diff --git a/docs/pq-handshake.md b/docs/pq-handshake.md
new file mode 100644
index 00000000..047aef88
--- /dev/null
+++ b/docs/pq-handshake.md
@@ -0,0 +1,274 @@
+# Post-quantum handshakes
+
+Status: draft. Decisions below were made on 2026-10-13; open items are listed at the end.
+Phase 1 is implemented (see [Implementation](#implementation-phase-1)).
+
+## Goals
+
+1. **Confidentiality against harvest-now-decrypt-later.** Traffic recorded today must stay
+   secret after a cryptographically relevant quantum computer exists. This needs a
+   post-quantum key exchange now.
+2. **Post-quantum authentication, later.** Needs certificates whose CA signature and host
+   keys are post-quantum. This requires reissuing certificates, so it ships second.
+3. **Never rely on IP fragmentation.** ML-KEM and ML-DSA do not fit in one packet, so the
+   handshake must carry messages larger than the path MTU itself.
+
+Non-goals: identity hiding beyond what IX offers today, and signatures at handshake time.
+The only signature in the system stays the CA signature inside the certificate.
+
+## Primitives and sizes
+
+| Primitive | Size |
+|---|---|
+| ML-KEM-1024 encapsulation key | 1568 B |
+| ML-KEM-1024 ciphertext | 1568 B |
+| ML-DSA-87 public key / signature | 2592 B / 4627 B |
+| Today's IX msg1 / msg2 (v2 cert, 151 B) | 252 B / 287 B |
+
+`crypto/mlkem` is available in Go 1.26 (the current `go.mod`); `crypto/mldsa` arrives in
+Go 1.27 and is only needed for phase 2.
+
+## Phase 1: ephemeral ML-KEM on today's certificates
+
+A new handshake subtype, `ix_psk2_mlkem1024` (2), IX-shaped (2 messages; the responder
+completes on msg1 as today), using the flynn/noise `psk2` modifier. The Noise protocol name
+becomes `Noise_IXpsk2_...`, which separates it from the classical subtype.
+
+- **msg1**: the initiator generates a fresh ML-KEM-1024 key pair and places the
+  encapsulation key in `NebulaHandshakeDetails.KEMKey` (field 9). msg1's payload is mixed
+  into the Noise transcript, so it cannot be stripped or replaced without msg2 failing.
+  With the psk modifier Noise also mixes `e` into the cipher key, so msg1's static key and
+  payload are encrypted, though only under a key derived from the public ephemeral.
+- **Responder**: after `ReadMessage(msg1)` it encapsulates to that key, obtaining
+  `(ct, ss)`, and calls `HandshakeState.SetPresharedKey(psk)` before writing msg2, with
+  `psk = HKDF-Extract(SHA-256, salt = "nebula ix_psk2_mlkem1024 v1", ikm = ss || ct)`.
+  The ciphertext is bound in because, unlike the encapsulation key, it is not part of the
+  Noise transcript.
+- **msg2** on the wire is `ct || noise_msg2`. The ciphertext travels outside the Noise
+  message because the initiator needs `ss` before processing msg2's `psk` token. Any change
+  to `ct` yields a different `ss` (ML-KEM implicit rejection) and the handshake fails.
+
+Transport keys depend on both the X25519 DH outputs and the ML-KEM shared secret.
+Authentication is unchanged and classical. With a minimal v2 certificate msg1 and msg2 are
+about 1.84 KB each, so each travels as 2 data and 2 repair fragments of about 936 bytes.
+A KEM key on the classical subtype, or a missing or malformed one on this subtype, is a fatal
+protocol error; a forged ciphertext only fails that packet.
+
+## Phase 2: pqIX hybrid with certificate v3
+
+Certificate v3 is hybrid: the classical static key and CA signature plus a static
+ML-KEM-1024 key and an ML-DSA-87 CA signature. Host authentication is by KEM rather than by
+signature, following PQNoise's pqIX, while keeping the classical DH tokens:
+
+- **msg1** (I to R): `e, s`, ephemeral ML-KEM key, initiator cert.
+- **msg2** (R to I): `e, s`, ciphertext to I's ephemeral KEM key, ciphertext to I's static
+  KEM key, responder cert.
+- **msg3** (I to R): ciphertext to R's static KEM key, plus first data. R authenticates I
+  when msg3 decrypts; I authenticates R implicitly when R's first reply decrypts.
+
+The initiator never caches the responder's certificate, so every handshake has three
+messages and carries both full certificates. With ML-DSA-87:
+
+| Message | Size | Data fragments (k) |
+|---|---|---|
+| msg1 | ~8.2 KB | 7 |
+| msg2 | ~9.8 KB | 9 |
+| msg3 | ~1.6 KB | 2 |
+
+flynn/noise cannot mix multiple KEM secrets into the chaining key, so phase 2 needs a
+KEM-aware Noise symmetric state written in this repository. Both subtypes coexist.
+
+Cryptographic review of the hybrid composition will be done by in-house cryptographers.
+
+## Fragment transport
+
+### Requirements
+
+- No IP fragmentation: every datagram's UDP payload is at most 1232 bytes **after** relay
+  wrapping (16 B header + 16 B tag), the IPv6 minimum MTU budget also used by DNS Flag Day
+  2020 and Solana's Turbine.
+- Every fragment of a handshake uses the same 5-tuple.
+- Bounded pre-authentication state, bounded amplification, selective loss recovery,
+  tolerance of reordering and duplication, and a strictly validated, fuzzable parser.
+
+### Wire format
+
+Every packet of a fragmented handshake subtype is:
+
+```
+Nebula header (16)   Version | Type=Handshake | Subtype | Reserved=0 | RemoteIndex | Counter
+Fragment header (8)  FragID u32 | MsgLen u16 | Index u8 | Flags u8
+Symbol               SymbolSize bytes
+```
+
+- `Counter` is the Noise message number (1, 2, 3). `RemoteIndex` is 0 for msg1, as today.
+- `FragID` is the sender's local index (random, non-zero).
+- `MsgLen` is the length of the whole message. Both ends derive the layout from it:
+  `K = ceil(MsgLen / 1176)`, `SymbolSize = ceil(MsgLen / K)`. Every symbol has the same
+  size, so a packet is at most 16 + 8 + 1176 = 1200 bytes.
+- `Index < K` is a data symbol (a slice of the message, the last one zero padded);
+  `Index >= K` is a repair symbol.
+- `Flags` is reserved for a cookie extension and must be zero.
+
+### Forward error correction
+
+Messages are protected with a systematic Reed-Solomon code over GF(2^8), implemented in
+`handshake/fec`:
+
+- The repair rows form a Cauchy matrix, `coefficient(row, col) = 1 / (row XOR col)` with
+  `row >= K > col`, so every square submatrix is invertible and **any K distinct symbols
+  reconstruct the message**. Naive identity-over-Vandermonde constructions are not MDS
+  (the PAR1 flaw and the 1997 Plank tutorial erratum).
+- A repair row depends only on its index, so repair symbols are generated on demand and
+  their indexes never need to be agreed in advance. Up to 256 symbols exist per message.
+- Known-answer vectors in `fec_test.go` pin the encoding and were cross-checked against an
+  independent implementation; changing them is a wire format change.
+- Cost for a 9.8 KB message (k=9): about 13 µs for three repair symbols, about 19 µs to
+  decode with three data symbols missing.
+
+**The sender chooses m**, the number of repair symbols per attempt. The receiver only needs
+any K symbols, so this is policy and can change without a protocol change. The starting
+rule is `m = max(2, ceil(K/3))`.
+
+**Retries send fresh symbols.** Attempt 0 sends indexes `0 .. K+m-1`; each retry sends
+the next `K+m` unused indexes. The receiver merges all distinct symbols across attempts,
+so no received fragment is wasted. Indexes are a deterministic function of the message,
+so retransmits stay idempotent: the responder caches msg2 itself and regenerates any
+symbol.
+
+**Repair symbols trail the data.** Data symbols go out back to back; repair symbols follow
+after a short delay, so a queue tail-drop at the end of the data run does not also take the
+repair. In simulation this lifts small messages substantially (k=2 at 10% loss, burst 8:
+91.4% to 98.2% first-attempt success). Pacing every fragment was also evaluated; it gains
+about 5 more points for k=7..9 on bursty paths, which is not worth the extra machinery,
+and the retry with fresh symbols covers the remainder.
+
+### Receiver rules
+
+Implemented in `handshake/fragment` (`Sender`, `Assembly`, and the capped `Table`).
+
+- Fragments are stored individually as they arrive and charged against a memory cap.
+  Nothing is allocated from a claimed size alone (FragmentSmack, game-engine "fragment
+  bombs").
+- `MsgLen` must be identical for every fragment of a `FragID`; a mismatch drops the whole
+  entry. msg1 is capped at `MsgLen <= 16384` (K <= 14).
+- The decoder keeps at most K distinct symbols, so an entry never exceeds about MsgLen bytes
+  however many repair symbols arrive.
+- When K symbols are present the message is decoded and handed to the handshake `Machine`
+  as a synthetic `[header | message]`. The Machine never sees fragments.
+- A completed entry is forgotten immediately. A late duplicate fragment starts a new entry
+  and, once enough arrive, reconstructs the same message again; the handshake layer's
+  duplicate detection recognises it.
+- If Noise rejects a decoded message, the entry is discarded and the next attempt
+  supplies fresh symbols. (Possible refinement: keep a few spare symbols so an injected
+  bad symbol can be identified by re-encoding instead of waiting for a retry.)
+- Pre-authentication msg1 entries live in a table keyed by (source address or relay index,
+  FragID) with a total memory cap, per-source quotas (/32 for IPv4, /64 for IPv6), a
+  sliding TTL of about 2 s refreshed by progress, and random eviction under pressure.
+  msg2 and msg3 reassembly attaches to the existing pending handshake found by RemoteIndex.
+
+### Retransmission and amplification
+
+- Only timers cause transmission. Reactive resends (the responder resending its cached
+  msg2 when a duplicate msg1 completes) are rate limited to one per handshake per retry
+  interval, to avoid TFTP's Sorcerer's Apprentice amplification.
+- Duplicate msg1 detection compares the decoded message to the cached one **before**
+  running Noise or certificate verification.
+- Before an address is validated, a responder sends at most 3x the bytes it received from
+  it (QUIC's rule). Normal handshakes sit near 1.1-1.2x.
+- In phase 2 the initiator retransmits msg3 until any authenticated packet arrives from
+  the responder; the responder sends a small confirmation on completion and holds tunnel
+  traffic until msg3 verifies.
+
+### Denial of service
+
+The responder can no longer complete statelessly on msg1, since msg1 arrives in pieces.
+Defenses are the capped table above plus a **cookie under load**: when the table is under
+pressure, a fragment from an unvalidated address is answered with a stateless cookie
+(a MAC over source address, FragID and a time bucket), and the initiator resends with the
+cookie. This costs a round trip only while under attack, in the style of WireGuard's
+mac2, QUIC Retry and the SCTP state cookie. A CA-keyed pre-filter MAC was considered and
+rejected: networks commonly trust many CAs.
+
+## Rollout
+
+- Separate settings for initiating and accepting PQ handshakes, both off by default:
+  `handshakes.pq.initiate`, `handshakes.pq.accept`. `handshakes.pq.repair_delay` (default
+  10ms) sets how long repair fragments trail the data fragments.
+- Old nodes silently drop unknown subtypes, so discovery is speculative: an initiator with
+  `initiate` enabled alternates PQ and classical attempts on successive retries, keeping
+  both pending, and the first to complete wins.
+- `handshakes.pq.sticky` (default off): once a peer has completed a PQ handshake, never fall
+  back to classical for it again during this process lifetime. This resists active
+  downgrade after first contact.
+- No lighthouse capability advertisement.
+
+## Simulation results
+
+Gilbert-Elliott channel (bad state drops everything), losses expressed as average rate and
+mean burst length in packets. First-attempt success for the whole handshake with
+`m = max(2, ceil(K/3))` and repair delayed 16 packet slots, compared with no FEC:
+
+| Handshake | Loss | Burst 1 | Burst 2 | Burst 4 | Burst 8 |
+|---|---|---|---|---|---|
+| phase 1 (2, 2) no FEC | 5% | 81.4% | 85.6% | 87.9% | 89.0% |
+| phase 1 (2, 2) FEC | 5% | 99.9% | 99.4% | 99.3% | 98.6% |
+| phase 1 (2, 2) FEC | 10% | 99.3% | 97.5% | 97.4% | 96.3% |
+| phase 2 (7, 9, 2) no FEC | 5% | 39.8% | 57.5% | 70.4% | 77.7% |
+| phase 2 (7, 9, 2) FEC | 5% | 99.6% | 94.2% | 90.4% | 88.8% |
+| phase 2 (7, 9, 2) FEC | 10% | 95.9% | 84.6% | 79.0% | 77.3% |
+
+With fresh repair symbols, phase 2 completes within two attempts at least 99.1% of the time
+in every scenario above, and within three at least 99.99%.
+
+## Prior art
+
+- **WireGuard / Rosenpass**: fit every message in one packet; cookies and biscuits for
+  statelessness. Not possible at our sizes.
+- **IKEv2**: RFC 7383 fragments only after keys exist, with each fragment authenticated;
+  RFC 9242/9370 move PQ key exchange into intermediate exchanges for that reason.
+- **QUIC / DTLS 1.3**: offset-based reassembly with ACKs; QUIC's 3x amplification limit and
+  1200-byte floor.
+- **Solana Turbine**: Reed-Solomon over 1232-byte UDP packets through untrusted relays;
+  moved to Merkle-authenticated shreds after unauthenticated repair symbols enabled
+  poisoning.
+- **kcp-go, UDPspeeder, CM256**: RS FEC over UDP; parity must scale up for small groups.
+- **gQUIC FEC (removed)**: FEC does not pay for bulk transfer under bursty loss; it does
+  for short, latency-critical transfers such as handshakes.
+- **TFTP, NFS duplicate request cache, memcached UDP, IP reassembly bugs**: retransmit
+  storms, ID-wrap misassembly, reflection amplification, and allocation from claimed sizes.
+
+## Implementation (phase 1)
+
+- `handshake`: the `ix_psk2_mlkem1024` subtype (`patterns.go`, `kem.go`), the `KEMKey`
+  payload field, and the psk2 plumbing in `Machine`.
+- `handshake/fec`, `handshake/fragment`: the code and the fragment transport.
+- `handshake_manager_pq.go` and the manager:
+  - **Initiator.** Attempts alternate PQ and classical, starting with PQ (sticky peers get
+    only PQ). The PQ attempt has its own local index registered alongside the classical
+    one; whichever response completes first wins, and the other index is released
+    (`settleIndex`). A timed out or deleted pending handshake releases both.
+  - **Responder.** msg1 fragments go into the capped pre-authentication table. A decoded
+    msg1 is looked up by hash among recently answered ones before any Noise, KEM or
+    certificate work: the trailing repair fragments of an attempt rebuild a msg1 its data
+    fragments already completed, and a retrying initiator offers fresh symbols for the
+    same msg1. A repeat goes straight to the cached-response resend, which offers fresh
+    msg2 symbols, is rate limited to one per retry interval and is held to the 3x
+    amplification budget.
+  - **Relays** carry fragments unchanged; repair fragments trail via a timer.
+- The pre-authentication table is capped at 4 MiB, 1024 entries, 4 per source (IPv4 /32,
+  IPv6 /64, or relay channel), 16 KiB messages and a 3 s idle timeout.
