diff --git a/quinn-proto/src/connection.rs b/quinn-proto/src/connection.rs
index 47d40190d..434f60139 100644
--- a/quinn-proto/src/connection.rs
+++ b/quinn-proto/src/connection.rs
@@ -71,6 +71,8 @@ pub struct Connection {
     events: VecDeque<Event>,
     /// Number of local connection IDs that have been issued in NEW_CONNECTION_ID frames.
     cids_issued: u64,
+    /// Outgoing spin bit state
+    spin: bool,
 
     //
     // Loss Detection
@@ -233,6 +235,7 @@ impl Connection {
             io: IoQueue::new(),
             events: VecDeque::new(),
             cids_issued: 0,
+            spin: false,
 
             crypto_count: 0,
             pto_count: 0,
@@ -733,6 +736,7 @@ impl Connection {
         now: u64,
         ecn: Option<EcnCodepoint>,
         packet: Option<u64>,
+        spin: bool,
     ) {
         self.reset_idle_timeout(now);
         self.receiving_ecn |= ecn.is_some();
@@ -753,6 +757,8 @@ impl Connection {
         if packet > self.rx_packet {
             self.rx_packet = packet;
             self.rx_packet_time = now;
+            // Update outgoing spin bit, inverting iff we're the client
+            self.spin = self.side.is_client() ^ spin;
         }
     }
 
@@ -850,7 +856,7 @@ impl Connection {
             &mut io::Cursor::new(self.tls.get_quic_transport_parameters().unwrap()),
         )?;
         self.set_params(params)?;
-        self.on_packet_authenticated(now, ecn, Some(packet_number));
+        self.on_packet_authenticated(now, ecn, Some(packet_number), false);
         self.write_tls();
         Ok(())
     }
@@ -960,7 +966,12 @@ impl Connection {
                     }
                 } else {
                     if !was_closed {
-                        self.on_packet_authenticated(now, ecn, number);
+                        let spin = if let Header::Short { spin, .. } = packet.header {
+                            spin
+                        } else {
+                            false
+                        };
+                        self.on_packet_authenticated(now, ecn, number, spin);
                     }
                     self.handle_connected_inner(now, number, packet)
                 }
@@ -1638,6 +1649,7 @@ impl Connection {
             let header = Header::Short {
                 dst_cid: self.rem_cid,
                 number: PacketNumber::new(number, self.largest_acked_packet),
+                spin: self.spin,
                 key_phase: self.key_phase,
             };
             //}
@@ -1940,6 +1952,7 @@ impl Connection {
         let header = Header::Short {
             dst_cid: self.rem_cid,
             number: PacketNumber::new(number, self.largest_acked_packet),
+            spin: self.spin,
             key_phase: self.key_phase,
         };
         let partial_encode = header.encode(&mut buf);
@@ -1980,6 +1993,7 @@ impl Connection {
             CryptoLevel::OneRtt => Header::Short {
                 dst_cid: self.rem_cid,
                 number,
+                spin: self.spin,
                 key_phase: self.key_phase,
             },
             CryptoLevel::Initial => Header::Long {
diff --git a/quinn-proto/src/packet.rs b/quinn-proto/src/packet.rs
index 9ae5ba041..b999cde26 100644
--- a/quinn-proto/src/packet.rs
+++ b/quinn-proto/src/packet.rs
@@ -93,12 +93,14 @@ impl PartialDecode {
 
                 let sample_offset = 1 + dst_cid.len() + 4;
                 let number = Self::decrypt_header(&mut buf, header_crypto, sample_offset)?;
+                let spin = buf.get_ref()[0] & SPIN_BIT != 0;
                 let key_phase = buf.get_ref()[0] & KEY_PHASE_BIT != 0;
                 (
                     buf.remaining(),
                     Header::Short {
                         dst_cid,
                         number,
+                        spin,
                         key_phase,
                     },
                     false,
@@ -279,6 +281,7 @@ pub enum Header {
     Short {
         dst_cid: ConnectionId,
         number: PacketNumber,
+        spin: bool,
         key_phase: bool,
     },
     VersionNegotiate {
@@ -350,9 +353,15 @@ impl Header {
             Short {
                 ref dst_cid,
                 number,
+                spin,
                 key_phase,
             } => {
-                w.write(FIXED_BIT | if key_phase { KEY_PHASE_BIT } else { 0 } | number.tag());
+                w.write(
+                    FIXED_BIT
+                        | if key_phase { KEY_PHASE_BIT } else { 0 }
+                        | if spin { SPIN_BIT } else { 0 }
+                        | number.tag(),
+                );
                 w.put_slice(dst_cid);
                 number.encode(w);
                 PartialEncode {
@@ -814,10 +823,11 @@ pub fn set_payload_length(packet: &mut [u8], header_len: usize, pn_len: usize) {
 pub const AEAD_TAG_SIZE: usize = 16;
 
 pub const LONG_HEADER_FORM: u8 = 0x80;
-const KEY_PHASE_BIT: u8 = 0x04;
 const FIXED_BIT: u8 = 0x40;
+pub const SPIN_BIT: u8 = 0x20;
 pub const SHORT_RESERVED_BITS: u8 = 0x18;
 pub const LONG_RESERVED_BITS: u8 = 0x0c;
+const KEY_PHASE_BIT: u8 = 0x04;
 
 /// Explicit congestion notification codepoint
 #[repr(u8)]
diff --git a/quinn-proto/src/tests.rs b/quinn-proto/src/tests.rs
index e8417efa9..313a33176 100644
--- a/quinn-proto/src/tests.rs
+++ b/quinn-proto/src/tests.rs
@@ -66,6 +66,9 @@ struct Pair {
     time: u64,
     // One-way
     latency: u64,
+    /// Number of spin bit flips
+    spins: u64,
+    last_spin: bool,
 }
 
 impl Default for Pair {
@@ -148,6 +151,8 @@ impl Pair {
             client: TestEndpoint::new(Side::Client, client, client_addr),
             time: 0,
             latency: 0,
+            spins: 0,
+            last_spin: false,
         }
     }
 
@@ -191,6 +196,11 @@ impl Pair {
         trace!(self.log, "client running");
         self.client.drive(&self.log, self.time, self.server.addr);
         for (ecn, packet) in self.client.outbound.drain(..) {
+            if packet[0] & packet::LONG_HEADER_FORM == 0 {
+                let spin = packet[0] & packet::SPIN_BIT != 0;
+                self.spins += (spin == self.last_spin) as u64;
+                self.last_spin = spin;
+            }
             if let Some(ref socket) = self.client.socket {
                 socket.send_to(&packet, self.server.addr).unwrap();
             }
@@ -441,6 +451,7 @@ fn lifecycle() {
     info!(pair.log, "closing");
     pair.client.close(pair.time, client_conn, 42, REASON.into());
     pair.drive();
+    assert!(pair.spins > 0);
     assert_matches!(pair.server.poll(),
                     Some((_, Event::ConnectionLost { reason: ConnectionError::ApplicationClosed {
                         reason: ApplicationClose { error_code: 42, ref reason }