diff --git a/core/src/muxing.rs b/core/src/muxing.rs
index 170a4537901..969c56c782e 100644
--- a/core/src/muxing.rs
+++ b/core/src/muxing.rs
@@ -51,19 +51,14 @@
 //! The upgrade process will take ownership of the connection, which makes it possible for the
 //! implementation of `StreamMuxer` to control everything that happens on the wire.
 
-use fnv::FnvHashMap;
 use futures::{future, prelude::*, task::Context, task::Poll};
 use multiaddr::Multiaddr;
-use parking_lot::Mutex;
-use std::{
-    fmt, io,
-    ops::Deref,
-    pin::Pin,
-    sync::atomic::{AtomicUsize, Ordering},
-};
+use std::{fmt, io, ops::Deref, pin::Pin};
 
+pub use self::boxed::StreamMuxerBox;
 pub use self::singleton::SingletonMuxer;
 
+mod boxed;
 mod singleton;
 
 /// Implemented on objects that can open and manage substreams.
@@ -491,251 +486,3 @@ where
             .destroy_substream(self.substream.take().expect("substream was empty"))
     }
 }
-
-/// Abstract `StreamMuxer`.
-pub struct StreamMuxerBox {
-    inner: Box<
-        dyn StreamMuxer<Substream = usize, OutboundSubstream = usize, Error = io::Error>
-            + Send
-            + Sync,
-    >,
-}
-
-impl StreamMuxerBox {
-    /// Turns a stream muxer into a `StreamMuxerBox`.
-    pub fn new<T>(muxer: T) -> StreamMuxerBox
-    where
-        T: StreamMuxer + Send + Sync + 'static,
-        T::OutboundSubstream: Send,
-        T::Substream: Send,
-    {
-        let wrap = Wrap {
-            inner: muxer,
-            substreams: Mutex::new(Default::default()),
-            next_substream: AtomicUsize::new(0),
-            outbound: Mutex::new(Default::default()),
-            next_outbound: AtomicUsize::new(0),
-        };
-
-        StreamMuxerBox {
-            inner: Box::new(wrap),
-        }
-    }
-}
-
-impl StreamMuxer for StreamMuxerBox {
-    type Substream = usize; // TODO: use a newtype
-    type OutboundSubstream = usize; // TODO: use a newtype
-    type Error = io::Error;
-
-    #[inline]
-    fn poll_event(
-        &self,
-        cx: &mut Context<'_>,
-    ) -> Poll<Result<StreamMuxerEvent<Self::Substream>, Self::Error>> {
-        self.inner.poll_event(cx)
-    }
-
-    #[inline]
-    fn open_outbound(&self) -> Self::OutboundSubstream {
-        self.inner.open_outbound()
-    }
-
-    #[inline]
-    fn poll_outbound(
-        &self,
-        cx: &mut Context<'_>,
-        s: &mut Self::OutboundSubstream,
-    ) -> Poll<Result<Self::Substream, Self::Error>> {
-        self.inner.poll_outbound(cx, s)
-    }
-
-    #[inline]
-    fn destroy_outbound(&self, substream: Self::OutboundSubstream) {
-        self.inner.destroy_outbound(substream)
-    }
-
-    #[inline]
-    fn read_substream(
-        &self,
-        cx: &mut Context<'_>,
-        s: &mut Self::Substream,
-        buf: &mut [u8],
-    ) -> Poll<Result<usize, Self::Error>> {
-        self.inner.read_substream(cx, s, buf)
-    }
-
-    #[inline]
-    fn write_substream(
-        &self,
-        cx: &mut Context<'_>,
-        s: &mut Self::Substream,
-        buf: &[u8],
-    ) -> Poll<Result<usize, Self::Error>> {
-        self.inner.write_substream(cx, s, buf)
-    }
-
-    #[inline]
-    fn flush_substream(
-        &self,
-        cx: &mut Context<'_>,
-        s: &mut Self::Substream,
-    ) -> Poll<Result<(), Self::Error>> {
-        self.inner.flush_substream(cx, s)
-    }
-
-    #[inline]
-    fn shutdown_substream(
-        &self,
-        cx: &mut Context<'_>,
-        s: &mut Self::Substream,
-    ) -> Poll<Result<(), Self::Error>> {
-        self.inner.shutdown_substream(cx, s)
-    }
-
-    #[inline]
-    fn destroy_substream(&self, s: Self::Substream) {
-        self.inner.destroy_substream(s)
-    }
-
-    #[inline]
-    fn poll_close(&self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
-        self.inner.poll_close(cx)
-    }
-}
-
-struct Wrap<T>
-where
-    T: StreamMuxer,
-{
-    inner: T,
-    substreams: Mutex<FnvHashMap<usize, T::Substream>>,
-    next_substream: AtomicUsize,
-    outbound: Mutex<FnvHashMap<usize, T::OutboundSubstream>>,
-    next_outbound: AtomicUsize,
-}
-
-impl<T> StreamMuxer for Wrap<T>
-where
-    T: StreamMuxer,
-{
-    type Substream = usize; // TODO: use a newtype
-    type OutboundSubstream = usize; // TODO: use a newtype