+- Tests: Noise-level KEM tests, codec and fragment tests and fuzzers, manager unit tests
+  for the response budget and rate limit, and e2e tests for a clean handshake, loss of
+  data fragments in both directions, fallback to a peer that does not accept PQ, a lost
+  msg2 recovered by fresh symbols, sticky mode under an attacker dropping PQ packets,
+  relays, and index cleanup on completion, fallback and timeout. ML-KEM and HKDF also
+  pass under `GOEXPERIMENT=boringcrypto` and `GODEBUG=fips140=on`.
+
+## Open items
+
+1. Cookie wire format and the load threshold that triggers it.
+2. Whether the decoder keeps spare symbols to identify injected symbols.
+3. msg3 confirmation message format (phase 2).
+4. Configuration reload: the `handshakes.pq` settings are read at startup only, like the
+   rest of the `handshakes` section.
diff --git a/e2e/pq_handshakes_test.go b/e2e/pq_handshakes_test.go
new file mode 100644
index 00000000..0519e38f
--- /dev/null
+++ b/e2e/pq_handshakes_test.go
@@ -0,0 +1,351 @@
+//go:build e2e_testing
+
+package e2e
+
+import (
+	"net/netip"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/slackhq/nebula"
+	"github.com/slackhq/nebula/cert"
+	"github.com/slackhq/nebula/cert_test"
+	"github.com/slackhq/nebula/e2e/router"
+	"github.com/slackhq/nebula/handshake/fragment"
+	"github.com/slackhq/nebula/header"
+	"github.com/slackhq/nebula/udp"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func pqConfig(initiate, accept, sticky bool) m {
+	return m{"handshakes": m{"pq": m{
+		"initiate":     initiate,
+		"accept":       accept,
+		"sticky":       sticky,
+		"repair_delay": "0s",
+	}}}
+}
+
+// handshakeOf returns the handshake header p carries, looking inside relay
+// wrapping, plus the fragment header when the handshake is fragmented.
+func handshakeOf(p *udp.Packet) (*header.H, *fragment.Header) {
+	b := p.Data
+	h := &header.H{}
+	if h.Parse(b) != nil {
+		return nil, nil
+	}
+	if h.Type == header.Message && h.Subtype == header.MessageRelay {
+		// Relays authenticate but do not encrypt the inner packet.
+		b = b[header.Len:]
+		if h.Parse(b) != nil {
+			return nil, nil
+		}
+	}
+	if h.Type != header.Handshake {
+		return nil, nil
+	}
+	if !header.IsFragmented(h.Subtype) {
+		return h, nil
+	}
+	// A relayed packet still has the relay's AEAD tag on the end.
+	for _, tag := range []int{0, 16} {
+		if len(b) < header.Len+tag {
+			break
+		}
+		if fh, _, err := fragment.Parse(b[header.Len : len(b)-tag]); err == nil {
+			return h, &fh
+		}
+	}
+	return h, nil
+}
+
+// handshakeTally counts handshake packets by style as the router moves them.
+type handshakeTally struct {
+	sync.Mutex
+	pq, classical int
+	maxPQLen      int
+}
+
+func (ht *handshakeTally) observe(p *udp.Packet) {
+	h, _ := handshakeOf(p)
+	if h == nil {
+		return
+	}
+	ht.Lock()
+	defer ht.Unlock()
+	if h.Subtype == header.HandshakeIXPSK2MLKEM1024 {
+		ht.pq++
+		ht.maxPQLen = max(ht.maxPQLen, len(p.Data))
+	} else {
+		ht.classical++
+	}
+}
+
+// isTunnelData reports whether p is ordinary tunnel traffic headed for to,
+// directly or as the last hop of a relay.
+func isTunnelData(p *udp.Packet, receiver, to *nebula.Control) bool {
+	h := &header.H{}
+	if receiver != to || h.Parse(p.Data) != nil || h.Type != header.Message {
+		return false
+	}
+	if h.Subtype == header.MessageRelay {
+		if h.Parse(p.Data[header.Len:]) != nil {
+			return false
+		}
+	}
+	return h.Type == header.Message && h.Subtype == header.MessageNone
+}
+
+func newPQPair(t *testing.T, mine, theirs m) (myControl, theirControl *nebula.Control, myVpn, theirVpn []netip.Prefix, myUdp, theirUdp netip.AddrPort, r *router.R) {
+	t.Helper()
+	ca, _, caKey, _ := cert_test.NewTestCaCert(cert.Version2, cert.Curve_CURVE25519, time.Now(), time.Now().Add(10*time.Minute), nil, nil, []string{})
+	myControl, myVpn, myUdp, _ = newSimpleServer(cert.Version2, ca, caKey, "me", "10.128.0.1/24", mine)
+	theirControl, theirVpn, theirUdp, _ = newSimpleServer(cert.Version2, ca, caKey, "them", "10.128.0.2/24", theirs)
+	myControl.InjectLightHouseAddr(theirVpn[0].Addr(), theirUdp)
+	r = router.NewR(t, myControl, theirControl)
+	myControl.Start()
+	theirControl.Start()
+	return
+}
+
+func TestPQHandshake(t *testing.T) {
+	t.Parallel()
+	// They only accept, which is all a PQ initiator needs.
+	myControl, theirControl, myVpn, theirVpn, myUdp, theirUdp, r := newPQPair(t, pqConfig(true, true, false), pqConfig(false, true, false))
+	defer r.RenderFlow()
+
+	var tally handshakeTally
+	myControl.InjectTunPacket(BuildTunUDPPacket(theirVpn[0].Addr(), 80, myVpn[0].Addr(), 80, []byte("Hi from me")))
+	require.True(t, r.RouteForAllExitFuncOrTimeout(5*time.Second, func(p *udp.Packet, receiver *nebula.Control) router.ExitType {
+		tally.observe(p)
+		if isTunnelData(p, receiver, theirControl) {
+			return router.RouteAndExit
+		}
+		return router.KeepRouting
+	}), "tunnel never came up")
+
+	assertUdpPacket(t, []byte("Hi from me"), theirControl.GetFromTun(true), myVpn[0].Addr(), theirVpn[0].Addr(), 80, 80)
+	assert.Zero(t, tally.classical, "no classical handshake should be needed")
+	assert.GreaterOrEqual(t, tally.pq, 4, "both messages travel as several fragments")
+	assert.LessOrEqual(t, tally.maxPQLen, fragment.MaxPacketLen)
+
+	assertHostInfoPair(t, myUdp, theirUdp, myVpn, theirVpn, myControl, theirControl)
+	assertTunnel(t, myVpn[0].Addr(), theirVpn[0].Addr(), myControl, theirControl, r)
+	assert.Empty(t, myControl.ListHostmapIndexes(true), "no pending index may outlive the handshake")
+	assert.Empty(t, theirControl.ListHostmapIndexes(true))
+
+	myControl.Stop()
+	theirControl.Stop()
+}
+
+// TestPQHandshakeLoss drops data fragments of both messages; the repair
+// fragments must carry the handshake without a retry.
+func TestPQHandshakeLoss(t *testing.T) {
+	t.Parallel()
+	myControl, theirControl, myVpn, theirVpn, _, _, r := newPQPair(t, pqConfig(true, true, false), pqConfig(false, true, false))
+	defer r.RenderFlow()
+
+	var tally handshakeTally
+	dropped := 0
+	myControl.InjectTunPacket(BuildTunUDPPacket(theirVpn[0].Addr(), 80, myVpn[0].Addr(), 80, []byte("Hi from me")))
+	require.True(t, r.RouteForAllExitFuncOrTimeout(5*time.Second, func(p *udp.Packet, receiver *nebula.Control) router.ExitType {
+		if h, fh := handshakeOf(p); fh != nil {
+			// Every data fragment of msg1, and the first of msg2.
+			if (h.MessageCounter == 1 && int(fh.Index) < 2) || (h.MessageCounter == 2 && fh.Index == 0) {
+				dropped++
+				return router.Drop
+			}
+		}
+		tally.observe(p)
+		if isTunnelData(p, receiver, theirControl) {
+			return router.RouteAndExit
+		}
+		return router.KeepRouting
+	}), "tunnel never came up")
+
+	assertUdpPacket(t, []byte("Hi from me"), theirControl.GetFromTun(true), myVpn[0].Addr(), theirVpn[0].Addr(), 80, 80)
+	assert.Equal(t, 3, dropped)
+	assert.Zero(t, tally.classical, "the first attempt should succeed from repair fragments")
+	assertTunnel(t, myVpn[0].Addr(), theirVpn[0].Addr(), myControl, theirControl, r)
+
+	myControl.Stop()
+	theirControl.Stop()
+}
+
+// TestPQFallback has a peer that does not accept PQ handshakes. The initiator's
+// next attempt is classical and the tunnel comes up anyway.
+func TestPQFallback(t *testing.T) {
+	t.Parallel()
+	myControl, theirControl, myVpn, theirVpn, myUdp, theirUdp, r := newPQPair(t, pqConfig(true, true, false), nil)
+	defer r.RenderFlow()
+
+	var tally handshakeTally
+	myControl.InjectTunPacket(BuildTunUDPPacket(theirVpn[0].Addr(), 80, myVpn[0].Addr(), 80, []byte("Hi from me")))
+	require.True(t, r.RouteForAllExitFuncOrTimeout(5*time.Second, func(p *udp.Packet, receiver *nebula.Control) router.ExitType {
+		tally.observe(p)
+		if isTunnelData(p, receiver, theirControl) {
+			return router.RouteAndExit
+		}
+		return router.KeepRouting
+	}), "tunnel never came up")
+
+	assertUdpPacket(t, []byte("Hi from me"), theirControl.GetFromTun(true), myVpn[0].Addr(), theirVpn[0].Addr(), 80, 80)
+	assert.Positive(t, tally.pq, "a PQ attempt comes first")
+	assert.Positive(t, tally.classical, "then a classical one")
+
+	assertHostInfoPair(t, myUdp, theirUdp, myVpn, theirVpn, myControl, theirControl)
+	assertTunnel(t, myVpn[0].Addr(), theirVpn[0].Addr(), myControl, theirControl, r)
+	assert.Empty(t, myControl.ListHostmapIndexes(true), "the losing PQ attempt's index must be released")
+
+	myControl.Stop()
+	theirControl.Stop()
+}
+
+// TestPQResendFreshSymbols loses the responder's whole first msg2. The
+// initiator's next PQ attempt offers fresh msg1 symbols, the responder
+// recognises the repeat and resends msg2 with symbols it has not sent before.
+func TestPQResendFreshSymbols(t *testing.T) {
+	t.Parallel()
+	myControl, theirControl, myVpn, theirVpn, _, _, r := newPQPair(t, pqConfig(true, true, false), pqConfig(false, true, false))
+	defer r.RenderFlow()
+
+	var firstMsg2, freshMsg2, freshMsg1 int
+	myControl.InjectTunPacket(BuildTunUDPPacket(theirVpn[0].Addr(), 80, myVpn[0].Addr(), 80, []byte("Hi from me")))
+	require.True(t, r.RouteForAllExitFuncOrTimeout(10*time.Second, func(p *udp.Packet, receiver *nebula.Control) router.ExitType {
+		h, fh := handshakeOf(p)
+		switch {
+		case h != nil && fh == nil:
+			// Keep the classical fallback out of the way.
+			return router.Drop
+		case fh != nil && h.MessageCounter == 2 && fh.Index < 4:
+			firstMsg2++
+			return router.Drop
+		case fh != nil && h.MessageCounter == 2:
+			freshMsg2++
+		case fh != nil && h.MessageCounter == 1 && fh.Index >= 4:
+			freshMsg1++
+		}
+		if isTunnelData(p, receiver, theirControl) {
+			return router.RouteAndExit
+		}
+		return router.KeepRouting
+	}), "tunnel never came up")
+
+	assertUdpPacket(t, []byte("Hi from me"), theirControl.GetFromTun(true), myVpn[0].Addr(), theirVpn[0].Addr(), 80, 80)
+	assert.Equal(t, 4, firstMsg2, "the first msg2 is 2 data and 2 repair fragments")
+	assert.Positive(t, freshMsg1, "the retry offers fresh msg1 symbols")
+	assert.Positive(t, freshMsg2, "the resend offers fresh msg2 symbols")
+	assertTunnel(t, myVpn[0].Addr(), theirVpn[0].Addr(), myControl, theirControl, r)
+
+	myControl.Stop()
+	theirControl.Stop()
+}
+
+// TestPQSticky checks a sticky initiator never offers a classical handshake to a
+// peer it has already completed a PQ handshake with.
+func TestPQSticky(t *testing.T) {
+	t.Parallel()
+	myControl, theirControl, myVpn, theirVpn, _, _, r := newPQPair(t, pqConfig(true, true, true), pqConfig(false, true, false))
+	defer r.RenderFlow()
+
+	myControl.InjectTunPacket(BuildTunUDPPacket(theirVpn[0].Addr(), 80, myVpn[0].Addr(), 80, []byte("Hi from me")))
+	require.True(t, r.RouteForAllExitFuncOrTimeout(5*time.Second, func(p *udp.Packet, receiver *nebula.Control) router.ExitType {
+		if isTunnelData(p, receiver, theirControl) {
+			return router.RouteAndExit
+		}
+		return router.KeepRouting
+	}), "tunnel never came up")
+	theirControl.GetFromTun(true)
+
+	t.Log("Rehandshake while an attacker drops every PQ packet")
+	var tally handshakeTally
+	myControl.ReHandshake(theirVpn[0].Addr())
+	r.RouteForAllExitFuncOrTimeout(time.Second, func(p *udp.Packet, receiver *nebula.Control) router.ExitType {
+		tally.observe(p)
+		if h, _ := handshakeOf(p); h != nil && h.Subtype == header.HandshakeIXPSK2MLKEM1024 {
+			return router.Drop
+		}
+		return router.KeepRouting
+	})
+	assert.Positive(t, tally.pq, "PQ attempts keep being made")
+	assert.Zero(t, tally.classical, "sticky must never fall back")
+
+	myControl.Stop()
+	theirControl.Stop()
+}
+
+func TestPQRelays(t *testing.T) {
+	t.Parallel()
+	ca, _, caKey, _ := cert_test.NewTestCaCert(cert.Version1, cert.Curve_CURVE25519, time.Now(), time.Now().Add(10*time.Minute), nil, nil, []string{})
+	useRelays := pqConfig(true, true, false)
+	useRelays["relay"] = m{"use_relays": true}
+	amRelay := pqConfig(true, true, false)
+	amRelay["relay"] = m{"am_relay": true}
+	myControl, myVpnIpNet, _, _ := newSimpleServer(cert.Version1, ca, caKey, "me     ", "10.128.0.1/24", useRelays)
+	relayControl, relayVpnIpNet, relayUdpAddr, _ := newSimpleServer(cert.Version1, ca, caKey, "relay  ", "10.128.0.128/24", amRelay)
+	theirControl, theirVpnIpNet, theirUdpAddr, _ := newSimpleServer(cert.Version1, ca, caKey, "them   ", "10.128.0.2/24", useRelays)
+
+	myControl.InjectLightHouseAddr(relayVpnIpNet[0].Addr(), relayUdpAddr)
+	myControl.InjectRelays(theirVpnIpNet[0].Addr(), []netip.Addr{relayVpnIpNet[0].Addr()})
+	relayControl.InjectLightHouseAddr(theirVpnIpNet[0].Addr(), theirUdpAddr)
+
+	r := router.NewR(t, myControl, relayControl, theirControl)
+	defer r.RenderFlow()
+
+	myControl.Start()
+	relayControl.Start()
+	theirControl.Start()
+
+	var tally handshakeTally
+	var relayedPQ int
+	myControl.InjectTunPacket(BuildTunUDPPacket(theirVpnIpNet[0].Addr(), 80, myVpnIpNet[0].Addr(), 80, []byte("Hi from me")))
+	require.True(t, r.RouteForAllExitFuncOrTimeout(10*time.Second, func(p *udp.Packet, receiver *nebula.Control) router.ExitType {
+		tally.observe(p)
+		outer := &header.H{}
+		if outer.Parse(p.Data) == nil && outer.Type == header.Message && outer.Subtype == header.MessageRelay {
+			if h, fh := handshakeOf(p); h != nil && fh != nil {
+				relayedPQ++
+				assert.LessOrEqual(t, len(p.Data), fragment.MaxPacketLen+32, "a relayed fragment fits the IPv6 minimum MTU")
+			}
+		}
+		if isTunnelData(p, receiver, theirControl) {
+			return router.RouteAndExit
+		}
+		return router.KeepRouting
+	}), "tunnel never came up")
+
+	assertUdpPacket(t, []byte("Hi from me"), theirControl.GetFromTun(true), myVpnIpNet[0].Addr(), theirVpnIpNet[0].Addr(), 80, 80)
+	assert.Positive(t, relayedPQ, "the end to end handshake is PQ through the relay")
+	r.RenderHostmaps("Final hostmaps", myControl, relayControl, theirControl)
+
+	myControl.Stop()
+	relayControl.Stop()
+	theirControl.Stop()
+}
+
+// TestPQTimeoutReleasesIndexes lets a handshake with both a PQ and a classical
+// attempt pending time out, and checks neither index is left behind.
+func TestPQTimeoutReleasesIndexes(t *testing.T) {
+	t.Parallel()
+	mine := pqConfig(true, true, false)
+	mine["handshakes"].(m)["retries"] = 2
+	myControl, theirControl, myVpn, theirVpn, _, _, r := newPQPair(t, mine, pqConfig(false, true, false))
+	defer r.RenderFlow()
+
+	var tally handshakeTally
+	myControl.InjectTunPacket(BuildTunUDPPacket(theirVpn[0].Addr(), 80, myVpn[0].Addr(), 80, []byte("Hi from me")))
+	r.RouteForAllExitFuncOrTimeout(1500*time.Millisecond, func(p *udp.Packet, receiver *nebula.Control) router.ExitType {
+		tally.observe(p)
+		return router.Drop
+	})
+
+	assert.Positive(t, tally.pq)
+	assert.Positive(t, tally.classical)
+	assert.Nil(t, myControl.GetHostInfoByVpnAddr(theirVpn[0].Addr(), true), "the handshake should have timed out")
+	assert.Empty(t, myControl.ListHostmapIndexes(true))
+
+	myControl.Stop()
+	theirControl.Stop()
+}
diff --git a/examples/config.yml b/examples/config.yml
index 0d1a9180..670e3f63 100644
--- a/examples/config.yml
+++ b/examples/config.yml
@@ -418,6 +418,21 @@ logging:
   # after receiving the response for lighthouse queries
   #trigger_buffer: 64
 
