7 files changed, 1088 insertions, 0 deletions
diff --git a/third_party/rust/neqo-transport/tests/sim/connection.rs b/third_party/rust/neqo-transport/tests/sim/connection.rs
new file mode 100644
index 0000000000..eafb0d9a3f
--- /dev/null
+++ b/third_party/rust/neqo-transport/tests/sim/connection.rs
@@ -0,0 +1,311 @@
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![allow(clippy::module_name_repetitions)]
+
+use super::{Node, Rng};
+use neqo_common::{event::Provider, qdebug, qtrace, Datagram};
+use neqo_crypto::AuthenticationStatus;
+use neqo_transport::{
+    Connection, ConnectionEvent, ConnectionParameters, Output, State, StreamId, StreamType,
+};
+use std::cmp::min;
+use std::fmt::{self, Debug};
+use std::time::Instant;
+
+/// The status of the processing of an event.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum GoalStatus {
+    /// The event didn't result in doing anything; the goal is waiting for something.
+    Waiting,
+    /// An action was taken as a result of the event.
+    Active,
+    /// The goal was accomplished.
+    Done,
+}
+
+/// A goal for the connection.
+/// Goals can be accomplished in any order.
+pub trait ConnectionGoal {
+    fn init(&mut self, _c: &mut Connection, _now: Instant) {}
+    /// Perform some processing.
+    fn process(&mut self, _c: &mut Connection, _now: Instant) -> GoalStatus {
+        GoalStatus::Waiting
+    }
+    /// Handle an event from the provided connection, returning `true` when the
+    /// goal is achieved.
+    fn handle_event(&mut self, c: &mut Connection, e: &ConnectionEvent, now: Instant)
+        -> GoalStatus;
+}
+
+pub struct ConnectionNode {
+    c: Connection,
+    goals: Vec<Box<dyn ConnectionGoal>>,
+}
+
+impl ConnectionNode {
+    pub fn new_client(
+        params: ConnectionParameters,
+        goals: impl IntoIterator<Item = Box<dyn ConnectionGoal>>,
+    ) -> Self {
+        Self {
+            c: test_fixture::new_client(params),
+            goals: goals.into_iter().collect(),
+        }
+    }
+
+    pub fn new_server(
+        params: ConnectionParameters,
+        goals: impl IntoIterator<Item = Box<dyn ConnectionGoal>>,
+    ) -> Self {
+        Self {
+            c: test_fixture::new_server(test_fixture::DEFAULT_ALPN, params),
+            goals: goals.into_iter().collect(),
+        }
+    }
+
+    pub fn default_client(goals: impl IntoIterator<Item = Box<dyn ConnectionGoal>>) -> Self {
+        Self::new_client(ConnectionParameters::default(), goals)
+    }
+
+    pub fn default_server(goals: impl IntoIterator<Item = Box<dyn ConnectionGoal>>) -> Self {
+        Self::new_server(ConnectionParameters::default(), goals)
+    }
+
+    #[allow(dead_code)]
+    pub fn clear_goals(&mut self) {
+        self.goals.clear();
+    }
+
+    #[allow(dead_code)]
+    pub fn add_goal(&mut self, goal: Box<dyn ConnectionGoal>) {
+        self.goals.push(goal);
+    }
+
+    /// Process all goals using the given closure and return whether any were active.
+    fn process_goals<F>(&mut self, mut f: F) -> bool
+    where
+        F: FnMut(&mut Box<dyn ConnectionGoal>, &mut Connection) -> GoalStatus,
+    {
+        // Waiting on drain_filter...
+        // self.goals.drain_filter(|g| f(g,  &mut self.c, &e)).count();
+        let mut active = false;
+        let mut i = 0;
+        while i < self.goals.len() {
+            let status = f(&mut self.goals[i], &mut self.c);
+            if status == GoalStatus::Done {
+                self.goals.remove(i);
+                active = true;
+            } else {
+                active |= status == GoalStatus::Active;
+                i += 1;
+            }
+        }
+        active
+    }
+}
+
+impl Node for ConnectionNode {
+    fn init(&mut self, _rng: Rng, now: Instant) {
+        for g in &mut self.goals {
+            g.init(&mut self.c, now);
+        }
+    }
+
+    fn process(&mut self, mut d: Option<Datagram>, now: Instant) -> Output {
+        let _ = self.process_goals(|goal, c| goal.process(c, now));
+        loop {
+            let res = self.c.process(d.take(), now);
+
+            let mut active = false;
+            while let Some(e) = self.c.next_event() {
+                qtrace!([self.c], "received event {:?}", e);
+
+                // Perform authentication automatically.
