diff --git a/bare_cnr/Cargo.toml b/bare_cnr/Cargo.toml
new file mode 100644
index 0000000..063b170
--- /dev/null
+++ b/bare_cnr/Cargo.toml
@@ -0,0 +1,22 @@
+[package]
+name = "bare_cnr"
+version = "0.1.0-alpha.1"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+thiserror = "1.0.31"
+serde = "1.0.137"
+serde_bare = "0.5.0"
+dashmap = "5.3.4"
+log = "0.4.17"
+
+[dependencies.tokio]
+# TODO optional = true
+version = "1.18.2"
+features = ["full"] # TODO restrict this
+
+
+[features]
+# TODO default = ["tokio"]
\ No newline at end of file
diff --git a/bare_cnr/src/lib.rs b/bare_cnr/src/lib.rs
new file mode 100644
index 0000000..13debef
--- /dev/null
+++ b/bare_cnr/src/lib.rs
@@ -0,0 +1,19 @@
+use thiserror::Error;
+
+mod multiplexer;
+mod transport;
+
+#[derive(Error, Debug)]
+pub enum CnrError {
+    #[error("Version mismatch.")]
+    VersionMismatch,
+
+    #[error("Ser/deserialisation error.")]
+    Serde(#[from] serde_bare::error::Error),
+
+    #[error("Input/output error.")]
+    Io(#[from] std::io::Error),
+
+    #[error("Channel closed.")]
+    Closed,
+}
diff --git a/bare_cnr/src/multiplexer.rs b/bare_cnr/src/multiplexer.rs
new file mode 100644
index 0000000..a3fa8e9
--- /dev/null
+++ b/bare_cnr/src/multiplexer.rs
@@ -0,0 +1,207 @@
+use crate::CnrError;
+use crate::CnrError::Closed;
+use dashmap::mapref::entry::Entry;
+use dashmap::DashMap;
+use log::error;
+use serde::de::DeserializeOwned;
+use serde::Serialize;
+use std::marker::PhantomData;
+use std::sync::Arc;
+use tokio::io::{AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt};
+use tokio::sync::mpsc::{Receiver, Sender};
+use tokio::sync::{Mutex, MutexGuard};
+use tokio::task::JoinHandle;
+
+pub struct TransportMultiplexer {
+    task_tx: JoinHandle<()>,
+    task_rx: JoinHandle<()>,
+
+    /// Whether this side is the initiator.
+    /// The initiator can open even-numbered channels.
+    initiator: bool,
+
+    /// Queue to send new bytes out on the wire
+    tx_queue: Sender<(u16, Vec<u8>)>,
+
+    /// Senders for sending messages from the wire to channels
+    channels: Arc<DashMap<u16, Sender<Vec<u8>>>>,
+
+    /// Channel receivers for channels that have received messages but haven't yet been claimed
+    unclaimed_channels: Arc<DashMap<u16, Receiver<Vec<u8>>>>,
+    // TODO channels:
+}
+
+impl TransportMultiplexer {
+    pub fn new<R: AsyncRead + Send + 'static + Unpin, W: AsyncWrite + Send + 'static + Unpin>(
+        rx: R,
+        tx: W,
+        initiator: bool,
+    ) -> Result<TransportMultiplexer, CnrError> {
+        let (txqueue_tx, txqueue_rx) = tokio::sync::mpsc::channel(8);
+
+        let channels = Arc::new(Default::default());
+        let unclaimed_channels = Arc::new(Default::default());
+
+        let task_tx = tokio::spawn(async move {
+            if let Err(err) = TransportMultiplexer::handle_tx(tx, txqueue_rx).await {
+                error!("TX handler failed: {:?}", err);
+            }
+        });
+        let channels2 = Arc::clone(&channels);
+        let unclaimed_channels2 = Arc::clone(&unclaimed_channels);
+        let task_rx = tokio::spawn(async move {
+            if let Err(err) =
+                TransportMultiplexer::handle_rx(rx, channels2, unclaimed_channels2).await
+            {
+                error!("RX handler failed: {:?}", err);
+            }
+        });
+
+        Ok(TransportMultiplexer {
+            task_tx,
+            task_rx,
+            initiator,
+            tx_queue: txqueue_tx,
+            channels,
+            unclaimed_channels,
+        })
+    }
+
+    pub fn open_channel_with_id<R, W>(&self, channel_id: u16) -> Option<Channel<R, W>> {
+        match self.channels.entry(channel_id) {
+            Entry::Occupied(oe) => {
+                match self.unclaimed_channels.remove(&channel_id) {
+                    Some((_k, chan_rx)) => Some(Channel {
+                        id: channel_id,
+                        inner: Arc::new(Mutex::new(ChannelInner {
+                            chan_id: channel_id,
+                            tx: self.tx_queue.clone(),
+                            rx: chan_rx,
+                        })),
+                        marker: Default::default(),
+                    }),
+                    None => {
+                        // Channel ID already in use.
+                        None
+                    }
+                }
+            }
+            Entry::Vacant(ve) => {
+                let (chan_tx, chan_rx) = tokio::sync::mpsc::channel(8);
+                ve.insert(chan_tx);
+                Some(Channel {
+                    id: channel_id,
+                    inner: Arc::new(Mutex::new(ChannelInner {
+                        chan_id: channel_id,
+                        tx: self.tx_queue.clone(),
+                        rx: chan_rx,
+                    })),
+                    marker: Default::default(),
+                })
+            }
+        }
+    }
+
+    async fn handle_tx<W: AsyncWrite + Unpin>(
+        mut tx: W,
+        mut txqueue_rx: Receiver<(u16, Vec<u8>)>,
+    ) -> Result<(), CnrError> {
+        while let Some((chan_id, next_msg)) = txqueue_rx.recv().await {
+            tx.write_u16(chan_id).await?;
+            tx.write_u32(next_msg.len().try_into().unwrap()).await?;
+            tx.write_all(&next_msg).await?;
+            // TODO(performance) would be nice not to flush if something else is just behind.
