diff --git a/tines-core/src/exchange.rs b/tines-core/src/exchange.rs
new file mode 100644
index 0000000..71308d5
--- /dev/null
+++ b/tines-core/src/exchange.rs
@@ -0,0 +1,647 @@
+//! Exchange state machine for tines-core.
+//!
+//! [`Exchange`] tracks the peers this node knows about and drives the
+//! content-defined gossip protocol: acknowledging received observations,
+//! summarizing what we hold, requesting what peers are missing, and delivering
+//! observations (fragmenting them across the wire when they exceed the MTU).
+//!
+//! Everything here is `#![no_std]`-clean and touches **no allocation**: peers
+//! live in a fixed [`heapless`] array, observation send queues are
+//! [`heapless`] vectors, and fragment buffers are stack arrays. No path reads
+//! untrusted wire bytes without first going through [`Frame::decode`], which
+//! turns any malformed input into an `Err` that the caller drops.
+
+use crate::consts;
+use crate::{Bloom, Event, Frame, NodeId, Observation, ObservationId, Reassembler, Store, Tick};
+use heapless::Vec;
+
+/// The maximum number of fragments an observation may be split into.
+///
+/// Matches [`crate::fragment`]: the reassembler tracks up to 64 indices via a
+/// bitset, so `count` is capped at 64 on both ends.
+const MAX_FRAGMENTS: usize = 64;
+
+/// A single peer the [`Exchange`] is tracking.
+pub struct Peer<A> {
+    /// Wire address of the peer.
+    pub addr: A,
+    /// Node id learned from the first frame received from this peer.
+    pub id: Option<NodeId>,
+    /// The peer's last-sent summary, if any. Used to queue only what they lack.
+    pub their_summary: Option<Bloom>,
+    /// Observations to send to this peer, youngest (lowest current age) first;
+    /// the front (`index 0`) is the next id to emit.
+    pub send_queue: Vec<ObservationId, { consts::SEND_QUEUE_CAP }>,
+    /// Tick of the last summary we sent to this peer, if any.
+    pub last_summary_sent: Option<Tick>,
+    /// Reassembler for inbound fragmented streams from this peer.
+    pub reassembler: Reassembler,
+    /// An observation currently mid-fragmentation: `(id, next fragment index)`.
+    pub in_flight: Option<(ObservationId, u8)>,
+}
+
+impl<A> Peer<A> {
+    /// Creates a peer with no known id, no summary, empty queue, and a fresh
+    /// reassembler — `last_summary_sent` is `None` so a summary is emitted on
+    /// the next [`Exchange::next_frame`].
+    fn new(addr: A) -> Self {
+        Peer {
+            addr,
+            id: None,
+            their_summary: None,
+            send_queue: Vec::new(),
+            last_summary_sent: None,
+            reassembler: Reassembler::new(),
+            in_flight: None,
+        }
+    }
+}
+
+/// The exchange state machine, tracking up to [`consts::MAX_PEERS`] peers.
+pub struct Exchange<A> {
+    me: NodeId,
+    salt: u32,
+    peers: Vec<Peer<A>, { consts::MAX_PEERS }>,
+    rr: usize,
+}
+
+impl<A: Clone + PartialEq + core::fmt::Debug> Exchange<A> {
+    /// Creates an empty exchange for local node `me`. The salt starts at 0.
+    pub fn new(me: NodeId) -> Self {
+        Exchange {
+            me,
+            salt: 0,
+            peers: Vec::new(),
+            rr: 0,
+        }
+    }
+
+    /// Handles one transport event.
+    ///
+    /// Returns `Some(obs)` when an inbound observation is newly stored (the
+    /// caller may react); otherwise `None`.
+    pub fn on_event<S: Store>(&mut self, ev: Event<A>, store: &mut S, now: Tick) -> Option<Observation> {
+        match ev {
+            Event::Contact(addr) => {
+                self.add_peer(addr);
+                None
+            }
+            Event::Lost(addr) => {
+                self.remove_peer(&addr);
+                None
+            }
+            Event::Frame(addr, bytes) => self.on_frame(addr, &bytes, store, now),
+        }
+    }
+
+    /// Produces at most one outbound frame, writing its bytes into `out` and
+    /// returning the destination address and byte count.