+                if matches!(e, ConnectionEvent::AuthenticationNeeded) {
+                    self.c.authenticated(AuthenticationStatus::Ok, now);
+                }
+
+                active |= self.process_goals(|goal, c| goal.handle_event(c, &e, now));
+            }
+            // Exit at this point if the connection produced a datagram.
+            // We also exit if none of the goals were active, as there is
+            // no point trying again if they did nothing.
+            if matches!(res, Output::Datagram(_)) || !active {
+                return res;
+            }
+            qdebug!([self.c], "no datagram and goal activity, looping");
+        }
+    }
+
+    fn done(&self) -> bool {
+        self.goals.is_empty()
+    }
+
+    fn print_summary(&self, test_name: &str) {
+        println!("{}: {:?}", test_name, self.c.stats());
+    }
+}
+
+impl Debug for ConnectionNode {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Display::fmt(&self.c, f)
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct ReachState {
+    target: State,
+}
+
+impl ReachState {
+    pub fn new(target: State) -> Self {
+        Self { target }
+    }
+}
+
+impl ConnectionGoal for ReachState {
+    fn handle_event(
+        &mut self,
+        _c: &mut Connection,
+        e: &ConnectionEvent,
+        _now: Instant,
+    ) -> GoalStatus {
+        if matches!(e, ConnectionEvent::StateChange(state) if *state == self.target) {
+            GoalStatus::Done
+        } else {
+            GoalStatus::Waiting
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct SendData {
+    remaining: usize,
+    stream_id: Option<StreamId>,
+}
+
+impl SendData {
+    pub fn new(amount: usize) -> Self {
+        Self {
+            remaining: amount,
+            stream_id: None,
+        }
+    }
+
+    fn make_stream(&mut self, c: &mut Connection) {
+        if self.stream_id.is_none() {
+            if let Ok(stream_id) = c.stream_create(StreamType::UniDi) {
+                qdebug!([c], "made stream {} for sending", stream_id);
+                self.stream_id = Some(stream_id);
+            }
+        }
+    }
+
+    fn send(&mut self, c: &mut Connection, stream_id: StreamId) -> GoalStatus {
+        const DATA: &[u8] = &[0; 4096];
+        let mut status = GoalStatus::Waiting;
+        loop {
+            let end = min(self.remaining, DATA.len());
+            let sent = c.stream_send(stream_id, &DATA[..end]).unwrap();
+            if sent == 0 {
+                return status;
+            }
+            self.remaining -= sent;
+            qtrace!("sent {} remaining {}", sent, self.remaining);
+            if self.remaining == 0 {
+                c.stream_close_send(stream_id).unwrap();
+                return GoalStatus::Done;
+            }
+            status = GoalStatus::Active;
+        }
+    }
+}
+
+impl ConnectionGoal for SendData {
+    fn init(&mut self, c: &mut Connection, _now: Instant) {
+        self.make_stream(c);
+    }
+
+    fn process(&mut self, c: &mut Connection, _now: Instant) -> GoalStatus {
+        self.stream_id
+            .map_or(GoalStatus::Waiting, |stream_id| self.send(c, stream_id))
+    }
+
+    fn handle_event(
+        &mut self,
+        c: &mut Connection,
+        e: &ConnectionEvent,
+        _now: Instant,
+    ) -> GoalStatus {
+        match e {
+            ConnectionEvent::SendStreamCreatable {
+                stream_type: StreamType::UniDi,
+            }
+            // TODO(mt): remove the second condition when #842 is fixed.
+            | ConnectionEvent::StateChange(_) => {
+                self.make_stream(c);
+                GoalStatus::Active
+            }
+
+            ConnectionEvent::SendStreamWritable { stream_id }
+                if Some(*stream_id) == self.stream_id =>
+            {
+                self.send(c, *stream_id)
+            }
+
+            // If we sent data in 0-RTT, then we didn't track how much we should
+            // have sent.  This is trivial to fix if 0-RTT testing is ever needed.
+            ConnectionEvent::ZeroRttRejected => panic!("not supported"),
+            _ => GoalStatus::Waiting,
+        }
+    }
+}
+
+/// Receive a prescribed amount of data from any stream.