-    type Error = io::Error;
-
-    #[inline]
-    fn poll_event(
-        &self,
-        cx: &mut Context<'_>,
-    ) -> Poll<Result<StreamMuxerEvent<Self::Substream>, Self::Error>> {
-        let substream = match self.inner.poll_event(cx) {
-            Poll::Pending => return Poll::Pending,
-            Poll::Ready(Ok(StreamMuxerEvent::AddressChange(a))) => {
-                return Poll::Ready(Ok(StreamMuxerEvent::AddressChange(a)))
-            }
-            Poll::Ready(Ok(StreamMuxerEvent::InboundSubstream(s))) => s,
-            Poll::Ready(Err(err)) => return Poll::Ready(Err(err.into())),
-        };
-
-        let id = self.next_substream.fetch_add(1, Ordering::Relaxed);
-        self.substreams.lock().insert(id, substream);
-        Poll::Ready(Ok(StreamMuxerEvent::InboundSubstream(id)))
-    }
-
-    #[inline]
-    fn open_outbound(&self) -> Self::OutboundSubstream {
-        let outbound = self.inner.open_outbound();
-        let id = self.next_outbound.fetch_add(1, Ordering::Relaxed);
-        self.outbound.lock().insert(id, outbound);
-        id
-    }
-
-    #[inline]
-    fn poll_outbound(
-        &self,
-        cx: &mut Context<'_>,
-        substream: &mut Self::OutboundSubstream,
-    ) -> Poll<Result<Self::Substream, Self::Error>> {
-        let mut list = self.outbound.lock();
-        let substream = match self
-            .inner
-            .poll_outbound(cx, list.get_mut(substream).unwrap())
-        {
-            Poll::Pending => return Poll::Pending,
-            Poll::Ready(Ok(s)) => s,
-            Poll::Ready(Err(err)) => return Poll::Ready(Err(err.into())),
-        };
-        let id = self.next_substream.fetch_add(1, Ordering::Relaxed);
-        self.substreams.lock().insert(id, substream);
-        Poll::Ready(Ok(id))
-    }
-
-    #[inline]
-    fn destroy_outbound(&self, substream: Self::OutboundSubstream) {
-        let mut list = self.outbound.lock();
-        self.inner
-            .destroy_outbound(list.remove(&substream).unwrap())
-    }
-
-    #[inline]
-    fn read_substream(
-        &self,
-        cx: &mut Context<'_>,
-        s: &mut Self::Substream,
-        buf: &mut [u8],
-    ) -> Poll<Result<usize, Self::Error>> {
-        let mut list = self.substreams.lock();
-        self.inner
-            .read_substream(cx, list.get_mut(s).unwrap(), buf)
-            .map_err(|e| e.into())
-    }
-
-    #[inline]
-    fn write_substream(
-        &self,
-        cx: &mut Context<'_>,
-        s: &mut Self::Substream,
-        buf: &[u8],
-    ) -> Poll<Result<usize, Self::Error>> {
-        let mut list = self.substreams.lock();
-        self.inner
-            .write_substream(cx, list.get_mut(s).unwrap(), buf)
-            .map_err(|e| e.into())
-    }
-
-    #[inline]
-    fn flush_substream(
-        &self,
-        cx: &mut Context<'_>,
-        s: &mut Self::Substream,
-    ) -> Poll<Result<(), Self::Error>> {
-        let mut list = self.substreams.lock();
-        self.inner
-            .flush_substream(cx, list.get_mut(s).unwrap())
-            .map_err(|e| e.into())
-    }
-
-    #[inline]
-    fn shutdown_substream(
-        &self,
-        cx: &mut Context<'_>,
-        s: &mut Self::Substream,
-    ) -> Poll<Result<(), Self::Error>> {
-        let mut list = self.substreams.lock();
-        self.inner
-            .shutdown_substream(cx, list.get_mut(s).unwrap())
-            .map_err(|e| e.into())
-    }
-
-    #[inline]
-    fn destroy_substream(&self, substream: Self::Substream) {
-        let mut list = self.substreams.lock();
-        self.inner
-            .destroy_substream(list.remove(&substream).unwrap())
-    }
-
-    #[inline]
-    fn poll_close(&self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
-        self.inner.poll_close(cx).map_err(|e| e.into())
-    }
-}
diff --git a/core/src/muxing/boxed.rs b/core/src/muxing/boxed.rs
new file mode 100644
index 00000000000..1284febe5dd
--- /dev/null
+++ b/core/src/muxing/boxed.rs
@@ -0,0 +1,255 @@
+use crate::muxing::StreamMuxerEvent;
+use crate::StreamMuxer;
+use fnv::FnvHashMap;
+use parking_lot::Mutex;
+use std::io;
+use std::sync::atomic::{AtomicUsize, Ordering};
+use std::task::{Context, Poll};
+
+/// Abstract `StreamMuxer`.
