ci: laxify codecoverage (#191)

* ci: laxify codecoverage

* reread coverage docs

* try this yaml out

* add project level codecov informational

* add a project wide coverage

* test project wide coverage status fail

* remove test code

.codecov.yml

@@ -1,3 +1,12 @@
+coverage:
+  status:
+    project:
+      default:
+        target: 62% # Ensure we don't accumulate too much debt.
+    patch:
+      default:
+        informational: true # Shows the patch metric but never turns red/fails the PR
+
 ignore:
   - "examples/**"
   - "**/stringers.go"

internal/ring.go

@@ -157,7 +157,7 @@ func (r *Ring) ReadDiscard(n int) error {
 	case n > buffered:
 		return errDiscardExceeds
 	case n == buffered:
-		r.Reset()
+		r.emptied()
 	case n+r.Off > len(r.Buf):
 		r.Off = n - (len(r.Buf) - r.Off)
 	default:
@@ -224,6 +224,18 @@ func (r *Ring) Reset() {
 	r.End = 0
 }
 
+// emptied marks the ring empty keeping the write position where it is, unlike
+// [Ring.Reset] which rewinds it to index 0. Bytes staged past that position with
+// [Ring.PeekWrite] are addressed relative to it, so moving it makes a later
+// [Ring.Commit] hand back the wrong bytes.
+func (r *Ring) emptied() {
+	off := r.End
+	if off == len(r.Buf) {
+		off = 0 // Tail exhausted, next write wraps.
+	}
+	r.Off, r.End = off, 0
+}
+
 // Size returns the capacity of the ring buffer.
 func (r *Ring) Size() int {
 	return len(r.Buf)
@@ -306,7 +318,7 @@ func (r *Ring) onReadEnd(totalRead int) {
 	}
 	newOff := r.addOff(r.Off, totalRead)
 	if newOff == r.End {
-		r.Reset()
+		r.emptied()
 	} else if newOff == len(r.Buf) {
 		r.Off = 0 // Optimization case.
 	} else {

internal/ring_test.go

@@ -661,3 +661,40 @@ func TestRingPeekWriteRejects(t *testing.T) {
 		t.Error("Commit beyond free must error")
 	}
 }
+
+// TestRingPeekWriteSurvivesEmptyRead checks bytes staged with [Ring.PeekWrite]
+// survive the ring being read empty, an event the stager does not control.
+func TestRingPeekWriteSurvivesEmptyRead(t *testing.T) {
+	r := &Ring{Buf: make([]byte, 16)}
+	if _, err := r.Write([]byte("AAAA")); err != nil {
+		t.Fatal("first write:", err)
+	}
+	// Stage "CCCC" one 4-byte gap past the write position.
+	if !r.PeekWrite([]byte("CCCC"), 4) {
+		t.Fatal("PeekWrite should fit")
+	}
+	// Drain everything readable: ring goes empty, staged bytes still pending.
+	got := make([]byte, 16)
+	n, err := r.Read(got)
+	if err != nil || string(got[:n]) != "AAAA" {
+		t.Fatalf("drain read %q (%v), want AAAA", got[:n], err)
+	}
+	if !r.IsEmpty() {
+		t.Fatal("ring should be empty after draining")
+	}
+	// Fill the gap and commit the staged tail.
+	if _, err := r.Write([]byte("BBBB")); err != nil {
+		t.Fatal("gap write:", err)
+	}
+	if err := r.Commit(4); err != nil {
+		t.Fatal("commit:", err)
+	}
+	n, err = r.Read(got)
+	if err != nil {
+		t.Fatal("read:", err)
+	}
+	if string(got[:n]) != "BBBBCCCC" {
+		t.Fatalf("read %q, want BBBBCCCC: the staged bytes were committed from the wrong offset", got[:n])
+	}
+	testRingSanity(t, r)
+}

tcp/conn.go

@@ -76,10 +76,16 @@ type ConnConfig struct {
 	// Logger sets the [Conn] logger.
 	// Lower level logging available at [Handler.SetLoggers] via [Conn.InternalHandler].
 	Logger *slog.Logger
-	// Policy is the optional transmit-steering algorithm (RTO, congestion
-	// control, ...) for the connection. nil disables it. A Policy needing time
-	// carries its own clock. See [Policy].
-	Policy Policy
+	// LossRecovery is the optional packet-loss recovery algorithm (RTO,
+	// congestion control, ...) for the connection. If set, Nanotime must also be
+	// set (else Configure returns an error). Leaving it nil disables loss
+	// recovery. See [LossRecovery].
+	LossRecovery LossRecovery
+	// Nanotime is the monotonic time source in nanoseconds (the func() int64
+	// convention used across lneto) that drives LossRecovery. It is required when
+	// LossRecovery is set and unused otherwise. The tcp package reads it only to
+	// stamp the loss-recovery hooks; it holds no clock itself.
+	Nanotime func() int64
 }
 
 // Configure should be called on any newly created connection before usage. See [ConnConfig].
@@ -87,6 +93,10 @@ func (conn *Conn) Configure(config ConnConfig) (err error) {
 	if config.RWBackoff == nil {
 		return lneto.ErrMissingHALConfig
 	}
+	if config.LossRecovery != nil && config.Nanotime == nil {
+		// The tcp package holds no clock: a loss-recovery algorithm cannot run without it.
+		return lneto.ErrInvalidConfig
+	}
 	conn.mu.Lock()
 	defer conn.mu.Unlock()
 	err = conn.h.SetBuffers(config.TxBuf, config.RxBuf, config.TxPacketQueueSize)
@@ -95,7 +105,7 @@ func (conn *Conn) Configure(config ConnConfig) (err error) {
 	}
 	conn._backoff = config.RWBackoff
 	conn.logger.log = config.Logger
-	conn.h.SetPolicy(config.Policy)
+	conn.h.SetLossRecovery(config.LossRecovery, config.Nanotime)
 	return nil
 }
 

tcp/control.go

@@ -95,12 +95,6 @@ func (tcb *ControlBlock) RecvWindow() Size { return tcb.rcv.WND }
 // ISS returns the initial sequence number of the connection that was defined on a call to Open by user.
 func (tcb *ControlBlock) ISS() Value { return tcb.snd.ISS }
 
-// SendUNA returns snd.UNA, the oldest sequence number not yet acked by the remote.
-func (tcb *ControlBlock) SendUNA() Value { return tcb.snd.UNA }
-
-// SendNext returns snd.NXT, one past the highest sequence number sent.
-func (tcb *ControlBlock) SendNext() Value { return tcb.snd.NXT }
-
 // MaxInFlightData returns the maximum size of a segment that can be sent by taking into account
 // the send window size and the unacked data. Returns 0 before StateSynRcvd.
 func (tcb *ControlBlock) MaxInFlightData() Size {
@@ -263,32 +257,14 @@ func (tcb *ControlBlock) HasPendingRetransmit() bool {
 	return tcb._state.TxDataOpen() && tcb.dupack >= retransmitAfterDupacks && tcb.nRetransmit <= tcb.dupack-retransmitAfterDupacks
 }
 
-// RetransmitFrom rewinds snd.NXT back to newNxt so the next PendingSegment and
-// Send calls retransmit unacknowledged data from that sequence number onwards.
-// It must be paired with ringTx.RetransmitFrom to rewind the transmit buffer to
-// the same point. Implements RFC 9293 §3.10.8 (RETRANSMISSION TIMEOUT).
-//
-// It reports false and changes nothing when newNxt falls outside the
-// unacknowledged range [snd.UNA, snd.NXT] or the connection cannot send data, so
-// a misbehaving [Policy] cannot corrupt the send sequence space.
-func (tcb *ControlBlock) RetransmitFrom(newNxt Value) bool {
-	if !tcb._state.TxDataOpen() {
-		return false
-	} else if newNxt.LessThan(tcb.snd.UNA) || tcb.snd.NXT.LessThan(newNxt) {
-		return false
-	}
-	tcb.snd.NXT = newNxt
-	tcb.dupack = 0
-	tcb.nRetransmit = 0
-	return true
-}
-
 // RetransmitAll rewinds snd.NXT back to snd.UNA so the next PendingSegment and
 // Send calls retransmit all unacknowledged data from the oldest sequence number
 // (go-back-N). It must be paired with ringTx.RetransmitFromUNA to rewind the
 // transmit buffer. Implements RFC 9293 §3.10.8 (RETRANSMISSION TIMEOUT).
 func (tcb *ControlBlock) RetransmitAll() {
-	tcb.RetransmitFrom(tcb.snd.UNA)
+	tcb.snd.NXT = tcb.snd.UNA
+	tcb.dupack = 0
+	tcb.nRetransmit = 0
 }
 
 // PendingSegment calculates a suitable next segment to send from a payload length.
@@ -303,10 +279,9 @@ func (tcb *ControlBlock) PendingSegment(payloadLen int) (_ Segment, ok bool) {
 		// Optimist Strategy: retransmit oldest data once.
 		return Segment{SEQ: tcb.snd.UNA, DATALEN: Size(payloadLen), ACK: tcb.rcv.NXT, WND: tcb.rcv.WND, Flags: FlagACK}, true
 	}
-	established := tcb._state == StateEstablished
-	canSendData := established || tcb._state == StateCloseWait
+	canSendData := tcb._state.txQueuedDataOpen()
 	if !canSendData {
-		payloadLen = 0 // Can't send data if not established or close-wait.
+		payloadLen = 0 // No send-buffer data may go out in this state.
 	}
 	if pending == 0 && payloadLen == 0 {
 		return Segment{}, false // No pending segment.
@@ -546,8 +521,12 @@ func (tcb *ControlBlock) validateOutgoingSegment(seg Segment) (err error) {
 			err = errSeqNotInWindow
 		}
 
-	case seg.DATALEN > 0 && (tcb._state == StateFinWait1 || tcb._state == StateFinWait2):
-		err = errConnectionClosing // Case 1: No further SENDs from the user will be accepted by the TCP implementation.
+	case seg.DATALEN > 0 && tcb._state == StateFinWait2:
+		// FIN-WAIT-2 means our FIN was acknowledged, so no data below it can be
+		// unacknowledged and data here is a caller error. FIN-WAIT-1 is excluded:
+		// its FIN sits above data the peer may still be missing, which must go out
+		// for either side to make progress (RFC 9293 §3.10.8).
+		err = errConnectionClosing
 
 	case checkSeq && tcb.snd.WND == 0 && seg.DATALEN > 0 && seg.SEQ == tcb.snd.NXT:
 		err = errZeroWindow

tcp/definitions.go

@@ -329,6 +329,14 @@ func (s State) TxDataOpen() bool {
 	return s == StateEstablished || s == StateCloseWait
 }
 
+// txQueuedDataOpen returns true if already-queued send-buffer data may still be
+// put on the wire. It stays true after a local close, where the FIN occupies a
+// sequence above data the peer has not acknowledged: until that data is
+// (re)transmitted the peer cannot reach the FIN. RFC 9293 §3.10.8.
+func (s State) txQueuedDataOpen() bool {
+	return s.TxDataOpen() || s == StateFinWait1 || s == StateClosing || s == StateLastAck
+}
+
 // RxDataOpen returns true if the state allows the receiving of incoming data segments.
 // Combine with [State.IsPreestablished] to know whether there is no more data to be received over the network.
 func (s State) RxDataOpen() bool {

tcp/handler.go

@@ -31,8 +31,14 @@ type Handler struct {
 	optcodec OptionCodec
 	// reasm tracks out-of-order segments staged in bufRx's free region. Always
 	// enabled once buffers are set (see [Handler.SetBuffers]).
-	reasm  reassembly
-	policy Policy
+	reasm reassembly
+	// loss is the optional packet-loss recovery algorithm (RTO, congestion
+	// control, ...) driven from the rx/tx hooks. nil disables loss recovery, in
+	// which case the connection behaves as if no timing existed. nanotime is the
+	// monotonic time source (nanoseconds) passed to those hooks; it is non-nil
+	// whenever loss is non-nil (enforced by [Conn.Configure]). See [LossRecovery].
+	loss     LossRecovery
+	nanotime func() int64
 
 	closing    bool
 	shutdownRx bool
@@ -73,16 +79,27 @@ func (h *Handler) SetBuffers(txbuf, rxbuf []byte, packets int) error {
 	return h.bufTx.ResetOrReuse(txbuf, packets, 0)
 }
 