+#[derive(Debug)]
+pub struct ReceiveData {
+    remaining: usize,
+}
+
+impl ReceiveData {
+    pub fn new(amount: usize) -> Self {
+        Self { remaining: amount }
+    }
+
+    fn recv(&mut self, c: &mut Connection, stream_id: StreamId) -> GoalStatus {
+        let mut buf = vec![0; 4096];
+        let mut status = GoalStatus::Waiting;
+        loop {
+            let end = min(self.remaining, buf.len());
+            let (recvd, _) = c.stream_recv(stream_id, &mut buf[..end]).unwrap();
+            qtrace!("received {} remaining {}", recvd, self.remaining);
+            if recvd == 0 {
+                return status;
+            }
+            self.remaining -= recvd;
+            if self.remaining == 0 {
+                return GoalStatus::Done;
+            }
+            status = GoalStatus::Active;
+        }
+    }
+}
+
+impl ConnectionGoal for ReceiveData {
+    fn handle_event(
+        &mut self,
+        c: &mut Connection,
+        e: &ConnectionEvent,
+        _now: Instant,
+    ) -> GoalStatus {
+        if let ConnectionEvent::RecvStreamReadable { stream_id } = e {
+            self.recv(c, *stream_id)
+        } else {
+            GoalStatus::Waiting
+        }
+    }
+}
diff --git a/third_party/rust/neqo-transport/tests/sim/delay.rs b/third_party/rust/neqo-transport/tests/sim/delay.rs
new file mode 100644
index 0000000000..95188c0562
--- /dev/null
+++ b/third_party/rust/neqo-transport/tests/sim/delay.rs
@@ -0,0 +1,98 @@
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![allow(clippy::module_name_repetitions)]
+
+use super::{Node, Rng};
+use neqo_common::Datagram;
+use neqo_transport::Output;
+use std::collections::BTreeMap;
+use std::convert::TryFrom;
+use std::fmt::{self, Debug};
+use std::ops::Range;
+use std::time::{Duration, Instant};
+
+/// An iterator that shares a `Random` instance and produces uniformly
+/// random `Duration`s within a specified range.
+pub struct RandomDelay {
+    start: Duration,
+    max: u64,
+    rng: Option<Rng>,
+}
+
+impl RandomDelay {
+    /// Make a new random `Duration` generator.  This panics if the range provided
+    /// is inverted (i.e., `bounds.start > bounds.end`), or spans 2^64
+    /// or more nanoseconds.
+    /// A zero-length range means that random values won't be taken from the Rng
+    pub fn new(bounds: Range<Duration>) -> Self {
+        let max = u64::try_from((bounds.end - bounds.start).as_nanos()).unwrap();
+        Self {
+            start: bounds.start,
+            max,
+            rng: None,
+        }
+    }
+
+    pub fn set_rng(&mut self, rng: Rng) {
+        self.rng = Some(rng);
+    }
+
+    pub fn next(&mut self) -> Duration {
+        let mut rng = self.rng.as_ref().unwrap().borrow_mut();
+        let r = rng.random_from(0..self.max);
+        self.start + Duration::from_nanos(r)
+    }
+}
+
+pub struct Delay {
+    random: RandomDelay,
+    queue: BTreeMap<Instant, Datagram>,
+}
+
+impl Delay {
+    pub fn new(bounds: Range<Duration>) -> Self {
+        Self {
+            random: RandomDelay::new(bounds),
+            queue: BTreeMap::default(),
+        }
+    }
+
+    fn insert(&mut self, d: Datagram, now: Instant) {
+        let mut t = now + self.random.next();
+        while self.queue.contains_key(&t) {
+            // This is a little inefficient, but it avoids drops on collisions,
+            // which are super-common for a fixed delay.
+            t += Duration::from_nanos(1);
+        }
+        self.queue.insert(t, d);
+    }
+}
+
+impl Node for Delay {
+    fn init(&mut self, rng: Rng, _now: Instant) {
+        self.random.set_rng(rng);
+    }
+
+    fn process(&mut self, d: Option<Datagram>, now: Instant) -> Output {
+        if let Some(dgram) = d {
+            self.insert(dgram, now);
+        }
+        if let Some((&k, _)) = self.queue.range(..=now).next() {
+            Output::Datagram(self.queue.remove(&k).unwrap())
+        } else if let Some(&t) = self.queue.keys().next() {
+            Output::Callback(t - now)
+        } else {
+            Output::None
+        }
+    }
+}
+
+impl Debug for Delay {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str("delay")
+    }
+}
diff --git a/third_party/rust/neqo-transport/tests/sim/drop.rs b/third_party/rust/neqo-transport/tests/sim/drop.rs
new file mode 100644
index 0000000000..d42913d99d
--- /dev/null
+++ b/third_party/rust/neqo-transport/tests/sim/drop.rs
@@ -0,0 +1,71 @@
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![allow(clippy::module_name_repetitions)]
+
+use super::{Node, Rng};
+use neqo_common::{qtrace, Datagram};
+use neqo_transport::Output;
+use std::fmt::{self, Debug};
+use std::time::Instant;
+
+/// A random dropper.
