from(&mut buf).await.expect("Error receiving");
///
/// let mut buf = &mut buf[..amt];
/// buf.reverse();
/// let _ = socket.send_to(buf).await.expect("Error sending");
///
/// // Close the socket. You can either call `close` on the socket,
/// // explicitly drop it or just let it go out of scope.
/// socket.close().await;
/// }); }
/// ```
#[derive(Debug)]
pub struct UtpSocket {
    /// The wrapped UDP socket
    socket: UdpSocket,

    /// Remote peer
    connected_to: SocketAddr,

    /// Sender connection identifier
    sender_connection_id: u16,

    /// Receiver connection identifier
    receiver_connection_id: u16,

    /// Sequence number for the next packet
    seq_nr: u16,

    /// Sequence number of the latest acknowledged packet sent by the remote peer
    ack_nr: u16,

    /// Socket state
    state: SocketState,

    /// Received but not acknowledged packets
    incoming_buffer: Vec<Packet>,

    /// Sent but not yet acknowledged packets
    send_window: Vec<Packet>,

    /// Packets not yet sent
    unsent_queue: VecDeque<Packet>,

    /// How many ACKs did the socket receive for packet with sequence number equal to `ack_nr`
    duplicate_ack_count: u32,

    /// Sequence number of the latest packet the remote peer acknowledged
    last_acked: u16,

    /// Timestamp of the latest packet the remote peer acknowledged
    last_acked_timestamp: Timestamp,

    /// Sequence number of the last packet removed from the incoming buffer
    last_dropped: u16,

    /// Round-trip time to remote peer
    rtt: i32,

    /// Variance of the round-trip time to the remote peer
    rtt_variance: i32,

    /// Data from the latest packet not yet returned in `recv_from`
    pending_data: Vec<u8>,

    /// Bytes in flight
    curr_window: u32,

    /// Window size of the remote peer
    remote_wnd_size: u32,

    /// Rolling window of packet delay to remote peer
    base_delays: VecDeque<Delay>,

    /// Rolling window of the difference between sending a packet and receiving its acknowledgement
    current_delays: Vec<DelayDifferenceSample>,

    /// Difference between timestamp of the latest packet received and time of reception
    their_delay: Delay,

    /// Start of the current minute for sampling purposes
    last_rollover: Timestamp,

    /// Current congestion timeout in milliseconds
    congestion_timeout: u64,

    /// Congestion window in bytes
    cwnd: u32,

    /// Maximum retransmission retries
    pub max_retransmission_retries: u32,
}

impl UtpSocket {
    /// Creates a new UTP socket from the given UDP socket and the remote peer's address.
    ///
    /// The connection identifier of the resulting socket is randomly generated.
    fn from_raw_parts(s: UdpSocket, src: SocketAddr) -> UtpSocket {
        let (receiver_id, sender_id) = generate_sequential_identifiers();

        UtpSocket {
            socket: s,
            connected_to: src,
            receiver_connection_id: receiver_id,
            sender_connection_id: sender_id,
            seq_nr: 1,
            ack_nr: 0,
            state: SocketState::New,
            incoming_buffer: Vec::new(),
            send_window: Vec::new(),
            unsent_queue: VecDeque::new(),
            duplicate_ack_count: 0,
            last_acked: 0,
            last_acked_timestamp: Timestamp::default(),
            last_dropped: 0,
            rtt: 0,
            rtt_variance: 0,
            pending_data: Vec::new(),
            curr_window: 0,
            remote_wnd_size: 0,
            current_delays: Vec::new(),
            base_delays: VecDeque::with_capacity(BASE_HISTORY),
            their_delay: Delay::default(),
            last_rollover: Timestamp::default(),
            congestion_timeout: INITIAL_CONGESTION_TIMEOUT,
            cwnd: INIT_CWND * MSS,
            max_retransmission_retries: MAX_RETRANSMISSION_RETRIES,
        }
    }

    /// Creates a new UTP socket from the given address.
    ///
    /// The address type can be any implementer of the `ToSocketAddr` trait. See its documentation
    /// for concrete examples.
    ///
    /// If more than one valid address is specified, only the first will be used.
    pub async fn bind<A: ToSocketAddrs>(addr: A) -> Result<UtpSocket> {
        let addr = take_address(addr).await?;
        let socket = UdpSocket::bind(addr).await?;
        Ok(UtpSocket::from_raw_parts(socket, addr))
    }

    /// Returns the socket address that this socket was created from.
    pub fn local_addr(&self) -> Result<SocketAddr> {
        self.socket.local_addr()
    }

    /// Returns the socket address of the remote peer of this UTP connection.
    pub fn peer_addr(&self) -> Result<SocketAddr> {
        if self.state == SocketState::Connected || self.state == SocketState::FinSent {
            Ok(self.connected_to)
        } else {
            Err(SocketError::NotConnected.into())
        }
    }

    /// Opens a connection to a remote host by hostname or IP address.
    ///
    /// The address type can be any implementer of the `ToSocketAddr` trait. See its documentation
    /// for concrete examples.
    ///
    /// If more than one valid address is specified, only the first will be used.
    pub async fn connect<A: ToSocketAddrs>(other: A) -> Result<UtpSocket> {
        let addr = take_address(other).await?;
        let my_addr = match addr {
            SocketAddr::V4(_) => "0.0.0.0:0",
            SocketAddr::V6(_) => "[::]:0",
        };
        let mut socket = UtpSocket::bind(my_addr).await?;
        socket.connected_to = addr;

        let mut packet = Packet::new();
        packet.set_type(PacketType::Syn);
        packet.set_connection_id(socket.receiver_connection_id);
        packet.set_seq_nr(socket.seq_nr);

        let mut len = 0;
        let mut buf = [0u8; BUF_SIZE];

        let mut syn_timeout = Duration::from_millis(socket.congestion_timeout);
        for _ in 0..MAX_SYN_RETRIES {
            packet.set_timestamp(now_microseconds());

            // Send packet
            debug!("Connecting to {}", socket.connected_to);
            socket
                .socket
                .send_to(packet.as_ref(), socket.connected_to)
                .await?;
            socket.state = SocketState::SynSent;
            debug!("sent {:?}", packet);

            // Validate response
            match io::timeout(syn_timeout, socket.socket.recv_from(&mut buf)).await {
                Ok((read, src)) => {
                    socket.connected_to = src;
                    len = read;
                    break;
                }
                Err(ref e)
                    if (e.kind() == ErrorKind::WouldBlock || e.kind() == ErrorKind::TimedOut) =>
                {
                    debug!("Timed out, retrying");
                    syn_timeout *= 2;
                    continue;
                }
                Err(e) => return Err(e),
            };
        }

        let addr = socket.connected_to;
        let packet = Packet::try_from(&buf[..len])?;
        debug!("received {:?}", packet);
        socket.handle_packet(&packet, addr).await?;

        debug!("connected to: {}", socket.connected_to);

        Ok(socket)
    }

    /// Gracefully closes connection to peer.
    ///
    /// This method allows both peers to receive all packets still in
    /// flight.
    pub async fn close(&mut self) -> Result<()> {
        // Nothing to do if the socket's already closed or not connected
        if self.state == SocketState::Closed
            || self.state == SocketState::New
            || self.state == SocketState::SynSent
        {
            return Ok(());
        }

        // Flush unsent and unacknowledged packets
        self.flush().await?;

        let mut packet = Packet::new();
        packet.set_connection_id(self.sender_connection_id);
        packet.set_seq_nr(self.seq_nr);
        packet.set_ack_nr(self.ack_nr);
        packet.set_timestamp(now_microseconds());
        packet.set_type(PacketType::Fin);

        // Send FIN
        self.socket
            .send_to(packet.as_ref(), self.connected_to)
            .await?;
        debug!("sent {:?}", packet);
        self.state = SocketState::FinSent;

        // Receive JAKE
        let mut buf = [0u8; BUF_SIZE];
        while self.state != SocketState::Closed {
            self.recv(&mut buf).await?;
        }

        Ok(())
    }

    /// Receives data from socket.
    ///
    /// On success, returns the number of bytes read and the sender's address.
    /// Returns 0 bytes read after receiving a FIN packet when the remaining
    /// in-flight packets are consumed.
    pub async fn recv_from(&mut self, buf: &mut [u8]) -> Result<(usize, SocketAddr)> {
        let read = self.flush_incoming_buffer(buf);

        if read > 0 {
            return Ok((read, self.connected_to));
        }

        // If the socket received a reset packet and all data has been flushed, then it can't
        // receive anything else
        if self.state == SocketState::ResetReceived {
            return Err(SocketError::ConnectionReset.into());
        }

        loop {
            // A closed socket with no pending data can only "read" 0 new bytes.
            if self.state == SocketState::Closed {
                return Ok((0, self.connected_to));
            }

            match self.recv(buf).await {
                Ok((0, _src)) => continue,
                Ok(x) => return Ok(x),
                Err(e) => return Err(e),
            }
        }
    }

    async fn recv(&mut self, buf: &mut [u8]) -> Result<(usize, SocketAddr)> {
        let mut b = [0; BUF_SIZE + HEADER_SIZE];
        let start = Instant::now();
        let (read, src);
        let mut retries = 0;

        // Try to receive a packet and handle timeouts
        loop {
            // Abort loop if the current try exceeds the maximum number of retransmission retries.
            if retries >= self.max_retransmission_retries {
                self.state = SocketState::Closed;
                return Err(SocketError::ConnectionTimedOut.into());
            }

            let timeout = if self.state != SocketState::New {
                debug!("setting read timeout of {} ms", self.congestion_timeout);
                Some(Duration::from_millis(self.congestion_timeout))
            } else {
                None
            };

            let response = match timeout {
                Some(timeout) => io::timeout(timeout, self.socket.recv_from(&mut b)).await,
                None => self.socket.recv_from(&mut b).await,
            };

            match response {
                Ok((r, s)) => {
                    read = r;
                    src = s;
                    break;
                }
                Err(ref e)
                    if (e.kind() == ErrorKind::WouldBlock || e.kind() == ErrorKind::TimedOut) =>
                {
                    debug!("recv_from timed out");
                    self.handle_receive_timeout().await?;
                }
                Err(e) => return Err(e),
            };

            let elapsed = start.elapsed();
            let elapsed_ms = elapsed.as_secs() * 1000 + elapsed.subsec_millis() as u64;
            debug!("{} ms elapsed", elapsed_ms);
            retries += 1;
        }

