feat(http2): add adaptive window size support using BDP (#2138)

This adds support for calculating the Bandwidth-delay product when using
HTTP2. When a DATA frame is received, a PING is sent to the remote.
While the PING acknoledgement is outstanding, the amount of bytes of all
received DATA frames is accumulated. Once we receive the PING
acknowledgement, we calculate the BDP based on the number of received
bytes and the round-trip-time of the PING. If we are near the current
maximum window size, the size is doubled.

It's disabled by default until tested more extensively.

src/proto/h2/bdp.rs

@@ -0,0 +1,190 @@
+// What should it do?
+//
+// # BDP Algorithm
+//
+// 1. When receiving a DATA frame, if a BDP ping isn't outstanding:
+//   1a. Record current time.
+//   1b. Send a BDP ping.
+// 2. Increment the number of received bytes.
+// 3. When the BDP ping ack is received:
+//   3a. Record duration from sent time.
+//   3b. Merge RTT with a running average.
+//   3c. Calculate bdp as bytes/rtt.
+//   3d. If bdp is over 2/3 max, set new max to bdp and update windows.
+//
+//
+// # Implementation
+//
+// - `hyper::Body::h2` variant includes a "bdp channel"
+//   - When the body's `poll_data` yields bytes, call `bdp.sample(bytes.len())`
+//
+
+use std::sync::{Arc, Mutex, Weak};
+use std::task::{self, Poll};
+use std::time::{Duration, Instant};
+
+use h2::{Ping, PingPong};
+
+type WindowSize = u32;
+
+/// Any higher than this likely will be hitting the TCP flow control.
+const BDP_LIMIT: usize = 1024 * 1024 * 16;
+
+pub(crate) fn disabled() -> Sampler {
+    Sampler {
+        shared: Weak::new(),
+    }
+}
+
+pub(super) fn channel(ping_pong: PingPong, initial_window: WindowSize) -> (Sampler, Estimator) {
+    let shared = Arc::new(Mutex::new(Shared {
+        bytes: 0,
+        ping_pong,
+        ping_sent: false,
+        sent_at: Instant::now(),
+    }));
+
+    (
+        Sampler {
+            shared: Arc::downgrade(&shared),
+        },
+        Estimator {
+            bdp: initial_window,
+            max_bandwidth: 0.0,
+            shared,
+            samples: 0,
+            rtt: 0.0,
+        },
+    )
+}
+
+#[derive(Clone)]
+pub(crate) struct Sampler {
+    shared: Weak<Mutex<Shared>>,
+}
+
+pub(super) struct Estimator {
+    shared: Arc<Mutex<Shared>>,
+
+    /// Current BDP in bytes
+    bdp: u32,
+    /// Largest bandwidth we've seen so far.
+    max_bandwidth: f64,
+    /// Count of samples made (ping sent and received)
+    samples: usize,
+    /// Round trip time in seconds
+    rtt: f64,
+}
+
+struct Shared {
+    bytes: usize,
+    ping_pong: PingPong,
+    ping_sent: bool,
+    sent_at: Instant,
+}
+
+impl Sampler {
+    pub(crate) fn sample(&self, bytes: usize) {
+        let shared = if let Some(shared) = self.shared.upgrade() {
+            shared
+        } else {
+            return;
+        };
+
+        let mut inner = shared.lock().unwrap();
+
+        if !inner.ping_sent {
+            if let Ok(()) = inner.ping_pong.send_ping(Ping::opaque()) {
+                inner.ping_sent = true;
+                inner.sent_at = Instant::now();
+                trace!("sending BDP ping");
+            } else {
+                return;
+            }
+        }
+
+        inner.bytes += bytes;
+    }
+
+    pub(crate) fn is_enabled(&self) -> bool {
+        self.shared.strong_count() > 0
+    }
+}
+
+impl Estimator {
+    pub(super) fn poll_estimate(&mut self, cx: &mut task::Context<'_>) -> Poll<WindowSize> {
+        let mut inner = self.shared.lock().unwrap();
+        if !inner.ping_sent {
+            // XXX: this doesn't register a waker...?
+            return Poll::Pending;
+        }
+
+        let (bytes, rtt) = match ready!(inner.ping_pong.poll_pong(cx)) {
+            Ok(_pong) => {
+                let rtt = inner.sent_at.elapsed();
+                let bytes = inner.bytes;
+                inner.bytes = 0;
+                inner.ping_sent = false;
+                self.samples += 1;
+                trace!("received BDP ack; bytes = {}, rtt = {:?}", bytes, rtt);
+                (bytes, rtt)
+            }
+            Err(e) => {
+                debug!("bdp pong error: {}", e);
+                return Poll::Pending;
+            }
+        };
+
+        drop(inner);
+
+        if let Some(bdp) = self.calculate(bytes, rtt) {
+            Poll::Ready(bdp)
+        } else {
+            // XXX: this doesn't register a waker...?
+            Poll::Pending
+        }
+    }
+
+    fn calculate(&mut self, bytes: usize, rtt: Duration) -> Option<WindowSize> {
+        // No need to do any math if we're at the limit.
+        if self.bdp as usize == BDP_LIMIT {
+            return None;
+        }
+
+        // average the rtt
+        let rtt = seconds(rtt);
+        if self.samples < 10 {
+            // Average the first 10 samples
+            self.rtt += (rtt - self.rtt) / (self.samples as f64);
+        } else {
+            self.rtt += (rtt - self.rtt) / 0.9;
+        }
+
+        // calculate the current bandwidth
+        let bw = (bytes as f64) / (self.rtt * 1.5);
+        trace!("current bandwidth = {:.1}B/s", bw);
+
+        if bw < self.max_bandwidth {
+            // not a faster bandwidth, so don't update
+            return None;
+        } else {
+            self.max_bandwidth = bw;
+        }
+
+        // if the current `bytes` sample is at least 2/3 the previous
+        // bdp, increase to double the current sample.
+        if (bytes as f64) >= (self.bdp as f64) * 0.66 {
+            self.bdp = (bytes * 2).min(BDP_LIMIT) as WindowSize;
+            trace!("BDP increased to {}", self.bdp);
+            Some(self.bdp)
+        } else {
+            None
+        }
+    }
+}
+
+fn seconds(dur: Duration) -> f64 {
+    const NANOS_PER_SEC: f64 = 1_000_000_000.0;
+    let secs = dur.as_secs() as f64;
+    secs + (dur.subsec_nanos() as f64) / NANOS_PER_SEC
+}