+pub struct Drop {
+    threshold: u64,
+    max: u64,
+    rng: Option<Rng>,
+}
+
+impl Drop {
+    /// Make a new random drop generator.  Each `drop` is called, this generates a
+    /// random value between 0 and `max` (exclusive).  If this value is less than
+    /// `threshold` a value of `true` is returned.
+    pub fn new(threshold: u64, max: u64) -> Self {
+        Self {
+            threshold,
+            max,
+            rng: None,
+        }
+    }
+
+    /// Generate random drops with the given percentage.
+    pub fn percentage(pct: u8) -> Self {
+        // Multiply by 10 so that the random number generator works more efficiently.
+        Self::new(u64::from(pct) * 10, 1000)
+    }
+
+    pub fn drop(&mut self) -> bool {
+        let mut rng = self.rng.as_ref().unwrap().borrow_mut();
+        let r = rng.random_from(0..self.max);
+        r < self.threshold
+    }
+}
+
+impl Node for Drop {
+    fn init(&mut self, rng: Rng, _now: Instant) {
+        self.rng = Some(rng);
+    }
+
+    // Pass any datagram provided directly out, but drop some of them.
+    fn process(&mut self, d: Option<Datagram>, _now: Instant) -> Output {
+        if let Some(dgram) = d {
+            if self.drop() {
+                qtrace!("drop {}", dgram.len());
+                Output::None
+            } else {
+                Output::Datagram(dgram)
+            }
+        } else {
+            Output::None
+        }
+    }
+}
+
+impl Debug for Drop {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str("drop")
+    }
+}
diff --git a/third_party/rust/neqo-transport/tests/sim/mod.rs b/third_party/rust/neqo-transport/tests/sim/mod.rs
new file mode 100644
index 0000000000..f7646aac56
--- /dev/null
+++ b/third_party/rust/neqo-transport/tests/sim/mod.rs
@@ -0,0 +1,232 @@
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Tests with simulated network
+#![cfg_attr(feature = "deny-warnings", deny(warnings))]
+#![warn(clippy::pedantic)]
+
+pub mod connection;
+mod delay;
+mod drop;
+pub mod rng;
+mod taildrop;
+
+use neqo_common::{qdebug, qinfo, qtrace, Datagram, Encoder};
+use neqo_transport::Output;
+use rng::Random;
+use std::cell::RefCell;
+use std::cmp::min;
+use std::convert::TryFrom;
+use std::fmt::Debug;
+use std::rc::Rc;
+use std::time::{Duration, Instant};
+use test_fixture::{self, now};
+
+use NodeState::{Active, Idle, Waiting};
+
+pub mod network {
+    pub use super::delay::Delay;
+    pub use super::drop::Drop;
+    pub use super::taildrop::TailDrop;
+}
+
+type Rng = Rc<RefCell<Random>>;
+
+/// A macro that turns a list of values into boxed versions of the same.
+#[macro_export]
+macro_rules! boxed {
+    [$($v:expr),+ $(,)?] => {
+        vec![ $( Box::new($v) as _ ),+ ]
+    };
+}
+
+/// Create a simulation test case.  This takes either two or three arguments.
+/// The two argument form takes a bare name (`ident`), a comma, and an array of
+/// items that implement `Node`.
+/// The three argument form adds a setup block that can be used to construct a
+/// complex value that is then shared between all nodes.  The values in the
+/// three-argument form have to be closures (or functions) that accept a reference
+/// to the value returned by the setup.
+#[macro_export]
+macro_rules! simulate {
+    ($n:ident, [ $($v:expr),+ $(,)? ] $(,)?) => {
+        simulate!($n, (), [ $(|_| $v),+ ]);
+    };
+    ($n:ident, $setup:expr, [ $( $v:expr ),+ $(,)? ] $(,)?) => {
+        #[test]
+        fn $n() {
+            let fixture = $setup;
+            let mut nodes: Vec<Box<dyn $crate::sim::Node>> = Vec::new();
+            $(
+                let f: Box<dyn FnOnce(&_) -> _> = Box::new($v);
+                nodes.push(Box::new(f(&fixture)));
+            )*
+            let mut sim = Simulator::new(stringify!($n), nodes);
+            if let Ok(seed) = std::env::var("SIMULATION_SEED") {
+                sim.seed_str(seed);
+            }
+            sim.run();
+        }
+    };
+}
+
+pub trait Node: Debug {
+    fn init(&mut self, _rng: Rng, _now: Instant) {}
+    /// Perform processing.  This optionally takes a datagram and produces either
+    /// another data, a time that the simulator needs to wait, or nothing.