-// SetPolicy installs the transmit-steering algorithm. nil disables it.
-// It should be set before the connection is opened. See [Policy].
-func (h *Handler) SetPolicy(policy Policy) {
-	h.policy = policy
+// SetLossRecovery installs the packet-loss recovery algorithm and the monotonic
+// time source (nanoseconds, the func() int64 convention used across lneto) that
+// drives it. The tcp package keeps no clock of its own; nanotime is read only to
+// stamp the rx/tx hooks (see [LossRecovery]). Passing loss == nil disables loss
+// recovery. It should be set before the connection is opened.
+func (h *Handler) SetLossRecovery(loss LossRecovery, nanotime func() int64) {
+	h.loss = loss
+	h.nanotime = nanotime
 }
-func (h *Handler) policyEnabled() bool { return h.policy != nil }
 
-// ControlBlock returns the state machine underlying the Handler, mainly so a
-// [Policy] can read the sequence spaces. Not for modification.
-func (h *Handler) ControlBlock() *ControlBlock { return &h.scb }
+func (h *Handler) lossEnabled() bool { return h.loss != nil }
+
+// NextDeadline returns the monotonic-nanosecond instant at which the connection
+// must next be serviced by a transmit attempt (e.g. an RTO expiry), or 0 when
+// there is no deadline or no loss recovery is configured. See [LossRecovery].
+func (h *Handler) NextDeadline() int64 {
+	if h.loss == nil {
+		return 0
+	}
+	return h.loss.NextDeadline()
+}
 
 // LocalPort returns the local port of the connection. Returns 0 if the connection is closed and uninitialized.
 func (h *Handler) LocalPort() uint16 {
@@ -148,15 +165,16 @@ func (h *Handler) reset(localPort, remotePort uint16, iss Value) {
 		shutdownRx: false,
 		// Persist configuration across reopen:
 		validator: h.validator,
-		policy:    h.policy,
+		loss:      h.loss,
+		nanotime:  h.nanotime,
 		logger:    h.logger,
 		// persist memory across repoen:
 		bufTx: h.bufTx,
 		bufRx: h.bufRx,
 		reasm: h.reasm,
 	}
-	if h.policyEnabled() {
-		h.policy.Reset()
+	if h.lossEnabled() {
+		h.loss.Reset()
 	}
 	h.reasm.clear() // preserve metadata capacity across reopen, drop held segments.
 	h.bufTx.ResetOrReuse(nil, 0, iss)
@@ -194,7 +212,9 @@ func (h *Handler) Recv(incomingPacket []byte) error {
 		return nil
 	}
 
-	if h.policyEnabled() && !h.policy.PreRx(h, tfrm) {
+	// Notify loss recovery of the received segment (RTT sampling, timer
+	// management) and let it drop the segment before processing if it asks to.
+	if h.lossEnabled() && !h.loss.PreRx(segIncoming, h.nanotime()).Keep {
 		return nil
 	}
 
@@ -225,9 +245,6 @@ func (h *Handler) Recv(incomingPacket []byte) error {
 	if prevState != h.scb.State() {
 		h.info("tcp.Handler:rx-statechange", slog.Uint64("port", uint64(h.localPort)), slog.String("old", prevState.String()), slog.String("new", h.scb.State().String()), slog.String("rxflags", segIncoming.Flags.String()))
 	}
-	if h.policyEnabled() {
-		h.policy.PostRx(h, prevState, tfrm)
-	}
 	if segIncoming.DATALEN != 0 && h.shutdownRx && (h.scb.State() == StateFinWait1 || h.scb.State() == StateFinWait2) {
 		// soypat/lneto#50: the application is done in both directions — read side
 		// shut down (CloseRead) and our FIN sent (Close) — so inbound data has no
@@ -363,28 +380,16 @@ func (h *Handler) Send(b []byte) (int, error) {
 	if h.IsTxOver() {
 		return 0, net.ErrClosed
 	}
-	tfrm, err := NewFrame(b)
-	if err != nil {
-		return 0, err
-	}
-	offset := uint8(5)
-	var holdNew bool
-	if h.policyEnabled() {
-		// Hand the Policy a defined frame: zeroed header at the minimum offset.
-		// It may append options and raise the offset, which is read back below.
-		tfrm.ClearHeader()
-		tfrm.SetOffsetAndFlags(offset, 0)
-		rtxFrom, doRtx, hold := h.policy.PreTx(h, tfrm)
-		holdNew = hold
-		if doRtx && h.scb.RetransmitFrom(rtxFrom) {
-			// Retransmission directed by the Policy: rewind the transmit buffer
-			// to match the send sequence so unacknowledged data is resent. Done
-			// before the early short-circuit below so an expired RTO
-			// retransmits even with no new data queued.
-			h.bufTx.RetransmitFrom(rtxFrom)
-		}
-		if o, _ := tfrm.OffsetAndFlags(); o > offset && int(o)*4 < len(b) {
-			offset = o
+	var now int64
+	if h.lossEnabled() {
+		now = h.nanotime()
+		if h.loss.PreTx(now).RetransmitAll {
+			// Go-back-N retransmission directed by loss recovery: rewind the
+			// send sequence and transmit buffer so unacknowledged data is resent
+			// from snd.UNA. Done before the early short-circuit below so an
+			// expired RTO retransmits even with no new data queued.
+			h.scb.RetransmitAll()
+			h.bufTx.RetransmitFromUNA()
 		}
 	}
 	awaitingSyn := h.AwaitingSynSend()
@@ -400,27 +405,29 @@ func (h *Handler) Send(b []byte) (int, error) {
 		// Early nop short circuit.
 		return 0, nil
 	}
+	tfrm, err := NewFrame(b)
+	if err != nil {
+		return 0, err
+	}
 	if buffered == 0 && h.closing && (h.scb.State() != StateCloseWait || !h.scb.HasPending()) {
 		// If Close called and no more data to be sent, terminate connection.
 		// In CLOSE-WAIT: wait until the pending ACK is sent first, since scb.Close()
 		// overwrites pending with [FIN|ACK] (unlike ESTABLISHED which merges via bitmask).
 		h.closing = false
-		err := h.scb.Close()
+		err = h.scb.Close()
 		if err != nil {
 			h.logerr("tcp.Handler.Close", slog.String("err", errstr(err)), slog.String("state", h.State().String()))
 			h.Abort()
 			return 0, io.EOF
 		}
 	}
-	// optHead is where the Handler's own options begin: after the fixed header
-	// and after any options the Policy already wrote, so neither clobbers the other.
-	optHead := int(offset) * 4
-	mss := uint16(len(b) - optHead)
+	offset := uint8(5)
+	mss := uint16(len(b) - sizeHeaderTCP)
 	var segment Segment
 	if awaitingSyn || requeueControl && h.scb.State() == StateSynSent {
 		// Handling init syn segment.
 		segment = ClientSynSegment(h.bufTx.iss, Size(h.bufRx.Size()))
-		h.optcodec.PutOption16(b[optHead:], OptMaxSegmentSize, mss)
+		h.optcodec.PutOption16(b[sizeHeaderTCP:], OptMaxSegmentSize, mss)
 		offset++
 		if requeueControl {
 			h.info("tcp.Handler:requeue-syn", slog.Uint64("port", uint64(h.localPort)), slog.Uint64("rport", uint64(h.remotePort)))
@@ -432,7 +439,7 @@ func (h *Handler) Send(b []byte) (int, error) {
 			WND:   Size(h.bufRx.Free()),
 			Flags: synack,
 		}
-		h.optcodec.PutOption16(b[optHead:], OptMaxSegmentSize, mss)
+		h.optcodec.PutOption16(b[sizeHeaderTCP:], OptMaxSegmentSize, mss)
 		offset++
 		h.info("tcp.Handler:requeue-synack", slog.Uint64("port", uint64(h.localPort)), slog.Uint64("rport", uint64(h.remotePort)))
 	} else if requeueControl {
@@ -440,22 +447,17 @@ func (h *Handler) Send(b []byte) (int, error) {
 		return 0, nil
 	} else {
 		var ok bool
-		maxPayload := len(b) - optHead
-		if holdNew && !h.nextSegmentIsRetransmit() {
-			// Policy is holding new data back (congestion window exhausted).
-			// A retransmission it directed in this same call still proceeds.
-			maxPayload = 0
-		}
+		maxPayload := len(b) - sizeHeaderTCP
 		segment, ok = h.scb.PendingSegment(maxPayload)
 		segment.WND = h.recvWindow()
 		if !ok {
 			// No pending control segment or data to send. Yield.
 			return 0, nil
 		} else if segment.Flags == synack {
-			h.optcodec.PutOption16(b[optHead:], OptMaxSegmentSize, mss)
+			h.optcodec.PutOption16(b[sizeHeaderTCP:], OptMaxSegmentSize, mss)
 			offset++
 		} else if segment.DATALEN > 0 {
-			n, err := h.bufTx.MakePacket(b[optHead:optHead+int(segment.DATALEN)], segment.SEQ)
+			n, err := h.bufTx.MakePacket(b[sizeHeaderTCP:sizeHeaderTCP+segment.DATALEN], segment.SEQ)
 			if err != nil {
 				return 0, err
 			}
@@ -472,19 +474,15 @@ func (h *Handler) Send(b []byte) (int, error) {
 	} else if prevState != h.scb.State() && h.logenabled(slog.LevelInfo) {
 		h.info("tcp.Handler:tx-statechange", slog.Uint64("port", uint64(h.localPort)), slog.String("oldState", prevState.String()), slog.String("newState", h.scb.State().String()), slog.String("txflags", segment.Flags.String()))
 	}
+	if h.lossEnabled() {
+		h.loss.PostTx(segment, now)
+	}
 	h.requeueControl = false
 	tfrm.SetSourcePort(h.localPort)
 	tfrm.SetDestinationPort(h.remotePort)
 	tfrm.SetSegment(segment, offset)
 	tfrm.SetUrgentPtr(0)
 	datalen := int(offset)*4 + int(segment.DATALEN)
-	if h.policyEnabled() {
-		// Frame trimmed to what is actually emitted so the Policy's Payload()
-		// is the segment data and nothing more.
-		if sent, err := NewFrame(b[:datalen]); err == nil {
-			h.policy.PostTx(h, sent)
-		}
-	}
 	closedSuccess := prevState == StateTimeWait && segment.Flags.HasAny(FlagACK)
 	if closedSuccess {
 		h.reset(0, 0, 0)
@@ -496,14 +494,6 @@ func (h *Handler) Send(b []byte) (int, error) {
 	return datalen, nil
 }
 