src/proto/h2/client.rs

@@ -4,7 +4,7 @@ use futures_util::stream::StreamExt as _;
 use h2::client::{Builder, SendRequest};
 use tokio::io::{AsyncRead, AsyncWrite};
 
-use super::{decode_content_length, PipeToSendStream, SendBuf};
+use super::{bdp, decode_content_length, PipeToSendStream, SendBuf};
 use crate::body::Payload;
 use crate::common::{task, Exec, Future, Never, Pin, Poll};
 use crate::headers;
@@ -21,17 +21,43 @@ type ConnDropRef = mpsc::Sender<Never>;
 ///// the "dispatch" task will be notified and can shutdown sooner.
 type ConnEof = oneshot::Receiver<Never>;
 
+// Our defaults are chosen for the "majority" case, which usually are not
+// resource constrained, and so the spec default of 64kb can be too limiting
+// for performance.
+const DEFAULT_CONN_WINDOW: u32 = 1024 * 1024 * 5; // 5mb
+const DEFAULT_STREAM_WINDOW: u32 = 1024 * 1024 * 2; // 2mb
+
+#[derive(Clone, Debug)]
+pub(crate) struct Config {
+    pub(crate) adaptive_window: bool,
+    pub(crate) initial_conn_window_size: u32,
+    pub(crate) initial_stream_window_size: u32,
+}
+
+impl Default for Config {
+    fn default() -> Config {
+        Config {
+            adaptive_window: false,
+            initial_conn_window_size: DEFAULT_CONN_WINDOW,
+            initial_stream_window_size: DEFAULT_STREAM_WINDOW,
+        }
+    }
+}
+
 pub(crate) async fn handshake<T, B>(
     io: T,
     req_rx: ClientRx<B>,
-    builder: &Builder,
+    config: &Config,
     exec: Exec,
 ) -> crate::Result<ClientTask<B>>
 where
     T: AsyncRead + AsyncWrite + Send + Unpin + 'static,
     B: Payload,
 {
-    let (h2_tx, conn) = builder
+    let (h2_tx, mut conn) = Builder::default()
+        .initial_window_size(config.initial_stream_window_size)
+        .initial_connection_window_size(config.initial_conn_window_size)
+        .enable_push(false)
         .handshake::<_, SendBuf<B::Data>>(io)
         .await
         .map_err(crate::Error::new_h2)?;
@@ -49,27 +75,34 @@ where
         }
     });
 
-    let conn = conn.map_err(|e| debug!("connection error: {}", e));
+    let sampler = if config.adaptive_window {
+        let (sampler, mut estimator) =
+            bdp::channel(conn.ping_pong().unwrap(), config.initial_stream_window_size);
 