+    fn process(&mut self, d: Option<Datagram>, now: Instant) -> Output;
+    /// An node can report when it considers itself "done".
+    fn done(&self) -> bool {
+        true
+    }
+    fn print_summary(&self, _test_name: &str) {}
+}
+
+/// The state of a single node.  Nodes will be activated if they are `Active`
+/// or if the previous node in the loop generated a datagram.  Nodes that return
+/// `true` from `Node::done` will be activated as normal.
+#[derive(Debug, PartialEq)]
+enum NodeState {
+    /// The node just produced a datagram.  It should be activated again as soon as possible.
+    Active,
+    /// The node is waiting.
+    Waiting(Instant),
+    /// The node became idle.
+    Idle,
+}
+
+#[derive(Debug)]
+struct NodeHolder {
+    node: Box<dyn Node>,
+    state: NodeState,
+}
+
+impl NodeHolder {
+    fn ready(&self, now: Instant) -> bool {
+        match self.state {
+            Active => true,
+            Waiting(t) => t >= now,
+            Idle => false,
+        }
+    }
+}
+
+pub struct Simulator {
+    name: String,
+    nodes: Vec<NodeHolder>,
+    rng: Rng,
+}
+
+impl Simulator {
+    pub fn new(name: impl AsRef<str>, nodes: impl IntoIterator<Item = Box<dyn Node>>) -> Self {
+        let name = String::from(name.as_ref());
+        // The first node is marked as Active, the rest are idle.
+        let mut it = nodes.into_iter();
+        let nodes = it
+            .next()
+            .map(|node| NodeHolder {
+                node,
+                state: Active,
+            })
+            .into_iter()
+            .chain(it.map(|node| NodeHolder { node, state: Idle }))
+            .collect::<Vec<_>>();
+        Self {
+            name,
+            nodes,
+            rng: Rc::default(),
+        }
+    }
+
+    pub fn seed(&mut self, seed: [u8; 32]) {
+        self.rng = Rc::new(RefCell::new(Random::new(seed)));
+    }
+
+    /// Seed from a hex string.
+    /// Though this is convenient, it panics if this isn't a 64 character hex string.
+    pub fn seed_str(&mut self, seed: impl AsRef<str>) {
+        let seed = Encoder::from_hex(seed);
+        self.seed(<[u8; 32]>::try_from(seed.as_ref()).unwrap());
+    }
+
+    fn next_time(&self, now: Instant) -> Instant {
+        let mut next = None;
+        for n in &self.nodes {
+            match n.state {
+                Idle => continue,
+                Active => return now,
+                Waiting(a) => next = Some(next.map_or(a, |b| min(a, b))),
+            }
+        }
+        next.expect("a node cannot be idle and not done")
+    }
+
+    /// Runs the simulation.