-// nextSegmentIsRetransmit reports whether the next data segment would resend
-// already-transmitted bytes rather than open new sequence space. Used to let a
-// retransmission through while a [Policy] holds new data back.
-func (h *Handler) nextSegmentIsRetransmit() bool {
-	endSeq, hasSent := h.bufTx.sentEndSeq()
-	return hasSent && h.scb.snd.NXT.LessThan(endSeq)
-}
-
 // Write implements [io.Writer] by copying b to a internal buffer to be sent over the network on the next
 // [Handler.Send] call that can send data to remote peer. Use [Handler.Free] to know the maximum length the argument slice can be before erroring.
 func (h *Handler) Write(b []byte) (int, error) {

tcp/loss.go

@@ -0,0 +1,83 @@
+package tcp
+
+// LossRecovery abstracts TCP packet-loss recovery: RTO, congestion control and
+// any similar algorithm that observes segment traffic and steers the
+// connection's transmit behaviour. As far as the tcp package is concerned these
+// are all the same thing — packet-loss recovery algorithms — so they share one
+// interface (see discussion #157).
+//
+// The tcp package stays free of any time source: the current monotonic time in
+// nanoseconds (the func() int64 convention used across lneto) is passed in at
+// each hook boundary. It originates from [ConnConfig.Nanotime] and satisfies the
+// "WHEN was this segment rx/tx'd" requirement without a clock living inside the
+// state machine, which also keeps implementations deterministic for testing
+// (see issue #140).
+//
+// The interface is intentionally free of errors: an implementation handles or
+// reports its own errors rather than propagating them into lneto internals.
+//
+// Introspection (smoothed RTT, current window, ...) is deliberately left off the
+// interface; expose it on the concrete implementation the caller constructs and
+// hands to [ConnConfig].
+type LossRecovery interface {
+	// Reset returns the implementation to its initial, pre-connection state. It
+	// is invoked whenever the connection is (re)opened or aborted so a single
+	// LossRecovery value can be reused across the lifetime of connection reuse
+	// (see discussion #115).
+	Reset()
+
+	// NextDeadline returns the monotonic-nanosecond instant at which the
+	// connection must next be serviced by a transmit attempt — typically the RTO
+	// expiry. A return of 0 means there is no pending deadline. It replaces a
+	// poll/atomic-flag scheme with a deadline the caller's event loop can
+	// schedule against.
+	NextDeadline() int64
+
+	// PreRx is called for every segment received on the TCP port before the
+	// state machine processes it, with the monotonic time the segment arrived. It
+	// returns whether the segment should be kept (processed) or dropped.
+	PreRx(incoming Segment, now int64) RxDirective
+
+	// PreTx is called on entering the transmit path (Encapsulate), before a
+	// segment is built, with the current monotonic time. Its directive tells the
+	// connection whether to retransmit unacknowledged data, rewind the send
+	// pointer, or hold back new data.
+	PreTx(now int64) TxDirective
+
+	// PostTx is called on leaving the transmit path with the segment that was
+	// actually emitted and the monotonic time it was sent. This is where segment
+	// timing (for RTT sampling and the retransmission timer) is recorded.
+	PostTx(outgoing Segment, now int64)
+}
+
+// TxDirective is returned by [LossRecovery.PreTx] to steer the transmit path.
+// The zero value directs the connection to proceed normally (send new data if
+// available, no retransmission).
+type TxDirective struct {
+	// RewindNXT is the number of sequence-space octets to rewind snd.NXT by
+	// before transmitting, for partial (e.g. selective) retransmission. Zero
+	// means no rewind. It is independent of Retransmit, which rewinds fully to
+	// snd.UNA.
+	// RewindNXT uint32
+
+	// RetransmitAll requests go-back-N retransmission: the connection rewinds
+	// snd.NXT to snd.UNA and resends unacknowledged data from the oldest
+	// sequence number.
+	RetransmitAll bool
+	// HoldNew pauses transmission of new data (for example when the congestion
+	// window is exhausted). Retransmissions already directed by this same
+	// directive still proceed.
+	// HoldNew bool
+}
+
+// RxDirective is returned by [LossRecovery.PreRx].
+//
+// NOTE: its shape is the minimum viable contract — it mirrors the original
+// PreRx "keep" boolean from discussion #157 — and is the one element of the
+// interface not yet fully settled there. It is a struct (rather than a bare
+// bool) so fields can be added without breaking implementations.
+type RxDirective struct {
+	// Keep reports whether the received segment should be handed to the state
+	// machine. A false value drops the segment before it is processed.
+	Keep bool
+}

tcp/loss_test.go

@@ -0,0 +1,262 @@
+package tcp
+
+import (
+	"math/rand"
+	"testing"
+
+	"github.com/soypat/lneto/ethernet"
+)
+
+// recordingLoss is a test LossRecovery that records every hook invocation and
+// lets the test steer the directives returned to the Handler. It is the
+// interface counterpart driven by the Handler under test.
+type recordingLoss struct {
+	resets   int
+	preRx    []hookCall
+	preTx    []int64
+	postTx   []hookCall
+	deadline int64 // value NextDeadline reports back.
+
+	// Directives handed back to the Handler.
+	keep bool        // PreRx result. Default true (see newRecordingLoss).
+	tx   TxDirective // PreTx result.
+}
+
+type hookCall struct {
+	seg Segment
+	now int64
+}
+
+func newRecordingLoss() *recordingLoss { return &recordingLoss{keep: true} }
+
+var _ LossRecovery = (*recordingLoss)(nil)
+
+func (l *recordingLoss) Reset()              { l.resets++ }
+func (l *recordingLoss) NextDeadline() int64 { return l.deadline }
+
+func (l *recordingLoss) PreRx(incoming Segment, now int64) RxDirective {
+	l.preRx = append(l.preRx, hookCall{seg: incoming, now: now})
+	return RxDirective{Keep: l.keep}
+}
+
+func (l *recordingLoss) PreTx(now int64) TxDirective {
+	l.preTx = append(l.preTx, now)
+	return l.tx
+}
+
+func (l *recordingLoss) PostTx(outgoing Segment, now int64) {
+	l.postTx = append(l.postTx, hookCall{seg: outgoing, now: now})
+}
+
+// TestLossRecovery_DisabledByDefault verifies the Handler runs normally with no
+// loss recovery installed: NextDeadline reports no deadline and the transmit/
+// receive paths never touch a nil LossRecovery.
+func TestLossRecovery_DisabledByDefault(t *testing.T) {
+	const mtu = ethernet.MaxMTU
+	rng := rand.New(rand.NewSource(1))
+	client, server := newHandler(t, mtu, 3), newHandler(t, mtu, 3)
+	setupClientServer(t, rng, client, server)
+
+	if d := client.NextDeadline(); d != 0 {
+		t.Fatalf("NextDeadline with no loss recovery = %d, want 0", d)
+	}
+	var buf [mtu]byte
+	establish(t, client, server, buf[:]) // must not panic on nil loss recovery.
+}
+
+// TestLossRecovery_HooksInvoked verifies the Handler drives the full hook
+// contract across a handshake: Reset on open, PreTx+PostTx on every transmit,
+// PreRx on every receive, each stamped with the configured monotonic clock.
+func TestLossRecovery_HooksInvoked(t *testing.T) {
+	const mtu = ethernet.MaxMTU
+	rng := rand.New(rand.NewSource(2))
+	client, server := newHandler(t, mtu, 3), newHandler(t, mtu, 3)
+
+	loss := newRecordingLoss()
+	const clockNow = 1_000_000
+	client.SetLossRecovery(loss, func() int64 { return clockNow })
+
+	setupClientServer(t, rng, client, server) // OpenActive → reset → Reset().
+	if loss.resets == 0 {
+		t.Fatal("Reset not called on open")
+	}
+
+	var buf [mtu]byte
+	establish(t, client, server, buf[:])
+
+	// Client emitted SYN and the final ACK: both paths must have hit PreTx/PostTx.
+	if len(loss.preTx) == 0 {
+		t.Fatal("PreTx never called on transmit")
+	}
+	if len(loss.postTx) == 0 {
+		t.Fatal("PostTx never called on transmit")
+	}
+	if len(loss.preTx) != len(loss.postTx) {
+		t.Fatalf("PreTx calls=%d, PostTx calls=%d, want equal", len(loss.preTx), len(loss.postTx))
+	}
+	// Client received the SYN-ACK: PreRx must have seen it.
+	if len(loss.preRx) == 0 {
+		t.Fatal("PreRx never called on receive")
+	}
+
+	// The Handler holds no clock: every hook must be stamped from the supplied
+	// nanotime source.
+	for i, c := range loss.postTx {
+		if c.now != clockNow {
+			t.Fatalf("PostTx[%d].now = %d, want clock %d", i, c.now, clockNow)
+		}
+	}
+	for i, now := range loss.preTx {
+		if now != clockNow {
+			t.Fatalf("PreTx[%d].now = %d, want clock %d", i, now, clockNow)
+		}
+	}
+	for i, c := range loss.preRx {
+		if c.now != clockNow {
+			t.Fatalf("PreRx[%d].now = %d, want clock %d", i, c.now, clockNow)
+		}
+	}
+
+	// PostTx receives the segment actually emitted: the first is the SYN.
+	if !loss.postTx[0].seg.Flags.HasAny(FlagSYN) {
+		t.Fatalf("first PostTx segment flags=%s, want SYN", loss.postTx[0].seg.Flags)
+	}
+}
+
+// TestLossRecovery_NextDeadlineDelegates verifies NextDeadline is forwarded to
+// the installed LossRecovery unchanged.
+func TestLossRecovery_NextDeadlineDelegates(t *testing.T) {
+	const mtu = ethernet.MaxMTU
+	rng := rand.New(rand.NewSource(3))
+	client, server := newHandler(t, mtu, 3), newHandler(t, mtu, 3)
+
+	loss := newRecordingLoss()
+	loss.deadline = 4242
+	client.SetLossRecovery(loss, func() int64 { return 1 })
+	setupClientServer(t, rng, client, server)
+
+	if d := client.NextDeadline(); d != 4242 {
+		t.Fatalf("NextDeadline = %d, want delegated 4242", d)
+	}
+}
+
+// TestLossRecovery_PreRxDropsSegment verifies a PreRx directive of Keep=false
+// drops the segment before the state machine sees it: the payload is not
+// buffered and connection state is untouched.
+func TestLossRecovery_PreRxDropsSegment(t *testing.T) {
+	const mtu = ethernet.MaxMTU
+	rng := rand.New(rand.NewSource(4))
+	client, server := newHandler(t, mtu, 3), newHandler(t, mtu, 3)
+
+	loss := newRecordingLoss()
+	server.SetLossRecovery(loss, func() int64 { return 1 })
+	setupClientServer(t, rng, client, server)
+	var buf [mtu]byte
+	establish(t, client, server, buf[:]) // keep=true so handshake completes.
+
+	// Now start dropping everything the server receives.
+	loss.keep = false
+	preRxBefore := len(loss.preRx)
+
+	data := []byte("dropme")
+	if _, err := client.Write(data); err != nil {
+		t.Fatal("client write:", err)
+	}
+	clear(buf[:])
+	n, err := client.Send(buf[:])
+	if err != nil {
+		t.Fatal("client send:", err)
+	}
+
+	if err := server.Recv(buf[:n]); err != nil {
+		t.Fatalf("dropped segment must return nil, got %v", err)
+	}
+	if len(loss.preRx) != preRxBefore+1 {
+		t.Fatalf("PreRx calls=%d, want %d (segment must reach PreRx)", len(loss.preRx), preRxBefore+1)
+	}
+	if server.BufferedInput() != 0 {
+		t.Fatalf("dropped segment must not be buffered, got %d bytes", server.BufferedInput())
+	}
+	if server.State() != StateEstablished {
+		t.Fatalf("dropped segment must not change state, got %s", server.State())
+	}
+}
+
+// TestLossRecovery_PreTxRetransmitAll verifies a PreTx directive of
+// RetransmitAll drives go-back-N: the Handler rewinds and re-emits already-sent,
+// unacknowledged data from snd.UNA on the next transmit.
+func TestLossRecovery_PreTxRetransmitAll(t *testing.T) {
+	const mtu = ethernet.MaxMTU
+	rng := rand.New(rand.NewSource(5))
+	client, server := newHandler(t, mtu, 3), newHandler(t, mtu, 3)
+
+	loss := newRecordingLoss()
+	client.SetLossRecovery(loss, func() int64 { return 1 })
+	setupClientServer(t, rng, client, server)
+	var buf [mtu]byte
+	establish(t, client, server, buf[:])
+
+	// Emit one data segment; server never ACKs, so it stays unacknowledged.
+	data := []byte("payload")
+	if _, err := client.Write(data); err != nil {
+		t.Fatal("client write:", err)
+	}
+	clear(buf[:])
+	n, err := client.Send(buf[:])
+	if err != nil {
+		t.Fatal("client send data:", err)
+	}
+	if n <= sizeHeaderTCP {
+		t.Fatal("expected data segment")
+	}
+	firstSeg := mustSegment(t, buf[:n], n-sizeHeaderTCP)
+
+	// Direct go-back-N on the next transmit.
+	loss.tx = TxDirective{RetransmitAll: true}
+	clear(buf[:])
+	n, err = client.Send(buf[:])
+	if err != nil {
+		t.Fatal("client send retransmit:", err)
+	}
+	if n <= sizeHeaderTCP {
+		t.Fatal("expected retransmitted data segment")
+	}
+	rtSeg := mustSegment(t, buf[:n], n-sizeHeaderTCP)
+
+	if rtSeg.SEQ != firstSeg.SEQ {
+		t.Fatalf("retransmit SEQ=%d, want original UNA SEQ=%d (go-back-N)", rtSeg.SEQ, firstSeg.SEQ)
+	}
+	if rtSeg.DATALEN != firstSeg.DATALEN {
+		t.Fatalf("retransmit DATALEN=%d, want %d", rtSeg.DATALEN, firstSeg.DATALEN)
+	}
+}
+
+// TestLossRecovery_ResetOnReopen verifies Reset fires on every (re)open and on
+// Abort, so a single LossRecovery value can be reused across connection reuse.
+func TestLossRecovery_ResetOnReopen(t *testing.T) {
+	const mtu = ethernet.MaxMTU
+	client := newHandler(t, mtu, 3)
+	loss := newRecordingLoss()
+	client.SetLossRecovery(loss, func() int64 { return 1 })
+
+	if err := client.OpenActive(1234, 5678, 0); err != nil {
+		t.Fatal("open 1:", err)
+	}
+	afterOpen := loss.resets
+	if afterOpen == 0 {
+		t.Fatal("Reset not called on first open")
+	}
+
+	client.Abort()
+	if loss.resets <= afterOpen {
+		t.Fatalf("Reset not called on Abort: resets=%d, want >%d", loss.resets, afterOpen)
+	}
+	afterAbort := loss.resets
+
+	if err := client.OpenActive(1234, 5678, 0); err != nil {
+		t.Fatal("open 2:", err)
+	}
+	if loss.resets <= afterAbort {
+		t.Fatalf("Reset not called on reopen: resets=%d, want >%d", loss.resets, afterAbort)
+	}
+}

