utp/

socket.rs

use std::cmp::{min, max};
use std::collections::VecDeque;
use std::net::{ToSocketAddrs, SocketAddr, UdpSocket};
use std::io::{Result, Error, ErrorKind};
use util::{now_microseconds, ewma, Sequence};
use packet::{Packet, PacketType, Encodable, Decodable, ExtensionType, HEADER_SIZE};
use rand::{self, Rng};
use time::SteadyTime;
use time;
use std::time::Duration;

// For simplicity's sake, let us assume no packet will ever exceed the
// Ethernet maximum transfer unit of 1500 bytes.
const BUF_SIZE: usize = 1500;
const GAIN: f64 = 1.0;
const ALLOWED_INCREASE: u32 = 1;
const TARGET: i64 = 100_000; // 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

// 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: i64 = 60_000_000;

#[derive(Debug)]
pub enum SocketError {
    ConnectionClosed,
    ConnectionReset,
    ConnectionTimedOut,
    UserTimedOut,
    InvalidAddress,
    InvalidPacket,
    InvalidReply,
    NotConnected,
}

impl From<SocketError> for Error {
    fn from(error: SocketError) -> Error {
        use self::SocketError::*;
        let (kind, message) = match error {
            ConnectionClosed => (ErrorKind::NotConnected, "The socket is closed"),
            ConnectionReset => {
                (ErrorKind::ConnectionReset,
                 "Connection reset by remote peer")
            }
            ConnectionTimedOut | UserTimedOut => (ErrorKind::TimedOut, "Connection timed out"),
            InvalidAddress => (ErrorKind::InvalidInput, "Invalid address"),
            InvalidPacket => (ErrorKind::Other, "Error parsing packet"),
            InvalidReply => {
                (ErrorKind::ConnectionRefused,
                 "The remote peer sent an invalid reply")
            }
            NotConnected => (ErrorKind::NotConnected, "The socket is not connected"),
        };
        Error::new(kind, message)
    }
}

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

struct DelayDifferenceSample {
    received_at: i64,
    difference: i64,
}

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

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
}

/// 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
/// use utp::UtpSocket;
///
/// let mut socket = UtpSocket::bind("127.0.0.1:1234").expect("Error binding socket");
///
/// let mut buf = [0; 1000];
/// let (amt, _src) = socket.recv_from(&mut buf).expect("Error receiving");
///
/// let mut buf = &mut buf[..amt];
/// buf.reverse();
/// let _ = socket.send_to(buf).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();
/// ```
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: u32,

    /// 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: VecDeque<u8>,

    /// Another buffer of data to be returned in recv_from
    /// this comes before pending_data
    read_ready_data: VecDeque<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<i64>,

    /// 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: u32,

    /// Start of the current minute for sampling purposes
    last_rollover: i64,

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

    /// Congestion window in bytes
    cwnd: u32,

    /// Maximum retransmission retries
    pub max_retransmission_retries: u32,

    /// Used by `set_read_timeout`.
    user_read_timeout: u64,

    /// The last time congestion algorithm was updated/handled-a-timeout
    last_congestion_update: SteadyTime,

    retries: u32,

    /// The first 'State' packet we sent if we are a server (it may
    /// need to be resent if the network dropped it).
    state_packet: Option<Packet>,

    /// The last time we've sent something over the wire
    last_msg_sent_timestamp: SteadyTime,
}

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 {
        // Safely generate the two sequential connection identifiers.
        // This avoids an overflow when the generated receiver identifier is the largest
        // representable value in u16 and it is incremented to yield the corresponding sender
        // identifier.
        let (receiver_id, sender_id) =
            || -> (u16, u16) {
                let mut rng = rand::thread_rng();
                loop {
                    let id = rng.gen::<u16>();
                    if id.checked_add(1).is_some() {
                        return (id, id + 1);
                    }
                }
            }();

        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: 0,
            last_dropped: 0,
            rtt: 0,
            rtt_variance: 0,
            read_ready_data: VecDeque::new(),
            pending_data: VecDeque::new(),
            curr_window: 0,
            remote_wnd_size: 0,
            current_delays: Vec::new(),
            base_delays: VecDeque::with_capacity(BASE_HISTORY),
            their_delay: 0,
            last_rollover: 0,
            congestion_timeout: INITIAL_CONGESTION_TIMEOUT,
            cwnd: INIT_CWND * MSS,
            max_retransmission_retries: MAX_RETRANSMISSION_RETRIES,
            user_read_timeout: 0,
            last_congestion_update: SteadyTime::now(),
            retries: 0,
            state_packet: None,
            last_msg_sent_timestamp: SteadyTime::now(),
        }
    }

    /// Creates a new UTP socket from the given UDP socket.
    pub fn bind_with_udp_socket(socket: UdpSocket) -> Result<UtpSocket> {
        socket.local_addr().map(|a| UtpSocket::from_raw_parts(socket, a))
    }

    /// 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 fn bind<A: ToSocketAddrs>(addr: A) -> Result<UtpSocket> {
        take_address(addr).and_then(|a| UdpSocket::bind(a).map(|s| UtpSocket::from_raw_parts(s, a)))
    }

    /// 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(Error::from(SocketError::NotConnected))
        }
    }

    /// 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 fn connect<A: ToSocketAddrs>(other: A) -> Result<UtpSocket> {
        let addr = try!(take_address(other));
        let my_addr = match addr {
            SocketAddr::V4(_) => "0.0.0.0:0",
            SocketAddr::V6(_) => ":::0",
        };
        let mut socket = try!(UtpSocket::bind(my_addr));
        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 buf = [0; BUF_SIZE];
        let mut syn_timeout = socket.congestion_timeout;
        let mut syn_retries = 0;

        while syn_retries < MAX_SYN_RETRIES {
            packet.set_timestamp_microseconds(now_microseconds());

            // Send packet
            debug!("Connecting to {}", socket.connected_to);
            try!(socket.socket.send_to(&packet.to_bytes()[..], socket.connected_to));
            socket.state = SocketState::SynSent;
            debug!("sent {:?}", packet);

            // Validate response
            socket.socket
                  .set_read_timeout(Some(Duration::from_millis(syn_timeout)))
                  .expect("Error setting read timeout");
            match socket.socket.recv_from(&mut buf) {
                Ok((read, addr)) => {
                    let packet = try!(Packet::from_bytes(&buf[..read]).or(Err(SocketError::InvalidPacket)));

                    socket.connected_to = addr;

                    if packet.get_type() != PacketType::State {
                        // The network might have dropped the `State` packet
                        // from the peer, so we need to ask for it again.
                        syn_retries += 1;
                        continue;
                    }

                    try!(socket.handle_packet(&packet, addr));

                    return Ok(socket);
                },
                Err(ref e) if (e.kind() == ErrorKind::WouldBlock ||
                               e.kind() == ErrorKind::TimedOut) => {
                    debug!("Timed out, retrying");
                    syn_timeout *= 2;
                    syn_retries += 1;
                    continue;
                }
                Err(e) => return Err(e),
            };
        }

