diff --git a/vortex-io/src/lib.rs b/vortex-io/src/lib.rs
index fabeb220934..f7b3cde6161 100644
--- a/vortex-io/src/lib.rs
+++ b/vortex-io/src/lib.rs
@@ -25,6 +25,7 @@ pub mod object_store;
 mod read_at;
 pub mod request;
 pub mod runtime;
+pub mod segments;
 pub mod session;
 #[cfg(not(target_arch = "wasm32"))]
 pub mod std_file;
diff --git a/vortex-io/src/segments.rs b/vortex-io/src/segments.rs
new file mode 100644
index 00000000000..390419528cb
--- /dev/null
+++ b/vortex-io/src/segments.rs
@@ -0,0 +1,382 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: Copyright the Vortex contributors
+
+//! Reads for scans that drive their own IO.
+//!
+//! A request names what to read by [`IoKey`], a registered segment source and a
+//! [`SourceRequest`] of it (one of its segments, or its size), and says how much it needs it with
+//! an [`IoIntent`]. One service serves many sources, so one session can read from all of them.
+//! A [`SegmentIoSession`] registers a whole batch of them before any can be read, and returns one
+//! [`IoHandle`] per request. The handle is the request: its result arrives in it. The session holds fetches and prefetches until they finish; an announcement is
+//! withdrawn by dropping every handle of it.
+//!
+//! A [`SegmentIoService`] serves every registered source. Its sessions share and coalesce reads,
+//! so one session's announcement can widen another session's read.
+//!
+//! This protocol is to replace the byte-range protocol in [`request`](crate::request), which the
+//! planner-driven scan still uses.
+
+use std::any::Any;
+use std::pin::Pin;
+use std::sync::Arc;
+use std::task::Context;
+use std::task::Poll;
+
+use futures::FutureExt;
+use futures::StreamExt;
+use futures::future::BoxFuture;
+use futures::future::Shared;
+use futures::stream::FuturesUnordered;
+use vortex_array::buffer::BufferHandle;
+use vortex_error::VortexError;
+use vortex_error::VortexResult;
+use vortex_error::vortex_bail;
+use vortex_error::vortex_err;
+
+/// A segment source registered with a [`SegmentIoService`], such as one opened file.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct SegmentSourceId(pub u32);
+
+/// What to read from a segment source.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub enum SourceRequest {
+    /// The source's total length in bytes, answered with [`SourceResult::Size`].
+    Size,
+    /// One segment, by its id within the source (the value of a
+    /// `vortex_layout::segments::SegmentId`), answered with [`SourceResult::Bytes`].
+    Segment(u32),
+}
+
+/// What one request reads: a source, and what to read from it.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct IoKey {
+    /// The source to read.
+    pub source: SegmentSourceId,
+    /// What to read from it.
+    pub request: SourceRequest,
+}
+
+impl IoKey {
+    /// The key of `request` of `source`.
+    pub fn new(source: SegmentSourceId, request: SourceRequest) -> Self {
+        Self { source, request }
+    }
+
+    /// The key of `segment` of `source`.
+    pub fn segment(source: SegmentSourceId, segment: u32) -> Self {
+        Self::new(source, SourceRequest::Segment(segment))
+    }
+
+    /// The key of the size of `source`.
+    pub fn size(source: SegmentSourceId) -> Self {
+        Self::new(source, SourceRequest::Size)
+    }
+}
+
+/// The answer to one request.
+#[derive(Clone, Debug)]
+pub enum SourceResult {
+    /// The source's total length in bytes.
+    Size(u64),
+    /// A segment's bytes.
+    Bytes(BufferHandle),
+}
+
+impl SourceResult {
+    /// The bytes of a segment read, failing for any other answer.
+    pub fn into_bytes(self) -> VortexResult<BufferHandle> {
+        match self {
+            Self::Bytes(bytes) => Ok(bytes),
+            Self::Size(_) => vortex_bail!("expected segment bytes, got a size"),
+        }
+    }
+
+    /// The size a size request was answered with, failing for any other answer.
+    pub fn size(&self) -> VortexResult<u64> {
+        match self {
+            Self::Size(size) => Ok(*size),
+            Self::Bytes(_) => vortex_bail!("expected a size, got segment bytes"),
+        }
+    }
+}
+
+/// How much a request needs what it reads.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub enum IoIntent {
+    /// Needed now. Its owner is reported by [`SegmentIoSession::poll_ready`] when it finishes.
+    /// A service may read fetches ahead of prefetches.
+    Fetch,
+    /// Needed soon: read, but its owner is not reported when it finishes.
+    Prefetch,
+    /// May be needed: only widens nearby reads, and is never read on its own. Dropping its
+    /// handles withdraws it.
+    Announce,
+}
+
+/// One read a planner or morsel asks for.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub struct IoRequest {
+    /// What to read.
+    pub key: IoKey,
+    /// How much it is needed.