tcp/policy.go

@@ -1,24 +0,0 @@
-package tcp
-
-// Policy observes segment traffic and steers transmit behaviour: RTO,
-// congestion control and the like (discussion #157). The tcp package holds no
-// clock, so a Policy needing time carries its own (issue #140).
-// Introspection stays off the interface; put it on the concrete type.
-type Policy interface {
-	// Reset returns the Policy to its pre-connection state. Called on every
-	// (re)open and Abort. Must preserve configuration such as a clock.
-	Reset()
-	// PreTx is called before writing to a frame.
-	// The outgoing frame options can be set by the Policy and will be respected if Frame offset >5.
-	// rtxFrom is ignored unless within [snd.UNA, snd.NXT]. Nothing is committed
-	// until PostTx: a transmit attempt may emit no segment at all.
-	PreTx(h *Handler, outgoingOpts Frame) (rtxFrom Value, retransmit, holdNew bool)
-	// PreRx is called by [Handler] on every incoming segment.
-	// PreRx can choose to drop segment if it returns keep=false.
-	PreRx(h *Handler, incoming Frame) (keep bool)
-	// PostRx is called by [Handler] after accepting an incoming segment.
-	// TODO: congestion control will also want the pre-Recv snd.UNA here.
-	PostRx(h *Handler, prevState State, accepted Frame)
-	// PostTx called on leaving the transmit path with the fully written frame.
-	PostTx(h *Handler, outgoing Frame)
-}