+    ///
+    /// Priority: (1) send a Summary to any peer due for one, then (2) emit one
+    /// Obs/Frag frame from a peer via round-robin.
+    pub fn next_frame<S: Store>(&mut self, store: &S, mtu: usize, now: Tick, out: &mut [u8]) -> Option<(A, usize)> {
+        let me = self.me;
+
+        // Priority 1: a summary is due for some peer.
+        for peer in self.peers.iter_mut() {
+            if !summary_due(peer, now) {
+                continue;
+            }
+            let mut bloom = Bloom::new(self.salt);
+            store.for_each_id(&mut |id| bloom.insert(id));
+            let bits = match Vec::<u8, { consts::SUMMARY_BYTES }>::from_slice(bloom.bits()) {
+                Ok(bits) => bits,
+                Err(_) => return None,
+            };
+            let frame = Frame::Summary {
+                from: me,
+                salt: self.salt,
+                bits,
+            };
+            let n = frame.encode(out).ok()?;
+            peer.last_summary_sent = Some(now);
+            self.salt = self.salt.wrapping_add(1);
+            return Some((peer.addr.clone(), n));
+        }
+
+        // Priority 2: round-robin, emit one frame from a single peer.
+        let len = self.peers.len();
+        if len == 0 {
+            return None;
+        }
+        for _ in 0..len {
+            let idx = self.rr % len;
+            self.rr = self.rr.wrapping_add(1);
+            let peer = &mut self.peers[idx];
+            if let Some(res) = emit_one(me, peer, store, mtu, now, out) {
+                return Some(res);
+            }
+        }
+        None
+    }
+
+    /// Returns the tracked peers.
+    pub fn peers(&self) -> &[Peer<A>] {
+        &self.peers
+    }
+
+    fn add_peer(&mut self, addr: A) {
+        if self.peers.iter().any(|p| p.addr == addr) {
+            return;
+        }
+        if self.peers.len() < consts::MAX_PEERS {
+            let _ = self.peers.push(Peer::new(addr));
+        }
+    }
+
+    fn remove_peer(&mut self, addr: &A) {
+        if let Some(idx) = self.peers.iter().position(|p| p.addr == *addr) {
+            self.peers.remove(idx);
+        }
+    }
+
+    /// Returns a mutable reference to the peer at `addr`, adding it as an
+    /// implicit contact if unknown and the table is not full.
+    fn get_peer_mut(&mut self, addr: &A) -> Option<&mut Peer<A>> {
+        if let Some(idx) = self.peers.iter().position(|p| &p.addr == addr) {
+            return self.peers.get_mut(idx);
+        }
+        if self.peers.len() < consts::MAX_PEERS {
+            let _ = self.peers.push(Peer::new(addr.clone()));
+            let idx = self.peers.len() - 1;
+            return self.peers.get_mut(idx);
+        }
+        None
+    }
+
+    fn on_frame<S: Store>(&mut self, addr: A, bytes: &[u8], store: &mut S, now: Tick) -> Option<Observation> {
+        let frame = match Frame::decode(bytes) {
+            Ok(frame) => frame,
+            Err(_) => return None,
+        };
+        let peer = self.get_peer_mut(&addr)?;
+        peer.id = Some(frame.sender());
+        match frame {
+            Frame::Summary { salt, bits, .. } => {
+                if let Some(bloom) = Bloom::from_parts(salt, &bits) {
+                    peer.their_summary = Some(bloom);
+                    rebuild_send_queue(peer, &*store, now);
+                }
+            }
+            Frame::Obs { from: _, obs } => {
+                let returned = obs.clone();
+                return match store.insert(obs, now) {
+                    Ok(true) => Some(returned),
+                    Ok(false) | Err(_) => None,
+                };
+            }
+            Frame::Frag { from: _, id, index, count, data } => {
+                if let Some(obs) = peer.reassembler.feed(id, index, count, &data, now) {
+                    let returned = obs.clone();
+                    return match store.insert(obs, now) {
+                        Ok(true) => Some(returned),
+                        Ok(false) | Err(_) => None,
+                    };
+                }
+            }
+        }
+        None
+    }
+}
+
+/// Returns `true` when a peer is due for a summary: none sent yet, or the
+/// summary interval has elapsed since the last one.