+    pub intent: IoIntent,
+}
+
+impl IoRequest {
+    /// A request for `key` with `intent`.
+    pub fn new(key: IoKey, intent: IoIntent) -> Self {
+        Self { key, intent }
+    }
+}
+
+/// Identifies the object that submitted a request, chosen by the caller.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct IoOwnerId(pub u64);
+
+/// The result of one read, shared by every handle of it.
+pub type SharedResult = Shared<BoxFuture<'static, Result<SourceResult, Arc<VortexError>>>>;
+
+/// One registered read.
+///
+/// Clones share the read. The read stays registered while any handle of it, from any session, is
+/// alive. A session also holds its fetches and prefetches until they finish, so dropping every
+/// handle withdraws a read only when it was announced alone.
+#[derive(Clone)]
+pub struct IoHandle {
+    result: SharedResult,
+    /// Keeps the read registered with its service while the handle lives.
+    _registration: Option<Arc<dyn Any + Send + Sync>>,
+}
+
+impl IoHandle {
+    /// A handle whose result arrives through `result`, keeping `registration` alive while it
+    /// lives.
+    pub fn new(result: SharedResult, registration: Arc<dyn Any + Send + Sync>) -> Self {
+        Self {
+            result,
+            _registration: Some(registration),
+        }
+    }
+
+    /// A handle that already holds its result, such as a cache hit or a source's size.
+    pub fn ready(result: VortexResult<SourceResult>) -> Self {
+        let result = futures::future::ready(result.map_err(Arc::new))
+            .boxed()
+            .shared();
+        // `Shared::peek` only sees a result once a poll has completed the future, so poll it once
+        // here for `try_get` to see it without a waker.
+        drop(result.clone().now_or_never());
+        Self {
+            result,
+            _registration: None,
+        }
+    }
+
+    /// The read's result once it has arrived. Never blocks and never reads.
+    pub fn try_get(&self) -> Option<VortexResult<SourceResult>> {
+        self.result
+            .peek()
+            .map(|result| result.clone().map_err(VortexError::from))
+    }
+
+    /// The shared future the result arrives through, for sessions that wake on it.
+    pub fn result(&self) -> SharedResult {
+        self.result.clone()
+    }
+}
+
+/// One scope's reads, such as the work descended from one root planner.
+///
+/// Registration does no IO and never blocks. Every read is driven by the service, so nothing has
+/// to be polled for reads to progress; polling only learns which reads finished.
+pub trait SegmentIoSession: Send + Sync {
+    /// Registers the whole batch before any of it can be read, and returns one handle per
+    /// request, in order. Every fetch is reported by [`poll_ready`](Self::poll_ready) once it
+    /// finishes, including one already finished at submit, such as a cache hit or an error.
+    fn submit(&self, owner: IoOwnerId, batch: &[IoRequest]) -> VortexResult<Vec<IoHandle>>;
+
+    /// Moves the owners of fetches that finished since the last call into `ready`, once per
+    /// fetch, and drives the session's prefetches. Returns `Pending`, registering `cx` to be
+    /// woken by the next one, if none have, and fails if no fetch is in flight, since nothing
+    /// would wake the caller. An owner is reported even if it dropped the fetch's handles.
+    fn poll_ready(
+        &self,
+        cx: &mut Context<'_>,
+        ready: &mut Vec<IoOwnerId>,
+    ) -> Poll<VortexResult<()>>;
+}
+
+/// Serves the reads of every registered segment source.
+pub trait SegmentIoService: Send + Sync {
+    /// A new session for one scope.
+    fn session(&self) -> Arc<dyn SegmentIoSession>;
+}
+
+/// A source registered with a [`SegmentIoService`]: the service, the source's id in it, and the
+/// registration. The service can read the source while any clone of this is alive.
+#[derive(Clone)]
+pub struct RegisteredSource {
+    service: Arc<dyn SegmentIoService>,
+    id: SegmentSourceId,
+    _registration: Arc<dyn Any + Send + Sync>,
+}
+
+impl RegisteredSource {
+    /// The registration of `id` with `service`, kept alive by `registration`.
+    pub fn new(
+        service: Arc<dyn SegmentIoService>,
+        id: SegmentSourceId,
+        registration: Arc<dyn Any + Send + Sync>,
+    ) -> Self {
+        Self {
+            service,
+            id,
+            _registration: registration,
+        }
+    }
+
+    /// The service that reads the source.
+    pub fn service(&self) -> &Arc<dyn SegmentIoService> {
+        &self.service
+    }
+
+    /// The source's id in its service.
+    pub fn id(&self) -> SegmentSourceId {
+        self.id
+    }
+}
+
+/// The fetches and prefetches of one session, waking the session when a fetch finishes.
+///
+/// Sessions keep one of these to implement [`SegmentIoSession::poll_ready`]. Prefetches are
+/// polled until they finish, so their reads start and their results land in their handles even
+/// though nobody awaits them.