+            tx.flush().await?;
+        }
+        Ok(())
+    }
+
+    async fn handle_rx<R: AsyncRead + Unpin>(
+        mut rx: R,
+        channels: Arc<DashMap<u16, Sender<Vec<u8>>>>,
+        unclaimed_channels: Arc<DashMap<u16, Receiver<Vec<u8>>>>,
+    ) -> Result<(), CnrError> {
+        loop {
+            // TODO(features): need to be able to support graceful EOF
+            let chan_id = rx.read_u16().await?;
+            let length = rx.read_u32().await? as usize;
+            // TODO(perf): use uninit?
+            let mut buf = vec![0u8; length];
+
+            rx.read_exact(&mut buf[..]).await?;
+
+            match channels.entry(chan_id) {
+                Entry::Occupied(oe) => {
+                    if let Err(err) = oe.get().send(buf).await {
+                        // TODO this channel has died. What can we do about it?
+                        todo!();
+                    }
+                }
+                Entry::Vacant(ve) => {
+                    let (chan_tx, chan_rx) = tokio::sync::mpsc::channel(8);
+                    unclaimed_channels.insert(chan_id, chan_rx);
+                    chan_tx.try_send(buf).expect("empty channel succeeds");
+                    ve.insert(chan_tx);
+                }
+            }
+        }
+    }
+}
+
+pub struct ChannelHandle<LTR, RTL, const initiator: bool> {
+    marker: PhantomData<(LTR, RTL)>,
+}
+
+struct ChannelInner {
+    chan_id: u16,
+    tx: Sender<(u16, Vec<u8>)>,
+    rx: Receiver<Vec<u8>>,
+}
+
+pub struct ChannelLock<'a, R, W> {
+    // TODO we could use an OwnedMutexGuard if needed
+    guard: MutexGuard<'a, ChannelInner>,
+    marker: PhantomData<(R, W)>,
+}
+
+impl<'a, R: DeserializeOwned, W: Serialize> ChannelLock<'a, R, W> {
+    pub async fn send(&mut self, message: &W) -> Result<(), CnrError> {
+        let bytes = serde_bare::to_vec(message)?;
+        self.guard
+            .tx
+            .send((self.guard.chan_id, bytes))
+            .await
+            .map_err(|_| CnrError::Closed)?;
+        Ok(())
+    }
+
+    pub async fn recv(&mut self) -> Result<R, CnrError> {
+        match self.guard.rx.recv().await {
+            Some(bytes) => Ok(serde_bare::from_slice(&bytes)?),
+            None => Err(CnrError::Closed),
+        }
+    }
+}
+
+pub struct Channel<R, W> {
+    pub id: u16,
+    inner: Arc<Mutex<ChannelInner>>,
+    marker: PhantomData<(R, W)>,
+}
+
+impl<R: Send + DeserializeOwned, W: Serialize> Channel<R, W> {
+    pub async fn lock(&self) -> ChannelLock<'_, R, W> {
+        ChannelLock {
+            guard: self.inner.lock().await,
+            marker: Default::default(),
+        }
+    }
+
+    pub async fn send(&mut self, message: &W) -> Result<(), CnrError> {
+        self.lock().await.send(message).await
+    }
+
+    pub async fn recv(&mut self) -> Result<R, CnrError> {
+        self.lock().await.recv().await
+    }
+}
diff --git a/bare_cnr/src/transport.rs b/bare_cnr/src/transport.rs
new file mode 100644
index 0000000..561e741
--- /dev/null
+++ b/bare_cnr/src/transport.rs
@@ -0,0 +1,43 @@
+use serde::de::DeserializeOwned;
+use serde::Serialize;
+use tokio::io::{AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt};
+
+use crate::CnrError;
+
+pub struct BareTransport<R, W> {
+    writer: W,
+    reader: R,
+}
+
+impl<W: AsyncWrite + Unpin, R: AsyncRead + Unpin> BareTransport<R, W> {
+    pub fn new(writer: W, reader: R) -> Self {
+        Self { writer, reader }
+    }
+
+    pub async fn write_one_message<M: Serialize>(&mut self, message: &M) -> Result<(), CnrError> {
+        let bytes = serde_bare::to_vec(message)?;
+        self.writer
+            .write_u32(bytes.len().try_into().unwrap())
+            .await?;
+        self.writer.write_all(&bytes).await?;
+        // TODO flush?
+        Ok(())
+    }
+
+    pub async fn read_one_message<M: DeserializeOwned>(&mut self) -> Result<Option<M>, CnrError> {
+        let length = self.reader.read_u32().await? as usize;
+        // TODO(perf): use uninit?
+        let mut buf = vec![0u8; length];
+
+        let first_read = self.reader.read(&mut buf[..]).await?;
+        if first_read == 0 {
+            return Ok(None);
+        }
+
+        if first_read != length {
+            self.reader.read_exact(&mut buf[first_read..]).await?;
+        }
+
+        return Ok(Some(serde_bare::from_slice(&buf)?));
+    }
+}