tcp/policy_test.go

@@ -1,434 +0,0 @@
-package tcp
-
-import (
-	"math/rand"
-	"testing"
-
-	"github.com/soypat/lneto/ethernet"
-)
-
-// recordingPolicy records every hook invocation and lets the test steer what is
-// returned to the Handler. It is the [Policy] counterpart driven by the Handler
-// under test.
-type recordingPolicy struct {
-	resets int
-	preRx  []Segment
-	preTx  int
-	postRx []Segment
-	postTx []txRecord
-
-	// Values handed back to the Handler.
-	keep       bool // PreRx result. Default true (see newRecordingPolicy).
-	rtxFrom    Value
-	retransmit bool
-	holdNew    bool
-	// writeOpts, when non-empty, is appended as TCP options by PreTx.
-	writeOpts []byte
-}
-
-// txRecord is what PostTx observed on the emitted frame.
-type txRecord struct {
-	seg    Segment
-	offset uint8
-	sport  uint16
-	dport  uint16
-}
-
-func newRecordingPolicy() *recordingPolicy { return &recordingPolicy{keep: true} }
-
-var _ Policy = (*recordingPolicy)(nil)
-
-func (p *recordingPolicy) Reset() { p.resets++ }
-
-func (p *recordingPolicy) PreRx(h *Handler, incoming Frame) bool {
-	p.preRx = append(p.preRx, incoming.Segment(len(incoming.Payload())))
-	return p.keep
-}
-
-func (p *recordingPolicy) PostRx(h *Handler, prevState State, accepted Frame) {
-	p.postRx = append(p.postRx, accepted.Segment(len(accepted.Payload())))
-}
-
-func (p *recordingPolicy) PreTx(h *Handler, outgoingOpts Frame) (Value, bool, bool) {
-	p.preTx++
-	if len(p.writeOpts) > 0 {
-		// Raise the offset first: Options() is sized from it.
-		words := uint8(5 + (len(p.writeOpts)+3)/4)
-		outgoingOpts.SetOffsetAndFlags(words, 0)
-		copy(outgoingOpts.Options(), p.writeOpts)
-	}
-	return p.rtxFrom, p.retransmit, p.holdNew
-}
-
-func (p *recordingPolicy) PostTx(h *Handler, outgoing Frame) {
-	offset, _ := outgoing.OffsetAndFlags()
-	p.postTx = append(p.postTx, txRecord{
-		seg:    outgoing.Segment(len(outgoing.Payload())),
-		offset: offset,
-		sport:  outgoing.SourcePort(),
-		dport:  outgoing.DestinationPort(),
-	})
-}
-
-// TestPolicy_DisabledByDefault verifies the Handler runs normally with no Policy
-// installed: the transmit and receive paths never touch a nil Policy.
-func TestPolicy_DisabledByDefault(t *testing.T) {
-	const mtu = ethernet.MaxMTU
-	rng := rand.New(rand.NewSource(1))
-	client, server := newHandler(t, mtu, 3), newHandler(t, mtu, 3)
-	setupClientServer(t, rng, client, server)
-
-	var buf [mtu]byte
-	establish(t, client, server, buf[:]) // must not panic on nil Policy.
-}
-
-// TestPolicy_HooksInvoked verifies the Handler drives the full hook contract
-// across a handshake: Reset on open, PreTx+PostTx on transmit, PreRx+PostRx on
-// receive.
-func TestPolicy_HooksInvoked(t *testing.T) {
-	const mtu = ethernet.MaxMTU
-	rng := rand.New(rand.NewSource(2))
-	client, server := newHandler(t, mtu, 3), newHandler(t, mtu, 3)
-
-	pol := newRecordingPolicy()
-	client.SetPolicy(pol)
-
-	setupClientServer(t, rng, client, server) // OpenActive → reset → Reset().
-	if pol.resets == 0 {
-		t.Fatal("Reset not called on open")
-	}
-
-	var buf [mtu]byte
-	establish(t, client, server, buf[:])
-
-	if pol.preTx == 0 {
-		t.Fatal("PreTx never called on transmit")
-	}
-	if len(pol.postTx) == 0 {
-		t.Fatal("PostTx never called on transmit")
-	}
-	if pol.preTx < len(pol.postTx) {
-		t.Fatalf("PreTx calls=%d < PostTx calls=%d: PostTx must never fire without PreTx", pol.preTx, len(pol.postTx))
-	}
-	// Client received the SYN-ACK and accepted it.
-	if len(pol.preRx) == 0 {
-		t.Fatal("PreRx never called on receive")
-	}
-	if len(pol.postRx) == 0 {
-		t.Fatal("PostRx never called on accepted receive")
-	}
-	// PostTx receives the segment actually emitted: the first is the SYN.
-	if !pol.postTx[0].seg.Flags.HasAny(FlagSYN) {
-		t.Fatalf("first PostTx segment flags=%s, want SYN", pol.postTx[0].seg.Flags)
-	}
-}
-
-// TestPolicy_PostTxSeesWrittenFrame verifies PostTx observes the fully populated
-// frame — ports, sequence numbers and payload length as emitted — and not the
-// frame as it stood before the segment was written into it.
-func TestPolicy_PostTxSeesWrittenFrame(t *testing.T) {
-	const mtu = ethernet.MaxMTU
-	rng := rand.New(rand.NewSource(6))
-	client, server := newHandler(t, mtu, 3), newHandler(t, mtu, 3)
-
-	pol := newRecordingPolicy()
-	client.SetPolicy(pol)
-	setupClientServer(t, rng, client, server)
-	var buf [mtu]byte
-	establish(t, client, server, buf[:])
-
-	data := []byte("payload")
-	if _, err := client.Write(data); err != nil {
-		t.Fatal("client write:", err)
-	}
-	clear(buf[:])
-	n, err := client.Send(buf[:])
-	if err != nil {
-		t.Fatal("client send:", err)
-	}
-	last := pol.postTx[len(pol.postTx)-1]
-	wantSeg := mustSegment(t, buf[:n], n-int(last.offset)*4)
-	if last.seg != wantSeg {
-		t.Fatalf("PostTx segment=%+v, want emitted %+v", last.seg, wantSeg)
-	}
-	if int(last.seg.DATALEN) != len(data) {
-		t.Fatalf("PostTx DATALEN=%d, want %d", last.seg.DATALEN, len(data))
-	}
-	if last.sport != client.LocalPort() || last.dport != client.RemotePort() {
-		t.Fatalf("PostTx ports=%d→%d, want %d→%d", last.sport, last.dport, client.LocalPort(), client.RemotePort())
-	}
-}
-
-// TestPolicy_NoPostTxWithoutSegment verifies a transmit attempt that emits
-// nothing still runs PreTx but never PostTx, so a Policy cannot mistake a
-// no-op Send for a segment on the wire.
-func TestPolicy_NoPostTxWithoutSegment(t *testing.T) {
-	const mtu = ethernet.MaxMTU
-	rng := rand.New(rand.NewSource(7))
-	client, server := newHandler(t, mtu, 3), newHandler(t, mtu, 3)
-
-	pol := newRecordingPolicy()
-	client.SetPolicy(pol)
-	setupClientServer(t, rng, client, server)
-	var buf [mtu]byte
-	establish(t, client, server, buf[:])
-
-	preTxBefore, postTxBefore := pol.preTx, len(pol.postTx)
-	n, err := client.Send(buf[:]) // Nothing queued: no segment.
-	if err != nil {
-		t.Fatal("client send:", err)
-	}
-	if n != 0 {
-		t.Fatalf("expected no segment, got %d bytes", n)
-	}
-	if pol.preTx != preTxBefore+1 {
-		t.Fatalf("PreTx calls=%d, want %d: PreTx must run on every attempt", pol.preTx, preTxBefore+1)
-	}
-	if len(pol.postTx) != postTxBefore {
-		t.Fatalf("PostTx calls=%d, want %d: no segment was emitted", len(pol.postTx), postTxBefore)
-	}
-}
-
-// TestPolicy_PreTxOptions verifies options written by PreTx survive to the wire:
-// the data offset accounts for them and the payload starts after them.
-func TestPolicy_PreTxOptions(t *testing.T) {
-	const mtu = ethernet.MaxMTU
-	rng := rand.New(rand.NewSource(8))
-	client, server := newHandler(t, mtu, 3), newHandler(t, mtu, 3)
-
-	pol := newRecordingPolicy()
-	client.SetPolicy(pol)
-	setupClientServer(t, rng, client, server)
-	var buf [mtu]byte
-	establish(t, client, server, buf[:])
-
-	// One 4-byte option word: NOP,NOP,NOP,EOL.
-	opts := []byte{1, 1, 1, 0}
-	pol.writeOpts = opts
-
-	data := []byte("payload")
-	if _, err := client.Write(data); err != nil {
-		t.Fatal("client write:", err)
-	}
-	clear(buf[:])
-	n, err := client.Send(buf[:])
-	if err != nil {
-		t.Fatal("client send:", err)
-	}
-	frm, err := NewFrame(buf[:n])
-	if err != nil {
-		t.Fatal("frame:", err)
-	}
-	offset, _ := frm.OffsetAndFlags()
-	if offset != 6 {
-		t.Fatalf("data offset=%d, want 6 (header + one option word)", offset)
-	}
-	if got := frm.Options(); string(got) != string(opts) {
-		t.Fatalf("options=%v, want %v", got, opts)
-	}
-	if got := frm.Payload(); string(got) != string(data) {
-		t.Fatalf("payload=%q, want %q: options must not overlap data", got, data)
-	}
-	if n != int(offset)*4+len(data) {
-		t.Fatalf("frame length=%d, want %d", n, int(offset)*4+len(data))
-	}
-}
-
-// TestPolicy_PreRxDropsSegment verifies keep=false drops the segment before the
-// state machine sees it: the payload is not buffered, connection state is
-// untouched and PostRx never fires.
-func TestPolicy_PreRxDropsSegment(t *testing.T) {
-	const mtu = ethernet.MaxMTU
-	rng := rand.New(rand.NewSource(4))
-	client, server := newHandler(t, mtu, 3), newHandler(t, mtu, 3)
-
-	pol := newRecordingPolicy()
-	server.SetPolicy(pol)
-	setupClientServer(t, rng, client, server)
-	var buf [mtu]byte
-	establish(t, client, server, buf[:]) // keep=true so handshake completes.
-
-	// Now start dropping everything the server receives.
-	pol.keep = false
-	preRxBefore, postRxBefore := len(pol.preRx), len(pol.postRx)
-
-	data := []byte("dropme")
-	if _, err := client.Write(data); err != nil {
-		t.Fatal("client write:", err)
-	}
-	clear(buf[:])
-	n, err := client.Send(buf[:])
-	if err != nil {
-		t.Fatal("client send:", err)
-	}
-
-	if err := server.Recv(buf[:n]); err != nil {
-		t.Fatalf("dropped segment must return nil, got %v", err)
-	}
-	if len(pol.preRx) != preRxBefore+1 {
-		t.Fatalf("PreRx calls=%d, want %d (segment must reach PreRx)", len(pol.preRx), preRxBefore+1)
-	}
-	if len(pol.postRx) != postRxBefore {
-		t.Fatalf("PostRx calls=%d, want %d: a dropped segment was never accepted", len(pol.postRx), postRxBefore)
-	}
-	if server.BufferedInput() != 0 {
-		t.Fatalf("dropped segment must not be buffered, got %d bytes", server.BufferedInput())
-	}
-	if server.State() != StateEstablished {
-		t.Fatalf("dropped segment must not change state, got %s", server.State())
-	}
-}
-
-// TestPolicy_PreTxRetransmit verifies a PreTx retransmit directive drives
-// go-back-N: the Handler rewinds the send sequence and the transmit buffer
-// together and re-emits already-sent, unacknowledged data from snd.UNA.
-func TestPolicy_PreTxRetransmit(t *testing.T) {
-	const mtu = ethernet.MaxMTU
-	rng := rand.New(rand.NewSource(5))
-	client, server := newHandler(t, mtu, 3), newHandler(t, mtu, 3)
-
-	pol := newRecordingPolicy()
-	client.SetPolicy(pol)
-	setupClientServer(t, rng, client, server)
-	var buf [mtu]byte
-	establish(t, client, server, buf[:])
-
-	// Emit one data segment; server never ACKs, so it stays unacknowledged.
-	data := []byte("payload")
-	if _, err := client.Write(data); err != nil {
-		t.Fatal("client write:", err)
-	}
-	clear(buf[:])
-	n, err := client.Send(buf[:])
-	if err != nil {
-		t.Fatal("client send data:", err)
-	}
-	if n <= sizeHeaderTCP {
-		t.Fatal("expected data segment")
-	}
-	firstSeg := mustSegment(t, buf[:n], n-sizeHeaderTCP)
-	firstData := append([]byte(nil), buf[sizeHeaderTCP:n]...)
-
-	// Direct go-back-N on the next transmit.
-	pol.rtxFrom, pol.retransmit = client.ControlBlock().SendUNA(), true
-	clear(buf[:])
-	n, err = client.Send(buf[:])
-	if err != nil {
-		t.Fatal("client send retransmit:", err)
-	}
-	if n <= sizeHeaderTCP {
-		t.Fatal("expected retransmitted data segment")
-	}
-	rtSeg := mustSegment(t, buf[:n], n-sizeHeaderTCP)
-
-	if rtSeg.SEQ != firstSeg.SEQ {
-		t.Fatalf("retransmit SEQ=%d, want original UNA SEQ=%d (go-back-N)", rtSeg.SEQ, firstSeg.SEQ)
-	}
-	if rtSeg.DATALEN != firstSeg.DATALEN {
-		t.Fatalf("retransmit DATALEN=%d, want %d", rtSeg.DATALEN, firstSeg.DATALEN)
-	}
-	if got := buf[sizeHeaderTCP:n]; string(got) != string(firstData) {
-		t.Fatalf("retransmit payload=%q, want %q", got, firstData)
-	}
-}
-
-// TestPolicy_PreTxRetransmitOutOfRange verifies an out-of-range rtxFrom is
-// refused, leaving the send sequence and transmit buffer untouched.
-func TestPolicy_PreTxRetransmitOutOfRange(t *testing.T) {
-	const mtu = ethernet.MaxMTU
-	rng := rand.New(rand.NewSource(9))
-	client, server := newHandler(t, mtu, 3), newHandler(t, mtu, 3)
-
-	pol := newRecordingPolicy()
-	client.SetPolicy(pol)
-	setupClientServer(t, rng, client, server)
-	var buf [mtu]byte
-	establish(t, client, server, buf[:])
-
-	if _, err := client.Write([]byte("payload")); err != nil {
-		t.Fatal("client write:", err)
-	}
-	clear(buf[:])
-	if _, err := client.Send(buf[:]); err != nil {
-		t.Fatal("client send data:", err)
-	}
-	nxtBefore := client.ControlBlock().SendNext()
-
-	// Well beyond snd.NXT: must be refused.
-	pol.rtxFrom, pol.retransmit = nxtBefore+1000, true
-	clear(buf[:])
-	if _, err := client.Send(buf[:]); err != nil {
-		t.Fatal("client send:", err)
-	}
-	if got := client.ControlBlock().SendNext(); got != nxtBefore {
-		t.Fatalf("snd.NXT=%d, want unchanged %d: out-of-range rtxFrom must be refused", got, nxtBefore)
-	}
-}
-
-// TestPolicy_HoldNew verifies holdNew suppresses new data while leaving control
-// segments free to go out.
-func TestPolicy_HoldNew(t *testing.T) {
-	const mtu = ethernet.MaxMTU
-	rng := rand.New(rand.NewSource(10))
-	client, server := newHandler(t, mtu, 3), newHandler(t, mtu, 3)
-
-	pol := newRecordingPolicy()
-	client.SetPolicy(pol)
-	setupClientServer(t, rng, client, server)
-	var buf [mtu]byte
-	establish(t, client, server, buf[:])
-
-	pol.holdNew = true
-	if _, err := client.Write([]byte("payload")); err != nil {
-		t.Fatal("client write:", err)
-	}
-	clear(buf[:])
-	n, err := client.Send(buf[:])
-	if err != nil {
-		t.Fatal("client send:", err)
-	}
-	if n > sizeHeaderTCP {
-		t.Fatalf("holdNew must suppress new data, got %d payload bytes", n-sizeHeaderTCP)
-	}
-
-	// Releasing the hold lets the same data out.
-	pol.holdNew = false
-	clear(buf[:])
-	n, err = client.Send(buf[:])
-	if err != nil {
-		t.Fatal("client send after hold:", err)
-	}
-	if n <= sizeHeaderTCP {
-		t.Fatal("data must flow once holdNew is cleared")
-	}
-}
-
-// TestPolicy_ResetOnReopen verifies Reset fires on every (re)open and on Abort,
-// so a single Policy value can be reused across connection reuse.
-func TestPolicy_ResetOnReopen(t *testing.T) {
-	const mtu = ethernet.MaxMTU
-	client := newHandler(t, mtu, 3)
-	pol := newRecordingPolicy()
-	client.SetPolicy(pol)
-
-	if err := client.OpenActive(1234, 5678, 0); err != nil {
-		t.Fatal("open 1:", err)
-	}
-	afterOpen := pol.resets
-	if afterOpen == 0 {
-		t.Fatal("Reset not called on first open")
-	}
-
-	client.Abort()
-	if pol.resets <= afterOpen {
-		t.Fatalf("Reset not called on Abort: resets=%d, want >%d", pol.resets, afterOpen)
-	}
-	afterAbort := pol.resets
-
-	if err := client.OpenActive(1234, 5678, 0); err != nil {
-		t.Fatal("open 2:", err)
-	}
-	if pol.resets <= afterAbort {
-		t.Fatalf("Reset not called on reopen: resets=%d, want >%d", pol.resets, afterAbort)
-	}
-}