        Err(Error::from(SocketError::ConnectionTimedOut))
    }

    /// If you have already prepared UDP sockets at each end (e.g. you're doing
    /// hole punching), then the rendezvous connection setup is your choice.
    ///
    /// Rendezvous connection will only use the specified socket and addresses,
    /// but each end must call `rendezvous_connect` itself.
    ///
    /// This is an unofficial extension to the uTP protocol. Both peers will try
    /// to act as initiator and acceptor sockets. Then, the connection id which
    /// is numerically lower decides which end will assume which role (initiator
    /// or acceptor).
    pub fn rendezvous_connect<A: ToSocketAddrs>(udp_socket: UdpSocket,
                                                other: A)
                                                -> Result<UtpSocket> {
        let addr = try!(take_address(other));
        let mut socket = try!(UtpSocket::bind_with_udp_socket(udp_socket));
        socket.rendezvous_connect_to(addr).map(|_| socket)
    }

    fn rendezvous_connect_to(&mut self, addr: SocketAddr) -> Result<()> {
        self.connected_to = addr;

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

        let mut buf = [0; BUF_SIZE];

        let mut syn_timeout = self.congestion_timeout;
        let mut retry_count = 0;

        let mut rx_syn: Option<Packet> = None;
        let mut rx_state: Option<Packet> = None;

        while retry_count < MAX_SYN_RETRIES {
            packet.set_timestamp_microseconds(now_microseconds());

            // Send packet
            debug!("Connecting to {}", self.connected_to);
            try!(self.socket.send_to(&packet.to_bytes()[..], self.connected_to));
            self.last_msg_sent_timestamp = SteadyTime::now();
            self.state = SocketState::SynSent;
            debug!("sent {:?}", packet);

            try!(self.socket.set_read_timeout(Some(Duration::from_millis(syn_timeout))));

            // Validate response
            match self.socket.recv_from(&mut buf) {
                Ok((read, src)) => {
                    let mut packet = match Packet::from_bytes(&buf[..read]) {
                        Ok(packet) => packet,
                        Err(_) => {
                            continue;
                        }
                    };

                    let cid = min(self.receiver_connection_id, packet.connection_id());

                    packet.set_connection_id(cid);

                    // Would be nicer to handle this in the handle_packet
                    // function, but we'd need to add new socket state
                    // not to interfere with SocketState::New and
                    // SocketState::SynSent states.
                    match packet.get_type() {
                        PacketType::Syn => {
                            self.receiver_connection_id = cid;
                            self.sender_connection_id = cid + 1;

                            let reply = self.prepare_reply(&packet, PacketType::State);
                            try!(self.socket.send_to(&reply.to_bytes()[..], self.connected_to));
                            self.last_msg_sent_timestamp = SteadyTime::now();

                            rx_syn = Some(packet);
                        }
                        PacketType::State => {
                            self.receiver_connection_id = cid;
                            self.sender_connection_id = cid + 1;

                            rx_state = Some(packet);
                        }
                        _ => continue,
                    }

                    match (&rx_syn, &rx_state) {
                        (&Some(ref _syn), &Some(ref state)) => {
                            try!(self.handle_packet(state, src));
                            return Ok(());
                        }
                        _ => continue,
                    }
                }
                Err(ref e) if (e.kind() == ErrorKind::WouldBlock ||
                               e.kind() == ErrorKind::TimedOut) => {
                    debug!("Timed out, retrying");
                    syn_timeout *= 2;
                    retry_count += 1;
                    continue;
                }
                Err(e) => return Err(e),
            };
        }

        Err(Error::from(SocketError::ConnectionTimedOut))
    }

    /// Gracefully closes connection to peer.
    ///
    /// This method allows both peers to receive all packets still in
    /// flight.
    pub 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
        try!(self.flush());

        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_microseconds(now_microseconds());
        packet.set_type(PacketType::Fin);

        // Send FIN
        try!(self.socket.send_to(&packet.to_bytes()[..], self.connected_to));
        self.last_msg_sent_timestamp = SteadyTime::now();
        debug!("sent {:?}", packet);
        self.state = SocketState::FinSent;

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

        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 fn recv_from(&mut self, buf: &mut [u8]) -> Result<(usize, SocketAddr)> {
        let read = {
            let (read_ready_0, read_ready_1) = self.read_ready_data.as_slices();
            let mut read = 0;
            if read_ready_0.len() > 0 {
                read = unsafe_copy(read_ready_0, buf)
            }
            if read_ready_1.len() > 0 {
                read += unsafe_copy(read_ready_1, &mut buf[read..])
            }
            read
        };
        if read > 0 {
            self.read_ready_data.drain(..read);
            return Ok((read, self.connected_to));
        }

        let read = self.flush_incoming_buffer(buf);

        if read > 0 {
            Ok((read, self.connected_to))
        } else {
            // 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(Error::from(SocketError::ConnectionReset));
            }

            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, true) {
                    Ok((0, _src)) => continue,
                    Ok(x) => return Ok(x),
                    Err(e) => return Err(e),
                }
            }
        }
    }

    /// Changes read operations to block for at most the specified number of
    /// milliseconds.
    pub fn set_read_timeout(&mut self, user_timeout: Option<u64>) {
        self.user_read_timeout = match user_timeout {
            Some(t) => {
                if t > 0 {
                    t
                } else {
                    0
                }
            }
            None => 0,
        }
    }

    #[cfg(windows)]
    fn ignore_udp_error(e: &Error) -> bool {
        // On Windows, the recv_from operation on the UDP socket may return the
        // following errors, which are expected and should be ignored:
        //
        // - 10054 (WSAECONNRESET): Windows can send this error if a previous
        //   send operation resulted in an ICMP Port Unreachable. And if it's a
        //   loopback interface, it can know whether there is already another
        //   end to communicate.
        // - 10040 (WSAEMSGSIZE): This error was randomly appearing in a test
        //   that I conducted. Not really sure why it's happening. The frequency
        //   decreased when I increased the receive buffer size, but it was not
        //   important to get a network up and running.
        //
        // Without these changes, it was impossible to get a relatively large
        // network running without issues. By large I mean a test that might be
        // too bursting for a single machine to run.
        //
        // More references:
        //
        // - http://stackoverflow.com/questions/30749423/is-winsock-error-10054-wsaeconnreset-normal-with-udp-to-from-localhost#comment49588739_30749423
        // - https://github.com/maidsafe/crust/pull/454
        const WSAECONNRESET: i32 = 10054;
        const WSAEMSGSIZE: i32 = 10040;
        match e.raw_os_error() {
            Some(e) => match e {
                WSAECONNRESET | WSAEMSGSIZE => true,
                _ => false,
            },
            None => false,
        }
    }