+  # pq enables handshakes that mix an ephemeral ML-KEM-1024 key exchange into the session keys, protecting
+  # recorded traffic against a future quantum computer. Authentication is unchanged. These handshakes are too
+  # large for one packet and travel as fragments with forward error correction.
+  #pq:
+    # initiate starts post-quantum handshakes. Attempts alternate between post-quantum and classical until one is
+    # answered, so peers that do not support them still connect.
+    #initiate: false
+    # accept answers post-quantum handshakes from peers.
+    #accept: false
+    # sticky never falls back to a classical handshake with a peer that has completed a post-quantum one since
+    # nebula started, which defeats an attacker who drops post-quantum handshakes to force a downgrade.
+    #sticky: false
+    # repair_delay is how long the forward error correction fragments trail the rest of a handshake message.
+    #repair_delay: 10ms
+
 # Tunnel manager settings
 #tunnels:
   # drop_inactive controls whether inactive tunnels are maintained or dropped after the inactive_timeout period has
diff --git a/handshake_manager.go b/handshake_manager.go
index ff80693c..76916725 100644
--- a/handshake_manager.go
+++ b/handshake_manager.go
@@ -15,6 +15,7 @@ import (
 	"github.com/rcrowley/go-metrics"
 	"github.com/slackhq/nebula/cert"
 	"github.com/slackhq/nebula/handshake"
+	"github.com/slackhq/nebula/handshake/fragment"
 	"github.com/slackhq/nebula/header"
 	"github.com/slackhq/nebula/udp"
 )
@@ -49,6 +50,7 @@ type HandshakeConfig struct {
 	tryInterval   time.Duration
 	retries       int64
 	triggerBuffer int
+	pq            pqHandshakeConfig
 