tcp/reorderstream_test.go

@@ -0,0 +1,122 @@
+package tcp
+
+import (
+	"math/rand"
+	"strconv"
+	"testing"
+
+	"github.com/soypat/lneto/ethernet"
+)
+
+// TestHandlerStreamIntegrityUnderReorder asserts byte identity of a reassembled
+// stream whose segments arrive out of order: arrival order is randomised within
+// each block of shuffleWindow segments, and nothing is lost or retransmitted, so
+// several segments sit staged in the receive ring at once. Reordering may cost
+// throughput; it may not change the bytes.
+func TestHandlerStreamIntegrityUnderReorder(t *testing.T) {
+	const (
+		mtu           = ethernet.MaxMTU
+		maxpackets    = 8
+		segSize       = 100
+		nsegs         = 8  // per round; 800 bytes through a 1500-byte ring
+		rounds        = 40 // enough for the ring to wrap many times
+		shuffleWindow = 4  // segments that may arrive in any order among themselves
+	)
+	rng := rand.New(rand.NewSource(3))
+	client, server := newHandler(t, mtu, maxpackets), newHandler(t, mtu, maxpackets)
+	setupClientServer(t, rng, client, server)
+	var rawbuf [mtu]byte
+	establish(t, client, server, rawbuf[:])
+
+	var want, got []byte
+	rb := make([]byte, mtu)
+	letter := byte('A')
+	for round := range rounds {
+		// Capture this round's segments on the wire, one segment per write.
+		segs := make([][]byte, 0, nsegs)
+		for range nsegs {
+			payload := make([]byte, segSize)
+			for j := range payload {
+				payload[j] = letter
+			}
+			letter++
+			if letter > 'Z' {
+				letter = 'A'
+			}
+			if n, err := client.Write(payload); err != nil || n != segSize {
+				t.Fatalf("round %d: client write: %d %v", round, n, err)
+			}
+			clear(rawbuf[:])
+			n, err := client.Send(rawbuf[:])
+			if err != nil {
+				t.Fatalf("round %d: client send: %v", round, err)
+			}
+			segs = append(segs, append([]byte(nil), rawbuf[:n]...))
+			want = append(want, payload...)
+		}
+
+		order := make([]int, 0, nsegs)
+		for i := 0; i < nsegs; i += shuffleWindow {
+			block := make([]int, 0, shuffleWindow)
+			for j := i; j < min(i+shuffleWindow, nsegs); j++ {
+				block = append(block, j)
+			}
+			rng.Shuffle(len(block), func(a, b int) { block[a], block[b] = block[b], block[a] })
+			order = append(order, block...)
+		}
+
+		for _, idx := range order {
+			if err := server.Recv(append([]byte(nil), segs[idx]...)); err != nil {
+				t.Logf("round %d segment %d refused: %v", round, idx, err)
+			}
+			// Drain as an application would, keeping the ring from filling.
+			for {
+				n, err := server.Read(rb)
+				if n > 0 {
+					got = append(got, rb[:n]...)
+				}
+				if n == 0 || err != nil {
+					break
+				}
+			}
+			// Feed ACKs back so the sender's window keeps opening; without this
+			// the test stalls on flow control instead of exercising reassembly.
+			clear(rawbuf[:])
+			if n, err := server.Send(rawbuf[:]); err == nil && n > 0 {
+				client.Recv(rawbuf[:n])
+			}
+		}
+
+		if string(got) != string(want) {
+			// Report the first divergence; later rounds only add noise.
+			i := 0
+			for i < len(got) && i < len(want) && got[i] == want[i] {
+				i++
+			}
+			t.Errorf("stream diverges in round %d at byte %d of %d; arrival order %v",
+				round, i, len(want), order)
+			lo := max(0, i-200)
+			t.Errorf("got  %s", summarizeRuns(got[lo:min(len(got), i+200)]))
+			t.Errorf("want %s", summarizeRuns(want[lo:min(len(want), i+200)]))
+			t.FailNow()
+		}
+	}
+	t.Logf("%d bytes intact across %d rounds of reordering (window %d)", len(got), rounds, shuffleWindow)
+}
+
+// summarizeRuns renders a byte stream as run-length pairs ("A*100 B*100") so a
+// duplicated or missing segment is visible at a glance.
+func summarizeRuns(b []byte) string {
+	out := make([]byte, 0, 64)
+	for i := 0; i < len(b); {
+		j := i
+		for j < len(b) && b[j] == b[i] {
+			j++
+		}
+		out = append(out, b[i], '*')
+		out = append(out, strconv.Itoa(j-i)...)
+		out = append(out, ' ')
+		i = j
+	}
+	return string(out)
+}

tcp/rto.go

@@ -1,11 +1,6 @@
-package rto
+package tcp
 
-import (
-	"time"
-
-	"github.com/soypat/lneto"
-	"github.com/soypat/lneto/tcp"
-)
+import "time"
 
 // RFC 6298 retransmission-timeout (RTO) parameters. The algorithm keeps a
 // single retransmission timer per connection (RFC 6298 §5): the timer is
@@ -35,80 +30,61 @@ const (
 	backoffMax = 12
 )
 
-// Timer implements the RFC 6298 round-trip-time estimator and the single
-// retransmission timer as a [tcp.Policy]. Construct it with [NewTimer] and hand
-// it to [tcp.ConnConfig.Policy].
+// RTO implements the RFC 6298 round-trip-time estimator and the single
+// retransmission timer as a [LossRecovery]. Construct it with new(RTO) and hand
+// it to [ConnConfig.LossRecovery]; the connection calls [RTO.Reset] on open, so
+// the zero value is ready to use.
 //
-// Timer is a pure, reactive state machine: it observes the segments a connection
-// sends and receives (via the tcp.Policy hooks) and from those alone derives RTT
-// estimates and retransmission decisions. The tcp package holds no clock, so the
-// Timer carries its own; injecting it keeps the estimator deterministic for unit
-// testing (see issue #140).
+// RTO is a pure, reactive state machine: it observes the segments a connection
+// sends and receives (via the LossRecovery hooks) and the monotonic time handed
+// in at each hook, and from those alone derives RTT estimates and retransmission
+// decisions. It holds no clock and allocates nothing, which keeps it
+// deterministic for unit testing (see issue #140).
 //
-// Timer tracks its own shadow of the send sequence space purely from the segments
-// it observes: [Timer.PostTx] advances the highest sequence sent and [Timer.PreRx]
+// RTO tracks its own shadow of the send sequence space purely from the segments
+// it observes: [RTO.PostTx] advances the highest sequence sent and [RTO.PreRx]
 // advances the highest sequence acknowledged. This is what lets it manage the
 // timer (RFC 6298 §5.2/§5.3) without reaching into the tcp state machine, and it
 // is also how retransmissions are distinguished for Karn's algorithm — a segment
 // whose sequence space is not beyond the shadow snd.NXT is a retransmission and
 // is never RTT-sampled.
-type Timer struct {
-	// nanotime is the monotonic time source in nanoseconds. Preserved by Reset.
-	nanotime func() int64
-
+type RTO struct {
 	srtt    time.Duration // smoothed round-trip time (SRTT).
 	rttvar  time.Duration // round-trip-time variation (RTTVAR).
 	rto     time.Duration // current retransmission timeout.
 	haveRTT bool          // false until the first RTT sample is taken.
 
 	// Shadow of the send sequence space, derived from observed segments.
-	haveSeq bool      // false until the first data segment is observed.
-	sndUNA  tcp.Value // highest acknowledged sequence number seen on the wire.
-	sndNXT  tcp.Value // one past the highest sequence number sent.
+	haveSeq bool  // false until the first data segment is observed.
+	sndUNA  Value // highest acknowledged sequence number seen on the wire.
+	sndNXT  Value // one past the highest sequence number sent.
 
 	// RTT sampling state (Karn's algorithm, RFC 6298 §3): at most one segment is
 	// timed at a time and retransmitted segments are never sampled.
 	timing   bool
-	timedSeq tcp.Value // ACK at or beyond this value completes the sample.
-	timedAt  int64     // send time (monotonic ns) of the timed segment.
+	timedSeq Value // ACK at or beyond this value completes the sample.
+	timedAt  int64 // send time (monotonic ns) of the timed segment.
 
 	// Retransmission timer state.
 	running  bool
 	deadline int64 // time (monotonic ns) at which the timer expires.
 	backoff  uint8 // consecutive timeouts, for exponential backoff.
-
-	// expirations counts timeouts since Reset. It exists so a policy sharing this
-	// timer can notice a timeout it did not itself drive: a congestion controller
-	// must collapse its window on one, and a policy that composes the timer as a
-	// peer never sees the timer's own directive.
-	expirations uint32
 }
 
-var _ tcp.Policy = (*Timer)(nil)
+var _ LossRecovery = (*RTO)(nil)
 
-// Configure prepares the Timer for use with nanotime, the monotonic time source
-// in nanoseconds (the func() int64 convention used across lneto). It must be
-// called before the connection is opened.
-func (r *Timer) Configure(nanotime func() int64) error {
-	if nanotime == nil {
-		return lneto.ErrMissingHALConfig // The estimator cannot run without a clock.
-	}
-	*r = Timer{rto: rtoInitial, nanotime: nanotime}
-	return nil
-}
-
-// Reset returns the estimator to its pre-connection state with the initial RTO,
-// preserving the configured clock. It implements [tcp.Policy] and is called when
-// the connection opens or aborts so the estimator survives connection reuse.
-func (r *Timer) Reset() { *r = Timer{rto: rtoInitial, nanotime: r.nanotime} }
+// Reset returns the estimator to its pre-connection state with the initial RTO.
+// It implements [LossRecovery] and is called when the connection opens or aborts
+// so the estimator can be reused across connection reuse.
+func (r *RTO) Reset() { *r = RTO{rto: rtoInitial} }
 
 // SmoothedRTT returns the current smoothed round-trip time (SRTT), or zero
 // before the first RTT measurement. It is concrete-type introspection and is
-// intentionally not part of [tcp.Policy].
-func (r *Timer) SmoothedRTT() time.Duration { return r.srtt }
+// intentionally not part of [LossRecovery].
+func (r *RTO) SmoothedRTT() time.Duration { return r.srtt }
 
 // CurrentRTO returns the timeout currently in effect, clamped to [rtoMin, rtoMax].
-func (r *Timer) CurrentRTO() time.Duration {
+func (r *RTO) CurrentRTO() time.Duration {
 	rto := r.rto
 	if rto < rtoMin {
 		rto = rtoMin
@@ -119,46 +95,23 @@ func (r *Timer) CurrentRTO() time.Duration {
 }
 
 // Running reports whether the retransmission timer is currently armed.
-func (r *Timer) Running() bool { return r.running }
-
-// Expirations returns how many times the retransmission timer has expired since
-// [Timer.Reset]. A policy that shares this timer rather than driving it watches
-// this for a change to learn that a timeout happened, since it never sees the
-// timer's own directive. It is concrete-type introspection and is intentionally
-// not part of [tcp.Policy].
-func (r *Timer) Expirations() uint32 { return r.expirations }
+func (r *RTO) Running() bool { return r.running }
 
 // NextDeadline returns the monotonic-nanosecond instant at which the timer
-// expires, or 0 when it is not armed. It is concrete-type introspection, not
-// part of [tcp.Policy]: an event loop that wants to schedule against the RTO
-// holds the Timer it configured and reads this.
-func (r *Timer) NextDeadline() int64 {
+// expires, or 0 when it is not armed. It implements [LossRecovery].
+func (r *RTO) NextDeadline() int64 {
 	if !r.running {
 		return 0
 	}
 	return r.deadline
 }
 