    #[cfg(not(windows))]
    fn ignore_udp_error(_: &Error) -> bool {
        false
    }

    fn recv(&mut self, buf: &mut [u8], use_user_timeout: bool) -> Result<(usize, SocketAddr)> {
        let mut b = [0; BUF_SIZE + HEADER_SIZE];
        let now = SteadyTime::now();
        let (read, src);
        let user_timeout = if use_user_timeout {
            self.user_read_timeout
        } else {
            0
        };
        let use_user_timeout = user_timeout != 0;

        // Try to receive a packet and handle timeouts
        loop {
            // Abort loop if the current try exceeds the maximum number of retransmission retries.
            if self.retries >= self.max_retransmission_retries {
                debug!("exceeds max_retransmission_retries : {} ; current connect state is : {:?}",
                       self.max_retransmission_retries,
                       self.state);
                self.state = SocketState::Closed;
                debug!("socket marked as closed from {:?} to {:?}",
                       self.local_addr(),
                       self.connected_to);
                return Err(Error::from(SocketError::ConnectionTimedOut));
            }

            let timeout;
            let congestion_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 user_timeout = Duration::from_millis(user_timeout);
                timeout = if use_user_timeout {
                    match congestion_timeout {
                        Some(congestion_timeout) => {
                            use std::cmp::min;
                            Some(min(congestion_timeout, user_timeout))
                        }
                        None => Some(user_timeout),
                    }
                } else {
                    congestion_timeout
                };
            }

            if use_user_timeout {
                let user_timeout = time::Duration::milliseconds(user_timeout as i64);
                if (SteadyTime::now() - now) >= user_timeout {
                    return Err(Error::from(SocketError::UserTimedOut));
                }
            }

            self.socket.set_read_timeout(timeout).expect("Error setting read timeout");
            match self.socket.recv_from(&mut b) {
                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");
                    let now = SteadyTime::now();
                    let congestion_timeout = {
                        time::Duration::milliseconds(self.congestion_timeout as i64)
                    };
                    if !use_user_timeout ||
                       ((now - self.last_congestion_update) >= congestion_timeout) {
                        self.last_congestion_update = now;
                        try!(self.handle_receive_timeout());
                        self.retries += 1;
                    }
                }
                Err(ref e) if Self::ignore_udp_error(e) => (),
                Err(e) => return Err(e),
            };

            let elapsed = (SteadyTime::now() - now).num_milliseconds();
            debug!("{} ms elapsed", elapsed);
        }

        self.last_congestion_update = SteadyTime::now();
        self.retries = 0;

        // Decode received data into a packet
        let packet = match Packet::from_bytes(&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) = try!(self.handle_packet(&packet, src)) {
            pkt.set_wnd_size(BUF_SIZE as u32);
            try!(self.socket.send_to(&pkt.to_bytes()[..], src));
            self.last_msg_sent_timestamp = SteadyTime::now();
            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 {
            if Sequence::less(self.last_dropped, packet.seq_nr()) {
                self.insert_into_buffer(packet);
            }
        }

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

        Ok((read, src))
    }

    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_microseconds(now_microseconds());
                packet.set_type(PacketType::Fin);

                // Send FIN
                try!(self.socket.send_to(&packet.to_bytes()[..], self.connected_to));
                self.last_msg_sent_timestamp = SteadyTime::now();
                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();
            }
        } 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 mut packet = &mut self.send_window[0];
            packet.set_timestamp_microseconds(now_microseconds());
            try!(self.socket.send_to(&packet.to_bytes()[..], self.connected_to));
            self.last_msg_sent_timestamp = SteadyTime::now();
            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: u32 = now_microseconds();
        let other_t_micro: u32 = original.timestamp_microseconds();
        resp.set_timestamp_microseconds(self_t_micro);
        resp.set_timestamp_difference_microseconds(self_t_micro.wrapping_sub(other_t_micro));
        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 {
        // Return pending data from a partially read packet
        if !self.pending_data.is_empty() {
            let flushed = {
                let (pending_0, pending_1) = self.pending_data.as_slices();
                let mut flushed = 0;
                if pending_0.len() > 0 {
                    flushed += unsafe_copy(pending_0, buf);
                }
                if pending_1.len() > 0 {
                    flushed += unsafe_copy(pending_1, &mut buf[flushed..]);
                }
                flushed
            };

            if flushed == self.pending_data.len() {
                self.pending_data.clear();
                self.advance_incoming_buffer();
            } else {
                self.pending_data.drain(..flushed);
            }

            return flushed;
        }

        if !self.incoming_buffer.is_empty() &&
           (self.ack_nr == self.incoming_buffer[0].seq_nr() ||
            self.ack_nr.wrapping_add(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.extend(self.incoming_buffer[0].payload.drain(flushed..));
            }

            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 fn send_to(&mut self, buf: &[u8]) -> Result<usize> {
        if self.state == SocketState::Closed {
            return Err(Error::from(SocketError::ConnectionClosed));
        }

        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);

            // `OverflowingOps` is marked unstable, so we can't use `overflowing_add` here
            if self.seq_nr == ::std::u16::MAX {
                self.seq_nr = 0;
            } else {
                self.seq_nr += 1;
            }
        }

        // Send every packet in the queue
        try!(self.send());

        Ok(total_length)
    }

    /// Consumes acknowledgements for every pending packet.
    pub 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());
            try!(self.recv(&mut buf, false));
        }

        Ok(())
    }

    fn send_state(&mut self) {
        let mut packet = Packet::new();
        packet.set_type(PacketType::State);
        let self_t_micro: u32 = now_microseconds();
        packet.set_timestamp_microseconds(self_t_micro);
        packet.set_timestamp_difference_microseconds(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.to_bytes()[..], self.connected_to);
        self.last_msg_sent_timestamp = SteadyTime::now();
    }

    /// Send a keepalive packet on the stream.
    pub fn send_keepalive(&mut self) {
        if (SteadyTime::now() - self.last_msg_sent_timestamp).num_milliseconds()
            >= 14_000 {
            self.send_state();
        }
    }

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

    /// Send one packet.
    #[inline]
    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 {
            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 = [0; BUF_SIZE];
            let (read, _) = try!(self.recv(&mut buf, false));
            self.read_ready_data.extend(&buf[..read]);
        }
        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_microseconds(now_microseconds());
        packet.set_timestamp_difference_microseconds(self.their_delay);
        try!(self.socket.send_to(&packet.to_bytes()[..], self.connected_to));
        self.last_msg_sent_timestamp = SteadyTime::now();
        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: i64, now: i64) {
        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: i64, now: i64) {
        // Remove samples more than one RTT old
        let rtt = self.rtt as i64 * 100;
        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) -> i64 {
        let input = self.current_delays.iter().map(|x| x.difference);
        ewma(input, 0.333) as i64
    }

    /// Calculates the lowest base delay in the current window.
    fn min_base_delay(&self) -> i64 {
        self.base_delays.iter().min().cloned().unwrap_or(0)
    }

    /// 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.
    fn send_fast_resend_request(&mut self) {
        for _ in 0..3 {
            self.send_state();
        }
    }

    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);