+fn summary_due<A>(peer: &Peer<A>, now: Tick) -> bool {
+    match peer.last_summary_sent {
+        None => true,
+        Some(sent) => now.saturating_sub(sent) >= consts::SUMMARY_INTERVAL_MS,
+    }
+}
+
+/// Emits at most one frame for `peer`: continuing an in-flight fragment if one
+/// is pending, otherwise draining the send queue (skipping ids no longer
+/// stored).
+fn emit_one<A: Clone, S: Store>(me: NodeId, peer: &mut Peer<A>, store: &S, mtu: usize, now: Tick, out: &mut [u8]) -> Option<(A, usize)> {
+    // in_flight takes precedence: continue the current fragmentation.
+    if let Some((id, index)) = peer.in_flight {
+        match store.get(&id, now) {
+            Some(obs) => return emit_fragment(me, peer, id, index, obs, mtu, out),
+            None => {
+                peer.in_flight = None;
+                return None;
+            }
+        }
+    }
+
+    while let Some(&front) = peer.send_queue.first() {
+        if !store.contains(&front) {
+            peer.send_queue.remove(0);
+            continue;
+        }
+        let Some(obs) = store.get(&front, now) else {
+            peer.send_queue.remove(0);
+            continue;
+        };
+        if let Some(res) = emit_fragment(me, peer, front, 0, obs, mtu, out) {
+            return Some(res);
+        }
+        peer.send_queue.remove(0);
+    }
+    None
+}
+
+/// Emits exactly one frame for `id`: a single Obs frame if it fits in `mtu`,
+/// otherwise one fragment of the postcard-encoded observation. Updates
+/// `in_flight`/`send_queue` state accordingly.
+fn emit_fragment<A: Clone>(
+    me: NodeId,
+    peer: &mut Peer<A>,
+    id: ObservationId,
+    index: u8,
+    obs: Observation,
+    mtu: usize,
+    out: &mut [u8],
+) -> Option<(A, usize)> {
+    // 1. Try the whole Observation as a single Obs frame.
+    let frame = Frame::Obs {
+        from: me,
+        obs: obs.clone(),
+    };
+    if let Ok(n) = frame.encode(out) {
+        if n <= mtu {
+            peer.send_queue.retain(|&x| x != id);
+            peer.in_flight = None;
+            return Some((peer.addr.clone(), n));
+        }
+    }
+
+    // 2. Otherwise emit a single fragment of the postcard-encoded observation.
+    let mut obs_buf = [0u8; consts::MAX_FRAME * 2];
+    let obs_bytes = postcard::to_slice(&obs, &mut obs_buf).ok()?;
+    let chunk = mtu.saturating_sub(crate::frag::FRAG_OVERHEAD);
+    if chunk == 0 {
+        return None;
+    }
+    #[allow(clippy::manual_div_ceil)]
+    let count = (obs_bytes.len() + chunk - 1) / chunk;
+    if count == 0 || count > MAX_FRAGMENTS {
+        return None;
+    }
+
+    let start = (index as usize).saturating_mul(chunk);
+    let end = core::cmp::min(start + chunk, obs_bytes.len());
+    let data = &obs_bytes[start..end];
+    let frag_data = Vec::<u8, { consts::MAX_FRAME }>::from_slice(data).ok()?;
+    let frame = Frame::Frag {
+        from: me,
+        id,
+        index,
+        count: count as u8,
+        data: frag_data,
+    };
+    let n = frame.encode(out).ok()?;
+
+    if (index as u32).wrapping_add(1) >= count as u32 {
+        peer.send_queue.retain(|&x| x != id);
+        peer.in_flight = None;
+    } else {
+        peer.in_flight = Some((id, index.wrapping_add(1)));
+    }
+    Some((peer.addr.clone(), n))
+}
+
+/// Rebuilds a peer's send queue to the ids in `store` they do not already hold
+/// (per their summary), kept youngest-first and capped at `SEND_QUEUE_CAP`.