        // Decode received data into a packet
        let packet = match Packet::try_from(&b[..read]) {
            Ok(packet) => packet,
            Err(e) => {
                debug!("{}", e);
                debug!("Ignoring invalid packet");
                return Ok((0, self.connected_to));
            }
        };
        debug!("received {:?}", packet);

        // Process packet, including sending a reply if necessary
        if let Some(mut pkt) = self.handle_packet(&packet, src).await? {
            pkt.set_wnd_size(WINDOW_SIZE);
            self.socket.send_to(pkt.as_ref(), src).await?;
            debug!("sent {:?}", pkt);
        }

        // Insert data packet into the incoming buffer if it isn't a duplicate of a previously
        // discarded packet
        if packet.get_type() == PacketType::Data
            && packet.seq_nr().wrapping_sub(self.last_dropped) > 0
        {
            self.insert_into_buffer(packet);
        }

        // Flush incoming buffer if possible
        let read = self.flush_incoming_buffer(buf);

        Ok((read, src))
    }

    async fn handle_receive_timeout(&mut self) -> Result<()> {
        self.congestion_timeout *= 2;
        self.cwnd = MSS;

        // There are three possible cases here:
        //
        // - If the socket is sending and waiting for acknowledgements (the send window is
        //   not empty), resend the first unacknowledged packet;
        //
        // - If the socket is not sending and it hasn't sent a FIN yet, then it's waiting
        //   for incoming packets: send a fast resend request;
        //
        // - If the socket sent a FIN previously, resend it.
        debug!(
            "self.send_window: {:?}",
            self.send_window
                .iter()
                .map(Packet::seq_nr)
                .collect::<Vec<u16>>()
        );

        if self.send_window.is_empty() {
            // The socket is trying to close, all sent packets were acknowledged, and it has
            // already sent a FIN: resend it.
            if self.state == SocketState::FinSent {
                let mut packet = Packet::new();
                packet.set_connection_id(self.sender_connection_id);
                packet.set_seq_nr(self.seq_nr);
                packet.set_ack_nr(self.ack_nr);
                packet.set_timestamp(now_microseconds());
                packet.set_type(PacketType::Fin);

                // Send FIN
                self.socket
                    .send_to(packet.as_ref(), self.connected_to)
                    .await?;
                debug!("resent FIN: {:?}", packet);
            } else if self.state != SocketState::New {
                // The socket is waiting for incoming packets but the remote peer is silent:
                // send a fast resend request.
                debug!("sending fast resend request");
                self.send_fast_resend_request().await;
            }
        } else {
            // The socket is sending data packets but there is no reply from the remote
            // peer: resend the first unacknowledged packet with the current timestamp.
            let packet = &mut self.send_window[0];
            packet.set_timestamp(now_microseconds());
            self.socket
                .send_to(packet.as_ref(), self.connected_to)
                .await?;
            debug!("resent {:?}", packet);
        }

        Ok(())
    }

    fn prepare_reply(&self, original: &Packet, t: PacketType) -> Packet {
        let mut resp = Packet::new();
        resp.set_type(t);
        let self_t_micro = now_microseconds();
        let other_t_micro = original.timestamp();
        let time_difference: Delay = abs_diff(self_t_micro, other_t_micro);
        resp.set_timestamp(self_t_micro);
        resp.set_timestamp_difference(time_difference);
        resp.set_connection_id(self.sender_connection_id);
        resp.set_seq_nr(self.seq_nr);
        resp.set_ack_nr(self.ack_nr);

        resp
    }

    /// Removes a packet in the incoming buffer and updates the current acknowledgement number.
    fn advance_incoming_buffer(&mut self) -> Option<Packet> {
        if !self.incoming_buffer.is_empty() {
            let packet = self.incoming_buffer.remove(0);
            debug!("Removed packet from incoming buffer: {:?}", packet);
            self.ack_nr = packet.seq_nr();
            self.last_dropped = self.ack_nr;
            Some(packet)
        } else {
            None
        }
    }

    /// Discards sequential, ordered packets in incoming buffer, starting from
    /// the most recently acknowledged to the most recent, as long as there are
    /// no missing packets. The discarded packets' payload is written to the
    /// slice `buf`, starting in position `start`.
    /// Returns the last written index.
    fn flush_incoming_buffer(&mut self, buf: &mut [u8]) -> usize {
        fn unsafe_copy(src: &[u8], dst: &mut [u8]) -> usize {
            let max_len = min(src.len(), dst.len());
            unsafe {
                use std::ptr::copy;
                copy(src.as_ptr(), dst.as_mut_ptr(), max_len);
            }
            max_len
        }

        // Return pending data from a partially read packet
        if !self.pending_data.is_empty() {
            let flushed = unsafe_copy(&self.pending_data[..], buf);

            if flushed == self.pending_data.len() {
                self.pending_data.clear();
                self.advance_incoming_buffer();
            } else {
                self.pending_data = self.pending_data[flushed..].to_vec();
            }

            return flushed;
        }

        if !self.incoming_buffer.is_empty()
            && (self.ack_nr == self.incoming_buffer[0].seq_nr()
                || self.ack_nr + 1 == self.incoming_buffer[0].seq_nr())
        {
            let flushed = unsafe_copy(&self.incoming_buffer[0].payload(), buf);

            if flushed == self.incoming_buffer[0].payload().len() {
                self.advance_incoming_buffer();
            } else {
                self.pending_data = self.incoming_buffer[0].payload()[flushed..].to_vec();
            }

            return flushed;
        }

        0
    }

    /// Sends data on the socket to the remote peer. On success, returns the number of bytes
    /// written.
    //
    // # Implementation details
    //
    // This method inserts packets into the send buffer and keeps trying to
    // advance the send window until an ACK corresponding to the last packet is
    // received.
    //
    // Note that the buffer passed to `send_to` might exceed the maximum packet
    // size, which will result in the data being split over several packets.
    pub async fn send_to(&mut self, buf: &[u8]) -> Result<usize> {
        if self.state == SocketState::Closed {
            return Err(SocketError::ConnectionClosed.into());
        }

        let total_length = buf.len();

        for chunk in buf.chunks(MSS as usize - HEADER_SIZE) {
            let mut packet = Packet::with_payload(chunk);
            packet.set_seq_nr(self.seq_nr);
            packet.set_ack_nr(self.ack_nr);
            packet.set_connection_id(self.sender_connection_id);

            self.unsent_queue.push_back(packet);

            // Intentionally wrap around sequence number
            self.seq_nr = self.seq_nr.wrapping_add(1);
        }

        // Send every packet in the queue
        self.send().await?;

        Ok(total_length)
    }

    /// Consumes acknowledgements for every pending packet.
    pub async fn flush(&mut self) -> Result<()> {
        let mut buf = [0u8; BUF_SIZE];
        while !self.send_window.is_empty() {
            debug!("packets in send window: {}", self.send_window.len());
            self.recv(&mut buf).await?;
        }

        Ok(())
    }

    /// Sends every packet in the unsent packet queue.
    async fn send(&mut self) -> Result<()> {
        while let Some(mut packet) = self.unsent_queue.pop_front() {
            self.send_packet(&mut packet).await?;
            self.curr_window += packet.len() as u32;
            self.send_window.push(packet);
        }
        Ok(())
    }

    /// Send one packet.
    async fn send_packet(&mut self, packet: &mut Packet) -> Result<()> {
        debug!("current window: {}", self.send_window.len());
        let max_inflight = min(self.cwnd, self.remote_wnd_size);
        let max_inflight = max(MIN_CWND * MSS, max_inflight);
        let now = now_microseconds();

        // Wait until enough in-flight packets are acknowledged for rate control purposes, but don't
        // wait more than 500 ms (PRE_SEND_TIMEOUT) before sending the packet.
        while self.curr_window >= max_inflight && now_microseconds() - now < PRE_SEND_TIMEOUT.into()
        {
            debug!("self.curr_window: {}", self.curr_window);
            debug!("max_inflight: {}", max_inflight);
            debug!("self.duplicate_ack_count: {}", self.duplicate_ack_count);
            debug!("now_microseconds() - now = {}", now_microseconds() - now);
            let mut buf = [0u8; BUF_SIZE];
            self.recv(&mut buf).await?;
        }
        debug!(
            "out: now_microseconds() - now = {}",
            now_microseconds() - now
        );

        // Check if it still makes sense to send packet, as we might be trying to resend a lost
        // packet acknowledged in the receive loop above.
        // If there were no wrapping around of sequence numbers, we'd simply check if the packet's
        // sequence number is greater than `last_acked`.
        let distance_a = packet.seq_nr().wrapping_sub(self.last_acked);
        let distance_b = self.last_acked.wrapping_sub(packet.seq_nr());
        if distance_a > distance_b {
            debug!("Packet already acknowledged, skipping...");
            return Ok(());
        }

        packet.set_timestamp(now_microseconds());
        packet.set_timestamp_difference(self.their_delay);
        self.socket
            .send_to(packet.as_ref(), self.connected_to)
            .await?;
        debug!("sent {:?}", packet);

        Ok(())
    }

    // Insert a new sample in the base delay list.
    //
    // The base delay list contains at most `BASE_HISTORY` samples, each sample is the minimum
    // measured over a period of a minute (MAX_BASE_DELAY_AGE).
    fn update_base_delay(&mut self, base_delay: Delay, now: Timestamp) {
        if self.base_delays.is_empty() || now - self.last_rollover > MAX_BASE_DELAY_AGE {
            // Update last rollover
            self.last_rollover = now;

            // Drop the oldest sample, if need be
            if self.base_delays.len() == BASE_HISTORY {
                self.base_delays.pop_front();
            }

            // Insert new sample
            self.base_delays.push_back(base_delay);
        } else {
            // Replace sample for the current minute if the delay is lower
            let last_idx = self.base_delays.len() - 1;
            if base_delay < self.base_delays[last_idx] {
                self.base_delays[last_idx] = base_delay;
            }
        }
    }