                // 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(|pkt| pkt.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.
    fn handle_packet(&mut self, packet: &Packet, src: SocketAddr) -> Result<Option<Packet>> {
        debug!("({:?}, {:?})", self.state, packet.get_type());

        let is_data_or_fin = packet.get_type() == PacketType::Data
                          || packet.get_type() == PacketType::Fin;

        // Acknowledge only if the packet strictly follows the previous one
        // and only if it is a payload packet. The restriction on PacketType
        // is due to all other (non Data) packets are assigned seq_nr the
        // same as the next Data packet, thus we could acknowledge what
        // we have not received yet.
        if is_data_or_fin && 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 = now.wrapping_sub(packet.timestamp_microseconds());
        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.last_acked = self.seq_nr.wrapping_sub(1);
                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;

                self.state_packet = Some(self.prepare_reply(packet, PacketType::State));

                // Advance the self.seq_nr (the sequence number of the next packet),
                // this is because the other end will use the `seq_nr` of this state
                // packet as his `self.last_acked`
                self.seq_nr = self.seq_nr.wrapping_add(1);

                Ok(self.state_packet.clone())
            }
            (SocketState::Connected, PacketType::Syn) if self.connected_to == src => {
                // The other end might have sent another Syn packet because
                // a reply to the first one did not arrive within a timeout
                // caused by network congestion.
                Ok(self.state_packet.clone())
            }
            (_, 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_dropped = packet.seq_nr();
                self.last_acked_timestamp = now_microseconds();
                Ok(None)
            }
            (SocketState::SynSent, _) => Err(Error::from(SocketError::InvalidReply)),
            (SocketState::Connected, PacketType::Data) |
            (SocketState::FinSent, PacketType::Data) => Ok(self.handle_data_packet(packet)),
            (SocketState::Connected, PacketType::State) => {
                self.handle_state_packet(packet);
                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.len() > 0 {
                        reply.set_sack(sack);
                    }
                }

                // Give up, the remote peer might not care about our missing packets
                self.state = SocketState::Closed;
                Ok(Some(reply))
            }
            (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(Error::from(SocketError::ConnectionReset))
            }
            (state, ty) => {
                let message = format!("Unimplemented handling for ({:?},{:?})", state, ty);
                debug!("{}", message);
                Err(Error::new(ErrorKind::Other, message))
            }
        }
    }

    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.len() > 0 {
                reply.set_sack(sack);
            }
        }

        Some(reply)
    }

    fn queuing_delay(&self) -> i64 {
        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);
    }

    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() as i64;
            let our_delay = now - self.send_window[index].timestamp_microseconds() as i64;
            debug!("our_delay: {}", our_delay);
            self.update_base_delay(our_delay, now);
            self.update_current_delay(our_delay, now);

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

            self.update_congestion_window(off_target, bytes_newly_acked as u32);

            // Update congestion timeout
            let rtt = (TARGET - off_target as i64) / 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.iter() {
            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);
                    packet_loss_detected = true;
                }

                if let Some(last_seq_nr) = self.send_window.last().map(Packet::seq_nr) {
                    for seq_nr in extension.iter()
                                           .enumerate()
                                           .filter(|&(_idx, received)| !received)
                                           .map(|(idx, _received)| {
                                               packet.ack_nr() + 2 + idx as u16
                                           })
                                           .take_while(|&seq_nr| seq_nr < last_seq_nr) {
                        debug!("SACK: packet {} lost", seq_nr);
                        self.resend_lost_packet(seq_nr);
                        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.iter().any(|ext| ext.get_type() == ExtensionType::SelectiveAck) {
            self.resend_lost_packet(packet.ack_nr().wrapping_add(1));
        }

        // 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(|p| packet.seq_nr() > p.seq_nr()).unwrap_or(false) {
            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(|p| p.seq_nr() != packet.seq_nr()).unwrap_or(true) {
                self.incoming_buffer.insert(i, packet);
            }
        }
    }
}

impl Drop for UtpSocket {
    fn drop(&mut self) {
        let _ = self.close();
    }
}

/// A structure representing a socket server.
///
/// # Examples
///
/// ```no_run
/// use utp::{UtpListener, UtpSocket};
/// use std::thread;
///
/// fn handle_client(socket: UtpSocket) {
///     // ...
/// }
///
/// fn main() {
///     // Create a listener
///     let addr = "127.0.0.1:8080";
///     let listener = UtpListener::bind(addr).expect("Error binding socket");
///
///     for connection in listener.incoming() {
///         // Spawn a new handler for each new connection
///         if let Ok((socket, _src)) = connection {
///             thread::spawn(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 fn bind<A: ToSocketAddrs>(addr: A) -> Result<UtpListener> {
        UdpSocket::bind(addr).and_then(|s| Ok(UtpListener { socket: s }))
    }

    /// 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 fn accept(&self) -> Result<(UtpSocket, SocketAddr)> {
        let mut buf = [0; BUF_SIZE];

        match self.socket.recv_from(&mut buf) {
            Ok((nread, src)) => {
                let packet = try!(Packet::from_bytes(&buf[..nread])
                                      .or(Err(SocketError::InvalidPacket)));

                // Ignore non-SYN packets
                if packet.get_type() != PacketType::Syn {
                    return Err(Error::from(SocketError::InvalidPacket));
                }

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

                let mut socket = try!(inner_socket.map(|s| UtpSocket::from_raw_parts(s, src)));

                // Establish connection with remote peer
                match socket.handle_packet(&packet, src) {
                    Ok(Some(reply)) => try!(socket.socket.send_to(&reply.to_bytes()[..], src)),
                    Ok(None) => return Err(Error::from(SocketError::InvalidPacket)),
                    Err(e) => return Err(e),
                };

                Ok((socket, src))
            }
            Err(e) => Err(e),
        }
    }

    /// 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 }
    }

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

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

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

    fn next(&mut self) -> Option<Result<(UtpSocket, SocketAddr)>> {
        Some(self.listener.accept())
    }
}

#[cfg(test)]
mod test {
    use std::thread;
    use std::net::ToSocketAddrs;
    use std::io::ErrorKind;
    use super::{UtpSocket, UtpListener, SocketState, BUF_SIZE, take_address};
    use packet::{Packet, PacketType, Encodable, Decodable};
    use util::now_microseconds;
    use rand;

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

    fn next_test_port() -> u16 {
        use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT, Ordering};
        static NEXT_OFFSET: AtomicUsize = ATOMIC_USIZE_INIT;
        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())
    }