 	messageMetrics *MessageMetrics
 }
@@ -74,6 +76,13 @@ type HandshakeManager struct {
 
 	// can be used to trigger outbound handshake for the given vpnIp
 	trigger chan netip.Addr
+
+	// pqTable reassembles post-quantum msg1s from senders that have not authenticated yet.
+	pqTable *fragment.Table[pqFragKey, pqFragSource]
+	// pqPeers holds vpn addresses that completed a post-quantum handshake, for sticky mode.
+	pqPeers sync.Map
+	// pqAnswered recognises repeats of post-quantum msg1s already answered.
+	pqAnswered *pqAnswered
 }
 
 type HandshakeHostInfo struct {
@@ -89,6 +98,7 @@ type HandshakeHostInfo struct {
 
 	hostinfo *HostInfo
 	machine  *handshake.Machine // The handshake state machine, set during stage 0 (initiator) or beginHandshake (responder multi-message)
+	pq       *pqAttempt         // The initiator's post-quantum attempt, pending alongside the classical one
 }
 
 func (hh *HandshakeHostInfo) cachePacket(l *slog.Logger, t header.MessageType, st header.MessageSubType, packet []byte, f packetCallback, m *cachedPacketMetrics) {
@@ -130,6 +140,8 @@ func NewHandshakeManager(l *slog.Logger, mainHostMap *HostMap, lightHouse *Light
 		metricInitiated:        metrics.GetOrRegisterCounter("handshake_manager.initiated", nil),
 		metricTimedOut:         metrics.GetOrRegisterCounter("handshake_manager.timed_out", nil),
 		l:                      l,
+		pqTable:                fragment.NewTable[pqFragKey, pqFragSource](pqTableConfig),
+		pqAnswered:             newPQAnswered(hsTimeout(config.retries, config.tryInterval)),
 	}
 }
 
@@ -145,6 +157,8 @@ func (hm *HandshakeManager) Run(ctx context.Context) {
 			hm.handleOutbound(vpnIP, true)
 		case now := <-clockSource.C:
 			hm.NextOutboundHandshakeTimerTick(now)
+			hm.pqTable.Expire(now)
+			hm.pqAnswered.expire(now)
 		}
 	}
 }
@@ -154,7 +168,7 @@ func (hm *HandshakeManager) HandleIncoming(via ViaSender, packet []byte, h *head
 	// don't yet support) are dropped here rather than silently routed through
 	// the IX path. Add a case when introducing a new pattern.
 	switch h.Subtype {
-	case header.HandshakeIXPSK0:
+	case header.HandshakeIXPSK0, pqSubtype:
 		// supported
 	default:
 		hm.l.Debug("dropping handshake with unsupported subtype",
@@ -170,6 +184,11 @@ func (hm *HandshakeManager) HandleIncoming(via ViaSender, packet []byte, h *head
 		}
 	}
 
+	if header.IsFragmented(h.Subtype) {
+		hm.handleFragment(via, packet, h)
+		return
+	}
+
 	// First message of a new handshake. The wire format requires RemoteIndex
 	// to be zero here (the initiator has no responder index to fill in yet),
 	// and generateIndex never allocates 0, so any non-zero RemoteIndex on a
@@ -181,7 +200,7 @@ func (hm *HandshakeManager) HandleIncoming(via ViaSender, packet []byte, h *head
 				"from", via, "remoteIndex", h.RemoteIndex)
 			return
 		}
-		hm.beginHandshake(via, packet, h)
+		hm.beginHandshake(via, packet, h, 0)
 		return
 	}
 
@@ -230,6 +249,9 @@ func (hm *HandshakeManager) handleOutbound(vpnIp netip.Addr, lighthouseTriggered
 				"style": header.SubTypeName(header.Handshake, hh.machine.Subtype()),
 			})
 		}
+		if hh.pq != nil {
+			fields = append(fields, "pqInitiatorIndex", hh.pq.index)
+		}
 		hh.hostinfo.logger(hm.l).Info("Handshake timed out", fields...)
 		hm.metricTimedOut.Inc(1)
 		hm.DeleteHostInfo(hostinfo)
@@ -239,25 +261,44 @@ func (hm *HandshakeManager) handleOutbound(vpnIp netip.Addr, lighthouseTriggered
 	// Increment the counter to increase our delay, linear backoff
 	hh.counter++
 
-	// Check if we have a handshake packet to transmit yet
-	if !hh.ready {
-		if !hm.buildStage0Packet(hh) {
+	// Pick this attempt's style and make sure its first message exists. A PQ
+	// attempt sends msg1 as fragments: now goes out immediately, later after the
+	// repair delay, and every attempt offers fresh repair symbols.
+	var now, later [][]byte
+	var machine *handshake.Machine
+	if hm.usePQ(hh, vpnIp) {
+		if hh.pq == nil && !hm.buildPQAttempt(hh) {
 			hm.OutboundHandshakeTimer.Add(vpnIp, hm.config.tryInterval*time.Duration(hh.counter))
 			return
 		}
+		var err error
+		now, later, err = hh.pq.sender.Attempt(fragment.DefaultRepair(hh.pq.sender.Layout().K))
+		if err != nil {
+			hostinfo.logger(hm.l).Error("Failed to fragment handshake", "error", err)
+			hm.OutboundHandshakeTimer.Add(vpnIp, hm.config.tryInterval*time.Duration(hh.counter))
+			return
+		}
+		machine = hh.pq.machine
+	} else {
+		if !hh.ready {
+			if !hm.buildStage0Packet(hh) {
+				hm.OutboundHandshakeTimer.Add(vpnIp, hm.config.tryInterval*time.Duration(hh.counter))
+				return
+			}
+		}
+		// TODO: this hardcodes "always retransmit stage 0", which is correct for
+		// IX (the initiator only ever sends one packet, msg1) but wrong the
+		// moment a 3+ message pattern lands. The retry loop should resend the
+		// most recent outgoing message, not always stage 0. That implies
+		// HandshakeHostInfo tracking a single "currentOutbound" packet (bytes +
+		// header metadata) that gets replaced as the handshake progresses,
+		// instead of indexing into HandshakePacket.
+		now = [][]byte{hostinfo.HandshakePacket[handshakePacketStage0]}
+		machine = hh.machine
 	}
-
-	// TODO: this hardcodes "always retransmit stage 0", which is correct for
-	// IX (the initiator only ever sends one packet, msg1) but wrong the
-	// moment a 3+ message pattern lands. The retry loop should resend the
-	// most recent outgoing message, not always stage 0. That implies
-	// HandshakeHostInfo tracking a single "currentOutbound" packet (bytes +
-	// header metadata) that gets replaced as the handshake progresses,
-	// instead of indexing into HandshakePacket.
-	stage0 := hostinfo.HandshakePacket[handshakePacketStage0]
 	hsFields := m{
-		"stage": uint64(hh.machine.MessageIndex()),
-		"style": header.SubTypeName(header.Handshake, hh.machine.Subtype()),
+		"stage": uint64(machine.MessageIndex()),
+		"style": header.SubTypeName(header.Handshake, machine.Subtype()),
 	}
 
 	remotes := hostinfo.remotes.CopyAddrs(hm.mainHostMap.GetPreferredRanges())
@@ -285,8 +326,10 @@ func (hm *HandshakeManager) handleOutbound(vpnIp netip.Addr, lighthouseTriggered
 	// Send the handshake to all known ips, stage 2 takes care of assigning the hostinfo.remote based on the first to reply
 	var sentTo []netip.AddrPort
 	hostinfo.remotes.ForEach(hm.mainHostMap.GetPreferredRanges(), func(addr netip.AddrPort, _ bool) {
-		hm.messageMetrics.Tx(header.Handshake, hh.machine.Subtype(), 1)
-		err := hm.outside.WriteTo(stage0, addr)
+		hm.messageMetrics.Tx(header.Handshake, machine.Subtype(), int64(len(now)+len(later)))
+		err := hm.sendFragmented(now, later, func(p []byte) error {
+			return hm.outside.WriteTo(p, addr)
+		})
 		if err != nil {
 			// These repeat every attempt, so match the success log below and only shout when the remotes changed
 			level := slog.LevelDebug
@@ -322,7 +365,7 @@ func (hm *HandshakeManager) handleOutbound(vpnIp netip.Addr, lighthouseTriggered
 		)
 	}
 