    /// Inserts a new sample in the current delay list after removing samples older than one RTT, as
    /// specified in RFC6817.
    fn update_current_delay(&mut self, v: Delay, now: Timestamp) {
        // Remove samples more than one RTT old
        let rtt = (self.rtt as i64 * 100).into();
        while !self.current_delays.is_empty() && now - self.current_delays[0].received_at > rtt {
            self.current_delays.remove(0);
        }

        // Insert new measurement
        self.current_delays.push(DelayDifferenceSample {
            received_at: now,
            difference: v,
        });
    }

    fn update_congestion_timeout(&mut self, current_delay: i32) {
        let delta = self.rtt - current_delay;
        self.rtt_variance += (delta.abs() - self.rtt_variance) / 4;
        self.rtt += (current_delay - self.rtt) / 8;
        self.congestion_timeout = max(
            (self.rtt + self.rtt_variance * 4) as u64,
            MIN_CONGESTION_TIMEOUT,
        );
        self.congestion_timeout = min(self.congestion_timeout, MAX_CONGESTION_TIMEOUT);

        debug!("current_delay: {}", current_delay);
        debug!("delta: {}", delta);
        debug!("self.rtt_variance: {}", self.rtt_variance);
        debug!("self.rtt: {}", self.rtt);
        debug!("self.congestion_timeout: {}", self.congestion_timeout);
    }

    /// Calculates the filtered current delay in the current window.
    ///
    /// The current delay is calculated through application of the exponential
    /// weighted moving average filter with smoothing factor 0.333 over the
    /// current delays in the current window.
    fn filtered_current_delay(&self) -> Delay {
        let input = self.current_delays.iter().map(|delay| &delay.difference);
        (ewma(input, 0.333) as i64).into()
    }

    /// Calculates the lowest base delay in the current window.
    fn min_base_delay(&self) -> Delay {
        self.base_delays.iter().min().cloned().unwrap_or_default()
    }

    /// Builds the selective acknowledgement extension data for usage in packets.
    fn build_selective_ack(&self) -> Vec<u8> {
        let stashed = self
            .incoming_buffer
            .iter()
            .filter(|pkt| pkt.seq_nr() > self.ack_nr + 1)
            .map(|pkt| (pkt.seq_nr() - self.ack_nr - 2) as usize)
            .map(|diff| (diff / 8, diff % 8));

        let mut sack = Vec::new();
        for (byte, bit) in stashed {
            // Make sure the amount of elements in the SACK vector is a
            // multiple of 4 and enough to represent the lost packets
            while byte >= sack.len() || sack.len() % 4 != 0 {
                sack.push(0u8);
            }

            sack[byte] |= 1 << bit;
        }

        sack
    }

    /// Sends a fast resend request to the remote peer.
    ///
    /// A fast resend request consists of sending three State packets (acknowledging the last
    /// received packet) in quick succession.
    async fn send_fast_resend_request(&self) {
        for _ in 0..3 {
            let mut packet = Packet::new();
            packet.set_type(PacketType::State);
            let self_t_micro = now_microseconds();
            packet.set_timestamp(self_t_micro);
            packet.set_timestamp_difference(self.their_delay);
            packet.set_connection_id(self.sender_connection_id);
            packet.set_seq_nr(self.seq_nr);
            packet.set_ack_nr(self.ack_nr);
            let _ = self
                .socket
                .send_to(packet.as_ref(), self.connected_to)
                .await;
        }
    }

    async fn resend_lost_packet(&mut self, lost_packet_nr: u16) {
        debug!("---> resend_lost_packet({}) <---", lost_packet_nr);
        match self
            .send_window
            .iter()
            .position(|pkt| pkt.seq_nr() == lost_packet_nr)
        {
            None => debug!("Packet {} not found", lost_packet_nr),
            Some(position) => {
                debug!("self.send_window.len(): {}", self.send_window.len());
                debug!("position: {}", position);
                let mut packet = self.send_window[position].clone();
                // FIXME: Unchecked result
                let _ = self.send_packet(&mut packet).await;

                // We intentionally don't increase `curr_window` because otherwise a packet's length
                // would be counted more than once
            }
        }
        debug!("---> END resend_lost_packet <---");
    }

    /// Forgets sent packets that were acknowledged by the remote peer.
    fn advance_send_window(&mut self) {
        // The reason I'm not removing the first element in a loop while its sequence number is
        // smaller than `last_acked` is because of wrapping sequence numbers, which would create the
        // sequence [..., 65534, 65535, 0, 1, ...]. If `last_acked` is smaller than the first
        // packet's sequence number because of wraparound (for instance, 1), no packets would be
        // removed, as the condition `seq_nr < last_acked` would fail immediately.
        //
        // On the other hand, I can't keep removing the first packet in a loop until its sequence
        // number matches `last_acked` because it might never match, and in that case no packets
        // should be removed.
        if let Some(position) = self
            .send_window
            .iter()
            .position(|packet| packet.seq_nr() == self.last_acked)
        {
            for _ in 0..position + 1 {
                let packet = self.send_window.remove(0);
                self.curr_window -= packet.len() as u32;
            }
        }
        debug!("self.curr_window: {}", self.curr_window);
    }

    /// Handles an incoming packet, updating socket state accordingly.
    ///
    /// Returns the appropriate reply packet, if needed.
    async fn handle_packet(&mut self, packet: &Packet, src: SocketAddr) -> Result<Option<Packet>> {
        debug!("({:?}, {:?})", self.state, packet.get_type());

        // Acknowledge only if the packet strictly follows the previous one
        if packet.seq_nr().wrapping_sub(self.ack_nr) == 1 {
            self.ack_nr = packet.seq_nr();
        }

        // Reset connection if connection id doesn't match and this isn't a SYN
        if packet.get_type() != PacketType::Syn
            && self.state != SocketState::SynSent
            && !(packet.connection_id() == self.sender_connection_id
                || packet.connection_id() == self.receiver_connection_id)
        {
            return Ok(Some(self.prepare_reply(packet, PacketType::Reset)));
        }

        // Update remote window size
        self.remote_wnd_size = packet.wnd_size();
        debug!("self.remote_wnd_size: {}", self.remote_wnd_size);

        // Update remote peer's delay between them sending the packet and us receiving it
        let now = now_microseconds();
        self.their_delay = abs_diff(now, packet.timestamp());
        debug!("self.their_delay: {}", self.their_delay);

        match (self.state, packet.get_type()) {
            (SocketState::New, PacketType::Syn) => {
                self.connected_to = src;
                self.ack_nr = packet.seq_nr();
                self.seq_nr = rand::random();
                self.receiver_connection_id = packet.connection_id() + 1;
                self.sender_connection_id = packet.connection_id();
                self.state = SocketState::Connected;
                self.last_dropped = self.ack_nr;

                Ok(Some(self.prepare_reply(packet, PacketType::State)))
            }
            (_, PacketType::Syn) => Ok(Some(self.prepare_reply(packet, PacketType::Reset))),
            (SocketState::SynSent, PacketType::State) => {
                self.connected_to = src;
                self.ack_nr = packet.seq_nr();
                self.seq_nr += 1;
                self.state = SocketState::Connected;
                self.last_acked = packet.ack_nr();
                self.last_acked_timestamp = now_microseconds();
                Ok(None)
            }
            (SocketState::SynSent, _) => Err(SocketError::InvalidReply.into()),
            (SocketState::Connected, PacketType::Data)
            | (SocketState::FinSent, PacketType::Data) => Ok(self.handle_data_packet(packet)),
            (SocketState::Connected, PacketType::State) => {
                self.handle_state_packet(packet).await;
                Ok(None)
            }
            (SocketState::Connected, PacketType::Fin) | (SocketState::FinSent, PacketType::Fin) => {
                if packet.ack_nr() < self.seq_nr {
                    debug!("FIN received but there are missing acknowledgements for sent packets");
                }
                let mut reply = self.prepare_reply(packet, PacketType::State);
                if packet.seq_nr().wrapping_sub(self.ack_nr) > 1 {
                    debug!(
                        "current ack_nr ({}) is behind received packet seq_nr ({})",
                        self.ack_nr,
                        packet.seq_nr()
                    );

                    // Set SACK extension payload if the packet is not in order
                    let sack = self.build_selective_ack();

                    if !sack.is_empty() {
                        reply.set_sack(sack);
                    }
                }

                // Give up, the remote peer might not care about our missing packets
                self.state = SocketState::Closed;
                Ok(Some(reply))
            }
            (SocketState::Closed, PacketType::Fin) => {
                Ok(Some(self.prepare_reply(packet, PacketType::State)))
            }
            (SocketState::FinSent, PacketType::State) => {
                if packet.ack_nr() == self.seq_nr {
                    self.state = SocketState::Closed;
                } else {
                    self.handle_state_packet(packet);
                }
                Ok(None)
            }
            (_, PacketType::Reset) => {
                self.state = SocketState::ResetReceived;
                Err(SocketError::ConnectionReset.into())
            }
            (state, ty) => {
                let message = format!("Unimplemented handling for ({:?},{:?})", state, ty);
                debug!("{}", message);
                Err(SocketError::Other(message).into())
            }
        }
    }

    fn handle_data_packet(&mut self, packet: &Packet) -> Option<Packet> {
        // If a FIN was previously sent, reply with a FIN packet acknowledging the received packet.
        let packet_type = if self.state == SocketState::FinSent {
            PacketType::Fin
        } else {
            PacketType::State
        };
        let mut reply = self.prepare_reply(packet, packet_type);

        if packet.seq_nr().wrapping_sub(self.ack_nr) > 1 {
            debug!(
                "current ack_nr ({}) is behind received packet seq_nr ({})",
                self.ack_nr,
                packet.seq_nr()
            );

            // Set SACK extension payload if the packet is not in order
            let sack = self.build_selective_ack();

            if !sack.is_empty() {
                reply.set_sack(sack);
            }
        }