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

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

        let child = thread::spawn(move || {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            assert!(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) {
            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!(server.state == SocketState::Closed);
        drop(server);

        assert!(child.join().is_ok());
    }

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

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

        let child = thread::spawn(move || {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            assert!(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) {
            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!(server.state == SocketState::Closed);
        drop(server);

        assert!(child.join().is_ok());
    }

    #[test]
    fn test_rendezvous_connect() {
        use std::net::{UdpSocket, Ipv4Addr, SocketAddrV4};

        let peer1_udp_socket = iotry!(UdpSocket::bind("0.0.0.0:0"));
        let peer2_udp_socket = iotry!(UdpSocket::bind("0.0.0.0:0"));

        let peer1_port = iotry!(peer1_udp_socket.local_addr()).port();
        let peer1_addr = SocketAddrV4::new(Ipv4Addr::new(127, 0, 0, 1), peer1_port);

        let peer2_port = iotry!(peer2_udp_socket.local_addr()).port();
        let peer2_addr = SocketAddrV4::new(Ipv4Addr::new(127, 0, 0, 1), peer2_port);

        const BUF_LEN: u32 = 16777216;

        let tx_buffer: Vec<u8> = (0..BUF_LEN).map(|_| rand::random::<u8>()).collect();

        let t = thread::spawn(move || {
            let mut peer1 = iotry!(UtpSocket::rendezvous_connect(peer1_udp_socket, peer2_addr));
            let mut sent_total = 0;
            while sent_total < tx_buffer.len() {
                let chunk_size = rand::random::<u16>() as usize + 1;
                let slice_end = ::std::cmp::min(tx_buffer.len(), sent_total + chunk_size);
                let sent = peer1.send_to(&tx_buffer[sent_total..slice_end]).unwrap();
                sent_total += sent;
            }
            let r = peer1.flush();
            r.unwrap();
            let _ = peer1.close();
            tx_buffer
        });

        let mut peer2 = iotry!(UtpSocket::rendezvous_connect(peer2_udp_socket, peer1_addr));
        let mut rx_buffer: Vec<u8> = (0..BUF_LEN).into_iter().map(|_| 0u8).collect();
        let mut received_total = 0;
        while received_total < rx_buffer.len() {
            let chunk_size = rand::random::<u16>() as usize + 1;
            ::std::thread::sleep(::std::time::Duration::from_millis(1));
            let slice_end = ::std::cmp::min(rx_buffer.len(), received_total + chunk_size);
            let (received, _) = peer2.recv_from(&mut rx_buffer[received_total..slice_end]).unwrap();
            received_total += received;
        }
        let tx_buffer = t.join().unwrap();
        assert_eq!(tx_buffer, rx_buffer);
        let _ = peer2.close();
    }

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

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

        let child = thread::spawn(move || {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            assert!(client.state == SocketState::Connected);
            assert!(client.close().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);
        assert!(server.state == SocketState::Closed);

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

        assert!(child.join().is_ok());
    }

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

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

        let child = thread::spawn(move || {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            assert!(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) {
            Err(ref e) if e.kind() == ErrorKind::NotConnected => (),
            v => panic!("expected {:?}, got {:?}", ErrorKind::NotConnected, v),
        }

        assert!(child.join().is_ok());
    }

    #[test]
    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 = thread::spawn(move || {
            // Make the server listen for incoming connections
            let mut buf = [0u8; BUF_SIZE];
            let _resp = server.recv(&mut buf, false);
            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!(client.state == SocketState::Connected);
        let sender_seq_nr = rx.recv().unwrap();
        let ack_nr = client.ack_nr;
        assert!(ack_nr != 0);
        assert!(ack_nr.wrapping_add(1) == sender_seq_nr);
        assert!(client.close().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!(client.ack_nr == ack_nr);
        drop(client);

        assert!(child.join().is_ok());
    }

    #[test]
    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);
        assert!(response.is_ok());
        let response = response.unwrap();
        assert!(response.is_some());

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

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

        // Response acknowledges SYN
        assert!(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);
        assert!(response.is_ok());
        let response = response.unwrap();
        assert!(response.is_some());

        let response = response.unwrap();
        assert!(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!(response.connection_id() == initial_connection_id);
        assert!(response.connection_id() == packet.connection_id() - 1);

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

        // Responses with no payload should not increase the sequence number
        // unless it's the State packet sent to acknowledge the Syn packet as
        // explained at
        // <http://www.bittorrent.org/beps/bep_0029.html#connection-setup>
        assert!(response.payload.is_empty());
        assert!(response.seq_nr() == old_response.seq_nr().wrapping_add(1));
        // }

        // 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);
        assert!(response.is_ok());
        let response = response.unwrap();
        assert!(response.is_some());

        let response = response.unwrap();

        assert!(response.get_type() == PacketType::State);

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

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

        // FIN should be acknowledged
        assert!(response.ack_nr() == packet.seq_nr());

        // }
    }

    #[test]
    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);
        assert!(response.is_ok());
        let response = response.unwrap();
        assert!(response.is_some());
        let response = response.unwrap();
        assert!(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);
        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);
        assert!(response.is_ok());
        let response = response.unwrap();
        assert!(response.is_none());

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

    #[test]
    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);
        assert!(response.is_ok());
        let response = response.unwrap();
        assert!(response.is_some());
        assert!(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);
        assert!(response.is_ok());
        let response = response.unwrap();
        assert!(response.is_some());

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

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

    #[test]
    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);
        assert!(response.is_ok());
        let response = response.unwrap();
        assert!(response.is_some());
        let response = response.unwrap();
        assert!(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::new();
        packet.set_wnd_size(BUF_SIZE as u32);
        packet.set_type(PacketType::Data);
        packet.set_connection_id(initial_connection_id);
        packet.set_seq_nr(old_packet.seq_nr() + 1);
        packet.set_ack_nr(old_response.seq_nr());
        packet.payload = vec![1, 2, 3];
        window.push(packet);

        let mut packet = Packet::new();
        packet.set_wnd_size(BUF_SIZE as u32);
        packet.set_type(PacketType::Data);
        packet.set_connection_id(initial_connection_id);
        packet.set_seq_nr(old_packet.seq_nr() + 2);
        packet.set_ack_nr(old_response.seq_nr());
        packet.payload = vec![4, 5, 6];
        window.push(packet);

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

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

    #[test]
    fn test_socket_unordered_packets() {
        let server_addr = next_test_ip4();