-	hm.f.relayManager.StartRelays(hm.f, vpnIp, hh, stage0)
+	hm.f.relayManager.StartRelays(hm.f, vpnIp, hh, now, later)
 
 	// If a lighthouse triggered this attempt then we are still in the timer wheel and do not need to re-add
 	if !lighthouseTriggered {
@@ -532,6 +575,10 @@ func (hm *HandshakeManager) DeleteHostInfo(hostinfo *HostInfo) {
 func (hm *HandshakeManager) unlockedDeleteHostInfo(hostinfo *HostInfo) {
 	for _, addr := range hostinfo.vpnAddrs {
 		if cur, ok := hm.vpnIps[addr]; ok && cur.hostinfo == hostinfo {
+			// A PQ attempt holds its own index alongside the hostinfo's.
+			if idx := cur.pqIndex(); idx != 0 && hm.indexes[idx] == cur {
+				delete(hm.indexes, idx)
+			}
 			delete(hm.vpnIps, addr)
 		}
 	}
@@ -642,9 +689,8 @@ func hsTimeout(tries int64, interval time.Duration) time.Duration {
 	return time.Duration(tries / 2 * ((2 * int64(interval)) + (tries-1)*int64(interval)))
 }
 
-// buildStage0Packet creates the initial handshake packet for the initiator.
-func (hm *HandshakeManager) buildStage0Packet(hh *HandshakeHostInfo) bool {
-	cs := hm.f.pki.getCertState()
+// initiatingVersion picks the certificate version to initiate a handshake with.
+func (hm *HandshakeManager) initiatingVersion(hh *HandshakeHostInfo, cs *CertState) cert.Version {
 	v := cs.DefaultVersion()
 	if hh.initiatingVersionOverride != cert.VersionPre1 {
 		v = hh.initiatingVersionOverride
@@ -656,6 +702,13 @@ func (hm *HandshakeManager) buildStage0Packet(hh *HandshakeHostInfo) bool {
 			}
 		}
 	}
+	return v
+}
+
+// buildStage0Packet creates the initial handshake packet for the initiator.
+func (hm *HandshakeManager) buildStage0Packet(hh *HandshakeHostInfo) bool {
+	cs := hm.f.pki.getCertState()
+	v := hm.initiatingVersion(hh, cs)
 
 	cred := cs.GetCredential(v)
 	if cred == nil {
@@ -694,7 +747,10 @@ func (hm *HandshakeManager) buildStage0Packet(hh *HandshakeHostInfo) bool {
 // beginHandshake handles an incoming handshake packet that doesn't match any
 // existing pending handshake. It creates a new responder Machine and processes
 // the first message.
-func (hm *HandshakeManager) beginHandshake(via ViaSender, packet []byte, h *header.H) {
+//
+// received is the number of bytes a fragmented msg1 arrived in, which bounds the
+// response sent back before the address is validated; 0 for unfragmented ones.
+func (hm *HandshakeManager) beginHandshake(via ViaSender, packet []byte, h *header.H, received int) {
 	f := hm.f
 	cs := f.pki.getCertState()
 
@@ -708,7 +764,7 @@ func (hm *HandshakeManager) beginHandshake(via ViaSender, packet []byte, h *head
 	machine, err := handshake.NewMachine(
 		v, cs.GetCredential,
 		hm.certVerifier(), func() (uint32, error) { return generateIndex(f.l) },
-		false, header.HandshakeIXPSK0,
+		false, h.Subtype,
 	)
 	if err != nil {
 		f.l.Error("Failed to create handshake machine", "from", via, "error", err)
@@ -798,11 +854,15 @@ func (hm *HandshakeManager) beginHandshake(via ViaSender, packet []byte, h *head
 
 	existing, err := hm.CheckAndComplete(hostinfo, handshakePacketStage0, f)
 	if err != nil {
-		hm.handleCheckAndCompleteError(err, existing, hostinfo, via)
+		hm.handleCheckAndCompleteError(err, existing, hostinfo, via, machine.Subtype(), received)
 		return
 	}
 
-	hm.sendHandshakeResponse(via, response, hostinfo, false)
+	hm.sendHandshakeResponse(via, response, hostinfo, false, received)
+	if header.IsFragmented(machine.Subtype()) {
+		hm.pqAnswered.add(packet[header.Len:], hostinfo, time.Now())
+	}
+	hm.notePQPeer(machine.Subtype(), vpnAddrs)
 	hostinfo.remotes.RefreshFromHandshake(vpnAddrs)
 
 	// Don't wait for UpdateWorker
@@ -813,23 +873,32 @@ func (hm *HandshakeManager) beginHandshake(via ViaSender, packet []byte, h *head
 
 // continueHandshake feeds an incoming packet to an existing pending handshake Machine.
 func (hm *HandshakeManager) continueHandshake(via ViaSender, hh *HandshakeHostInfo, packet []byte) {
-	f := hm.f
-
 	hh.Lock()
 	defer hh.Unlock()
 
-	// Re-verify hh is still tracked. Between queryIndex returning and us taking
-	// hh.Lock, handleOutbound may have timed out and deleted it. Once we hold
-	// hh.Lock no other deleter can race our index: handleOutbound also takes
-	// hh.Lock first, and handleRecvError targets a main-hostmap entry with a
-	// different localIndexId.
-	hm.RLock()
-	cur, ok := hm.indexes[hh.hostinfo.localIndexId]
-	hm.RUnlock()
-	if !ok || cur != hh {
+	if !hm.stillPending(hh, hh.hostinfo.localIndexId) {
 		return
 	}
+	hm.processResponse(via, hh, hh.machine, packet)
+}
 
+// stillPending re-verifies hh is still tracked under index. Between queryIndex
+// returning and the caller taking hh.Lock, handleOutbound may have timed out and
+// deleted it. Once hh.Lock is held no other deleter can race the index:
+// handleOutbound also takes hh.Lock first, and handleRecvError targets a
+// main-hostmap entry with a different localIndexId. The caller must hold hh.Lock.
+func (hm *HandshakeManager) stillPending(hh *HandshakeHostInfo, index uint32) bool {
+	hm.RLock()
+	defer hm.RUnlock()
+	cur, ok := hm.indexes[index]
+	return ok && cur == hh
+}
+
+// processResponse runs a responder's message through one of the initiator's
+// pending attempts and completes the tunnel when the handshake finishes. The
+// caller must hold hh.Lock.
+func (hm *HandshakeManager) processResponse(via ViaSender, hh *HandshakeHostInfo, machine *handshake.Machine, packet []byte) {
+	f := hm.f
 	hostinfo := hh.hostinfo
 	if !via.IsRelayed {
 		if !f.lightHouse.GetRemoteAllowList().AllowAll(hostinfo.vpnAddrs, via.UdpAddr.Addr()) {
@@ -839,7 +908,6 @@ func (hm *HandshakeManager) continueHandshake(via ViaSender, hh *HandshakeHostIn
 		}
 	}
 
-	machine := hh.machine
 	if machine == nil {
 		f.l.Error("No handshake machine available for continuation",
 			"vpnAddrs", hostinfo.vpnAddrs, "from", via)
@@ -862,7 +930,7 @@ func (hm *HandshakeManager) continueHandshake(via ViaSender, hh *HandshakeHostIn
 	}
 
 	if response != nil {
-		hm.sendHandshakeResponse(via, response, hostinfo, false)
+		hm.sendHandshakeResponse(via, response, hostinfo, false, 0)
 	}
 
 	if result == nil {
@@ -975,7 +1043,10 @@ func (hm *HandshakeManager) continueHandshake(via ViaSender, hh *HandshakeHostIn
 	hostinfo.vpnAddrs = vpnAddrs
 	hostinfo.buildNetworks(f.myVpnNetworksTable, remoteCert.Certificate)
 
+	// The classical and PQ attempts each hold an index; keep the one that won.
+	hm.settleIndex(hh, result.LocalIndex)
 	hm.Complete(hostinfo, f)
+	hm.notePQPeer(machine.Subtype(), vpnAddrs)
 
 	if len(hh.packetStore) > 0 {
 		if f.l.Enabled(context.Background(), slog.LevelDebug) {
@@ -1054,20 +1125,23 @@ func (hm *HandshakeManager) validatePeerCert(via ViaSender, remoteCert *cert.Cac
 // cached is true when msg is a stored response being retransmitted because
 // the peer's stage-1 retransmit landed (the ErrAlreadySeen path); false on a
 // fresh response.
-func (hm *HandshakeManager) sendHandshakeResponse(via ViaSender, msg []byte, hostinfo *HostInfo, cached bool) {
+//
+// received is the number of bytes a fragmented msg1 arrived in; it bounds what a
+// fragmented response may send back. It is ignored for unfragmented subtypes.
+func (hm *HandshakeManager) sendHandshakeResponse(via ViaSender, msg []byte, hostinfo *HostInfo, cached bool, received int) {
 	if msg == nil {
 		return
 	}
 
 	f := hm.f
-	f.messageMetrics.Tx(header.Handshake, header.MessageSubType(msg[1]), 1)
+	subtype := header.MessageSubType(msg[1])
 
 	// Common log fields. peerCert may be nil during intermediate
 	// multi-message flows (handshake hasn't completed yet); skip the cert
 	// block if so.
 	logFields := []any{
 		"vpnAddrs", hostinfo.vpnAddrs,
-		"handshake", m{"stage": uint64(2), "style": header.SubTypeName(header.Handshake, header.HandshakeIXPSK0)},
+		"handshake", m{"stage": uint64(2), "style": header.SubTypeName(header.Handshake, subtype)},
 		"cached", cached,
 		"initiatorIndex", hostinfo.remoteIndexId,
 		"responderIndex", hostinfo.localIndexId,
@@ -1081,6 +1155,17 @@ func (hm *HandshakeManager) sendHandshakeResponse(via ViaSender, msg []byte, hos
 		)
 	}
 