-// PreRx keeps every segment: the estimator never drops traffic and records
-// nothing before the connection has decided whether the segment counts. It
-// implements [tcp.Policy].
-func (r *Timer) PreRx(h *tcp.Handler, incoming tcp.Frame) bool {
-	return true
-}
-
-// PostRx samples the RTT and manages the retransmission timer from a segment the
-// connection accepted (RFC 6298 §5.2/§5.3). It implements [tcp.Policy].
-//
-// Only accepted segments reach here. Acting on a refused one would let an
-// acknowledgement the state machine rejected, for data never sent, collapse the
-// backoff and take a bogus RTT sample.
-func (r *Timer) PostRx(h *tcp.Handler, prevState tcp.State, accepted tcp.Frame) {
-	r.postRx(accepted.Segment(len(accepted.Payload())), r.nanotime())
-}
-
-func (r *Timer) postRx(incoming tcp.Segment, now int64) {
-	if !r.haveSeq || !incoming.Flags.HasAny(tcp.FlagACK) {
-		return
+// PreRx samples the RTT and manages the retransmission timer from a received
+// segment (RFC 6298 §5.2/§5.3). It implements [LossRecovery] and always keeps
+// the segment (the estimator never drops traffic).
+func (r *RTO) PreRx(incoming Segment, now int64) RxDirective {
+	if !r.haveSeq || !incoming.Flags.HasAny(FlagACK) {
+		return RxDirective{Keep: true}
 	}
 	ack := incoming.ACK
 	if r.timing && !ack.LessThan(r.timedSeq) {
@@ -179,22 +132,18 @@ func (r *Timer) postRx(incoming tcp.Segment, now int64) {
 		r.running = true
 		r.deadline = now + int64(r.CurrentRTO())
 	}
+	return RxDirective{Keep: true}
 }
 
 // PreTx reports whether the retransmission timer has expired and, if so, applies
 // the RFC 6298 §5.4–§5.6 timeout response — discard the outstanding RTT sample
-// (Karn), back the RTO off exponentially and restart the timer — and asks the
-// connection to retransmit from snd.UNA (go-back-N). It writes no TCP options:
-// retransmission timing needs none of its own. It implements [tcp.Policy].
-func (r *Timer) PreTx(h *tcp.Handler, outgoingOpts tcp.Frame) (rtxFrom tcp.Value, retransmit, holdNew bool) {
-	return r.preTx(r.nanotime(), h.ControlBlock().SendUNA())
-}
-
-func (r *Timer) preTx(now int64, una tcp.Value) (rtxFrom tcp.Value, retransmit, holdNew bool) {
+// (Karn), back the RTO off exponentially and restart the timer — returning a
+// directive that asks the connection to retransmit from snd.UNA (go-back-N). It
+// implements [LossRecovery].
+func (r *RTO) PreTx(now int64) TxDirective {
 	if !r.running || now < r.deadline || r.sndUNA == r.sndNXT {
-		return 0, false, false
+		return TxDirective{}
 	}
-	r.expirations++
 	r.timing = false // §5.4: do not sample a retransmitted segment.
 	if r.backoff < backoffMax {
 		r.backoff++
@@ -202,24 +151,20 @@ func (r *Timer) preTx(now int64, una tcp.Value) (rtxFrom tcp.Value, retransmit,
 	}
 	r.running = true
 	r.deadline = now + int64(r.CurrentRTO())
-	return una, true, false
+	return TxDirective{RetransmitAll: true}
 }
 
 // PostTx records an emitted segment: it advances the shadow send sequence,
 // begins timing newly transmitted data (RFC 6298 §3) and arms the timer (§5.1).
 // Segments that do not extend the send sequence are retransmissions and are
 // never RTT-sampled (Karn's algorithm). Control-only segments (no data) are
-// ignored. It implements [tcp.Policy].
-func (r *Timer) PostTx(h *tcp.Handler, outgoing tcp.Frame) {
-	r.postTx(outgoing.Segment(len(outgoing.Payload())), r.nanotime())
-}
-
-func (r *Timer) postTx(outgoing tcp.Segment, now int64) {
+// ignored. It implements [LossRecovery].
+func (r *RTO) PostTx(outgoing Segment, now int64) {
 	if outgoing.DATALEN == 0 {
 		return // only data segments are timed / arm the RTO.
 	}
 	segStart := outgoing.SEQ
-	segEnd := segStart + tcp.Value(outgoing.LEN())
+	segEnd := segStart + Value(outgoing.LEN())
 	if !r.haveSeq {
 		r.haveSeq = true
 		r.sndUNA = segStart
@@ -244,20 +189,9 @@ func (r *Timer) postTx(outgoing tcp.Segment, now int64) {
 	}
 }
 
-// ObserveRTT folds a round-trip measurement taken by other means into the
-// estimator, for a policy that composes this timer and can measure the round trip
-// more accurately than acknowledgement timing allows. The RFC 7323 timestamp echo
-// is the case this exists for.
-//
-// Unlike the timer's own sampling this does not apply Karn's algorithm, because a
-// sample derived from an echoed timestamp is unambiguous even when the segment
-// carrying it was a retransmission (RFC 7323 §4.1). Non-positive samples are
-// ignored.
-func (r *Timer) ObserveRTT(rtt time.Duration) { r.updateRTT(rtt) }
-
 // updateRTT folds a round-trip measurement into SRTT/RTTVAR/RTO using the
 // integer-shift form of RFC 6298 §2.2/§2.3.
-func (r *Timer) updateRTT(sample time.Duration) {
+func (r *RTO) updateRTT(sample time.Duration) {
 	if sample <= 0 {
 		return
 	}