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

        let child = thread::spawn(move || {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            assert!(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);
            let s = client.socket.try_clone().ok().expect("Error cloning internal UDP socket");
            let mut window: Vec<Packet> = Vec::new();

            for data in (1..13u8).collect::<Vec<u8>>()[..].chunks(3) {
                let mut packet = Packet::new();
                packet.set_wnd_size(BUF_SIZE as u32);
                packet.set_type(PacketType::Data);
                packet.set_connection_id(client.sender_connection_id);
                packet.set_seq_nr(client.seq_nr);
                packet.set_ack_nr(client.ack_nr);
                packet.payload = data.to_vec();
                window.push(packet.clone());
                client.send_window.push(packet.clone());
                client.seq_nr += 1;
                client.curr_window += packet.len() as u32;
            }

            let mut packet = Packet::new();
            packet.set_wnd_size(BUF_SIZE as u32);
            packet.set_type(PacketType::Fin);
            packet.set_connection_id(client.sender_connection_id);
            packet.set_seq_nr(client.seq_nr);
            packet.set_ack_nr(client.ack_nr);
            window.push(packet);
            client.seq_nr += 1;

            iotry!(s.send_to(&window[3].to_bytes()[..], server_addr));
            iotry!(s.send_to(&window[2].to_bytes()[..], server_addr));
            iotry!(s.send_to(&window[1].to_bytes()[..], server_addr));
            iotry!(s.send_to(&window[0].to_bytes()[..], server_addr));
            iotry!(s.send_to(&window[4].to_bytes()[..], server_addr));

            for _ in 0u8..2 {
                let mut buf = [0; BUF_SIZE];
                iotry!(s.recv_from(&mut buf));
            }
        });

        let mut buf = [0; BUF_SIZE];
        let expected: Vec<u8> = (1..13u8).collect();
        let mut received: Vec<u8> = vec![];
        loop {
            match server.recv_from(&mut buf) {
                Ok((0, _src)) => break,
                Ok((len, _src)) => received.extend(buf[..len].to_vec()),
                Err(e) => panic!("{:?}", 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);
        assert_eq!(received.len(), expected.len());
        assert_eq!(received, expected);

        assert!(child.join().is_ok());
    }

    #[test]
    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 = thread::spawn(move || {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            iotry!(client.send_to(&d[..]));
            iotry!(client.close());
        });

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

        // Receive data
        let data_packet = match server.socket.recv_from(&mut buf) {
            Ok((read, _src)) => iotry!(Packet::from_bytes(&buf[..read])),
            Err(e) => panic!("{}", e),
        };
        assert_eq!(data_packet.get_type(), PacketType::Data);
        assert_eq!(data_packet.payload, data);
        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 0u8..3 {
            iotry!(server.socket.send_to(&packet.to_bytes()[..], 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) {
            Ok((0, _)) => panic!("Received 0 bytes from socket"),
            Ok((read, _src)) => {
                let packet = iotry!(Packet::from_bytes(&buf[..read]));
                assert_eq!(packet.get_type(), PacketType::Data);
                assert_eq!(packet.seq_nr(), data_packet.seq_nr());
                assert!(packet.payload == data_packet.payload);
                let response = server.handle_packet(&packet, client_addr);
                assert!(response.is_ok());
                let response = response.unwrap();
                assert!(response.is_some());
                let response = response.unwrap();
                iotry!(server.socket.send_to(&response.to_bytes()[..], server.connected_to));
            }
            Err(e) => panic!("{}", e),
        }

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

        assert!(child.join().is_ok());
    }

    #[test]
    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!(server.state == SocketState::New);
        assert!(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 = thread::spawn(move || {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            assert!(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, false).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!(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 {
            match server.recv_from(&mut buf) {
                Ok((0, _)) => continue,
                Ok(_) => break,
                Err(e) => panic!("{}", e),
            }
        }

        drop(server);

        assert!(child.join().is_ok());
    }

    #[test]
    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_microseconds(128);

        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_microseconds(), 128);

        packet.set_seq_nr(3);
        packet.set_timestamp_microseconds(256);

        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_microseconds(), 256);

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

        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_microseconds(), 128);
    }

    #[test]
    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!(server.state == SocketState::New);
        assert!(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 = thread::spawn(move || {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            assert!(client.state == SocketState::Connected);

            let mut packet = Packet::new();
            packet.set_wnd_size(BUF_SIZE as u32);
            packet.set_type(PacketType::Data);
            packet.set_connection_id(client.sender_connection_id);
            packet.set_seq_nr(client.seq_nr);
            packet.set_ack_nr(client.ack_nr);
            packet.payload = vec![1, 2, 3];

            // Send two copies of the packet, with different timestamps
            for _ in 0u8..2 {
                packet.set_timestamp_microseconds(now_microseconds());
                iotry!(client.socket.send_to(&packet.to_bytes()[..], server_addr));
            }
            client.seq_nr += 1;

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

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

        let mut buf = [0u8; BUF_SIZE];
        iotry!(server.recv(&mut buf, false));
        // 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!(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) {
                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);

        assert!(child.join().is_ok());
    }

    // #[test]
    // #[ignore]
    // fn test_selective_ack_response() {
    //     let (server_addr, client_addr) = (next_test_ip4(), next_test_ip4());
    //     const LEN: usize = 1024 * 10;
    //     let data = (0..LEN).map(|idx| idx as u8).collect::<Vec<u8>>();
    //     let to_send = data.clone();

    //     // Client
    //     thread::spawn(move || {
    //         let client = iotry!(UtpSocket::bind(client_addr));
    //         let mut client = iotry!(UtpSocket::connect(server_addr));
    //         client.congestion_timeout = 50;

    //         iotry!(client.send_to(&to_send[..]));
    //         iotry!(client.close());
    //     });

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

    // let mut buf = [0; BUF_SIZE];

    //     // Connect
    //     iotry!(server.recv_from(&mut buf));

    //     // Discard packets
    //     iotry!(server.socket.recv_from(&mut buf));
    //     iotry!(server.socket.recv_from(&mut buf));
    //     iotry!(server.socket.recv_from(&mut buf));

    //     // Generate SACK
    //     let mut packet = Packet::new();
    //     packet.set_seq_nr(server.seq_nr);
    //     packet.set_ack_nr(server.ack_nr - 1);
    //     packet.set_connection_id(server.sender_connection_id);
    //     packet.set_timestamp_microseconds(now_microseconds());
    //     packet.set_type(PacketType::State);
    //     packet.set_sack(vec!(12, 0, 0, 0));

    //     // Send SACK
    //     iotry!(server.socket.send_to(&packet.to_bytes()[..], server.connected_to.clone()));

    //     // Expect to receive "missing" packets
    //     let mut received: Vec<u8> = vec!();
    //     loop {
    //         match server.recv_from(&mut buf) {
    //             Ok((0, _src)) => break,
    //             Ok((len, _src)) => received.extend(buf[..len].to_vec()),
    //             Err(e) => panic!("{:?}", e)
    //         }
    //     }
    //     assert!(!received.is_empty());
    //     assert_eq!(received.len(), data.len());
    //     assert_eq!(received, data);
    // }

    #[test]
    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 = thread::spawn(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::new();
                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_microseconds(now_microseconds());
                packet.payload = chunk.to_vec();
                packet.set_type(PacketType::Data);