+	if header.IsFragmented(subtype) {
+		fields := append(logFields, "from", via)
+		if err := hm.sendFragmentedResponse(via, msg, hostinfo, cached, received); err != nil {
+			f.l.Debug("Did not send handshake message", append(fields, "error", err)...)
+		} else {
+			f.l.Info("Handshake message sent", fields...)
+		}
+		return
+	}
+
+	f.messageMetrics.Tx(header.Handshake, subtype, 1)
 	if !via.IsRelayed {
 		fields := append(logFields, "from", via)
 		err := f.outside.WriteTo(msg, via.UdpAddr)
@@ -1107,10 +1192,10 @@ func (hm *HandshakeManager) sendHandshakeResponse(via ViaSender, msg []byte, hos
 // This only fires from the responder-side beginHandshake path, after the
 // peer cert has been validated and ConnectionState populated, so peerCert
 // is always non-nil for the cases that log it.
-func (hm *HandshakeManager) handleCheckAndCompleteError(err error, existing, hostinfo *HostInfo, via ViaSender) {
+func (hm *HandshakeManager) handleCheckAndCompleteError(err error, existing, hostinfo *HostInfo, via ViaSender, subtype header.MessageSubType, received int) {
 	f := hm.f
 	peerCert := hostinfo.ConnectionState.peerCert
-	hsFields := m{"stage": uint64(1), "style": header.SubTypeName(header.Handshake, header.HandshakeIXPSK0)}
+	hsFields := m{"stage": uint64(1), "style": header.SubTypeName(header.Handshake, subtype)}
 
 	switch err {
 	case ErrAlreadySeen:
@@ -1121,7 +1206,7 @@ func (hm *HandshakeManager) handleCheckAndCompleteError(err error, existing, hos
 		// ephemeral keys; a freshly-built one would have different keys and break
 		// the tunnel even though both sides "completed" the handshake.
 		if msg := existing.HandshakePacket[handshakePacketStage2]; msg != nil {
-			hm.sendHandshakeResponse(via, msg, existing, true)
+			hm.sendHandshakeResponse(via, msg, existing, true, received)
 		}
 