-    let conn_task = async move {
-        match future::select(conn, conn_drop_rx).await {
-            Either::Left(_) => {
-                // ok or err, the `conn` has finished
+        let conn = future::poll_fn(move |cx| {
+            match estimator.poll_estimate(cx) {
+                Poll::Ready(wnd) => {
+                    conn.set_target_window_size(wnd);
+                    conn.set_initial_window_size(wnd)?;
+                }
+                Poll::Pending => {}
             }
-            Either::Right(((), conn)) => {
-                // mpsc has been dropped, hopefully polling
-                // the connection some more should start shutdown
-                // and then close
-                trace!("send_request dropped, starting conn shutdown");
-                drop(cancel_tx);
-                let _ = conn.await;
-            }
-        }
+
+            Pin::new(&mut conn).poll(cx)
+        });
+        let conn = conn.map_err(|e| debug!("connection error: {}", e));
+
+        exec.execute(conn_task(conn, conn_drop_rx, cancel_tx));
+        sampler
+    } else {
+        let conn = conn.map_err(|e| debug!("connection error: {}", e));
+
+        exec.execute(conn_task(conn, conn_drop_rx, cancel_tx));
+        bdp::disabled()
     };
 
-    exec.execute(conn_task);
-
     Ok(ClientTask {
+        bdp: sampler,
         conn_drop_ref,
         conn_eof,
         executor: exec,
@@ -78,10 +111,31 @@ where
     })
 }
 
+async fn conn_task<C, D>(conn: C, drop_rx: D, cancel_tx: oneshot::Sender<Never>)
+where
+    C: Future + Unpin,
+    D: Future<Output = ()> + Unpin,
+{
+    match future::select(conn, drop_rx).await {
+        Either::Left(_) => {
+            // ok or err, the `conn` has finished
+        }
+        Either::Right(((), conn)) => {
+            // mpsc has been dropped, hopefully polling
+            // the connection some more should start shutdown
+            // and then close
+            trace!("send_request dropped, starting conn shutdown");
+            drop(cancel_tx);
+            let _ = conn.await;
+        }
+    }
+}
+
 pub(crate) struct ClientTask<B>
 where
     B: Payload,
 {
+    bdp: bdp::Sampler,
     conn_drop_ref: ConnDropRef,
     conn_eof: ConnEof,
     executor: Exec,
@@ -156,10 +210,12 @@ where
                         }
                     }
 
+                    let bdp = self.bdp.clone();
                     let fut = fut.map(move |result| match result {
                         Ok(res) => {
                             let content_length = decode_content_length(res.headers());
-                            let res = res.map(|stream| crate::Body::h2(stream, content_length));
+                            let res =
+                                res.map(|stream| crate::Body::h2(stream, content_length, bdp));
                             Ok(res)
                         }
                         Err(err) => {

src/proto/h2/mod.rs

@@ -12,12 +12,16 @@ use crate::body::Payload;
 use crate::common::{task, Future, Pin, Poll};
 use crate::headers::content_length_parse_all;
 
+pub(crate) mod bdp;
 pub(crate) mod client;
 pub(crate) mod server;
 
 pub(crate) use self::client::ClientTask;
 pub(crate) use self::server::Server;
 
+/// Default initial stream window size defined in HTTP2 spec.
+pub(crate) const SPEC_WINDOW_SIZE: u32 = 65_535;
+
 fn strip_connection_headers(headers: &mut HeaderMap, is_request: bool) {
     // List of connection headers from:
     // https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Connection

src/proto/h2/server.rs

@@ -1,12 +1,12 @@
 use std::error::Error as StdError;
 use std::marker::Unpin;
 
-use h2::server::{Builder, Connection, Handshake, SendResponse};
+use h2::server::{Connection, Handshake, SendResponse};
 use h2::Reason;
 use pin_project::{pin_project, project};
 use tokio::io::{AsyncRead, AsyncWrite};
 
-use super::{decode_content_length, PipeToSendStream, SendBuf};
+use super::{bdp, decode_content_length, PipeToSendStream, SendBuf};
 use crate::body::Payload;
 use crate::common::exec::H2Exec;
 use crate::common::{task, Future, Pin, Poll};
@@ -16,6 +16,34 @@ use crate::service::HttpService;
 
 use crate::{Body, Response};
 