+fn rebuild_send_queue<A, S: Store>(peer: &mut Peer<A>, store: &S, now: Tick) {
+    let Some(their) = peer.their_summary.as_ref() else {
+        return;
+    };
+
+    // Top-K youngest-first: a sorted (age ascending) buffer capped at the queue
+    // capacity. When full, only a candidate younger than the current oldest
+    // (last element) is admitted, dropping that oldest.
+    let mut heap: Vec<(ObservationId, u64), { consts::SEND_QUEUE_CAP }> = Vec::new();
+    store.for_each_id(&mut |id| {
+        if their.might_contain(id) {
+            return;
+        }
+        let obs = match store.get(id, now) {
+            Some(obs) => obs,
+            None => return,
+        };
+        insert_youngest(&mut heap, *id, obs.age_ms);
+    });
+
+    peer.send_queue.clear();
+    for (id, _) in heap {
+        peer.send_queue.push(id).expect("heap is capped at SEND_QUEUE_CAP");
+    }
+}
+
+/// Inserts `(id, age)` into the age-ascending top-K buffer, keeping it sorted
+/// and capped at `SEND_QUEUE_CAP`.
+fn insert_youngest(heap: &mut Vec<(ObservationId, u64), { consts::SEND_QUEUE_CAP }>, id: ObservationId, age: u64) {
+    if heap.len() < consts::SEND_QUEUE_CAP {
+        let pos = insert_pos(heap, age);
+        let _ = heap.insert(pos, (id, age));
+        return;
+    }
+    // Full: admit only if younger than the current oldest (last element).
+    if let Some((_, oldest)) = heap.last() {
+        if age < *oldest {
+            heap.pop();
+            let pos = insert_pos(heap, age);
+            let _ = heap.insert(pos, (id, age));
+        }
+    }
+}
+
+/// Returns the index at which `age` belongs to keep `heap` age-ascending.
+fn insert_pos(heap: &Vec<(ObservationId, u64), { consts::SEND_QUEUE_CAP }>, age: u64) -> usize {
+    let mut pos = heap.len();
+    while pos > 0 && heap[pos - 1].1 > age {
+        pos -= 1;
+    }
+    pos
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::{MemStore, Observation, Payload, Topic};
+    use std::vec::Vec;
+
+    const ME: NodeId = [42; 16];
+
+    fn obs(observer: [u8; 16], seq: u64, age: u64, at: Option<u64>, topic: &[u8], payload: &[u8]) -> Observation {
+        Observation {
+            observer,
+            seq,
+            age_ms: age,
+            observed_at: at,
+            topic: Topic::from_slice(topic).unwrap(),
+            payload: Payload::from_slice(payload).unwrap(),
+        }
+    }
+
+    /// Encodes a frame into a stack [`FrameBuf`] for feeding [`Event::Frame`].