 	case ErrExistingHostInfo:
diff --git a/handshake_manager_pq.go b/handshake_manager_pq.go
new file mode 100644
index 00000000..71557ba5
--- /dev/null
+++ b/handshake_manager_pq.go
@@ -0,0 +1,446 @@
+package nebula
+
+import (
+	"context"
+	"crypto/sha256"
+	"errors"
+	"log/slog"
+	"net/netip"
+	"sync"
+	"time"
+
+	"github.com/slackhq/nebula/handshake"
+	"github.com/slackhq/nebula/handshake/fragment"
+	"github.com/slackhq/nebula/header"
+)
+
+const (
+	// pqSubtype is the post-quantum handshake this node speaks.
+	pqSubtype = header.HandshakeIXPSK2MLKEM1024
+
+	// DefaultPQRepairDelay is how long repair fragments trail the data fragments.
+	DefaultPQRepairDelay = 10 * time.Millisecond
+
+	// pqAmplification bounds what a responder sends in reply to an unvalidated
+	// address, as a multiple of what it received from it.
+	pqAmplification = 3
+)
+
+// pqTableConfig bounds the responder's reassembly of msg1 fragments from senders
+// that have not authenticated yet.
+var pqTableConfig = fragment.TableConfig{
+	MaxBytes:     4 << 20,
+	MaxEntries:   1024,
+	MaxPerSource: 4,
+	MaxMsgLen:    16384,
+	TTL:          3 * time.Second,
+}
+
+type pqHandshakeConfig struct {
+	initiate    bool          // start PQ handshakes
+	accept      bool          // answer PQ handshakes
+	sticky      bool          // never fall back to classical for a peer that completed PQ
+	repairDelay time.Duration // delay between data and repair fragments
+}
+
+// pqAttempt is the initiator's post-quantum attempt for a pending handshake. It
+// lives alongside the classical attempt in HandshakeHostInfo and has its own
+// local index, so whichever answer arrives first wins.
+type pqAttempt struct {
+	machine *handshake.Machine
+	index   uint32
+	sender  *fragment.Sender
+	msg2    *fragment.Assembly
+}
+
+// pqFragKey identifies one msg1 being reassembled.
+type pqFragKey struct {
+	addr   netip.AddrPort
+	relay  uint32
+	fragID uint32
+}
+
+// pqFragSource groups msg1 reassemblies for the per-source quota: an IPv4 /32,
+// an IPv6 /64, or one relay channel.
+type pqFragSource struct {
+	prefix netip.Prefix
+	relay  uint32
+}
+
+func pqFragKeyFor(via ViaSender, fragID uint32) (pqFragKey, pqFragSource) {
+	if via.IsRelayed {
+		return pqFragKey{relay: via.relay.LocalIndex, fragID: fragID},
+			pqFragSource{relay: via.relay.LocalIndex}
+	}
+	addr := via.UdpAddr.Addr().Unmap()
+	bits := 64
+	if addr.Is4() {
+		bits = 32
+	}
+	prefix, _ := addr.Prefix(bits)
+	return pqFragKey{addr: via.UdpAddr, fragID: fragID}, pqFragSource{prefix: prefix}
+}
+
+// reassembled rebuilds the packet the handshake Machine expects from the header
+// of a fragment and the decoded message.
+func reassembled(h *header.H, msg []byte) []byte {
+	p := make([]byte, header.Len, header.Len+len(msg))
+	header.Encode(p, header.Version, header.Handshake, h.Subtype, h.RemoteIndex, h.MessageCounter)
+	return append(p, msg...)
+}
+
+// usePQ decides whether this outbound attempt is post-quantum. Attempts
+// alternate, starting with PQ, because a peer that does not speak PQ silently
+// drops it. A peer that already completed a PQ handshake never gets a classical
+// attempt when sticky is on.
+func (hm *HandshakeManager) usePQ(hh *HandshakeHostInfo, vpnIp netip.Addr) bool {
+	if !hm.config.pq.initiate {
+		return false
+	}
+	if hm.config.pq.sticky {
+		if _, ok := hm.pqPeers.Load(vpnIp); ok {
+			return true
+		}
+	}
+	return hh.counter%2 == 1
+}
+
+// buildPQAttempt creates the initiator's PQ Machine and msg1 sender.
+func (hm *HandshakeManager) buildPQAttempt(hh *HandshakeHostInfo) bool {
+	cs := hm.f.pki.getCertState()
+	v := hm.initiatingVersion(hh, cs)
+	if cs.GetCredential(v) == nil {
+		hm.f.l.Error("Unable to handshake with host because no certificate is available",
+			"vpnAddrs", hh.hostinfo.vpnAddrs, "certVersion", v)
+		return false
+	}
+
+	pq := &pqAttempt{}
+	machine, err := handshake.NewMachine(
+		v, cs.GetCredential,
+		hm.certVerifier(), func() (uint32, error) { return hm.allocatePQIndex(hh, pq) },
+		true, pqSubtype,
+	)
+	if err != nil {
+		hm.f.l.Error("Failed to create handshake machine",
+			"vpnAddrs", hh.hostinfo.vpnAddrs, "error", err)
+		return false
+	}
+
+	msg, err := machine.Initiate(nil)
+	if err != nil {
+		hm.f.l.Error("Failed to initiate handshake",
+			"vpnAddrs", hh.hostinfo.vpnAddrs, "error", err)
+		hm.releasePQIndex(hh, pq)
+		return false
+	}
+
+	sender, err := fragment.NewSender(msg[header.Len:], pqSubtype, 0, 1, pq.index)
+	if err != nil {
+		hm.f.l.Error("Failed to fragment handshake",
+			"vpnAddrs", hh.hostinfo.vpnAddrs, "error", err)
+		hm.releasePQIndex(hh, pq)
+		return false
+	}
+
+	pq.machine = machine
+	pq.sender = sender
+	hh.pq = pq
+	return true
+}
+
+// allocatePQIndex registers a local index for the PQ attempt. Unlike
+// allocateIndex it leaves hostinfo.localIndexId alone; that is settled when one
+// of the attempts completes.
+func (hm *HandshakeManager) allocatePQIndex(hh *HandshakeHostInfo, pq *pqAttempt) (uint32, error) {
+	hm.mainHostMap.RLock()
+	defer hm.mainHostMap.RUnlock()
+	hm.Lock()
+	defer hm.Unlock()
+
+	for range 32 {
+		index, err := generateIndex(hm.l)
+		if err != nil {
+			return 0, err
+		}
+
+		_, inPending := hm.indexes[index]
+		_, inMain := hm.mainHostMap.Indexes[index]
+
+		if !inMain && !inPending {
+			pq.index = index
+			hm.indexes[index] = hh
+			return index, nil
+		}
+	}
+
+	return 0, errors.New("failed to generate unique localIndexId")
+}
+
+func (hm *HandshakeManager) releasePQIndex(hh *HandshakeHostInfo, pq *pqAttempt) {
+	hm.Lock()
+	defer hm.Unlock()
+	if pq.index != 0 && hm.indexes[pq.index] == hh {
+		delete(hm.indexes, pq.index)
+	}
+}
+
+// settleIndex makes winner the hostinfo's local index and releases the index of
+// the attempt that lost.
+func (hm *HandshakeManager) settleIndex(hh *HandshakeHostInfo, winner uint32) {
+	hm.Lock()
+	defer hm.Unlock()
+	for _, idx := range []uint32{hh.hostinfo.localIndexId, hh.pqIndex()} {
+		if idx != 0 && idx != winner && hm.indexes[idx] == hh {
+			delete(hm.indexes, idx)
+		}
+	}
+	hh.hostinfo.localIndexId = winner
+}
+
+func (hh *HandshakeHostInfo) pqIndex() uint32 {
+	if hh.pq == nil {
+		return 0
+	}
+	return hh.pq.index
+}
+
+// handleFragment routes one fragment of a PQ handshake message.
+func (hm *HandshakeManager) handleFragment(via ViaSender, packet []byte, h *header.H) {
+	fh, sym, err := fragment.Parse(packet[header.Len:])
+	if err != nil {
+		hm.l.Debug("dropping malformed handshake fragment", "from", via, "error", err)
+		return
+	}
+
+	switch h.MessageCounter {
+	case 1:
+		if !hm.config.pq.accept {
+			hm.l.Debug("dropping post-quantum handshake, not accepting them", "from", via)
+			return
+		}
+		if h.RemoteIndex != 0 {
+			hm.l.Debug("dropping stage-1 handshake with non-zero RemoteIndex",
+				"from", via, "remoteIndex", h.RemoteIndex)
+			return
+		}
+		key, src := pqFragKeyFor(via, fh.FragID)
+		res, err := hm.pqTable.Add(key, src, time.Now(), fh, sym)
+		if err != nil {
+			hm.l.Debug("dropping handshake fragment", "from", via, "error", err)
+			return
+		}
+		if res == nil {
+			return
+		}
+		if existing := hm.pqAnswered.lookup(res.Msg, time.Now()); existing != nil {
+			// Already answered: resend, rate limited, with fresh symbols.
+			if msg := existing.HandshakePacket[handshakePacketStage2]; msg != nil {
+				hm.sendHandshakeResponse(via, msg, existing, true, res.Received)
+			}
+			return
+		}
+		hm.beginHandshake(via, reassembled(h, res.Msg), h, res.Received)
+
+	case 2:
+		if hh := hm.queryIndex(h.RemoteIndex); hh != nil {
+			hm.continuePQ(via, hh, h, fh, sym)
+		}
+	}
+}
+
+// continuePQ collects msg2 fragments for the initiator's PQ attempt.
+func (hm *HandshakeManager) continuePQ(via ViaSender, hh *HandshakeHostInfo, h *header.H, fh fragment.Header, sym []byte) {
+	hh.Lock()
+	defer hh.Unlock()
+
+	pq := hh.pq
+	if pq == nil || pq.index != h.RemoteIndex || !hm.stillPending(hh, pq.index) {
+		return
+	}
+
+	if pq.msg2 == nil {
+		a, err := fragment.NewAssembly(fh)
+		if err != nil {
+			return
+		}
+		pq.msg2 = a
+	}
+	ready, err := pq.msg2.Add(fh, sym)
+	if errors.Is(err, fragment.ErrMismatch) {
+		// The responder's FragID and MsgLen never change, so this is a stray or a
+		// forgery. Start over from this fragment rather than wedge.
+		pq.msg2 = nil
+		return
+	}
+	if err != nil || !ready {
+		return
+	}
+
+	msg, err := pq.msg2.Decode()
+	pq.msg2 = nil
+	if err != nil {
+		hm.l.Debug("failed to decode handshake fragments", "from", via, "error", err)
+		return
+	}
+	hm.processResponse(via, hh, pq.machine, reassembled(h, msg))
+}
+
+// sendFragmented sends packets to a peer: now immediately and later after the
+// repair delay. It returns the first error from the immediate sends; errors from
+// delayed sends are only logged.
+func (hm *HandshakeManager) sendFragmented(now, later [][]byte, send func([]byte) error) error {
+	var first error
+	for _, p := range now {
+		if err := send(p); err != nil && first == nil {
+			first = err
+		}
+	}
+	sendLater := func() {
+		for _, p := range later {
+			if err := send(p); err != nil {
+				hm.l.Debug("Failed to send handshake repair fragment", "error", err)
+			}
+		}
+	}
+	switch {
+	case len(later) == 0:
+	case hm.config.pq.repairDelay <= 0:
+		sendLater()
+	default:
+		time.AfterFunc(hm.config.pq.repairDelay, sendLater)
+	}
+	return first
+}
+
+// sendFragmentedResponse sends the responder's msg2 as fragments. A resend of a
+// cached response (cached) offers fresh symbols and is rate limited to one per
+// retry interval. received is how many bytes the triggering msg1 took, which
+// bounds what may be sent back before the address is validated.
+func (hm *HandshakeManager) sendFragmentedResponse(via ViaSender, msg []byte, hostinfo *HostInfo, cached bool, received int) error {
+	now := time.Now()
+	if cached {
+		last := hostinfo.handshakeResendAt.Load()
+		if now.UnixNano()-last < int64(hm.config.tryInterval) ||
+			!hostinfo.handshakeResendAt.CompareAndSwap(last, now.UnixNano()) {
+			return errors.New("handshake response resend rate limited")
+		}
+	} else {
+		hostinfo.handshakeResendAt.Store(now.UnixNano())
+	}
+
+	var h header.H
+	if err := h.Parse(msg); err != nil {
+		return err
+	}
+	s, err := fragment.NewSender(msg[header.Len:], h.Subtype, h.RemoteIndex, h.MessageCounter, hostinfo.localIndexId)
+	if err != nil {
+		return err
+	}
+
+	l := s.Layout()
+	repair := fragment.DefaultRepair(l.K)
+	if received > 0 {
+		pktLen := header.Len + fragment.HeaderLen + l.SymbolSize
+		budget := pqAmplification * received / pktLen
+		if budget < l.K {
+			return errors.New("handshake response exceeds the amplification budget")
+		}
+		repair = min(repair, budget-l.K)
+	}
+
+	n := uint32(l.K + repair)
+	s.SetNext(int(hostinfo.handshakeNextSymbol.Add(n) - n))
+	pkts, later, err := s.Attempt(repair)
+	if err != nil {
+		return err
+	}
+
+	f := hm.f
+	var send func([]byte) error
+	if !via.IsRelayed {
+		send = func(p []byte) error {
+			return f.outside.WriteTo(p, via.UdpAddr)
+		}
+	} else {
+		if via.relay == nil {
+			return errors.New("both addr and via.relay are nil")
+		}
+		hostinfo.relayState.InsertRelayTo(via.relayHI.vpnAddrs[0])
+		via.relayHI.relayState.UpdateRelayForByIdxState(via.relay.LocalIndex, Established)
+		send = func(p []byte) error {
+			f.SendVia(via.relayHI, via.relay, p, make([]byte, 12), make([]byte, mtu), false, 0)
+			return nil
+		}
+	}
+	f.messageMetrics.Tx(header.Handshake, h.Subtype, int64(len(pkts)+len(later)))
+	return hm.sendFragmented(pkts, later, send)
+}
+
+// notePQPeer remembers peers that completed a PQ handshake, for sticky mode.
+func (hm *HandshakeManager) notePQPeer(subtype header.MessageSubType, vpnAddrs []netip.Addr) {
+	if !hm.config.pq.sticky || subtype != pqSubtype {
+		return
+	}
+	for _, a := range vpnAddrs {
+		hm.pqPeers.Store(a, struct{}{})
+	}
+	if hm.l.Enabled(context.Background(), slog.LevelDebug) {
+		hm.l.Debug("Remembering post-quantum peer", "vpnAddrs", vpnAddrs)
+	}
+}
+
+// pqAnswered remembers the msg1s a responder recently answered, keyed by hash,
+// so a repeat is recognised before any noise, KEM or certificate work. Repeats
+// are routine: the trailing repair fragments of an attempt rebuild a msg1 that
+// its data fragments already completed, and a retrying initiator offers fresh
+// symbols for the same msg1. Entries only exist for completed handshakes, which
+// need a valid certificate, and live as long as an initiator keeps retrying.
+type pqAnswered struct {
+	sync.Mutex
+	ttl     time.Duration
+	entries map[[sha256.Size]byte]pqAnsweredEntry
+}
+
+type pqAnsweredEntry struct {
+	hostinfo *HostInfo
+	at       time.Time
+}
+
+const pqAnsweredMax = 4096
+
+func newPQAnswered(ttl time.Duration) *pqAnswered {
+	return &pqAnswered{ttl: ttl, entries: map[[sha256.Size]byte]pqAnsweredEntry{}}
+}
+
+func (a *pqAnswered) add(msg1 []byte, hostinfo *HostInfo, now time.Time) {
+	a.Lock()
+	defer a.Unlock()
+	if len(a.entries) >= pqAnsweredMax {
+		for k := range a.entries {
+			delete(a.entries, k)
+			break
+		}
+	}
+	a.entries[sha256.Sum256(msg1)] = pqAnsweredEntry{hostinfo: hostinfo, at: now}
+}
+
+func (a *pqAnswered) lookup(msg1 []byte, now time.Time) *HostInfo {
+	a.Lock()
+	defer a.Unlock()
+	e, ok := a.entries[sha256.Sum256(msg1)]
+	if !ok || now.Sub(e.at) > a.ttl {
+		return nil
+	}
+	return e.hostinfo
+}
+
+func (a *pqAnswered) expire(now time.Time) {
+	a.Lock()
+	defer a.Unlock()
+	for k, e := range a.entries {
+		if now.Sub(e.at) > a.ttl {
+			delete(a.entries, k)
+		}
+	}
+}
diff --git a/handshake_manager_pq_test.go b/handshake_manager_pq_test.go
new file mode 100644
index 00000000..832429c1
--- /dev/null
+++ b/handshake_manager_pq_test.go
@@ -0,0 +1,155 @@
+package nebula
+
+import (
+	"math/rand/v2"
+	"net/netip"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/slackhq/nebula/handshake/fragment"
+	"github.com/slackhq/nebula/header"
+	"github.com/slackhq/nebula/test"
+	"github.com/slackhq/nebula/udp"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// recordingConn captures what is written to it.
+type recordingConn struct {
+	udp.NoopConn
+	sync.Mutex
+	sent [][]byte
+}
+
+func (c *recordingConn) WriteTo(b []byte, _ netip.AddrPort) error {
+	c.Lock()
+	defer c.Unlock()
+	c.sent = append(c.sent, append([]byte(nil), b...))
+	return nil
+}
+
+func (c *recordingConn) take() [][]byte {
+	c.Lock()
+	defer c.Unlock()
+	s := c.sent
+	c.sent = nil
+	return s
+}
+
+func newPQTestManager(t *testing.T) (*HandshakeManager, *recordingConn) {
+	l := test.NewLogger()
+	conn := &recordingConn{}
+	cfg := defaultHandshakeConfig
+	cfg.pq = pqHandshakeConfig{initiate: true, accept: true}
+	hm := NewHandshakeManager(l, newHostMap(l), newTestLighthouse(), conn, cfg)
+	hm.f = &Interface{
+		handshakeManager: hm,
+		outside:          conn,
+		messageMetrics:   newMessageMetricsOnlyRecvError(),
+		l:                l,
+	}
+	return hm, conn
+}
+
+// fragmentIndexes checks every packet is a fragment of the expected response and
+// returns their symbol indexes.
+func fragmentIndexes(t *testing.T, pkts [][]byte, fragID, remoteIndex uint32) []int {
+	t.Helper()
+	var out []int
+	for _, p := range pkts {
+		require.LessOrEqual(t, len(p), fragment.MaxPacketLen)
+		var h header.H
+		require.NoError(t, h.Parse(p))
+		assert.Equal(t, pqSubtype, h.Subtype)
+		assert.Equal(t, remoteIndex, h.RemoteIndex)
+		assert.Equal(t, uint64(2), h.MessageCounter)
+		fh, _, err := fragment.Parse(p[header.Len:])
+		require.NoError(t, err)
+		assert.Equal(t, fragID, fh.FragID)
+		out = append(out, int(fh.Index))
+	}
+	return out
+}
+
+func TestSendFragmentedResponse(t *testing.T) {
+	hm, conn := newPQTestManager(t)
+	via := ViaSender{UdpAddr: netip.MustParseAddrPort("192.0.2.1:4242")}
+	hostinfo := &HostInfo{localIndexId: 77}
+
+	// A 2000 byte message is 2 symbols of 1000, so packets of 1024 bytes.
+	msg := make([]byte, header.Len, header.Len+2000)
+	header.Encode(msg, header.Version, header.Handshake, pqSubtype, 5, 2)
+	r := rand.New(rand.NewPCG(1, 1))
+	for range 2000 {
+		msg = append(msg, byte(r.Uint32()))
+	}
+	const pktLen = header.Len + fragment.HeaderLen + 1000
+
+	t.Run("first send", func(t *testing.T) {
+		require.NoError(t, hm.sendFragmentedResponse(via, msg, hostinfo, false, 0))
+		assert.Equal(t, []int{0, 1, 2, 3}, fragmentIndexes(t, conn.take(), 77, 5))
+	})
+
+	t.Run("resends are rate limited", func(t *testing.T) {
+		require.Error(t, hm.sendFragmentedResponse(via, msg, hostinfo, true, 10*pktLen))
+		assert.Empty(t, conn.take())
+	})
+
+	allowResend := func() {
+		hostinfo.handshakeResendAt.Store(time.Now().Add(-time.Second).UnixNano())
+	}
+
+	t.Run("resend offers fresh symbols", func(t *testing.T) {
+		allowResend()
+		require.NoError(t, hm.sendFragmentedResponse(via, msg, hostinfo, true, 10*pktLen))
+		assert.Equal(t, []int{4, 5, 6, 7}, fragmentIndexes(t, conn.take(), 77, 5))
+	})
+
+	t.Run("amplification budget trims repair", func(t *testing.T) {
+		allowResend()
+		// 3x of a little over one packet allows 3 packets: both data and 1 repair.
+		require.NoError(t, hm.sendFragmentedResponse(via, msg, hostinfo, true, pktLen+10))
+		assert.Equal(t, []int{8, 9, 10}, fragmentIndexes(t, conn.take(), 77, 5))
+	})
+
+	t.Run("amplification budget below the data refuses", func(t *testing.T) {
+		allowResend()
+		require.Error(t, hm.sendFragmentedResponse(via, msg, hostinfo, true, pktLen/2))
+		assert.Empty(t, conn.take())
+	})
+}
+
+func TestPQAnswered(t *testing.T) {
+	now := time.Unix(1000, 0)
+	a := newPQAnswered(5 * time.Second)
+	hi := &HostInfo{}
+
+	assert.Nil(t, a.lookup([]byte("msg1"), now))
+	a.add([]byte("msg1"), hi, now)
+	assert.Same(t, hi, a.lookup([]byte("msg1"), now.Add(time.Second)))
+	assert.Nil(t, a.lookup([]byte("other"), now))
+	assert.Nil(t, a.lookup([]byte("msg1"), now.Add(6*time.Second)), "stale entries are not answered")
+
+	a.expire(now.Add(6 * time.Second))
+	assert.Empty(t, a.entries)
+
+	for i := range pqAnsweredMax + 10 {
+		a.add([]byte{byte(i), byte(i >> 8)}, hi, now)
+	}
+	assert.Len(t, a.entries, pqAnsweredMax)
+}
+
+func TestPQFragKeyFor(t *testing.T) {
+	k1, s1 := pqFragKeyFor(ViaSender{UdpAddr: netip.MustParseAddrPort("192.0.2.1:4242")}, 9)
+	k2, s2 := pqFragKeyFor(ViaSender{UdpAddr: netip.MustParseAddrPort("192.0.2.1:4243")}, 9)
+	assert.NotEqual(t, k1, k2, "different ports are different messages")
+	assert.Equal(t, s1, s2, "but share an IPv4 /32 quota")
+
+	_, s3 := pqFragKeyFor(ViaSender{UdpAddr: netip.MustParseAddrPort("[2001:db8::1]:1")}, 9)
+	_, s4 := pqFragKeyFor(ViaSender{UdpAddr: netip.MustParseAddrPort("[2001:db8::ffff]:1")}, 9)
+	assert.Equal(t, s3, s4, "an IPv6 /64 shares a quota")
+
+	_, s5 := pqFragKeyFor(ViaSender{UdpAddr: netip.MustParseAddrPort("[::ffff:192.0.2.1]:1")}, 9)
+	assert.Equal(t, s1, s5, "v4-mapped addresses count as IPv4")
+}
diff --git a/hostmap.go b/hostmap.go
index 71814a2f..7427053a 100644
--- a/hostmap.go
+++ b/hostmap.go
@@ -260,6 +260,11 @@ type HostInfo struct {
 	// We need these to avoid replayed handshake packets creating new hostinfos which causes churn
 	HandshakePacket map[uint8][]byte
 