+pub struct StreamMuxerBox {
+    inner: Box<
+        dyn StreamMuxer<Substream = usize, OutboundSubstream = usize, Error = io::Error>
+            + Send
+            + Sync,
+    >,
+}
+
+struct Wrap<T>
+where
+    T: StreamMuxer,
+{
+    inner: T,
+    substreams: Mutex<FnvHashMap<usize, T::Substream>>,
+    next_substream: AtomicUsize,
+    outbound: Mutex<FnvHashMap<usize, T::OutboundSubstream>>,
+    next_outbound: AtomicUsize,
+}
+
+impl<T> StreamMuxer for Wrap<T>
+where
+    T: StreamMuxer,
+{
+    type Substream = usize; // TODO: use a newtype
+    type OutboundSubstream = usize; // TODO: use a newtype
+    type Error = io::Error;
+
+    #[inline]
+    fn poll_event(
+        &self,
+        cx: &mut Context<'_>,
+    ) -> Poll<Result<StreamMuxerEvent<Self::Substream>, Self::Error>> {
+        let substream = match self.inner.poll_event(cx) {
+            Poll::Pending => return Poll::Pending,
+            Poll::Ready(Ok(StreamMuxerEvent::AddressChange(a))) => {
+                return Poll::Ready(Ok(StreamMuxerEvent::AddressChange(a)))
+            }
+            Poll::Ready(Ok(StreamMuxerEvent::InboundSubstream(s))) => s,
+            Poll::Ready(Err(err)) => return Poll::Ready(Err(err.into())),
+        };
+
+        let id = self.next_substream.fetch_add(1, Ordering::Relaxed);
+        self.substreams.lock().insert(id, substream);
+        Poll::Ready(Ok(StreamMuxerEvent::InboundSubstream(id)))
+    }
+
+    #[inline]
+    fn open_outbound(&self) -> Self::OutboundSubstream {
+        let outbound = self.inner.open_outbound();
+        let id = self.next_outbound.fetch_add(1, Ordering::Relaxed);
+        self.outbound.lock().insert(id, outbound);
+        id
+    }
+
+    #[inline]
+    fn poll_outbound(
+        &self,
+        cx: &mut Context<'_>,
+        substream: &mut Self::OutboundSubstream,
+    ) -> Poll<Result<Self::Substream, Self::Error>> {
+        let mut list = self.outbound.lock();
+        let substream = match self
+            .inner
+            .poll_outbound(cx, list.get_mut(substream).unwrap())
+        {
+            Poll::Pending => return Poll::Pending,
+            Poll::Ready(Ok(s)) => s,
+            Poll::Ready(Err(err)) => return Poll::Ready(Err(err.into())),
+        };
+        let id = self.next_substream.fetch_add(1, Ordering::Relaxed);
+        self.substreams.lock().insert(id, substream);
+        Poll::Ready(Ok(id))
+    }
+
+    #[inline]
+    fn destroy_outbound(&self, substream: Self::OutboundSubstream) {
+        let mut list = self.outbound.lock();
+        self.inner
+            .destroy_outbound(list.remove(&substream).unwrap())
+    }
+
+    #[inline]
+    fn read_substream(
+        &self,
+        cx: &mut Context<'_>,
+        s: &mut Self::Substream,
+        buf: &mut [u8],
+    ) -> Poll<Result<usize, Self::Error>> {
+        let mut list = self.substreams.lock();
+        self.inner
+            .read_substream(cx, list.get_mut(s).unwrap(), buf)
+            .map_err(|e| e.into())
+    }
+
+    #[inline]
+    fn write_substream(
+        &self,
+        cx: &mut Context<'_>,
+        s: &mut Self::Substream,
+        buf: &[u8],
+    ) -> Poll<Result<usize, Self::Error>> {
+        let mut list = self.substreams.lock();
+        self.inner
+            .write_substream(cx, list.get_mut(s).unwrap(), buf)
+            .map_err(|e| e.into())
+    }
+
+    #[inline]
+    fn flush_substream(
+        &self,
+        cx: &mut Context<'_>,
+        s: &mut Self::Substream,
+    ) -> Poll<Result<(), Self::Error>> {