        Some(reply)
    }

    fn queuing_delay(&self) -> Delay {
        let filtered_current_delay = self.filtered_current_delay();
        let min_base_delay = self.min_base_delay();
        let queuing_delay = filtered_current_delay - min_base_delay;

        debug!("filtered_current_delay: {}", filtered_current_delay);
        debug!("min_base_delay: {}", min_base_delay);
        debug!("queuing_delay: {}", queuing_delay);

        queuing_delay
    }

    /// Calculates the new congestion window size, increasing it or decreasing it.
    ///
    /// This is the core of uTP, the [LEDBAT][ledbat_rfc] congestion algorithm. It depends on
    /// estimating the queuing delay between the two peers, and adjusting the congestion window
    /// accordingly.
    ///
    /// `off_target` is a normalized value representing the difference between the current queuing
    /// delay and a fixed target delay (`TARGET`). `off_target` ranges between -1.0 and 1.0. A
    /// positive value makes the congestion window increase, while a negative value makes the
    /// congestion window decrease.
    ///
    /// `bytes_newly_acked` is the number of bytes acknowledged by an inbound `State` packet. It may
    /// be the size of the packet explicitly acknowledged by the inbound packet (i.e., with sequence
    /// number equal to the inbound packet's acknowledgement number), or every packet implicitly
    /// acknowledged (every packet with sequence number between the previous inbound `State`
    /// packet's acknowledgement number and the current inbound `State` packet's acknowledgement
    /// number).
    ///
    ///[ledbat_rfc]: https://tools.ietf.org/html/rfc6817
    fn update_congestion_window(&mut self, off_target: f64, bytes_newly_acked: u32) {
        let flightsize = self.curr_window;

        let cwnd_increase = GAIN * off_target * bytes_newly_acked as f64 * MSS as f64;
        let cwnd_increase = cwnd_increase / self.cwnd as f64;
        debug!("cwnd_increase: {}", cwnd_increase);

        self.cwnd = (self.cwnd as f64 + cwnd_increase) as u32;
        let max_allowed_cwnd = flightsize + ALLOWED_INCREASE * MSS;
        self.cwnd = min(self.cwnd, max_allowed_cwnd);
        self.cwnd = max(self.cwnd, MIN_CWND * MSS);

        debug!("cwnd: {}", self.cwnd);
        debug!("max_allowed_cwnd: {}", max_allowed_cwnd);
    }

    #[async_recursion::async_recursion]
    async fn handle_state_packet(&mut self, packet: &Packet) {
        if packet.ack_nr() == self.last_acked {
            self.duplicate_ack_count += 1;
        } else {
            self.last_acked = packet.ack_nr();
            self.last_acked_timestamp = now_microseconds();
            self.duplicate_ack_count = 1;
        }

        // Update congestion window size
        if let Some(index) = self
            .send_window
            .iter()
            .position(|p| packet.ack_nr() == p.seq_nr())
        {
            // Calculate the sum of the size of every packet implicitly and explicitly acknowledged
            // by the inbound packet (i.e., every packet whose sequence number precedes the inbound
            // packet's acknowledgement number, plus the packet whose sequence number matches)
            let bytes_newly_acked = self
                .send_window
                .iter()
                .take(index + 1)
                .fold(0, |acc, p| acc + p.len());

            // Update base and current delay
            let now = now_microseconds();
            let our_delay = now - self.send_window[index].timestamp();
            debug!("our_delay: {}", our_delay);
            self.update_base_delay(our_delay, now);
            self.update_current_delay(our_delay, now);

            let off_target: f64 = (TARGET - u32::from(self.queuing_delay()) as f64) / TARGET;
            debug!("off_target: {}", off_target);

            self.update_congestion_window(off_target, bytes_newly_acked as u32);

            // Update congestion timeout
            let rtt = u32::from(our_delay - self.queuing_delay()) / 1000; // in milliseconds
            self.update_congestion_timeout(rtt as i32);
        }

        let mut packet_loss_detected: bool =
            !self.send_window.is_empty() && self.duplicate_ack_count == 3;

        // Process extensions, if any
        for extension in packet.extensions() {
            if extension.get_type() == ExtensionType::SelectiveAck {
                // If three or more packets are acknowledged past the implicit missing one,
                // assume it was lost.
                if extension.iter().count_ones() >= 3 {
                    self.resend_lost_packet(packet.ack_nr() + 1).await;
                    packet_loss_detected = true;
                }

                if let Some(last_seq_nr) = self.send_window.last().map(Packet::seq_nr) {
                    let lost_packets = extension
                        .iter()
                        .enumerate()
                        .filter(|&(_, received)| !received)
                        .map(|(idx, _)| packet.ack_nr() + 2 + idx as u16)
                        .take_while(|&seq_nr| seq_nr < last_seq_nr);

                    for seq_nr in lost_packets {
                        debug!("SACK: packet {} lost", seq_nr);
                        self.resend_lost_packet(seq_nr).await;
                        packet_loss_detected = true;
                    }
                }
            } else {
                debug!("Unknown extension {:?}, ignoring", extension.get_type());
            }
        }

        // Three duplicate ACKs mean a fast resend request. Resend the first unacknowledged packet
        // if the incoming packet doesn't have a SACK extension. If it does, the lost packets were
        // already resent.
        if !self.send_window.is_empty()
            && self.duplicate_ack_count == 3
            && !packet
                .extensions()
                .any(|ext| ext.get_type() == ExtensionType::SelectiveAck)
        {
            self.resend_lost_packet(packet.ack_nr() + 1).await;
        }

        // Packet lost, halve the congestion window
        if packet_loss_detected {
            debug!("packet loss detected, halving congestion window");
            self.cwnd = max(self.cwnd / 2, MIN_CWND * MSS);
            debug!("cwnd: {}", self.cwnd);
        }

        // Success, advance send window
        self.advance_send_window();
    }

    /// Inserts a packet into the socket's buffer.
    ///
    /// The packet is inserted in such a way that the packets in the buffer are sorted according to
    /// their sequence number in ascending order. This allows storing packets that were received out
    /// of order.
    ///
    /// Trying to insert a duplicate of a packet will silently fail.
    /// it's more recent (larger timestamp).
    fn insert_into_buffer(&mut self, packet: Packet) {
        // Immediately push to the end if the packet's sequence number comes after the last
        // packet's.
        if self
            .incoming_buffer
            .last()
            .map_or(false, |p| packet.seq_nr() > p.seq_nr())
        {
            self.incoming_buffer.push(packet);
        } else {
            // Find index following the most recent packet before the one we wish to insert
            let i = self
                .incoming_buffer
                .iter()
                .filter(|p| p.seq_nr() < packet.seq_nr())
                .count();

            if self
                .incoming_buffer
                .get(i)
                .map_or(true, |p| p.seq_nr() != packet.seq_nr())
            {
                self.incoming_buffer.insert(i, packet);
            }
        }
    }
}

impl Drop for UtpSocket {
    fn drop(&mut self) {
        task::block_on(async {
            drop(self.close().await);
        });
    }
}

/// A structure representing a socket server.
///
/// # Examples
///
/// ```no_run
/// use async_std_utp::{UtpListener, UtpSocket};
/// use async_std::{prelude::*, task};
///
/// async fn handle_client(socket: UtpSocket) {
///     // ...
/// }
///
/// # fn main() { async_std::task::block_on(async {
///     // Create a listener
///     let addr = "127.0.0.1:8080";
///     let listener = UtpListener::bind(addr).await.expect("Error binding socket");
///     let mut incoming = listener.incoming();
///     while let Some(connection) = incoming.next().await {
///         // Spawn a new handler for each new connection
///         if let Ok((socket, _src)) = connection {
///             task::spawn(async move { handle_client(socket) });
///         }
///     }
/// # }); }
/// ```
pub struct UtpListener {
    /// The public facing UDP socket
    socket: UdpSocket,
}

impl UtpListener {
    /// Creates a new `UtpListener` bound to a specific address.
    ///
    /// The resulting listener is ready for accepting connections.
    ///
    /// The address type can be any implementer of the `ToSocketAddr` trait. See its documentation
    /// for concrete examples.
    ///
    /// If more than one valid address is specified, only the first will be used.
    pub async fn bind<A: ToSocketAddrs>(addr: A) -> Result<UtpListener> {
        let socket = UdpSocket::bind(addr).await?;
        Ok(UtpListener { socket })
    }

    /// Accepts a new incoming connection from this listener.
    ///
    /// This function will block the caller until a new uTP connection is established. When
    /// established, the corresponding `UtpSocket` and the peer's remote address will be returned.
    ///
    /// Notice that the resulting `UtpSocket` is bound to a different local port than the public
    /// listening port (which `UtpListener` holds). This may confuse the remote peer!
    pub async fn accept(&self) -> Result<(UtpSocket, SocketAddr)> {
        let mut buf = [0; BUF_SIZE];

        let (nread, src) = self.socket.recv_from(&mut buf).await?;
        let packet = Packet::try_from(&buf[..nread])?;

        // Ignore non-SYN packets
        if packet.get_type() != PacketType::Syn {
            let message = format!("Expected SYN packet, got {:?} instead", packet.get_type());
            return Err(SocketError::Other(message).into());
        }

        // The address of the new socket will depend on the type of the listener.
        let local_addr = self.socket.local_addr()?;
        let inner_socket = match local_addr {
            SocketAddr::V4(_) => UdpSocket::bind("0.0.0.0:0"),
            SocketAddr::V6(_) => UdpSocket::bind("[::]:0"),
        }
        .await?;

        let mut socket = UtpSocket::from_raw_parts(inner_socket, src);

        // Establish connection with remote peer
        if let Ok(Some(reply)) = socket.handle_packet(&packet, src).await {
            socket
                .socket
                .send_to(reply.as_ref(), src)
                .await
                .and(Ok((socket, src)))
        } else {
            Err(SocketError::Other("Reached unreachable statement".to_owned()).into())
        }
    }

    /// Returns an iterator over the connections being received by this listener.
    ///
    /// The returned iterator will never return `None`.
    pub fn incoming(&self) -> Incoming<'_> {
        Incoming {
            listener: self,
            accept: None,
        }
    }

    /// Returns the local socket address of this listener.
    pub fn local_addr(&self) -> Result<SocketAddr> {
        self.socket.local_addr()
    }
}

type AcceptFuture<'a> = Option<BoxFuture<'a, io::Result<(UtpSocket, SocketAddr)>>>;

pub struct Incoming<'a> {
    listener: &'a UtpListener,
    accept: AcceptFuture<'a>,
}

impl<'a> futures::Stream for Incoming<'a> {
    type Item = Result<(UtpSocket, SocketAddr)>;

    fn poll_next(
        mut self: std::pin::Pin<&mut Self>,
        cx: &mut std::task::Context<'_>,
    ) -> std::task::Poll<Option<Self::Item>> {
        loop {
            if self.accept.is_none() {
                self.accept = Some(Box::pin(self.listener.accept()));
            }

            if let Some(f) = &mut self.accept {
                let res = ready!(f.as_mut().poll(cx));
                self.accept = None;
                return Poll::Ready(Some(res));
            }
        }
    }
}