+    pub fn run(mut self) -> Duration {
+        let start = now();
+        let mut now = start;
+        let mut dgram = None;
+
+        for n in &mut self.nodes {
+            n.node.init(self.rng.clone(), now);
+        }
+        println!("{}: seed {}", self.name, self.rng.borrow().seed_str());
+
+        let real_start = Instant::now();
+        loop {
+            for n in &mut self.nodes {
+                if dgram.is_none() && !n.ready(now) {
+                    qdebug!([self.name], "skipping {:?}", n.node);
+                    continue;
+                }
+
+                qdebug!([self.name], "processing {:?}", n.node);
+                let res = n.node.process(dgram.take(), now);
+                n.state = match res {
+                    Output::Datagram(d) => {
+                        qtrace!([self.name], " => datagram {}", d.len());
+                        dgram = Some(d);
+                        Active
+                    }
+                    Output::Callback(delay) => {
+                        qtrace!([self.name], " => callback {:?}", delay);
+                        assert_ne!(delay, Duration::new(0, 0));
+                        Waiting(now + delay)
+                    }
+                    Output::None => {
+                        qtrace!([self.name], " => nothing");
+                        assert!(n.node.done(), "nodes have to be done when they go idle");
+                        Idle
+                    }
+                };
+            }
+
+            if self.nodes.iter().all(|n| n.node.done()) {
+                let real_elapsed = real_start.elapsed();
+                println!("{}: real elapsed time: {:?}", self.name, real_elapsed);
+                let elapsed = now - start;
+                println!("{}: simulated elapsed time: {:?}", self.name, elapsed);
+                for n in &self.nodes {
+                    n.node.print_summary(&self.name);
+                }
+                return elapsed;
+            }
+
+            if dgram.is_none() {
+                let next = self.next_time(now);
+                if next > now {
+                    qinfo!(
+                        [self.name],
+                        "advancing time by {:?} to {:?}",
+                        next - now,
+                        next - start
+                    );
+                    now = next;
+                }
+            }
+        }
+    }
+}
diff --git a/third_party/rust/neqo-transport/tests/sim/net.rs b/third_party/rust/neqo-transport/tests/sim/net.rs
new file mode 100644
index 0000000000..754426f895
--- /dev/null
+++ b/third_party/rust/neqo-transport/tests/sim/net.rs
@@ -0,0 +1,111 @@
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use super::rng::RandomDuration;
+use super::{Node, Rng};
+use neqo_common::Datagram;
+use neqo_transport::Output;
+use std::collections::BTreeMap;
+use std::fmt::{self, Debug};
+use std::iter;
+use std::ops::Range;
+use std::time::{Duration, Instant};
+
+/// 
+pub struct RandomDrop {
+    threshold: u64,
+    max: u64,
+    rng: Rng,
+}
+
+impl RandomDuration {
+    /// Make a new random `Duration` generator.  This asserts if the range provided
+    /// is inverted (i.e., `bounds.start > bounds.end`), or spans 2^64
+    /// or more nanoseconds.
+    /// A zero-length range means that random values won't be taken from the Rng
+    pub fn new(bounds: Range<Duration>, rng: Rng) -> Self {
+        let max = u64::try_from((bounds.end - bounds.start).as_nanos()).unwrap();
+        Self {
+            start: bounds.start,
+            max,
+            rng,
+        }
+    }
+
+    fn next(&mut self) -> Duration {
+        let r = if self.max == 0 {
+            Duration::new(0, 0)
+        } else {
+            self.rng.borrow_mut().random_from(0..self.max)
+        }
+        self.start + Duration::from_nanos(r)
+    }
+}
+
+enum DelayState {
+    New(Range<Duration>),
+    Ready(RandomDuration),
+}
+
+pub struct Delay {
+    state: DelayState,
+    queue: BTreeMap<Instant, Datagram>,
+}
+
+impl Delay
+{
+    pub fn new(bounds: Range<Duration>) -> Self
+    {
+        Self {
+            State: DelayState::New(bounds),
+            queue: BTreeMap::default(),
+        }
+    }
+
+    fn insert(&mut self, d: Datagram, now: Instant) {
+        let mut t = if let State::Ready(r) = self.state {
+        now + self.source.next()
+        } else {
+            unreachable!();
+        }
+        while self.queue.contains_key(&t) {
+            // This is a little inefficient, but it avoids drops on collisions,
+            // which are super-common for a fixed delay.
+            t += Duration::from_nanos(1);
+        }
+        self.queue.insert(t, d);
+    }
+}
+
+impl Node for Delay
+{
+    fn init(&mut self, rng: Rng, now: Instant) {
+        if let DelayState::New(bounds) = self.state {
+            self.state = RandomDuration::new(bounds);
+        } else {
+            unreachable!();
+        }
+    }
+
+    fn process(&mut self, d: Option<Datagram>, now: Instant) -> Output {
+        if let Some(dgram) = d {
+            self.insert(dgram, now);
+        }
+        if let Some((&k, _)) = self.queue.range(..now).nth(0) {
+            Output::Datagram(self.queue.remove(&k).unwrap())
+        } else if let Some(&t) = self.queue.keys().nth(0) {
+            Output::Callback(t - now)
+        } else {
+            Output::None
+        }
+    }
+}
+
+impl<T> Debug for Delay<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str("delay")
+    }
+}
diff --git a/third_party/rust/neqo-transport/tests/sim/rng.rs b/third_party/rust/neqo-transport/tests/sim/rng.rs
new file mode 100644
index 0000000000..d314e8b36f
--- /dev/null
+++ b/third_party/rust/neqo-transport/tests/sim/rng.rs
@@ -0,0 +1,81 @@
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use neqo_common::Decoder;
+use std::convert::TryFrom;
+use std::ops::Range;
+
+/// An implementation of a xoshiro256** pseudorandom generator.