+    fn frame_bytes(f: &Frame) -> heapless::Vec<u8, { consts::MAX_FRAME }> {
+        let mut buf = [0u8; consts::MAX_FRAME];
+        let n = f.encode(&mut buf).expect("frame fits in MAX_FRAME");
+        heapless::Vec::from_slice(&buf[..n]).expect("encoded frame fits")
+    }
+
+    #[test]
+    fn contact_triggers_summary_frame() {
+        let mut ex = Exchange::new(ME);
+        let mut store = MemStore::new();
+        ex.on_event(Event::Contact(1), &mut store, 0);
+
+        let mut buf = [0u8; consts::MAX_FRAME];
+        let (addr, n) = ex.next_frame(&store, consts::MAX_FRAME, 0, &mut buf).expect("summary on contact");
+        assert_eq!(addr, 1);
+        match Frame::decode(&buf[..n]).unwrap() {
+            Frame::Summary { from, .. } => assert_eq!(from, ME),
+            other => panic!("expected a Summary frame, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn summary_from_peer_queues_only_what_they_lack() {
+        let mut ex = Exchange::new(ME);
+        let mut store = MemStore::new();
+        let a = obs([2; 16], 1, 100, None, b"t", b"a");
+        let b = obs([2; 16], 2, 50, None, b"t", b"b");
+        let c = obs([2; 16], 3, 10, None, b"t", b"c");
+        let id_a = a.id();
+        let id_b = b.id();
+        let id_c = c.id();
+        store.insert(a, 0).unwrap();
+        store.insert(b, 0).unwrap();
+        store.insert(c, 0).unwrap();
+
+        // Peer contacts us, then sends a summary containing only id B.
+        ex.on_event(Event::Contact(1), &mut store, 0);
+        let mut their = Bloom::new(0);
+        their.insert(&id_b);
+        let bits = heapless::Vec::from_slice(their.bits()).unwrap();
+        let sum = Frame::Summary {
+            from: [2; 16],
+            salt: 0,
+            bits,
+        };
+        ex.on_event(Event::Frame(1, frame_bytes(&sum)), &mut store, 0);
+
+        // First poll still emits our own Summary (peer never saw ours).
+        let mut emitted = Vec::new();
+        for _ in 0..10 {
+            let mut buf = [0u8; consts::MAX_FRAME];
+            match ex.next_frame(&store, consts::MAX_FRAME, 0, &mut buf) {
+                Some((_, n)) => emitted.push(Frame::decode(&buf[..n]).unwrap()),
+                None => break,
+            }
+        }
+
+        assert_eq!(emitted.len(), 3, "our summary + Obs for A and C only");
+        match &emitted[0] {
+            Frame::Summary { from, .. } => assert_eq!(*from, ME),
+            other => panic!("expected our Summary first, got {other:?}"),
+        }
+        // Youngest-first among {A, C}: C (age 10) before A (age 100).
+        assert_obs_id(&emitted[1], id_c);
+        assert_obs_id(&emitted[2], id_a);
+        assert!(
+            !emitted.iter().any(|f| matches!(f, Frame::Obs { obs, .. } if obs.id() == id_b)),
+            "B must be omitted"
+        );
+        assert!(ex.next_frame(&store, consts::MAX_FRAME, 0, &mut [0u8; consts::MAX_FRAME]).is_none());
+    }
+
+    fn assert_obs_id(frame: &Frame, id: ObservationId) {
+        match frame {
+            Frame::Obs { obs, .. } => assert_eq!(obs.id(), id),
+            other => panic!("expected an Obs frame, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn received_obs_is_stored_and_returned() {
+        let mut ex = Exchange::new(ME);
+        let mut store = MemStore::new();
+        ex.on_event(Event::Contact(1), &mut store, 0);
+        let obs = obs([2; 16], 7, 0, None, b"t", b"hi");
+        let frame = Frame::Obs {
+            from: [2; 16],
+            obs: obs.clone(),
+        };
+        let returned = ex.on_event(Event::Frame(1, frame_bytes(&frame)), &mut store, 0).expect("obs returned");
+        assert_eq!(returned, obs);
+        assert!(store.contains(&obs.id()));
+    }
+
+    #[test]
+    fn duplicate_obs_returns_none() {
+        let mut ex = Exchange::new(ME);
+        let mut store = MemStore::new();
+        ex.on_event(Event::Contact(1), &mut store, 0);
+        let obs = obs([2; 16], 7, 0, None, b"t", b"hi");
+        let frame = Frame::Obs {
+            from: [2; 16],
+            obs: obs.clone(),
+        };
+        assert!(ex.on_event(Event::Frame(1, frame_bytes(&frame)), &mut store, 0).is_some());
+        let dup = Frame::Obs {
+            from: [2; 16],
+            obs: obs.clone(),
+        };
+        assert!(ex.on_event(Event::Frame(1, frame_bytes(&dup)), &mut store, 0).is_none());
+    }
+
+    #[test]
+    fn malformed_frame_is_dropped() {
+        let mut ex = Exchange::new(ME);
+        let mut store = MemStore::new();
+        ex.on_event(Event::Contact(1), &mut store, 0);
+        let before = store.len();
+        let bogus = Event::Frame(1, heapless::Vec::from_slice(&[0xFFu8; 50]).unwrap());
+        assert_eq!(ex.on_event(bogus, &mut store, 0), None);
+        assert_eq!(store.len(), before, "store must be unchanged by a malformed frame");
+    }
+
+    #[test]
+    fn frame_from_unknown_addr_creates_peer() {
+        let mut ex = Exchange::new(ME);
+        let mut store = MemStore::new();
+        // No Contact: a Summary from addr 9 must implicitly create the peer.