#[cfg(test)]
mod test {
    use crate::packet::*;
    use crate::socket::{take_address, SocketState, UtpListener, UtpSocket, BUF_SIZE};
    use crate::time::now_microseconds;
    use async_std::task;
    use rand;
    use std::io::ErrorKind;
    use std::net::ToSocketAddrs;

    macro_rules! iotry {
        ($e:expr) => {
            match $e.await {
                Ok(e) => e,
                Err(e) => panic!("{:?}", e),
            }
        };
    }

    fn next_test_port() -> u16 {
        use std::sync::atomic::{AtomicUsize, Ordering};
        static NEXT_OFFSET: AtomicUsize = AtomicUsize::new(0);
        const BASE_PORT: u16 = 9600;
        BASE_PORT + NEXT_OFFSET.fetch_add(1, Ordering::Relaxed) as u16
    }

    fn next_test_ip4<'a>() -> (&'a str, u16) {
        ("127.0.0.1", next_test_port())
    }

    fn next_test_ip6<'a>() -> (&'a str, u16) {
        ("::1", next_test_port())
    }

    #[async_std::test]
    async fn test_socket_ipv4() {
        let server_addr = next_test_ip4();

        let mut server = iotry!(UtpSocket::bind(server_addr));
        assert_eq!(server.state, SocketState::New);

        let child = task::spawn(async move {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            assert_eq!(client.state, SocketState::Connected);
            // Check proper difference in client's send connection id and receive connection id
            assert_eq!(
                client.sender_connection_id,
                client.receiver_connection_id + 1
            );
            assert_eq!(
                client.connected_to,
                server_addr.to_socket_addrs().unwrap().next().unwrap()
            );
            iotry!(client.close());
            drop(client);
        });

        let mut buf = [0; BUF_SIZE];
        match server.recv_from(&mut buf).await {
            e => println!("{:?}", e),
        }
        // After establishing a new connection, the server's ids are a mirror of the client's.
        assert_eq!(
            server.receiver_connection_id,
            server.sender_connection_id + 1
        );

        assert_eq!(server.state, SocketState::Closed);
        drop(server);

        child.await;
    }

    #[async_std::test]
    async fn test_socket_ipv6() {
        let server_addr = next_test_ip6();

        let mut server = iotry!(UtpSocket::bind(server_addr));
        assert_eq!(server.state, SocketState::New);

        let child = task::spawn(async move {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            assert_eq!(client.state, SocketState::Connected);
            // Check proper difference in client's send connection id and receive connection id
            assert_eq!(
                client.sender_connection_id,
                client.receiver_connection_id + 1
            );
            assert_eq!(
                client.connected_to,
                server_addr.to_socket_addrs().unwrap().next().unwrap()
            );
            iotry!(client.close());
            drop(client);
        });

        let mut buf = [0u8; BUF_SIZE];
        match server.recv_from(&mut buf).await {
            e => println!("{:?}", e),
        }
        // After establishing a new connection, the server's ids are a mirror of the client's.
        assert_eq!(
            server.receiver_connection_id,
            server.sender_connection_id + 1
        );

        assert_eq!(server.state, SocketState::Closed);
        drop(server);

        child.await;
    }

    #[async_std::test]
    async fn test_recvfrom_on_closed_socket() {
        let server_addr = next_test_ip4();

        let mut server = iotry!(UtpSocket::bind(server_addr));
        assert_eq!(server.state, SocketState::New);

        let child = task::spawn(async move {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            assert_eq!(client.state, SocketState::Connected);
            assert!(client.close().await.is_ok());
        });

        // Make the server listen for incoming connections until the end of the input
        let mut buf = [0u8; BUF_SIZE];
        let _resp = server.recv_from(&mut buf).await;
        assert_eq!(server.state, SocketState::Closed);

        // Trying to receive again returns `Ok(0)` (equivalent to the old `EndOfFile`)
        match server.recv_from(&mut buf).await {
            Ok((0, _src)) => {}
            e => panic!("Expected Ok(0), got {:?}", e),
        }
        assert_eq!(server.state, SocketState::Closed);

        child.await;
    }

    #[async_std::test]
    async fn test_sendto_on_closed_socket() {
        let server_addr = next_test_ip4();

        let mut server = iotry!(UtpSocket::bind(server_addr));
        assert_eq!(server.state, SocketState::New);

        let child = task::spawn(async move {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            assert_eq!(client.state, SocketState::Connected);
            iotry!(client.close());
        });

        // Make the server listen for incoming connections
        let mut buf = [0u8; BUF_SIZE];
        let (_read, _src) = iotry!(server.recv_from(&mut buf));
        assert_eq!(server.state, SocketState::Closed);

        // Trying to send to the socket after closing it raises an error
        match server.send_to(&buf).await {
            Err(ref e) if e.kind() == ErrorKind::NotConnected => (),
            v => panic!("expected {:?}, got {:?}", ErrorKind::NotConnected, v),
        }

        child.await;
    }

    #[async_std::test]
    async fn test_acks_on_socket() {
        use std::sync::mpsc::channel;
        let server_addr = next_test_ip4();
        let (tx, rx) = channel();

        let mut server = iotry!(UtpSocket::bind(server_addr));

        let child = task::spawn(async move {
            // Make the server listen for incoming connections
            let mut buf = [0u8; BUF_SIZE];
            let _resp = server.recv(&mut buf).await;
            tx.send(server.seq_nr).unwrap();

            // Close the connection
            iotry!(server.recv_from(&mut buf));

            drop(server);
        });

        let mut client = iotry!(UtpSocket::connect(server_addr));
        assert_eq!(client.state, SocketState::Connected);
        let sender_seq_nr = rx.recv().unwrap();
        let ack_nr = client.ack_nr;
        assert_eq!(ack_nr, sender_seq_nr);
        assert!(client.close().await.is_ok());

        // The reply to both connect (SYN) and close (FIN) should be
        // STATE packets, which don't increase the sequence number
        // and, hence, the receiver's acknowledgement number.
        assert_eq!(client.ack_nr, ack_nr);
        drop(client);

        child.await;
    }

    #[async_std::test]
    async fn test_handle_packet() {
        //fn test_connection_setup() {
        let initial_connection_id: u16 = rand::random();
        let sender_connection_id = initial_connection_id + 1;
        let (server_addr, client_addr) = (
            next_test_ip4().to_socket_addrs().unwrap().next().unwrap(),
            next_test_ip4().to_socket_addrs().unwrap().next().unwrap(),
        );
        let mut socket = iotry!(UtpSocket::bind(server_addr));

        let mut packet = Packet::new();
        packet.set_wnd_size(BUF_SIZE as u32);
        packet.set_type(PacketType::Syn);
        packet.set_connection_id(initial_connection_id);

        // Do we have a response?
        let response = socket.handle_packet(&packet, client_addr).await;
        assert!(response.is_ok());
        let response = response.unwrap();
        assert!(response.is_some());

        // Is is of the correct type?
        let response = response.unwrap();
        assert_eq!(response.get_type(), PacketType::State);

        // Same connection id on both ends during connection establishment
        assert_eq!(response.connection_id(), packet.connection_id());

        // Response acknowledges SYN
        assert_eq!(response.ack_nr(), packet.seq_nr());

        // No payload?
        assert!(response.payload().is_empty());
        //}

        // ---------------------------------

        // fn test_connection_usage() {
        let old_packet = packet;
        let old_response = response;

        let mut packet = Packet::new();
        packet.set_type(PacketType::Data);
        packet.set_connection_id(sender_connection_id);
        packet.set_seq_nr(old_packet.seq_nr() + 1);
        packet.set_ack_nr(old_response.seq_nr());

        let response = socket.handle_packet(&packet, client_addr).await;
        assert!(response.is_ok());
        let response = response.unwrap();
        assert!(response.is_some());

        let response = response.unwrap();
        assert_eq!(response.get_type(), PacketType::State);

        // Sender (i.e., who the initiated connection and sent a SYN) has connection id equal to
        // initial connection id + 1
        // Receiver (i.e., who accepted connection) has connection id equal to initial connection id
        assert_eq!(response.connection_id(), initial_connection_id);
        assert_eq!(response.connection_id(), packet.connection_id() - 1);

        // Previous packets should be ack'ed
        assert_eq!(response.ack_nr(), packet.seq_nr());

        // Responses with no payload should not increase the sequence number
        assert!(response.payload().is_empty());
        assert_eq!(response.seq_nr(), old_response.seq_nr());
        // }

        //fn test_connection_teardown() {
        let old_packet = packet;
        let old_response = response;

        let mut packet = Packet::new();
        packet.set_type(PacketType::Fin);
        packet.set_connection_id(sender_connection_id);
        packet.set_seq_nr(old_packet.seq_nr() + 1);
        packet.set_ack_nr(old_response.seq_nr());

        let response = socket.handle_packet(&packet, client_addr).await;
        assert!(response.is_ok());
        let response = response.unwrap();
        assert!(response.is_some());

        let response = response.unwrap();

        assert_eq!(response.get_type(), PacketType::State);

        // FIN packets have no payload but the sequence number shouldn't increase
        assert_eq!(packet.seq_nr(), old_packet.seq_nr() + 1);

        // Nor should the ACK packet's sequence number
        assert_eq!(response.seq_nr(), old_response.seq_nr());

        // FIN should be acknowledged
        assert_eq!(response.ack_nr(), packet.seq_nr());

        //}
    }

    #[async_std::test]
    async fn test_response_to_keepalive_ack() {
        // Boilerplate test setup
        let initial_connection_id: u16 = rand::random();
        let (server_addr, client_addr) = (
            next_test_ip4().to_socket_addrs().unwrap().next().unwrap(),
            next_test_ip4().to_socket_addrs().unwrap().next().unwrap(),
        );
        let mut socket = iotry!(UtpSocket::bind(server_addr));