+#[derive(Default)]
+pub struct FetchTracker {
+    fetches: FuturesUnordered<Pin<Box<dyn Future<Output = IoOwnerId> + Send>>>,
+    prefetches: FuturesUnordered<SharedResult>,
+}
+
+impl FetchTracker {
+    /// Tracks a prefetch until it finishes, without reporting it.
+    pub fn track_prefetch(&mut self, handle: &IoHandle) {
+        self.prefetches.push(handle.result());
+    }
+
+    /// Tracks a fetch submitted by `owner`.
+    pub fn track(&mut self, owner: IoOwnerId, handle: &IoHandle) {
+        let bytes = handle.result();
+        self.fetches.push(Box::pin(async move {
+            // Errors are taken from the handle by its owner.
+            drop(bytes.await);
+            owner
+        }));
+    }
+
+    /// Moves the owners of finished fetches into `ready`, as [`SegmentIoSession::poll_ready`].
+    pub fn poll_ready(
+        &mut self,
+        cx: &mut Context<'_>,
+        ready: &mut Vec<IoOwnerId>,
+    ) -> Poll<VortexResult<()>> {
+        // Finished prefetches are dropped; their handles hold the results.
+        while let Poll::Ready(Some(_)) = self.prefetches.poll_next_unpin(cx) {}
+        let before = ready.len();
+        loop {
+            match self.fetches.poll_next_unpin(cx) {
+                Poll::Ready(Some(owner)) => ready.push(owner),
+                Poll::Ready(None) if ready.len() > before => return Poll::Ready(Ok(())),
+                Poll::Ready(None) => {
+                    return Poll::Ready(Err(vortex_err!(
+                        "waiting for segment reads with no fetch in flight"
+                    )));
+                }
+                Poll::Pending if ready.len() > before => return Poll::Ready(Ok(())),
+                Poll::Pending => return Poll::Pending,
+            }
+        }
+    }
+
+    /// Drops every tracked fetch and prefetch.
+    pub fn clear(&mut self) {
+        self.fetches.clear();
+        self.prefetches.clear();
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::task::Waker;
+
+    use futures::channel::oneshot;
+    use vortex_buffer::ByteBuffer;
+
+    use super::*;
+
+    fn pending() -> (oneshot::Sender<BufferHandle>, IoHandle) {
+        let (send, recv) = oneshot::channel::<BufferHandle>();
+        let result = recv
+            .map(|result| {
+                result
+                    .map(SourceResult::Bytes)
+                    .map_err(|_| Arc::new(vortex_err!("cancelled")))
+            })
+            .boxed()
+            .shared();
+        (send, IoHandle::new(result, Arc::new(())))
+    }
+
+    #[test]
+    fn ready_handle_needs_no_poll() -> VortexResult<()> {
+        let handle = IoHandle::ready(Ok(SourceResult::Bytes(BufferHandle::new_host(
+            ByteBuffer::copy_from(b"abc"),
+        ))));
+        let result = handle.try_get().ok_or_else(|| vortex_err!("not ready"))??;
+        assert_eq!(result.into_bytes()?.len(), 3);
+        Ok(())
+    }
+
+    #[test]
+    fn results_are_checked_against_what_was_asked() -> VortexResult<()> {
+        assert_eq!(SourceResult::Size(7).size()?, 7);
+        assert!(SourceResult::Size(7).into_bytes().is_err());
+        let bytes = SourceResult::Bytes(BufferHandle::new_host(ByteBuffer::copy_from(b"a")));
+        assert!(bytes.size().is_err());
+        Ok(())
+    }
+
+    #[test]
+    fn tracker_reports_owners_as_fetches_finish() -> VortexResult<()> {
+        let mut tracker = FetchTracker::default();
+        let (send_a, a) = pending();
+        let (_send_b, b) = pending();
+        tracker.track(IoOwnerId(1), &a);
+        tracker.track(IoOwnerId(2), &b);
+        let mut cx = Context::from_waker(Waker::noop());
+        let mut ready = Vec::new();
+        assert!(tracker.poll_ready(&mut cx, &mut ready).is_pending());
+        assert!(a.try_get().is_none());
+
+        send_a
+            .send(BufferHandle::new_host(ByteBuffer::copy_from(b"a")))
+            .map_err(|_| vortex_err!("receiver dropped"))?;
+        assert!(matches!(
+            tracker.poll_ready(&mut cx, &mut ready),
+            Poll::Ready(Ok(()))
+        ));
+        assert_eq!(ready, vec![IoOwnerId(1)]);
+        assert!(a.try_get().is_some());
+        assert!(b.try_get().is_none());
+        Ok(())
+    }
+
+    #[test]
+    fn tracker_fails_with_nothing_in_flight() {
+        let mut tracker = FetchTracker::default();
+        let mut cx = Context::from_waker(Waker::noop());
+        assert!(matches!(
+            tracker.poll_ready(&mut cx, &mut Vec::new()),
+            Poll::Ready(Err(_))
+        ));
+    }
+}