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

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

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

        let mut buf = [0; BUF_SIZE];
        let mut received: Vec<u8> = vec![];
        loop {
            match server.recv_from(&mut buf) {
                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);

        assert!(child.join().is_ok());
    }

    #[test]
    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 = thread::spawn(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) {
                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);

        assert!(child.join().is_ok());
    }

    #[test]
    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 = thread::spawn(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 = [0; BUF_SIZE];
        let mut received: Vec<u8> = vec![];
        loop {
            match server.recv_from(&mut buf) {
                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);

        assert!(child.join().is_ok());
    }

    #[test]
    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);
    }

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

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

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

        assert!(child.join().is_ok());
    }

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

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

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

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

        assert!(child.join().is_ok());
    }

    #[test]
    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 = thread::spawn(move || {
            let mut buf = [0; BUF_SIZE];
            loop {
                match server.recv_from(&mut buf) {
                    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);

        let other_socket = iotry!(::std::net::UdpSocket::bind("0.0.0.0:0"));

        iotry!(other_socket.send_to(&packet.to_bytes()[..], server_addr));

        let mut buf = [0; BUF_SIZE];
        match other_socket.recv_from(&mut buf) {
            Ok((len, _src)) => {
                let reply = Packet::from_bytes(&buf[..len]).ok().unwrap();
                assert_eq!(reply.get_type(), PacketType::Reset);
            }
            Err(e) => panic!("{:?}", e),
        }
        iotry!(client.close());

        assert!(child.join().is_ok());
    }

    #[test]
    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 = thread::spawn(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.to_bytes()[..], server_addr));
            let mut buf = [0; BUF_SIZE];
            match client.socket.recv_from(&mut buf) {
                Ok((_len, _src)) => (),
                Err(e) => panic!("{:?}", e),
            }
        });

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

    #[test]
    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 = thread::spawn(move || {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            iotry!(client.send_to(&to_send[..]));
            iotry!(client.close());
        });

        let mut buf = [0; BUF_SIZE];

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

        // 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_microseconds(now_microseconds());
        packet.set_type(PacketType::Fin);
        iotry!(server.socket.send_to(&packet.to_bytes()[..], client_addr));

        // Receive until end
        let mut received: Vec<u8> = vec![];
        loop {
            match server.recv_from(&mut buf) {
                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);

        assert!(child.join().is_ok());
    }

    #[test]
    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).unwrap();

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

        let expected = vec![7, 9];
        let actual = socket.base_delays.iter().map(|&x| x).collect::<Vec<_>>();
        assert_eq!(expected, actual);
        assert_eq!(socket.min_base_delay(), 7);
    }

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

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

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

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

    #[test]
    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).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 = thread::spawn(move || {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            let mut buf = [0; 1024];
            iotry!(tx.send(client.local_addr()));
            iotry!(client.recv_from(&mut buf));
        });

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

        // `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());

        assert!(child.join().is_ok());
    }

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

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

    // Test reaction to connection loss when sending data packets
    #[test]
    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 = thread::spawn(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 socket = client.socket.try_clone().unwrap();
            let mut buf = [0; BUF_SIZE];
            iotry!(socket.recv_from(&mut buf));
            for _ in 0..attempts {
                match socket.recv_from(&mut buf) {
                    Ok((len, _src)) => {
                        assert_eq!(Packet::from_bytes(&buf[..len]).unwrap().get_type(),
                                   PacketType::Data)
                    }
                    Err(e) => panic!("{}", e),
                }
            }
        });

        // Drain incoming packets
        let mut buf = [0; 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 = [0; BUF_SIZE];
        match server.recv(&mut buf, false) {
            Err(ref e) if e.kind() == ErrorKind::TimedOut => (),
            x => panic!("Expected Err(TimedOut), got {:?}", x),
        }

        assert!(child.join().is_ok());
    }

    // Test reaction to connection loss when sending FIN
    #[test]
    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 = thread::spawn(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 socket = client.socket.try_clone().unwrap();
            let mut buf = [0; BUF_SIZE];
            iotry!(socket.recv_from(&mut buf));
            for _ in 0..attempts {
                match socket.recv_from(&mut buf) {
                    Ok((len, _src)) => {
                        assert_eq!(Packet::from_bytes(&buf[..len]).unwrap().get_type(),
                                   PacketType::Fin)
                    }
                    Err(e) => panic!("{}", e),
                }
            }
        });

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

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

    // Test reaction to connection loss when waiting for data packets
    #[test]
    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;

        let child = thread::spawn(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 socket = client.socket.try_clone().unwrap();
            let seq_nr = client.seq_nr;
            let mut buf = [0; BUF_SIZE];
            for _ in 0..(3 * attempts) {
                match socket.recv_from(&mut buf) {
                    Ok((len, _src)) => {
                        let packet = iotry!(Packet::from_bytes(&buf[..len]));
                        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) {
            Err(ref e) if e.kind() == ErrorKind::TimedOut => (),
            x => panic!("Expected Err(TimedOut), got {:?}", x),
        }
        assert!(child.join().is_ok());
    }

    const NETWORK_NODE_COUNT: usize = 20;
    const NETWORK_MSG_COUNT: usize = 5;

    fn test_network(exchange: fn(&mut UtpSocket) -> ()) {
        use std::net::SocketAddr;
        use std::thread::{JoinHandle, spawn};

        const NODE_COUNT: usize = NETWORK_NODE_COUNT;

        struct Node {
            listener: UtpListener,
        }

        impl Node {
            fn new() -> Node {
                Node { listener: iotry!(UtpListener::bind("127.0.0.1:0")) }
            }

            fn run(&mut self, exchange: fn(&mut UtpSocket) -> (), peer_addrs: Vec<SocketAddr>) {
                let connect_cnt = peer_addrs.len();

                let connect_join_handle = spawn(move || {
                    let mut send_jhs = Vec::<JoinHandle<()>>::new();

                    for peer_addr in peer_addrs {
                        send_jhs.push(spawn(move || {
                            let mut socket = iotry!(UtpSocket::connect(peer_addr));
                            exchange(&mut socket);
                        }));
                    }

                    for jh in send_jhs {
                        iotry!(jh.join());
                    }
                });

                let mut recv_jhs = Vec::<JoinHandle<()>>::new();

                for _ in 0..NODE_COUNT-1-connect_cnt {
                    let mut socket = iotry!(self.listener.accept()).0;
                    recv_jhs.push(spawn(move || {
                        exchange(&mut socket);
                    }));
                }

                for jh in recv_jhs {
                    iotry!(jh.join());
                }

                iotry!(connect_join_handle.join());
            }
        }

        let mut nodes = Vec::<Node>::new();

        for _ in 0..NODE_COUNT {
            nodes.push(Node::new());
        }

        let listening_addrs = nodes.iter()
                                   .map(|n| iotry!(n.listener.local_addr()))
                                   .collect::<Vec<_>>();

        let mut join_handles = Vec::<JoinHandle<()>>::new();

        let mut ni: usize = 0;
        for mut node in nodes {
            let mut addrs = Vec::<SocketAddr>::new();

            for ai in 0..listening_addrs.len() {
                if ai <= ni { continue }
                addrs.push(listening_addrs[ai].clone());
            }

            join_handles.push(spawn(move || {
                node.run(exchange, addrs);
            }));

            ni += 1;
        }

        for handle in join_handles {
            iotry!(handle.join());
        }
    }

    #[test]
    fn test_network_no_timeout() {
        static MSG_COUNT: usize  = NETWORK_MSG_COUNT;