+        let bits = heapless::Vec::from_slice(&[0u8; consts::SUMMARY_BYTES]).unwrap();
+        let sum = Frame::Summary {
+            from: [9; 16],
+            salt: 0,
+            bits,
+        };
+        ex.on_event(Event::Frame(9, frame_bytes(&sum)), &mut store, 0);
+        let peers = ex.peers();
+        assert_eq!(peers.len(), 1);
+        assert_eq!(peers[0].id, Some([9; 16]));
+    }
+
+    #[test]
+    fn lost_removes_peer() {
+        let mut ex = Exchange::new(ME);
+        let mut store = MemStore::new();
+        ex.on_event(Event::Contact(1), &mut store, 0);
+        assert_eq!(ex.peers().len(), 1);
+        ex.on_event(Event::Lost(1), &mut store, 0);
+        assert_eq!(ex.peers().len(), 0);
+    }
+
+    #[test]
+    fn peer_table_full_ignores_new_contact() {
+        let mut ex = Exchange::new(ME);
+        let mut store = MemStore::new();
+        for i in 1..=consts::MAX_PEERS as u8 {
+            ex.on_event(Event::Contact(i), &mut store, 0);
+        }
+        assert_eq!(ex.peers().len(), consts::MAX_PEERS);
+        // Overflow contact is ignored, not a panic.
+        ex.on_event(Event::Contact(200), &mut store, 0);
+        assert_eq!(ex.peers().len(), consts::MAX_PEERS);
+    }
+
+    #[test]
+    fn summary_resent_after_interval() {
+        let mut ex = Exchange::new(ME);
+        let mut store = MemStore::new();
+        ex.on_event(Event::Contact(1), &mut store, 0);
+
+        let mut buf = [0u8; consts::MAX_FRAME];
+        let (_, n) = ex.next_frame(&store, consts::MAX_FRAME, 0, &mut buf).expect("first summary");
+        match Frame::decode(&buf[..n]).unwrap() {
+            Frame::Summary { salt, .. } => assert_eq!(salt, 0),
+            other => panic!("expected a Summary, got {other:?}"),
+        }
+
+        // Immediately again: our summary is fresh, nothing else queued -> None.
+        assert!(ex
+            .next_frame(&store, consts::MAX_FRAME, 0, &mut [0u8; consts::MAX_FRAME])
+            .is_none());
+
+        // After the interval: another Summary, now rotated to salt 1.