+pub struct Random {
+    state: [u64; 4],
+}
+
+impl Random {
+    pub fn new(seed: [u8; 32]) -> Self {
+        assert!(seed.iter().any(|&x| x != 0));
+        let mut dec = Decoder::from(&seed);
+        Self {
+            state: [
+                dec.decode_uint(8).unwrap(),
+                dec.decode_uint(8).unwrap(),
+                dec.decode_uint(8).unwrap(),
+                dec.decode_uint(8).unwrap(),
+            ],
+        }
+    }
+
+    pub fn random(&mut self) -> u64 {
+        let result = (self.state[1].overflowing_mul(5).0)
+            .rotate_right(7)
+            .overflowing_mul(9)
+            .0;
+        let t = self.state[1] << 17;
+
+        self.state[2] ^= self.state[0];
+        self.state[3] ^= self.state[1];
+        self.state[1] ^= self.state[2];
+        self.state[0] ^= self.state[3];
+
+        self.state[2] ^= t;
+        self.state[3] = self.state[3].rotate_right(45);
+
+        result
+    }
+
+    /// Generate a random value from the range.
+    /// If the range is empty or inverted (`range.start > range.end`), then
+    /// this returns the value of `range.start` without generating any random values.
+    pub fn random_from(&mut self, range: Range<u64>) -> u64 {
+        let max = range.end.saturating_sub(range.start);
+        if max == 0 {
+            return range.start;
+        }
+
+        let shift = (max - 1).leading_zeros();
+        assert_ne!(max, 0);
+        loop {
+            let r = self.random() >> shift;
+            if r < max {
+                return range.start + r;
+            }
+        }
+    }
+
+    /// Get the seed necessary to continue from this point.
+    pub fn seed_str(&self) -> String {
+        format!(
+            "{:8x}{:8x}{:8x}{:8x}",
+            self.state[0], self.state[1], self.state[2], self.state[3],
+        )
+    }
+}
+
+impl Default for Random {
+    fn default() -> Self {
+        let buf = neqo_crypto::random(32);
+        Random::new(<[u8; 32]>::try_from(&buf[..]).unwrap())
+    }
+}
diff --git a/third_party/rust/neqo-transport/tests/sim/taildrop.rs b/third_party/rust/neqo-transport/tests/sim/taildrop.rs
new file mode 100644
index 0000000000..7346b27178
--- /dev/null
+++ b/third_party/rust/neqo-transport/tests/sim/taildrop.rs
@@ -0,0 +1,184 @@
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![allow(clippy::module_name_repetitions)]
+
+use super::Node;
+use neqo_common::{qtrace, Datagram};
+use neqo_transport::Output;
+use std::cmp::max;
+use std::collections::VecDeque;
+use std::convert::TryFrom;
+use std::fmt::{self, Debug};
+use std::time::{Duration, Instant};
+
+/// One second in nanoseconds.
+const ONE_SECOND_NS: u128 = 1_000_000_000;
+
+/// This models a link with a tail drop router at the front of it.
+pub struct TailDrop {
+    /// An overhead associated with each entry.  This accounts for
+    /// layer 2, IP, and UDP overheads.
+    overhead: usize,
+    /// The rate at which bytes egress the link, in bytes per second.
+    rate: usize,
+    /// The depth of the queue, in bytes.
+    capacity: usize,
+
+    /// A counter for how many bytes are enqueued.
+    used: usize,
+    /// A queue of unsent bytes.
+    queue: VecDeque<Datagram>,
+    /// The time that the next datagram can enter the link.
+    next_deque: Option<Instant>,
+
+    /// Any sub-ns delay from the last enqueue.
+    sub_ns_delay: u32,
+    /// The time it takes a byte to exit the other end of the link.
+    delay: Duration,
+    /// The packets that are on the link and when they can be delivered.
+    on_link: VecDeque<(Instant, Datagram)>,
+
+    /// The number of packets received.
+    received: usize,
+    /// The number of packets dropped.
+    dropped: usize,
+    /// The number of packets delivered.
+    delivered: usize,
+    /// The maximum amount of queue capacity ever used.
+    /// As packets leave the queue as soon as they start being used, this doesn't
+    /// count them.
+    maxq: usize,
+}
+
+impl TailDrop {
+    /// Make a new taildrop node with the given rate, queue capacity, and link delay.