        fn make_buf(i: usize) -> [u8; 10] {
            let mut buf = [0; 10];
            for j in 0..10 {
                buf[j] = (i + j) as u8;
            }
            buf
        }

        fn sequential_exchange(socket: &mut UtpSocket) {
            let mut i = 0;
            let from = socket.socket.local_addr().map(|addr| addr.port()).unwrap_or(0);
            let to   = socket.connected_to.port();

            while i < MSG_COUNT {
                let tx_buf = make_buf(i);
                assert_eq!(iotry!(socket.send_to(&tx_buf)), tx_buf.len());
                let mut buf = [0; 10];

                match socket.recv_from(&mut buf) {
                    Ok((cnt, _)) => {
                        if cnt == 0 {
                            if socket.state != SocketState::Connected {
                                panic!("socket is in an invalid state \"{:?}\" from {:?} to {:?}",
                                         socket.state, from, to);
                            }
                        }
                        assert_eq!(cnt, 10);
                        if buf != make_buf(i) {
                            panic!("expected {:?} but received {:?} in recv step {}",
                                   make_buf(i),
                                   buf,
                                   i);
                        }
                    },
                    Err(err) => {
                        panic!("Recv error {:?}; from {:?} to {:?}", err, from, to);
                    }
                }
                i += 1;
            }
        }

        for i in 0..100 {
            println!("------ Testing Network iteration {}", i);
            test_network(sequential_exchange);
        }
    }

    #[test]
    fn test_network_with_timeout() {
        static MSG_COUNT: usize  = NETWORK_MSG_COUNT;

        fn make_buf(i: usize) -> [u8; 10] {
            let mut buf = [0; 10];
            for j in 0..10 {
                buf[j] = (i + j) as u8;
            }
            buf
        }

        fn timeout_exchange(socket: &mut UtpSocket) {
            socket.set_read_timeout(Some(50));
            let mut recv_cnt = 0;
            let mut send_cnt = 0;

            let from = socket.socket.local_addr().map(|addr| addr.port()).unwrap_or(0);
            let to   = socket.connected_to.port();

            loop {
                if send_cnt < MSG_COUNT {
                    let tx_buf = make_buf(send_cnt);

                    match socket.send_to(&tx_buf) {
                        Ok(cnt) => {
                            assert_eq!(cnt, tx_buf.len());
                            send_cnt += 1;
                        }
                        Err(ref e) if e.kind() == ErrorKind::TimedOut => {}
                        Err(e) => {
                            panic!("{:?}", e);
                        }
                    }
                }
                if recv_cnt < MSG_COUNT {
                    let exp_buf = make_buf(recv_cnt);

                    let mut buf = [0; 10];
                    match socket.recv_from(&mut buf) {
                        Ok((cnt, _)) => {
                            if cnt == 0 {
                                if socket.state != SocketState::Connected {
                                    panic!("socket is in an invalid state \"{:?}\" \
                                           from {:?} to {:?} in receive #{}",
                                             socket.state, from, to, recv_cnt);
                                }
                            } else {
                                assert_eq!(cnt, exp_buf.len());
                                assert_eq!(buf, exp_buf);
                                recv_cnt += 1;
                            }
                        },
                        Err(ref e) if e.kind() == ErrorKind::TimedOut => {
                        },
                        Err(e) => {
                            panic!("{:?} recv_cnt={} send_cnt={}", e, recv_cnt, send_cnt);
                        }
                    }
                }

                if send_cnt == MSG_COUNT && recv_cnt == MSG_COUNT {
                    break;
                }
            }
        }


        for i in 0..100 {
            println!("------ Testing Network iteration {}", i);
            test_network(timeout_exchange);
        }
    }

    #[test]
    fn test_send_client_to_server() {
        let listener = iotry!(UtpListener::bind("127.0.0.1:0"));
        let server_addr = iotry!(listener.local_addr());

        static TX_BUF: [u8; 10] = [0,1,2,3,4,5,6,7,8,9];

        let client_t = thread::spawn(move || {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            assert_eq!(iotry!(client.send_to(&TX_BUF)), TX_BUF.len());
        });

        let mut server = iotry!(listener.accept()).0;

        let mut buf = [0; 10];
        iotry!(server.recv_from(&mut buf));
        assert_eq!(buf, TX_BUF);

        assert!(client_t.join().is_ok());
    }

    // Test data exchange
    #[test]
    fn test_send_server_to_client() {
        let listener = iotry!(UtpListener::bind("127.0.0.1:0"));
        let server_addr = iotry!(listener.local_addr());

        static TX_BUF: [u8; 10] = [0,1,2,3,4,5,6,7,8,9];

        let client_t = thread::spawn(move || {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            let mut buf = [0; 10];
            iotry!(client.recv_from(&mut buf));
            assert_eq!(buf, TX_BUF);
        });

        let mut server = iotry!(listener.accept()).0;

        assert_eq!(iotry!(server.send_to(&TX_BUF)), TX_BUF.len());
        let fr = server.flush();
        assert!(fr.is_ok());

        assert!(client_t.join().is_ok());
    }

    // Test data exchange
    #[test]
    fn test_data_exchange_utp() {
        let listener = iotry!(UtpListener::bind("127.0.0.1:0"));
        let server_addr = iotry!(listener.local_addr());

        static TX_BUF: [u8; 10] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];

        let client_t = thread::spawn(move || {
            let mut client = iotry!(UtpSocket::connect(server_addr));
            assert_eq!(iotry!(client.send_to(&TX_BUF)), TX_BUF.len());
            let mut buf = [0; 10];
            iotry!(client.recv_from(&mut buf));
            assert_eq!(buf, TX_BUF);
        });

        let mut server = iotry!(listener.accept()).0;

        assert_eq!(iotry!(server.send_to(&TX_BUF)), TX_BUF.len());
        let mut buf = [0; 10];
        iotry!(server.recv_from(&mut buf));
        assert_eq!(buf, TX_BUF);
        let _ = server.flush();

        assert!(client_t.join().is_ok());
    }

    /// Analogous to the above, but with TCP sockets.
    #[test]
    fn test_data_exchange_tcp() {
        use std::net::{TcpListener, TcpStream};
        use std::io::{Read, Write};

        static TX_BUF: [u8; 10] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];

        let listener = iotry!(TcpListener::bind("127.0.0.1:0"));
        let server_addr = iotry!(listener.local_addr());

        let client_t = thread::spawn(move || {
            let mut client = iotry!(TcpStream::connect(server_addr));
            assert_eq!(iotry!(client.write(&TX_BUF)), TX_BUF.len());
            let mut buf = [0; 10];
            iotry!(client.read(&mut buf));
            assert_eq!(buf, TX_BUF);
        });

        let mut server = iotry!(listener.accept()).0;

        assert_eq!(iotry!(server.write(&TX_BUF)), TX_BUF.len());
        let mut buf = [0; 10];
        iotry!(server.read(&mut buf));
        assert_eq!(buf, TX_BUF);

        assert!(client_t.join().is_ok());
    }

}