+        let mut buf = [0u8; consts::MAX_FRAME];
+        let (_, n) = ex
+            .next_frame(&store, consts::MAX_FRAME, consts::SUMMARY_INTERVAL_MS, &mut buf)
+            .expect("second summary");
+        match Frame::decode(&buf[..n]).unwrap() {
+            Frame::Summary { salt, .. } => assert_eq!(salt, 1),
+            other => panic!("expected a Summary, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn salt_rotates_each_summary() {
+        let mut ex = Exchange::new(ME);
+        let mut store = MemStore::new();
+        ex.on_event(Event::Contact(1), &mut store, 0);
+        let s0 = summary_salt(&mut ex, &store, 0);
+        let s1 = summary_salt(&mut ex, &store, consts::SUMMARY_INTERVAL_MS);
+        assert_eq!(s0, 0);
+        assert_eq!(s1, s0 + 1);
+    }
+
+    fn summary_salt(ex: &mut Exchange<u8>, store: &MemStore, now: Tick) -> u32 {
+        let mut buf = [0u8; consts::MAX_FRAME];
+        let (_, n) = ex.next_frame(store, consts::MAX_FRAME, now, &mut buf).expect("summary");
+        match Frame::decode(&buf[..n]).unwrap() {
+            Frame::Summary { salt, .. } => salt,
+            other => panic!("expected a Summary, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn large_obs_goes_out_as_frags() {
+        let mut ex = Exchange::new(ME);
+        let mut store = MemStore::new();
+        let big = obs(ME, 1, 0, None, b"t", &[0u8; 1024]);
+        let id = big.id();
+        store.insert(big, 0).unwrap();
+
+        // Peer sends an empty summary (holds nothing) so it lacks everything.
+        ex.on_event(Event::Contact(1), &mut store, 0);
+        let bits = heapless::Vec::from_slice(&[0u8; consts::SUMMARY_BYTES]).unwrap();
+        let sum = Frame::Summary {
+            from: [1; 16],
+            salt: 0,
+            bits,
+        };
+        ex.on_event(Event::Frame(1, frame_bytes(&sum)), &mut store, 0);
+
+        // mtu 200 forces fragmentation; first poll still emits our Summary.
+        let mtu = 200usize;
+        let mut indices = Vec::new();
+        for _ in 0..consts::MAX_FRAME {
+            let mut buf = [0u8; consts::MAX_FRAME];
+            match ex.next_frame(&store, mtu, 0, &mut buf) {
+                Some((_, n)) => match Frame::decode(&buf[..n]).unwrap() {
+                    Frame::Summary { .. } => {}
+                    Frame::Frag { index, count, .. } => indices.push((index, count)),
+                    other => panic!("expected only Frag frames, got {other:?}"),
+                },
+                None => break,
+            }
+        }
+
+        assert!(!indices.is_empty(), "the large observation must be fragmented");
+        let count = indices[0].1;
+        assert!(count >= 2, "a 1024-byte payload must span multiple fragments");
+        for (i, &(index, c)) in indices.iter().enumerate() {
+            assert_eq!(index, i as u8, "fragments must carry increasing indices");
+            assert_eq!(c, count, "all fragments share the same count");
+        }
+        let last = indices.last().copied().unwrap();
+        assert_eq!(last.0 + 1, last.1, "fragment indices are contiguous to count");
+        assert_eq!(indices.len(), count as usize);
+        // After draining, in_flight clears and the queue drops the id.
+        assert!(ex.peers()[0].in_flight.is_none());
+        assert!(!ex.peers()[0].send_queue.contains(&id));
+    }
+}
diff --git a/tines-core/src/lib.rs b/tines-core/src/lib.rs
index 8257be3..8a08ccd 100644
--- a/tines-core/src/lib.rs
+++ b/tines-core/src/lib.rs
@@ -6,6 +6,7 @@ extern crate alloc;
 
 pub mod bloom;
 pub mod consts;
+pub mod exchange;
 pub mod frag;
 pub mod observation;
 pub mod store;
@@ -15,6 +16,7 @@ pub mod transport;
 pub mod wire;
 
 pub use bloom::Bloom;
+pub use exchange::{Exchange, Peer};
 pub use frag::{fragment, Reassembler};
 pub use observation::{Observation, NodeId, ObservationId, Payload, Tick, Topic};
 pub use store::{Store, TrimPolicy, StoreError};