+    pub fn new(rate: usize, capacity: usize, delay: Duration) -> Self {
+        Self {
+            overhead: 64,
+            rate,
+            capacity,
+            used: 0,
+            queue: VecDeque::new(),
+            next_deque: None,
+            sub_ns_delay: 0,
+            delay,
+            on_link: VecDeque::new(),
+            received: 0,
+            dropped: 0,
+            delivered: 0,
+            maxq: 0,
+        }
+    }
+
+    /// A tail drop queue on a 10Mbps link (approximated to 1 million bytes per second)
+    /// with a fat 32k buffer (about 30ms), and the default forward delay of 50ms.
+    pub fn dsl_uplink() -> Self {
+        TailDrop::new(1_000_000, 32_768, Duration::from_millis(50))
+    }
+
+    /// Cut downlink to one fifth of the uplink (2Mbps), and reduce the buffer to 1/4.
+    pub fn dsl_downlink() -> Self {
+        TailDrop::new(200_000, 8_192, Duration::from_millis(50))
+    }
+
+    /// How "big" is this datagram, accounting for overheads.
+    /// This approximates by using the same overhead for storing in the queue
+    /// and for sending on the wire.
+    fn size(&self, d: &Datagram) -> usize {
+        d.len() + self.overhead
+    }
+
+    /// Start sending a datagram.
+    fn send(&mut self, d: Datagram, now: Instant) {
+        // How many bytes are we "transmitting"?
+        let sz = u128::try_from(self.size(&d)).unwrap();
+
+        // Calculate how long it takes to put the packet on the link.
+        // Perform the calculation based on 2^32 seconds and save any remainder.
+        // This ensures that high rates and small packets don't result in rounding
+        // down times too badly.
+        // Duration consists of a u64 and a u32, so we have 32 high bits to spare.
+        let t = sz * (ONE_SECOND_NS << 32) / u128::try_from(self.rate).unwrap()
+            + u128::from(self.sub_ns_delay);
+        let send_ns = u64::try_from(t >> 32).unwrap();
+        assert_ne!(send_ns, 0, "sending a packet takes <1ns");
+        self.sub_ns_delay = u32::try_from(t & u128::from(u32::MAX)).unwrap();
+        let deque_time = now + Duration::from_nanos(send_ns);
+        self.next_deque = Some(deque_time);
+
+        // Now work out when the packet is fully received at the other end of
+        // the link. Setup to deliver the packet then.
+        let delivery_time = deque_time + self.delay;
+        self.on_link.push_back((delivery_time, d));
+    }
+
+    /// Enqueue for sending.  Maybe.  If this overflows the queue, drop it instead.
+    fn maybe_enqueue(&mut self, d: Datagram, now: Instant) {
+        self.received += 1;
+        if self.next_deque.is_none() {
+            // Nothing in the queue and nothing still sending.
+            debug_assert!(self.queue.is_empty());
+            self.send(d, now);
+        } else if self.used + self.size(&d) <= self.capacity {
+            self.used += self.size(&d);
+            self.maxq = max(self.maxq, self.used);
+            self.queue.push_back(d);
+        } else {
+            qtrace!("taildrop dropping {} bytes", d.len());
+            self.dropped += 1;
+        }
+    }
+
+    /// If the last packet that was sending has been sent, start sending
+    /// the next one.
+    fn maybe_send(&mut self, now: Instant) {
+        if self.next_deque.as_ref().map_or(false, |t| *t <= now) {
+            if let Some(d) = self.queue.pop_front() {
+                self.used -= self.size(&d);
+                self.send(d, now);
+            } else {
+                self.next_deque = None;
+                self.sub_ns_delay = 0;
+            }
+        }
+    }
+}
+
+impl Node for TailDrop {
+    fn process(&mut self, d: Option<Datagram>, now: Instant) -> Output {
+        if let Some(dgram) = d {
+            self.maybe_enqueue(dgram, now);
+        }
+
+        self.maybe_send(now);
+
+        if let Some((t, _)) = self.on_link.front() {
+            if *t <= now {
+                let (_, d) = self.on_link.pop_front().unwrap();
+                self.delivered += 1;
+                Output::Datagram(d)
+            } else {
+                Output::Callback(*t - now)
+            }
+        } else {
+            Output::None
+        }
+    }
+
+    fn print_summary(&self, test_name: &str) {
+        println!(
+            "{}: taildrop: rx {} drop {} tx {} maxq {}",
+            test_name, self.received, self.dropped, self.delivered, self.maxq,
+        );
+    }
+}
+
+impl Debug for TailDrop {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str("taildrop")
+    }
+}