diff --git a/tines-core/tests/common/mod.rs b/tines-core/tests/common/mod.rs
new file mode 100644
index 0000000..50e9c89
--- /dev/null
+++ b/tines-core/tests/common/mod.rs
@@ -0,0 +1,97 @@
+//! Shared, test-only helper: two `PairTransport`s wired back-to-back as if they
+//! were peers on a datagram link. A `send` on one side surfaces as a `Frame`
+//! on the other; `connect`/`disconnect` broadcast `Contact`/`Lost`.
+//!
+//! Each side owns its own inbound queue (`inbox`) and holds a shared handle to
+//! the other side's queue (`peer_inbox`), so outbound frames land in the peer's
+//! queue — just like frames crossing a real link. `std` is fine here; this is a
+//! test fixture, not `tines-core` proper.
+
+use std::cell::RefCell;
+use std::collections::VecDeque;
+use std::rc::Rc;
+
+use tines_core::transport::{Event, FrameBuf, SendError, Transport};
+
+/// A pair of in-memory transports connected back-to-back for tests.
+///
+/// Addresses are `0` and `1`; the first handle handed back by [`PairTransport::pair`]
+/// is `0`, the second is `1`.
+pub struct PairTransport {
+    /// Ourselves: `0` or `1`.
+    me: u8,
+    /// Address of the peer this side talks to.
+    other: u8,
+    /// Our own inbound queue.
+    inbox: Rc<RefCell<VecDeque<Event<u8>>>>,
+    /// The peer's inbound queue, where our `send`s land.
+    peer_inbox: Rc<RefCell<VecDeque<Event<u8>>>>,
+    /// Configured MTU.
+    mtu: usize,
+}
+
+impl PairTransport {
+    /// Build a connected pair. Returns `(addr 0, addr 1)`.
+    pub fn pair(mtu: usize) -> (PairTransport, PairTransport) {
+        let inbox0: Rc<RefCell<VecDeque<Event<u8>>>> = Rc::new(RefCell::new(VecDeque::new()));
+        let inbox1: Rc<RefCell<VecDeque<Event<u8>>>> = Rc::new(RefCell::new(VecDeque::new()));
+
+        let a = PairTransport {
+            me: 0,
+            other: 1,
+            inbox: inbox0.clone(),
+            peer_inbox: inbox1.clone(),
+            mtu,
+        };
+        let b = PairTransport {
+            me: 1,
+            other: 0,
+            inbox: inbox1.clone(),
+            peer_inbox: inbox0.clone(),
+            mtu,
+        };
+        (a, b)
+    }
+
+    /// Announce that the peer is reachable.
+    ///
+    /// A real contact is symmetric: when two peers meet, each independently
+    /// observes it and calls `connect()` on its own transport, so this enqueues
+    /// `Contact(other)` on *this* side's queue only. (Broadcasting onto the
+    /// peer's queue as well would crowd out the peer's own `Contact` and any
+    /// in-flight frames — the paired peer learns of the contact via its own
+    /// `connect()`, as the test in `tests/pair.rs` exercises.)
+    pub fn connect(&mut self) {
+        let other = self.other;
+        self.inbox.borrow_mut().push_back(Event::Contact(other));
+    }
+
+    /// Announce that the peer is unreachable.
+    #[allow(dead_code)] // required by the transport interface; used by sim harness
+    pub fn disconnect(&mut self) {
+        let other = self.other;
+        self.inbox.borrow_mut().push_back(Event::Lost(other));
+    }
+}
+
+impl Transport for PairTransport {
+    type Addr = u8;
+
+    fn poll(&mut self) -> Option<Event<Self::Addr>> {
+        self.inbox.borrow_mut().pop_front()
+    }
+
+    fn send(&mut self, to: &Self::Addr, frame: &[u8]) -> Result<(), SendError> {
+        if *to != self.other {
+            return Err(SendError::Unreachable);
+        }
+        let buf = FrameBuf::from_slice(frame).map_err(|_| SendError::TooLarge)?;
+        let event = Event::Frame(self.me, buf);
+        self.peer_inbox.borrow_mut().push_back(event);
+        Ok(())
+    }
+
+    fn mtu(&self) -> usize {
+        self.mtu
+    }
+}
diff --git a/tines-core/tests/pair.rs b/tines-core/tests/pair.rs
new file mode 100644
index 0000000..ad9d93a
--- /dev/null
+++ b/tines-core/tests/pair.rs
@@ -0,0 +1,16 @@
+mod common;
+use common::PairTransport;
+
+use tines_core::transport::{Event, FrameBuf, Transport};
+
+#[test]
+fn pair_transport_connect_then_frame() {
+    let (mut a, mut b) = PairTransport::pair(1500);
+    a.connect();
+    assert_eq!(a.poll(), Some(Event::Contact(1)));
+    a.send(&1, b"hi").unwrap();
+    assert_eq!(
+        b.poll(),
+        Some(Event::Frame(0, FrameBuf::from_slice(b"hi").unwrap()))
+    );
+}