+// Our defaults are chosen for the "majority" case, which usually are not
+// resource constrained, and so the spec default of 64kb can be too limiting
+// for performance.
+//
+// At the same time, a server more often has multiple clients connected, and
+// so is more likely to use more resources than a client would.
+const DEFAULT_CONN_WINDOW: u32 = 1024 * 1024; // 1mb
+const DEFAULT_STREAM_WINDOW: u32 = 1024 * 1024; // 1mb
+
+#[derive(Clone, Debug)]
+pub(crate) struct Config {
+    pub(crate) adaptive_window: bool,
+    pub(crate) initial_conn_window_size: u32,
+    pub(crate) initial_stream_window_size: u32,
+    pub(crate) max_concurrent_streams: Option<u32>,
+}
+
+impl Default for Config {
+    fn default() -> Config {
+        Config {
+            adaptive_window: false,
+            initial_conn_window_size: DEFAULT_CONN_WINDOW,
+            initial_stream_window_size: DEFAULT_STREAM_WINDOW,
+            max_concurrent_streams: None,
+        }
+    }
+}
+
 #[pin_project]
 pub(crate) struct Server<T, S, B, E>
 where
@@ -31,7 +59,13 @@ enum State<T, B>
 where
     B: Payload,
 {
-    Handshaking(Handshake<T, SendBuf<B::Data>>),
+    Handshaking {
+        /// If Some, bdp is enabled with the initial size.
+        ///
+        /// If None, bdp is disabled.
+        bdp_initial_size: Option<u32>,
+        hs: Handshake<T, SendBuf<B::Data>>,
+    },
     Serving(Serving<T, B>),
     Closed,
 }
@@ -40,6 +74,7 @@ struct Serving<T, B>
 where
     B: Payload,
 {
+    bdp: Option<(bdp::Sampler, bdp::Estimator)>,
     conn: Connection<T, SendBuf<B::Data>>,
     closing: Option<crate::Error>,
 }
@@ -52,11 +87,28 @@ where
     B: Payload,
     E: H2Exec<S::Future, B>,
 {
-    pub(crate) fn new(io: T, service: S, builder: &Builder, exec: E) -> Server<T, S, B, E> {
+    pub(crate) fn new(io: T, service: S, config: &Config, exec: E) -> Server<T, S, B, E> {
+        let mut builder = h2::server::Builder::default();
+        builder
+            .initial_window_size(config.initial_stream_window_size)
+            .initial_connection_window_size(config.initial_conn_window_size);
+        if let Some(max) = config.max_concurrent_streams {
+            builder.max_concurrent_streams(max);
+        }
         let handshake = builder.handshake(io);
+
+        let bdp = if config.adaptive_window {
+            Some(config.initial_stream_window_size)
+        } else {
+            None
+        };
+
         Server {
             exec,
-            state: State::Handshaking(handshake),
+            state: State::Handshaking {
+                bdp_initial_size: bdp,
+                hs: handshake,
+            },
             service,
         }
     }
@@ -64,7 +116,7 @@ where
     pub fn graceful_shutdown(&mut self) {
         trace!("graceful_shutdown");
         match self.state {
-            State::Handshaking(..) => {
+            State::Handshaking { .. } => {
                 // fall-through, to replace state with Closed
             }
             State::Serving(ref mut srv) => {
@@ -95,9 +147,15 @@ where
         let me = &mut *self;
         loop {
             let next = match me.state {
-                State::Handshaking(ref mut h) => {
-                    let conn = ready!(Pin::new(h).poll(cx).map_err(crate::Error::new_h2))?;
+                State::Handshaking {
+                    ref mut hs,
+                    ref bdp_initial_size,
+                } => {
+                    let mut conn = ready!(Pin::new(hs).poll(cx).map_err(crate::Error::new_h2))?;
+                    let bdp = bdp_initial_size
+                        .map(|wnd| bdp::channel(conn.ping_pong().expect("ping_pong"), wnd));
                     State::Serving(Serving {
+                        bdp,
                         conn,
                         closing: None,
                     })
@@ -135,7 +193,12 @@ where
     {
         if self.closing.is_none() {
             loop {
-                // At first, polls the readiness of supplied service.
+                self.poll_bdp(cx);
+
+                // Check that the service is ready to accept a new request.
+                //
+                // - If not, just drive the connection some.
+                // - If ready, try to accept a new request from the connection.
                 match service.poll_ready(cx) {
                     Poll::Ready(Ok(())) => (),
                     Poll::Pending => {
@@ -168,7 +231,14 @@ where
                     Some(Ok((req, respond))) => {
                         trace!("incoming request");
                         let content_length = decode_content_length(req.headers());
-                        let req = req.map(|stream| crate::Body::h2(stream, content_length));
+                        let bdp_sampler = self
+                            .bdp
+                            .as_ref()
+                            .map(|bdp| bdp.0.clone())
+                            .unwrap_or_else(bdp::disabled);
+
+                        let req =
+                            req.map(|stream| crate::Body::h2(stream, content_length, bdp_sampler));
                         let fut = H2Stream::new(service.call(req), respond);
                         exec.execute_h2stream(fut);
                     }
@@ -193,6 +263,18 @@ where
 
         Poll::Ready(Err(self.closing.take().expect("polled after error")))
     }
+
+    fn poll_bdp(&mut self, cx: &mut task::Context<'_>) {
+        if let Some((_, ref mut estimator)) = self.bdp {
+            match estimator.poll_estimate(cx) {
+                Poll::Ready(wnd) => {
+                    self.conn.set_target_window_size(wnd);
+                    let _ = self.conn.set_initial_window_size(wnd);
+                }
+                Poll::Pending => {}
+            }
+        }
+    }
 }
 
 #[allow(missing_debug_implementations)]