        // Establish connection
        let mut packet = Packet::new();
        packet.set_wnd_size(BUF_SIZE as u32);
        packet.set_type(PacketType::Syn);
        packet.set_connection_id(initial_connection_id);

        let response = socket.handle_packet(&packet, client_addr).await;
        assert!(response.is_ok());
        let response = response.unwrap();
        assert!(response.is_some());
        let response = response.unwrap();
        assert_eq!(response.get_type(), PacketType::State);

        let old_packet = packet;
        let old_response = response;

        // Now, send a keepalive packet
        let mut packet = Packet::new();
        packet.set_wnd_size(BUF_SIZE as u32);
        packet.set_type(PacketType::State);
        packet.set_connection_id(initial_connection_id);
        packet.set_seq_nr(old_packet.seq_nr() + 1);
        packet.set_ack_nr(old_response.seq_nr());

        let response = socket.handle_packet(&packet, client_addr).await;
        assert!(response.is_ok());
        let response = response.unwrap();
        assert!(response.is_none());

        // Send a second keepalive packet, identical to the previous one
        let response = socket.handle_packet(&packet, client_addr).await;
        assert!(response.is_ok());
        let response = response.unwrap();
        assert!(response.is_none());

        // Mark socket as closed
        socket.state = SocketState::Closed;
    }

    #[async_std::test]
    async fn test_response_to_wrong_connection_id() {
        // Boilerplate test setup
        let initial_connection_id: u16 = rand::random();
        let (server_addr, client_addr) = (
            next_test_ip4().to_socket_addrs().unwrap().next().unwrap(),
            next_test_ip4().to_socket_addrs().unwrap().next().unwrap(),
        );
        let mut socket = iotry!(UtpSocket::bind(server_addr));

        // Establish connection
        let mut packet = Packet::new();
        packet.set_wnd_size(BUF_SIZE as u32);
        packet.set_type(PacketType::Syn);
        packet.set_connection_id(initial_connection_id);

        let response = socket.handle_packet(&packet, client_addr).await;
        assert!(response.is_ok());
        let response = response.unwrap();
        assert!(response.is_some());
        assert_eq!(response.unwrap().get_type(), PacketType::State);

        // Now, disrupt connection with a packet with an incorrect connection id
        let new_connection_id = initial_connection_id.wrapping_mul(2);

        let mut packet = Packet::new();
        packet.set_wnd_size(BUF_SIZE as u32);
        packet.set_type(PacketType::State);
        packet.set_connection_id(new_connection_id);

        let response = socket.handle_packet(&packet, client_addr).await;
        assert!(response.is_ok());
        let response = response.unwrap();
        assert!(response.is_some());

        let response = response.unwrap();
        assert_eq!(response.get_type(), PacketType::Reset);
        assert_eq!(response.ack_nr(), packet.seq_nr());

        // Mark socket as closed
        socket.state = SocketState::Closed;
    }

    #[async_std::test]
    async fn test_unordered_packets() {
        // Boilerplate test setup
        let initial_connection_id: u16 = rand::random();
        let (server_addr, client_addr) = (
            next_test_ip4().to_socket_addrs().unwrap().next().unwrap(),
            next_test_ip4().to_socket_addrs().unwrap().next().unwrap(),
        );
        let mut socket = iotry!(UtpSocket::bind(server_addr));

        // Establish connection
        let mut packet = Packet::new();
        packet.set_wnd_size(BUF_SIZE as u32);
        packet.set_type(PacketType::Syn);
        packet.set_connection_id(initial_connection_id);

        let response = socket.handle_packet(&packet, client_addr).await;
        assert!(response.is_ok());
        let response = response.unwrap();
        assert!(response.is_some());
        let response = response.unwrap();
        assert_eq!(response.get_type(), PacketType::State);

        let old_packet = packet;
        let old_response = response;

        let mut window: Vec<Packet> = Vec::new();

        // Now, send a keepalive packet
        let mut packet = Packet::with_payload(&[1, 2, 3]);
        packet.set_wnd_size(BUF_SIZE as u32);
        packet.set_connection_id(initial_connection_id);
        packet.set_seq_nr(old_packet.seq_nr() + 1);
        packet.set_ack_nr(old_response.seq_nr());
        window.push(packet);

        let mut packet = Packet::with_payload(&[4, 5, 6]);
        packet.set_wnd_size(BUF_SIZE as u32);
        packet.set_connection_id(initial_connection_id);
        packet.set_seq_nr(old_packet.seq_nr() + 2);
        packet.set_ack_nr(old_response.seq_nr());
        window.push(packet);

        // Send packets in reverse order
        let response = socket.handle_packet(&window[1], client_addr).await;
        assert!(response.is_ok());
        let response = response.unwrap();
        assert!(response.is_some());
        let response = response.unwrap();
        assert!(response.ack_nr() != window[1].seq_nr());

        let response = socket.handle_packet(&window[0], client_addr).await;
        assert!(response.is_ok());
        let response = response.unwrap();
        assert!(response.is_some());

        // Mark socket as closed
        socket.state = SocketState::Closed;
    }

    #[async_std::test]
    async fn test_response_to_triple_ack() {
        let server_addr = next_test_ip4();
        let mut server = iotry!(UtpSocket::bind(server_addr));

        // Fits in a packet
        const LEN: usize = 1024;
        let data = (0..LEN).map(|idx| idx as u8).collect::<Vec<u8>>();
        let d = data.clone();
        assert_eq!(LEN, data.len());

        let child = task::spawn(async move {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            iotry!(client.send_to(&d[..]));
            iotry!(client.close());
        });

        let mut buf = [0u8; BUF_SIZE];
        // Expect SYN
        iotry!(server.recv(&mut buf));

        // Receive data
        let data_packet = match server.socket.recv_from(&mut buf).await {
            Ok((read, _src)) => Packet::try_from(&buf[..read]).unwrap(),
            Err(e) => panic!("{}", e),
        };
        assert_eq!(data_packet.get_type(), PacketType::Data);
        assert_eq!(&data_packet.payload(), &data.as_slice());
        assert_eq!(data_packet.payload().len(), data.len());

        // Send triple ACK
        let mut packet = Packet::new();
        packet.set_wnd_size(BUF_SIZE as u32);
        packet.set_type(PacketType::State);
        packet.set_seq_nr(server.seq_nr);
        packet.set_ack_nr(data_packet.seq_nr() - 1);
        packet.set_connection_id(server.sender_connection_id);

        for _ in 0..3u8 {
            iotry!(server.socket.send_to(packet.as_ref(), server.connected_to));
        }

        // Receive data again and check that it's the same we reported as missing
        let client_addr = server.connected_to;
        match server.socket.recv_from(&mut buf).await {
            Ok((0, _)) => panic!("Received 0 bytes from socket"),
            Ok((read, _src)) => {
                let packet = Packet::try_from(&buf[..read]).unwrap();
                assert_eq!(packet.get_type(), PacketType::Data);
                assert_eq!(packet.seq_nr(), data_packet.seq_nr());
                assert_eq!(packet.payload(), data_packet.payload());
                let response = server.handle_packet(&packet, client_addr).await;
                assert!(response.is_ok());
                let response = response.unwrap();
                assert!(response.is_some());
                let response = response.unwrap();
                iotry!(server
                    .socket
                    .send_to(response.as_ref(), server.connected_to));
            }
            Err(e) => panic!("{}", e),
        }

        // Receive close
        iotry!(server.recv_from(&mut buf));
        child.await;
    }

    #[async_std::test]
    async fn test_socket_timeout_request() {
        let (server_addr, client_addr) = (
            next_test_ip4().to_socket_addrs().unwrap().next().unwrap(),
            next_test_ip4().to_socket_addrs().unwrap().next().unwrap(),
        );

        let client = iotry!(UtpSocket::bind(client_addr));
        let mut server = iotry!(UtpSocket::bind(server_addr));
        const LEN: usize = 512;
        let data = (0..LEN).map(|idx| idx as u8).collect::<Vec<u8>>();
        let d = data.clone();

        assert_eq!(server.state, SocketState::New);
        assert_eq!(client.state, SocketState::New);

        // Check proper difference in client's send connection id and receive connection id
        assert_eq!(
            client.sender_connection_id,
            client.receiver_connection_id + 1
        );

        let child = task::spawn(async move {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            assert_eq!(client.state, SocketState::Connected);
            assert_eq!(client.connected_to, server_addr);
            iotry!(client.send_to(&d[..]));
            drop(client);
        });

        let mut buf = [0u8; BUF_SIZE];
        server.recv(&mut buf).await.unwrap();
        // After establishing a new connection, the server's ids are a mirror of the client's.
        assert_eq!(
            server.receiver_connection_id,
            server.sender_connection_id + 1
        );

        assert_eq!(server.state, SocketState::Connected);

        // Purposefully read from UDP socket directly and discard it, in order
        // to behave as if the packet was lost and thus trigger the timeout
        // handling in the *next* call to `UtpSocket.recv_from`.
        iotry!(server.socket.recv_from(&mut buf));

        // Set a much smaller than usual timeout, for quicker test completion
        server.congestion_timeout = 50;

        // Now wait for the previously discarded packet
        loop {
            let response = server.recv_from(&mut buf).await;
            match response {
                Ok((0, _)) => continue,
                Ok(_) => break,
                Err(e) => panic!("{}", e),
            }
        }

        drop(server);
        child.await;
    }

    #[async_std::test]
    async fn test_sorted_buffer_insertion() {
        let server_addr = next_test_ip4();
        let mut socket = iotry!(UtpSocket::bind(server_addr));

        let mut packet = Packet::new();
        packet.set_seq_nr(1);

        assert!(socket.incoming_buffer.is_empty());

        socket.insert_into_buffer(packet.clone());
        assert_eq!(socket.incoming_buffer.len(), 1);

        packet.set_seq_nr(2);
        packet.set_timestamp(128.into());

        socket.insert_into_buffer(packet.clone());
        assert_eq!(socket.incoming_buffer.len(), 2);
        assert_eq!(socket.incoming_buffer[1].seq_nr(), 2);
        assert_eq!(socket.incoming_buffer[1].timestamp(), 128.into());

        packet.set_seq_nr(3);
        packet.set_timestamp(256.into());

        socket.insert_into_buffer(packet.clone());
        assert_eq!(socket.incoming_buffer.len(), 3);
        assert_eq!(socket.incoming_buffer[2].seq_nr(), 3);
        assert_eq!(socket.incoming_buffer[2].timestamp(), 256.into());