+	// A responder resending a fragmented handshake response offers symbols it has
+	// not sent before, at most once per retry interval.
+	handshakeResendAt   atomic.Int64  // unix nanos of the last send
+	handshakeNextSymbol atomic.Uint32 // next fragment symbol index to offer
+
 	// nextLHQuery is the earliest we can ask the lighthouse for new information.
 	// This is used to limit lighthouse re-queries in chatty clients
 	nextLHQuery atomic.Int64
diff --git a/main.go b/main.go
index a8d01779..cc91615b 100644
--- a/main.go
+++ b/main.go
@@ -232,6 +232,12 @@ func Main(c *config.C, configTest bool, buildVersion string, l *slog.Logger, dev
 		retries:        int64(c.GetInt("handshakes.retries", DefaultHandshakeRetries)),
 		triggerBuffer:  c.GetInt("handshakes.trigger_buffer", DefaultHandshakeTriggerBuffer),
 		messageMetrics: messageMetrics,
+		pq: pqHandshakeConfig{
+			initiate:    c.GetBool("handshakes.pq.initiate", false),
+			accept:      c.GetBool("handshakes.pq.accept", false),
+			sticky:      c.GetBool("handshakes.pq.sticky", false),
+			repairDelay: c.GetDuration("handshakes.pq.repair_delay", DefaultPQRepairDelay),
+		},
 	}
 
 	handshakeManager := NewHandshakeManager(l, hostMap, lightHouse, udpConns[0], handshakeConfig)
diff --git a/relay_manager.go b/relay_manager.go
index 46d7a2bb..a6e3f22c 100644
--- a/relay_manager.go
+++ b/relay_manager.go
@@ -57,8 +57,9 @@ func (rm *relayManager) GetUseRelays() bool {
 // StartRelays drives the relay-establishment side of an outbound handshake attempt.
 // For each candidate relay it either kicks off a handshake to the relay, sends a CreateRelayRequest, retransmits
 // one that may have been lost, or, once the relay is Established, forwards the in-progress
-// stage 0 handshake packet for vpnIp through it.
-func (rm *relayManager) StartRelays(f *Interface, vpnIp netip.Addr, hh *HandshakeHostInfo, stage0 []byte) {
+// stage 0 handshake packets for vpnIp through it: now immediately and later after the
+// handshake repair delay.
+func (rm *relayManager) StartRelays(f *Interface, vpnIp netip.Addr, hh *HandshakeHostInfo, now, later [][]byte) {
 	hostinfo := hh.hostinfo
 	if !rm.GetUseRelays() || len(hostinfo.remotes.relays) == 0 {
 		hh.lastRelays = nil
@@ -161,7 +162,10 @@ func (rm *relayManager) StartRelays(f *Interface, vpnIp netip.Addr, hh *Handshak
 		switch existingRelay.State {
 		case Established:
 			hl.Log(context.Background(), level, "Send handshake via relay", "relay", relay.String())
-			f.SendVia(relayHostInfo, existingRelay, stage0, make([]byte, 12), make([]byte, mtu), false, 0)
+			_ = f.handshakeManager.sendFragmented(now, later, func(p []byte) error {
+				f.SendVia(relayHostInfo, existingRelay, p, make([]byte, 12), make([]byte, mtu), false, 0)
+				return nil
+			})
 		case Disestablished:
 			// Mark this relay as 'requested'
 			relayHostInfo.relayState.UpdateRelayForByIpState(vpnIp, Requested)
diff --git a/relay_manager_test.go b/relay_manager_test.go
index 8da38940..9e9dd99c 100644
--- a/relay_manager_test.go
+++ b/relay_manager_test.go
@@ -50,7 +50,7 @@ func TestStartRelaysLogDedupe(t *testing.T) {
 		var buf bytes.Buffer
 		rm := newRM(&buf)
 		hh := newHH()
-		rm.StartRelays(f, vpnIp, hh, nil)
+		rm.StartRelays(f, vpnIp, hh, nil, nil)
 		assert.Equal(t, []netip.Addr{vpnIp}, hh.lastRelays, "lastRelays should record the relay set we just attempted")
 		assert.Contains(t, buf.String(), "level=INFO "+msg, "expected Info level on first attempt")
 	})
@@ -59,10 +59,10 @@ func TestStartRelaysLogDedupe(t *testing.T) {
 		var buf bytes.Buffer
 		rm := newRM(&buf)
 		hh := newHH()
-		rm.StartRelays(f, vpnIp, hh, nil)
+		rm.StartRelays(f, vpnIp, hh, nil, nil)
 		first := append([]netip.Addr(nil), hh.lastRelays...)
 		buf.Reset()
-		rm.StartRelays(f, vpnIp, hh, nil)
+		rm.StartRelays(f, vpnIp, hh, nil, nil)
 		assert.Equal(t, first, hh.lastRelays)
 		assert.Contains(t, buf.String(), "level=DEBUG "+msg, "expected Debug level on identical retry")
 		assert.NotContains(t, buf.String(), "level=INFO "+msg, "Info should not fire on identical retry")
@@ -72,13 +72,13 @@ func TestStartRelaysLogDedupe(t *testing.T) {
 		var buf bytes.Buffer
 		rm := newRM(&buf)
 		hh := newHH()
-		rm.StartRelays(f, vpnIp, hh, nil)
+		rm.StartRelays(f, vpnIp, hh, nil, nil)
 		buf.Reset()
 
 		// The lighthouse handed us a new set this round.
 		hh.hostinfo.remotes.relays = []netip.Addr{vpnIp, otherRelay}
 
-		rm.StartRelays(f, vpnIp, hh, nil)
+		rm.StartRelays(f, vpnIp, hh, nil, nil)
 		assert.Equal(t, []netip.Addr{vpnIp, otherRelay}, hh.lastRelays)
 		assert.Contains(t, buf.String(), "level=INFO "+msg, "expected Info when the relay list changes")
 	})
@@ -90,7 +90,7 @@ func TestStartRelaysLogDedupe(t *testing.T) {
 		hh := newHH()
 		hh.lastRelays = []netip.Addr{vpnIp}
 
-		rm.StartRelays(f, vpnIp, hh, nil)
+		rm.StartRelays(f, vpnIp, hh, nil, nil)
 		assert.Nil(t, hh.lastRelays, "with relays disabled lastRelays should be cleared")
 		assert.NotContains(t, buf.String(), msg, "should not log when we shortcut out")
 	})