+        let mut list = self.substreams.lock();
+        self.inner
+            .flush_substream(cx, list.get_mut(s).unwrap())
+            .map_err(|e| e.into())
+    }
+
+    #[inline]
+    fn shutdown_substream(
+        &self,
+        cx: &mut Context<'_>,
+        s: &mut Self::Substream,
+    ) -> Poll<Result<(), Self::Error>> {
+        let mut list = self.substreams.lock();
+        self.inner
+            .shutdown_substream(cx, list.get_mut(s).unwrap())
+            .map_err(|e| e.into())
+    }
+
+    #[inline]
+    fn destroy_substream(&self, substream: Self::Substream) {
+        let mut list = self.substreams.lock();
+        self.inner
+            .destroy_substream(list.remove(&substream).unwrap())
+    }
+
+    #[inline]
+    fn poll_close(&self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        self.inner.poll_close(cx).map_err(|e| e.into())
+    }
+}
+
+impl StreamMuxerBox {
+    /// Turns a stream muxer into a `StreamMuxerBox`.
+    pub fn new<T>(muxer: T) -> StreamMuxerBox
+    where
+        T: StreamMuxer + Send + Sync + 'static,
+        T::OutboundSubstream: Send,
+        T::Substream: Send,
+    {
+        let wrap = Wrap {
+            inner: muxer,
+            substreams: Mutex::new(Default::default()),
+            next_substream: AtomicUsize::new(0),
+            outbound: Mutex::new(Default::default()),
+            next_outbound: AtomicUsize::new(0),
+        };
+
+        StreamMuxerBox {
+            inner: Box::new(wrap),
+        }
+    }
+}
+
+impl StreamMuxer for StreamMuxerBox {
+    type Substream = usize; // TODO: use a newtype
+    type OutboundSubstream = usize; // TODO: use a newtype
+    type Error = io::Error;
+
+    #[inline]
+    fn poll_event(
+        &self,
+        cx: &mut Context<'_>,
+    ) -> Poll<Result<StreamMuxerEvent<Self::Substream>, Self::Error>> {
+        self.inner.poll_event(cx)
+    }
+
+    #[inline]
+    fn open_outbound(&self) -> Self::OutboundSubstream {
+        self.inner.open_outbound()
+    }
+
+    #[inline]
+    fn poll_outbound(
+        &self,
+        cx: &mut Context<'_>,
+        s: &mut Self::OutboundSubstream,
+    ) -> Poll<Result<Self::Substream, Self::Error>> {
+        self.inner.poll_outbound(cx, s)
+    }
+
+    #[inline]
+    fn destroy_outbound(&self, substream: Self::OutboundSubstream) {
+        self.inner.destroy_outbound(substream)
+    }
+
+    #[inline]
+    fn read_substream(
+        &self,
+        cx: &mut Context<'_>,
+        s: &mut Self::Substream,
+        buf: &mut [u8],
+    ) -> Poll<Result<usize, Self::Error>> {
+        self.inner.read_substream(cx, s, buf)
+    }
+
+    #[inline]
+    fn write_substream(
+        &self,
+        cx: &mut Context<'_>,
+        s: &mut Self::Substream,
+        buf: &[u8],
+    ) -> Poll<Result<usize, Self::Error>> {
+        self.inner.write_substream(cx, s, buf)
+    }
+
+    #[inline]
+    fn flush_substream(
+        &self,
+        cx: &mut Context<'_>,
+        s: &mut Self::Substream,
+    ) -> Poll<Result<(), Self::Error>> {
+        self.inner.flush_substream(cx, s)
+    }
+
+    #[inline]
+    fn shutdown_substream(
+        &self,
+        cx: &mut Context<'_>,
+        s: &mut Self::Substream,
+    ) -> Poll<Result<(), Self::Error>> {
+        self.inner.shutdown_substream(cx, s)
+    }
+
+    #[inline]
+    fn destroy_substream(&self, s: Self::Substream) {
+        self.inner.destroy_substream(s)
+    }
+
+    #[inline]
+    fn poll_close(&self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        self.inner.poll_close(cx)
+    }
+}