        // Replacing a packet with a more recent version doesn't work
        packet.set_seq_nr(2);
        packet.set_timestamp(456.into());

        socket.insert_into_buffer(packet.clone());
        assert_eq!(socket.incoming_buffer.len(), 3);
        assert_eq!(socket.incoming_buffer[1].seq_nr(), 2);
        assert_eq!(socket.incoming_buffer[1].timestamp(), 128.into());
    }

    #[async_std::test]
    async fn test_duplicate_packet_handling() {
        let (server_addr, client_addr) = (next_test_ip4(), next_test_ip4());

        let client = iotry!(UtpSocket::bind(client_addr));
        let mut server = iotry!(UtpSocket::bind(server_addr));

        assert_eq!(server.state, SocketState::New);
        assert_eq!(client.state, SocketState::New);

        // Check proper difference in client's send connection id and receive connection id
        assert_eq!(
            client.sender_connection_id,
            client.receiver_connection_id + 1
        );

        let child = task::spawn(async move {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            assert_eq!(client.state, SocketState::Connected);

            let mut packet = Packet::with_payload(&[1, 2, 3]);
            packet.set_wnd_size(BUF_SIZE as u32);
            packet.set_connection_id(client.sender_connection_id);
            packet.set_seq_nr(client.seq_nr);
            packet.set_ack_nr(client.ack_nr);

            // Send two copies of the packet, with different timestamps
            for _ in 0..2 {
                packet.set_timestamp(now_microseconds());
                iotry!(client.socket.send_to(packet.as_ref(), server_addr));
            }
            client.seq_nr += 1;

            // Receive one ACK
            for _ in 0..1 {
                let mut buf = [0u8; BUF_SIZE];
                iotry!(client.socket.recv_from(&mut buf));
            }

            iotry!(client.close());
        });

        let mut buf = [0u8; BUF_SIZE];
        iotry!(server.recv(&mut buf));
        // After establishing a new connection, the server's ids are a mirror of the client's.
        assert_eq!(
            server.receiver_connection_id,
            server.sender_connection_id + 1
        );

        assert_eq!(server.state, SocketState::Connected);

        let expected: Vec<u8> = vec![1, 2, 3];
        let mut received: Vec<u8> = vec![];
        loop {
            match server.recv_from(&mut buf).await {
                Ok((0, _src)) => break,
                Ok((len, _src)) => received.extend(buf[..len].to_vec()),
                Err(e) => panic!("{:?}", e),
            }
        }
        assert_eq!(received.len(), expected.len());
        assert_eq!(received, expected);

        child.await;
    }

    #[async_std::test]
    async fn test_correct_packet_loss() {
        let server_addr = next_test_ip4();

        let mut server = iotry!(UtpSocket::bind(server_addr));
        const LEN: usize = 1024 * 10;
        let data = (0..LEN).map(|idx| idx as u8).collect::<Vec<u8>>();
        let to_send = data.clone();

        let child = task::spawn(async move {
            let mut client = iotry!(UtpSocket::connect(server_addr));

            // Send everything except the odd chunks
            let chunks = to_send[..].chunks(BUF_SIZE);
            let dst = client.connected_to;
            for (index, chunk) in chunks.enumerate() {
                let mut packet = Packet::with_payload(chunk);
                packet.set_seq_nr(client.seq_nr);
                packet.set_ack_nr(client.ack_nr);
                packet.set_connection_id(client.sender_connection_id);
                packet.set_timestamp(now_microseconds());

                if index % 2 == 0 {
                    iotry!(client.socket.send_to(packet.as_ref(), dst));
                }

                client.curr_window += packet.len() as u32;
                client.send_window.push(packet);
                client.seq_nr += 1;
            }

            iotry!(client.close());
        });

        let mut buf = [0u8; BUF_SIZE];
        let mut received: Vec<u8> = vec![];
        loop {
            match server.recv_from(&mut buf).await {
                Ok((0, _src)) => break,
                Ok((len, _src)) => received.extend(buf[..len].to_vec()),
                Err(e) => panic!("{}", e),
            }
        }
        assert_eq!(received.len(), data.len());
        assert_eq!(received, data);

        child.await;
    }

    #[async_std::test]
    async fn test_tolerance_to_small_buffers() {
        let server_addr = next_test_ip4();
        let mut server = iotry!(UtpSocket::bind(server_addr));
        const LEN: usize = 1024;
        let data = (0..LEN).map(|idx| idx as u8).collect::<Vec<u8>>();
        let to_send = data.clone();

        let child = task::spawn(async move {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            iotry!(client.send_to(&to_send[..]));
            iotry!(client.close());
        });

        let mut read = Vec::new();
        while server.state != SocketState::Closed {
            let mut small_buffer = [0; 512];
            match server.recv_from(&mut small_buffer).await {
                Ok((0, _src)) => break,
                Ok((len, _src)) => read.extend(small_buffer[..len].to_vec()),
                Err(e) => panic!("{}", e),
            }
        }

        assert_eq!(read.len(), data.len());
        assert_eq!(read, data);

        child.await;
    }

    #[async_std::test]
    async fn test_sequence_number_rollover() {
        let (server_addr, client_addr) = (next_test_ip4(), next_test_ip4());

        let mut server = iotry!(UtpSocket::bind(server_addr));

        const LEN: usize = BUF_SIZE * 4;
        let data = (0..LEN).map(|idx| idx as u8).collect::<Vec<u8>>();
        let to_send = data.clone();

        let child = task::spawn(async move {
            let mut client = iotry!(UtpSocket::bind(client_addr));

            // Advance socket's sequence number
            client.seq_nr = ::std::u16::MAX - (to_send.len() / (BUF_SIZE * 2)) as u16;

            let mut client = iotry!(UtpSocket::connect(server_addr));
            // Send enough data to rollover
            iotry!(client.send_to(&to_send[..]));
            // Check that the sequence number did rollover
            assert!(client.seq_nr < 50);
            // Close connection
            iotry!(client.close());
        });

        let mut buf = [0u8; BUF_SIZE];
        let mut received: Vec<u8> = vec![];
        loop {
            match server.recv_from(&mut buf).await {
                Ok((0, _src)) => break,
                Ok((len, _src)) => received.extend(buf[..len].to_vec()),
                Err(e) => panic!("{}", e),
            }
        }
        assert_eq!(received.len(), data.len());
        assert_eq!(received, data);

        child.await;
    }

    #[async_std::test]
    async fn test_drop_unused_socket() {
        let server_addr = next_test_ip4();
        let server = iotry!(UtpSocket::bind(server_addr));

        // Explicitly dropping socket. This test should not hang.
        drop(server);
    }

    #[async_std::test]
    async fn test_invalid_packet_on_connect() {
        use async_std::net::UdpSocket;
        let server_addr = next_test_ip4();
        let server = iotry!(UdpSocket::bind(server_addr));

        let child = task::spawn(async move {
            let mut buf = [0u8; BUF_SIZE];
            match server.recv_from(&mut buf).await {
                Ok((_len, client_addr)) => {
                    iotry!(server.send_to(&[], client_addr));
                }
                _ => panic!(),
            }
        });

        match UtpSocket::connect(server_addr).await {
            Err(ref e) if e.kind() == ErrorKind::Other => (), // OK
            Err(e) => panic!("Expected ErrorKind::Other, got {:?}", e),
            Ok(_) => panic!("Expected Err, got Ok"),
        }

        child.await;
    }

    #[async_std::test]
    async fn test_receive_unexpected_reply_type_on_connect() {
        use async_std::net::UdpSocket;
        let server_addr = next_test_ip4();
        let server = iotry!(UdpSocket::bind(server_addr));

        let child = task::spawn(async move {
            let mut buf = [0u8; BUF_SIZE];
            let mut packet = Packet::new();
            packet.set_type(PacketType::Data);

            match server.recv_from(&mut buf).await {
                Ok((_len, client_addr)) => {
                    iotry!(server.send_to(packet.as_ref(), client_addr));
                }
                _ => panic!(),
            }
        });

        match UtpSocket::connect(server_addr).await {
            Err(ref e) if e.kind() == ErrorKind::ConnectionRefused => (), // OK
            Err(e) => panic!("Expected ErrorKind::ConnectionRefused, got {:?}", e),
            Ok(_) => panic!("Expected Err, got Ok"),
        }

        child.await;
    }

    #[async_std::test]
    async fn test_receiving_syn_on_established_connection() {
        // Establish connection
        let server_addr = next_test_ip4();
        let mut server = iotry!(UtpSocket::bind(server_addr));

        let child = task::spawn(async move {
            let mut buf = [0; BUF_SIZE];
            loop {
                match server.recv_from(&mut buf).await {
                    Ok((0, _src)) => break,
                    Ok(_) => (),
                    Err(e) => panic!("{:?}", e),
                }
            }
        });

        let mut client = iotry!(UtpSocket::connect(server_addr));
        let mut packet = Packet::new();
        packet.set_wnd_size(BUF_SIZE as u32);
        packet.set_type(PacketType::Syn);
        packet.set_connection_id(client.sender_connection_id);
        packet.set_seq_nr(client.seq_nr);
        packet.set_ack_nr(client.ack_nr);
        iotry!(client.socket.send_to(packet.as_ref(), server_addr));
        let mut buf = [0u8; BUF_SIZE];
        match client.socket.recv_from(&mut buf).await {
            Ok((len, _src)) => {
                let reply = Packet::try_from(&buf[..len]).ok().unwrap();
                assert_eq!(reply.get_type(), PacketType::Reset);
            }
            Err(e) => panic!("{:?}", e),
        }
        iotry!(client.close());

        child.await;
    }

    #[async_std::test]
    async fn test_receiving_reset_on_established_connection() {
        // Establish connection
        let server_addr = next_test_ip4();
        let mut server = iotry!(UtpSocket::bind(server_addr));

        let child = task::spawn(async move {
            let client = iotry!(UtpSocket::connect(server_addr));
            let mut packet = Packet::new();
            packet.set_wnd_size(BUF_SIZE as u32);
            packet.set_type(PacketType::Reset);
            packet.set_connection_id(client.sender_connection_id);
            packet.set_seq_nr(client.seq_nr);
            packet.set_ack_nr(client.ack_nr);
            iotry!(client.socket.send_to(packet.as_ref(), server_addr));
            let mut buf = [0u8; BUF_SIZE];
            match client.socket.recv_from(&mut buf).await {
                Ok((_len, _src)) => (),
                Err(e) => panic!("{:?}", e),
            }
        });

        let mut buf = [0u8; BUF_SIZE];
        loop {
            match server.recv_from(&mut buf).await {
                Ok((0, _src)) => break,
                Ok(_) => (),
                Err(ref e) if e.kind() == ErrorKind::ConnectionReset => return,
                Err(e) => panic!("{:?}", e),
            }
        }
        child.await;
        panic!("Should have received Reset");
    }

    #[cfg(not(windows))]
    #[async_std::test]
    async fn test_premature_fin() {
        let (server_addr, client_addr) = (next_test_ip4(), next_test_ip4());
        let mut server = iotry!(UtpSocket::bind(server_addr));

        const LEN: usize = BUF_SIZE * 4;
        let data = (0..LEN).map(|idx| idx as u8).collect::<Vec<u8>>();
        let to_send = data.clone();

        let child = task::spawn(async move {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            iotry!(client.send_to(&to_send[..]));
            iotry!(client.close());
        });

        let mut buf = [0u8; BUF_SIZE];

        // Accept connection
        iotry!(server.recv(&mut buf));

        // Send FIN without acknowledging packets received
        let mut packet = Packet::new();
        packet.set_connection_id(server.sender_connection_id);
        packet.set_seq_nr(server.seq_nr);
        packet.set_ack_nr(server.ack_nr);
        packet.set_timestamp(now_microseconds());
        packet.set_type(PacketType::Fin);
        iotry!(server.socket.send_to(packet.as_ref(), client_addr));

        // Receive until end
        let mut received: Vec<u8> = vec![];
        loop {
            match server.recv_from(&mut buf).await {
                Ok((0, _src)) => break,
                Ok((len, _src)) => received.extend(buf[..len].to_vec()),
                Err(e) => panic!("{}", e),
            }
        }
        assert_eq!(received.len(), data.len());
        assert_eq!(received, data);

        child.await;
    }

    #[async_std::test]
    async fn test_base_delay_calculation() {
        let minute_in_microseconds = 60 * 10i64.pow(6);
        let samples = vec![
            (0, 10),
            (1, 8),
            (2, 12),
            (3, 7),
            (minute_in_microseconds + 1, 11),
            (minute_in_microseconds + 2, 19),
            (minute_in_microseconds + 3, 9),
        ];
        let addr = next_test_ip4();
        let mut socket = UtpSocket::bind(addr).await.unwrap();

        for (timestamp, delay) in samples {
            socket.update_base_delay(delay.into(), ((timestamp + delay) as u32).into());
        }

        let expected = vec![7i64, 9i64]
            .into_iter()
            .map(Into::into)
            .collect::<Vec<_>>();
        let actual = socket.base_delays.iter().cloned().collect::<Vec<_>>();
        assert_eq!(expected, actual);
        assert_eq!(
            socket.min_base_delay(),
            expected.iter().min().cloned().unwrap_or_default()
        );
    }

    #[async_std::test]
    async fn test_local_addr() {
        let addr = next_test_ip4();
        let addr = addr.to_socket_addrs().unwrap().next().unwrap();
        let socket = UtpSocket::bind(addr).await.unwrap();

        assert!(socket.local_addr().is_ok());
        assert_eq!(socket.local_addr().unwrap(), addr);
    }

    #[async_std::test]
    async fn test_listener_local_addr() {
        let addr = next_test_ip4();
        let addr = addr.to_socket_addrs().unwrap().next().unwrap();
        let listener = UtpListener::bind(addr).await.unwrap();

        assert!(listener.local_addr().is_ok());
        assert_eq!(listener.local_addr().unwrap(), addr);
    }

    #[async_std::test]
    async fn test_peer_addr() {
        use std::sync::mpsc::channel;
        let addr = next_test_ip4();
        let server_addr = addr.to_socket_addrs().unwrap().next().unwrap();
        let mut server = UtpSocket::bind(server_addr).await.unwrap();
        let (tx, rx) = channel();

        // `peer_addr` should return an error because the socket isn't connected yet
        assert!(server.peer_addr().is_err());

        let child = task::spawn(async move {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            let mut buf = [0; 1024];
            tx.send(client.local_addr()).unwrap();
            iotry!(client.recv_from(&mut buf));
        });

        // Wait for a connection to be established
        let mut buf = [0; 1024];
        iotry!(server.recv(&mut buf));

        // `peer_addr` should succeed and be equal to the client's address
        assert!(server.peer_addr().is_ok());
        // The client is expected to be bound to "0.0.0.0", so we can only check if the port is
        // correct
        let client_addr = rx.recv().unwrap().unwrap();
        assert_eq!(server.peer_addr().unwrap().port(), client_addr.port());

        // Close the connection
        iotry!(server.close());

        // `peer_addr` should now return an error because the socket is closed
        assert!(server.peer_addr().is_err());

        child.await;
    }

    #[async_std::test]
    async fn test_take_address() {
        // Expected successes
        assert!(take_address("0.0.0.0:0").await.is_ok());
        assert!(take_address("[::]:0").await.is_ok());
        assert!(take_address(("0.0.0.0", 0)).await.is_ok());
        assert!(take_address(("::", 0)).await.is_ok());
        assert!(take_address(("1.2.3.4", 5)).await.is_ok());

        // Expected failures
        assert!(take_address("999.0.0.0:0").await.is_err());
        assert!(take_address("1.2.3.4:70000").await.is_err());
        assert!(take_address("").await.is_err());
        assert!(take_address("this is not an address").await.is_err());
        assert!(take_address("no.dns.resolution.com").await.is_err());
    }

    // Test reaction to connection loss when sending data packets
    #[async_std::test]
    async fn test_connection_loss_data() {
        let server_addr = next_test_ip4();
        let mut server = iotry!(UtpSocket::bind(server_addr));
        // Decrease timeouts for faster tests
        server.congestion_timeout = 1;
        let attempts = server.max_retransmission_retries;

        let child = task::spawn(async move {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            iotry!(client.send_to(&[0]));
            // Simulate connection loss by killing the socket.
            client.state = SocketState::Closed;
            let ref socket = client.socket;
            let mut buf = [0u8; BUF_SIZE];
            iotry!(socket.recv_from(&mut buf));
            for _ in 0..attempts {
                match socket.recv_from(&mut buf).await {
                    Ok((len, _src)) => assert_eq!(
                        Packet::try_from(&buf[..len]).unwrap().get_type(),
                        PacketType::Data
                    ),
                    Err(e) => panic!("{}", e),
                }
            }
        });

        // Drain incoming packets
        let mut buf = [0u8; BUF_SIZE];
        iotry!(server.recv_from(&mut buf));

        iotry!(server.send_to(&[0]));

        // Try to receive ACKs, time out too many times on flush, and fail with `TimedOut`
        let mut buf = [0u8; BUF_SIZE];
        match server.recv(&mut buf).await {
            Err(ref e) if e.kind() == ErrorKind::TimedOut => (),
            x => panic!("Expected Err(TimedOut), got {:?}", x),
        }

        child.await;
    }

    // Test reaction to connection loss when sending FIN
    #[async_std::test]
    async fn test_connection_loss_fin() {
        let server_addr = next_test_ip4();
        let mut server = iotry!(UtpSocket::bind(server_addr));
        // Decrease timeouts for faster tests
        server.congestion_timeout = 1;
        let attempts = server.max_retransmission_retries;

        let child = task::spawn(async move {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            iotry!(client.send_to(&[0]));
            // Simulate connection loss by killing the socket.
            client.state = SocketState::Closed;
            let ref socket = client.socket;
            let mut buf = [0u8; BUF_SIZE];
            iotry!(socket.recv_from(&mut buf));
            for _ in 0..attempts {
                match socket.recv_from(&mut buf).await {
                    Ok((len, _src)) => assert_eq!(
                        Packet::try_from(&buf[..len]).unwrap().get_type(),
                        PacketType::Fin
                    ),
                    Err(e) => panic!("{}", e),
                }
            }
        });

        // Drain incoming packets
        let mut buf = [0u8; BUF_SIZE];
        iotry!(server.recv_from(&mut buf));

        // Send FIN, time out too many times, and fail with `TimedOut`
        match server.close().await {
            Err(ref e) if e.kind() == ErrorKind::TimedOut => (),
            x => panic!("Expected Err(TimedOut), got {:?}", x),
        }
        child.await;
    }

    // Test reaction to connection loss when waiting for data packets
    #[async_std::test]
    async fn test_connection_loss_waiting() {
        let server_addr = next_test_ip4();
        let mut server = iotry!(UtpSocket::bind(server_addr));
        // Decrease timeouts for faster tests
        server.congestion_timeout = 1;
        let attempts = server.max_retransmission_retries;

        task::spawn(async move {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            iotry!(client.send_to(&[0]));
            // Simulate connection loss by killing the socket.
            client.state = SocketState::Closed;
            let ref socket = client.socket;
            let seq_nr = client.seq_nr;
            let mut buf = [0u8; BUF_SIZE];
            for _ in 0..(3 * attempts) {
                match socket.recv_from(&mut buf).await {
                    Ok((len, _src)) => {
                        let packet = Packet::try_from(&buf[..len]).unwrap();
                        assert_eq!(packet.get_type(), PacketType::State);
                        assert_eq!(packet.ack_nr(), seq_nr - 1);
                    }
                    Err(e) => panic!("{}", e),
                }
            }
        });

        // Drain incoming packets
        let mut buf = [0; BUF_SIZE];
        iotry!(server.recv_from(&mut buf));

        // Try to receive data, time out too many times, and fail with `TimedOut`
        let mut buf = [0; BUF_SIZE];
        match server.recv_from(&mut buf).await {
            Err(ref e) if e.kind() == ErrorKind::TimedOut => (),
            x => panic!("Expected Err(TimedOut), got {:?}", x),
        }
    }
}