tcp/rto/timer_test.go

@@ -1,311 +0,0 @@
-package rto
-
-import (
-	"testing"
-	"time"
-
-	"github.com/soypat/lneto/tcp"
-)
-
-const rtoMs = int64(time.Millisecond)
-
-// dataSeg builds a data segment of datalen octets starting at seq.
-func dataSeg(seq uint32, datalen int) tcp.Segment {
-	return tcp.Segment{SEQ: tcp.Value(seq), DATALEN: tcp.Size(datalen), Flags: tcp.FlagPSH | tcp.FlagACK}
-}
-
-// ackSeg builds a bare ACK acknowledging up to ack.
-func ackSeg(ack uint32) tcp.Segment {
-	return tcp.Segment{ACK: tcp.Value(ack), Flags: tcp.FlagACK}
-}
-
-func newRTO() *Timer {
-	var r Timer
-	if err := r.Configure(func() int64 { return 0 }); err != nil {
-		panic(err)
-	}
-	return &r
-}
-
-// frameOf renders a segment as the wire frame the [tcp.Policy] hooks receive.
-func frameOf(t *testing.T, s tcp.Segment) tcp.Frame {
-	t.Helper()
-	frm, err := tcp.NewFrame(make([]byte, 20+int(s.DATALEN)))
-	if err != nil {
-		t.Fatal(err)
-	}
-	frm.SetSegment(s, 5)
-	return frm
-}
-
-func TestRTO_Configure(t *testing.T) {
-	var r Timer
-	if err := r.Configure(nil); err == nil {
-		t.Error("Configure must reject a nil clock")
-	}
-	if err := r.Configure(func() int64 { return 0 }); err != nil {
-		t.Fatal(err)
-	}
-	if r.nanotime == nil {
-		t.Fatal("clock not stored")
-	}
-	r.Reset()
-	if r.nanotime == nil {
-		t.Error("Reset must preserve the configured clock")
-	}
-}
-
-func TestRTO_Reset(t *testing.T) {
-	r := newRTO()
-	r.Reset()
-	if r.rto != rtoInitial {
-		t.Errorf("initial rto=%v, want %v", r.rto, rtoInitial)
-	}
-	if r.CurrentRTO() != rtoInitial {
-		t.Errorf("CurrentRTO=%v, want %v", r.CurrentRTO(), rtoInitial)
-	}
-	if r.haveRTT {
-		t.Error("haveRTT should be false before first sample")
-	}
-	if r.Running() || r.NextDeadline() != 0 {
-		t.Error("timer must be disarmed after Reset")
-	}
-}
-
-// TestRTO_ArmOnSendSampleOnAck sends data, verifies the timer arms, then acks it
-// and verifies an RTT sample is taken and the timer stops once all data is acked.
-func TestRTO_ArmOnSendSampleOnAck(t *testing.T) {
-	r := newRTO()
-	const iss = uint32(1000)
-
-	r.postTx(dataSeg(iss, 100), 0)
-	if !r.Running() {
-		t.Fatal("timer must arm after sending data")
-	}
-	if r.NextDeadline() != int64(rtoInitial) {
-		t.Errorf("deadline=%d, want %d", r.NextDeadline(), int64(rtoInitial))
-	}
-
-	// ACK arrives one RTT (40ms) later covering all sent data.
-	if !r.PreRx(nil, frameOf(t, ackSeg(iss+100))) {
-		t.Error("PreRx must keep the segment")
-	}
-	r.postRx(ackSeg(iss+100), 40*rtoMs)
-	if r.Running() {
-		t.Error("timer must stop once all data is acknowledged")
-	}
-	if r.SmoothedRTT() != 40*time.Millisecond {
-		t.Errorf("srtt=%v, want 40ms", r.SmoothedRTT())
-	}
-}
-
-// TestRTO_RetransmitOnTimeout verifies PreTx directs a go-back-N retransmit once
-// the deadline passes with data outstanding, and backs the RTO off.
-func TestRTO_RetransmitOnTimeout(t *testing.T) {
-	r := newRTO()
-	const iss = uint32(1000)
-	r.postTx(dataSeg(iss, 100), 0)
-
-	if _, rtx, _ := r.preTx(int64(rtoInitial)-1, tcp.Value(iss)); rtx {
-		t.Fatal("must not retransmit before the deadline")
-	}
-	from, rtx, hold := r.preTx(int64(rtoInitial), tcp.Value(iss))
-	if !rtx {
-		t.Fatal("RTO must fire at the deadline with data outstanding")
-	}
-	if hold {
-		t.Error("the estimator never holds new data back")
-	}
-	if from != tcp.Value(iss) {
-		t.Errorf("retransmit from %d, want snd.UNA=%d", from, iss)
-	}
-	if r.CurrentRTO() != 2*rtoInitial {
-		t.Errorf("rto=%v after one backoff, want %v", r.CurrentRTO(), 2*rtoInitial)
-	}
-	// The connection resends from snd.UNA; postTx sees a retransmission.
-	r.postTx(dataSeg(iss, 100), int64(rtoInitial))
-	if r.timing {
-		t.Error("retransmitted segment must not be RTT-sampled (Karn)")
-	}
-}
-
-// TestRTO_KarnNoSampleOnRetransmittedAck verifies that after a retransmission the
-// ACK does not produce an RTT sample (Karn's algorithm).
-func TestRTO_KarnNoSampleOnRetransmittedAck(t *testing.T) {
-	r := newRTO()
-	const iss = uint32(1000)
-	r.postTx(dataSeg(iss, 100), 0)
-	// Timeout and retransmit.
-	r.preTx(int64(rtoInitial), tcp.Value(iss))
-	r.postTx(dataSeg(iss, 100), int64(rtoInitial))
-	// ACK now arrives; no sample should be taken since timing was discarded.
-	r.postRx(ackSeg(iss+100), int64(rtoInitial)+10*rtoMs)
-	if r.haveRTT {
-		t.Error("no RTT sample should exist after a retransmission (Karn)")
-	}
-}
-
-// TestRTO_TimerRestartsWhilePartiallyAcked verifies the timer restarts (not
-// stops) when an ACK advances UNA but data remains in flight (RFC 6298 §5.3).
-func TestRTO_TimerRestartsWhilePartiallyAcked(t *testing.T) {
-	r := newRTO()
-	const iss = uint32(1000)
-	r.postTx(dataSeg(iss, 100), 0)
-	r.postTx(dataSeg(iss+100, 100), 0) // 200 octets outstanding, iss..iss+200.
-
-	r.postRx(ackSeg(iss+100), 40*rtoMs) // acks first 100 only.
-	if !r.Running() {
-		t.Fatal("timer must remain armed while data is still in flight")
-	}
-	if r.NextDeadline() != 40*rtoMs+int64(r.CurrentRTO()) {
-		t.Errorf("deadline=%d, want %d", r.NextDeadline(), 40*rtoMs+int64(r.CurrentRTO()))
-	}
-}
-
-// TestRTO_NoArmWithoutData verifies control-only segments neither arm the timer
-// nor start an RTT sample.
-func TestRTO_NoArmWithoutData(t *testing.T) {
-	r := newRTO()
-	r.postTx(tcp.Segment{SEQ: 1000, Flags: tcp.FlagACK}, 0) // pure ACK, DATALEN==0.
-	if r.Running() || r.timing {
-		t.Error("pure control segment must not arm the timer or start a sample")
-	}
-}
-
-// TestRTO_BackoffCollapsesOnValidSample verifies a valid RTT measurement
-// collapses the exponential backoff counter (RFC 6298 §5.7).
-func TestRTO_BackoffCollapsesOnValidSample(t *testing.T) {
-	r := newRTO()
-	const iss = uint32(1000)
-	r.postTx(dataSeg(iss, 100), 0)
-	r.preTx(int64(rtoInitial), tcp.Value(iss))     // one timeout: backoff=1.
-	r.postTx(dataSeg(iss, 100), int64(rtoInitial)) // retransmit (no sample).
-	if r.backoff != 1 {
-		t.Fatalf("backoff=%d, want 1 after a timeout", r.backoff)
-	}
-	// New data sent and freshly sampled, then acked.
-	r.postTx(dataSeg(iss+100, 100), int64(rtoInitial)+rtoMs)
-	r.postRx(ackSeg(iss+200), int64(rtoInitial)+30*rtoMs)
-	if r.backoff != 0 {
-		t.Errorf("backoff=%d, want 0 after a valid RTT sample", r.backoff)
-	}
-}
-
-// TestRTO_Clamped verifies CurrentRTO is clamped to [rtoMin, rtoMax].
-func TestRTO_Clamped(t *testing.T) {
-	r := newRTO()
-	r.rto = time.Nanosecond
-	if got := r.CurrentRTO(); got != rtoMin {
-		t.Errorf("CurrentRTO=%v, want floor %v", got, rtoMin)
-	}
-	r.rto = time.Hour
-	if got := r.CurrentRTO(); got != rtoMax {
-		t.Errorf("CurrentRTO=%v, want ceiling %v", got, rtoMax)
-	}
-}
-
-// TestRTO_UpdateRTTFirstSample verifies the first-measurement initialization of
-// SRTT/RTTVAR (RFC 6298 §2.2).
-func TestRTO_UpdateRTTFirstSample(t *testing.T) {
-	r := newRTO()
-	r.updateRTT(100 * time.Millisecond)
-	if r.srtt != 100*time.Millisecond {
-		t.Errorf("srtt=%v, want 100ms", r.srtt)
-	}
-	if r.rttvar != 50*time.Millisecond {
-		t.Errorf("rttvar=%v, want 50ms", r.rttvar)
-	}
-	// RTO = SRTT + K*RTTVAR = 100 + 4*50 = 300ms.
-	if r.rto != 300*time.Millisecond {
-		t.Errorf("rto=%v, want 300ms", r.rto)
-	}
-}
-
-// TestRTO_PolicyHooksDeriveFromFrame exercises Timer through the [tcp.Policy]
-// hooks, verifying it reads the segment out of the frame it is handed: sending
-// data arms a deadline and a full ACK disarms it and yields the RTT sample.
-func TestRTO_PolicyHooksDeriveFromFrame(t *testing.T) {
-	var clock int64
-	var r Timer
-	if err := r.Configure(func() int64 { return clock }); err != nil {
-		t.Fatal(err)
-	}
-	var pol tcp.Policy = &r
-	pol.Reset()
-
-	pol.PostTx(nil, frameOf(t, dataSeg(1000, 100)))
-	if r.NextDeadline() == 0 {
-		t.Fatal("expected an armed deadline after sending data")
-	}
-	clock = 10 * rtoMs
-	if !pol.PreRx(nil, frameOf(t, ackSeg(1100))) {
-		t.Error("PreRx must keep")
-	}
-	pol.PostRx(nil, tcp.StateEstablished, frameOf(t, ackSeg(1100)))
-	if r.NextDeadline() != 0 {
-		t.Error("expected disarmed timer after full ack")
-	}
-	if r.SmoothedRTT() != 10*time.Millisecond {
-		t.Errorf("srtt=%v, want 10ms sampled through the hooks", r.SmoothedRTT())
-	}
-}
-
-// TestRTO_PreRxNeverDrops verifies the estimator keeps every segment and records
-// nothing at PreRx time. Dropping is not its business, and the connection has not
-// yet judged the segment: an acknowledgement for data never sent would otherwise
-// collapse the backoff and take a bogus round-trip sample. Only accepted segments
-// reach PostRx, which the Handler guarantees.
-func TestRTO_PreRxNeverDrops(t *testing.T) {
-	r := newRTO()
-	const iss = uint32(1000)
-	r.postTx(dataSeg(iss, 100), 0)
-	armed := r.NextDeadline()
-	if armed == 0 {
-		t.Fatal("timer must be armed after sending data")
-	}
-
-	// An acknowledgement far beyond anything sent, which the connection refuses.
-	if !r.PreRx(nil, frameOf(t, ackSeg(iss+100000))) {
-		t.Error("PreRx must keep: dropping is not the estimator's business")
-	}
-	if r.NextDeadline() != armed {
-		t.Errorf("deadline moved to %d at PreRx, want it left at %d", r.NextDeadline(), armed)
-	}
-	if r.SmoothedRTT() != 0 {
-		t.Errorf("took an RTT sample of %v at PreRx", r.SmoothedRTT())
-	}
-	if !r.Running() {
-		t.Error("timer disarmed at PreRx")
-	}
-}
-
-// TestRTO_RetransmitsZeroWindowProbe verifies the timer takes over the periodic
-// probing of a closed send window. A zero-window probe is a single octet the peer
-// cannot accept, so it goes unacknowledged; the timer must keep resending it, with
-// exponential backoff, which is the persist-timer behaviour of RFC 9293 §3.8.6.1.
-// The tcp package relies on this and refuses to probe without a policy installed.
-func TestRTO_RetransmitsZeroWindowProbe(t *testing.T) {
-	r := newRTO()
-	const iss = uint32(5000)
-	probe := dataSeg(iss, 1) // The one-octet probe.
-	r.postTx(probe, 0)
-
-	now := int64(rtoInitial)
-	prevRTO := r.CurrentRTO()
-	for attempt := 1; attempt <= 4; attempt++ {
-		from, rtx, _ := r.preTx(now, tcp.Value(iss))
-		if !rtx {
-			t.Fatalf("attempt %d: timer did not fire; the probe would never be resent", attempt)
-		}
-		if from != tcp.Value(iss) {
-			t.Errorf("attempt %d: retransmit from %d, want the probe octet at %d", attempt, from, iss)
-		}
-		if got := r.CurrentRTO(); got <= prevRTO {
-			t.Errorf("attempt %d: rto %v did not back off past %v", attempt, got, prevRTO)
-		}
-		prevRTO = r.CurrentRTO()
-		// The peer still cannot accept the octet, so it stays unacknowledged.
-		r.postTx(probe, now)
-		now += int64(prevRTO)
-	}
-}

tcp/rto_test.go

@@ -0,0 +1,206 @@
+package tcp
+
+import (
+	"testing"
+	"time"
+)
+
+const rtoMs = int64(time.Millisecond)
+
+// rtoDataSeg builds a data segment of datalen octets starting at seq.
+func rtoDataSeg(seq uint32, datalen int) Segment {
+	return Segment{SEQ: Value(seq), DATALEN: Size(datalen), Flags: FlagPSH | FlagACK}
+}
+
+// rtoAckSeg builds a bare ACK acknowledging up to ack.
+func rtoAckSeg(ack uint32) Segment {
+	return Segment{ACK: Value(ack), Flags: FlagACK}
+}
+
+func newRTO() *RTO {
+	var r RTO
+	r.Reset()
+	return &r
+}
+
+func TestRTO_Reset(t *testing.T) {
+	var r RTO
+	r.Reset()
+	if r.rto != rtoInitial {
+		t.Errorf("initial rto=%v, want %v", r.rto, rtoInitial)
+	}
+	if r.CurrentRTO() != rtoInitial {
+		t.Errorf("CurrentRTO=%v, want %v", r.CurrentRTO(), rtoInitial)
+	}
+	if r.haveRTT {
+		t.Error("haveRTT should be false before first sample")
+	}
+	if r.Running() || r.NextDeadline() != 0 {
+		t.Error("timer must be disarmed after Reset")
+	}
+}
+
+// TestRTO_ArmOnSendSampleOnAck sends data, verifies the timer arms, then acks it
+// and verifies an RTT sample is taken and the timer stops once all data is acked.
+func TestRTO_ArmOnSendSampleOnAck(t *testing.T) {
+	r := newRTO()
+	const iss = uint32(1000)
+
+	r.PostTx(rtoDataSeg(iss, 100), 0)
+	if !r.Running() {
+		t.Fatal("timer must arm after sending data")
+	}
+	if r.NextDeadline() != int64(rtoInitial) {
+		t.Errorf("deadline=%d, want %d", r.NextDeadline(), int64(rtoInitial))
+	}
+
+	// ACK arrives one RTT (40ms) later covering all sent data.
+	dir := r.PreRx(rtoAckSeg(iss+100), 40*rtoMs)
+	if !dir.Keep {
+		t.Error("PreRx must keep the segment")
+	}
+	if r.Running() {
+		t.Error("timer must stop once all data is acknowledged")
+	}
+	if r.SmoothedRTT() != 40*time.Millisecond {
+		t.Errorf("srtt=%v, want 40ms", r.SmoothedRTT())
+	}
+}
+
+// TestRTO_RetransmitOnTimeout verifies PreTx directs a go-back-N retransmit once
+// the deadline passes with data outstanding, and backs the RTO off.
+func TestRTO_RetransmitOnTimeout(t *testing.T) {
+	r := newRTO()
+	const iss = uint32(1000)
+	r.PostTx(rtoDataSeg(iss, 100), 0)
+
+	if r.PreTx(int64(rtoInitial) - 1).RetransmitAll {
+		t.Fatal("must not retransmit before the deadline")
+	}
+	dir := r.PreTx(int64(rtoInitial))
+	if !dir.RetransmitAll {
+		t.Fatal("RTO must fire at the deadline with data outstanding")
+	}
+	if r.CurrentRTO() != 2*rtoInitial {
+		t.Errorf("rto=%v after one backoff, want %v", r.CurrentRTO(), 2*rtoInitial)
+	}
+	// The connection resends from snd.UNA; PostTx sees a retransmission.
+	r.PostTx(rtoDataSeg(iss, 100), int64(rtoInitial))
+	if r.timing {
+		t.Error("retransmitted segment must not be RTT-sampled (Karn)")
+	}
+}
+
+// TestRTO_KarnNoSampleOnRetransmittedAck verifies that after a retransmission the
+// ACK does not produce an RTT sample (Karn's algorithm).
+func TestRTO_KarnNoSampleOnRetransmittedAck(t *testing.T) {
+	r := newRTO()
+	const iss = uint32(1000)
+	r.PostTx(rtoDataSeg(iss, 100), 0)
+	// Timeout and retransmit.
+	r.PreTx(int64(rtoInitial))
+	r.PostTx(rtoDataSeg(iss, 100), int64(rtoInitial))
+	// ACK now arrives; no sample should be taken since timing was discarded.
+	r.PreRx(rtoAckSeg(iss+100), int64(rtoInitial)+10*rtoMs)
+	if r.haveRTT {
+		t.Error("no RTT sample should exist after a retransmission (Karn)")
+	}
+}
+
+// TestRTO_TimerRestartsWhilePartiallyAcked verifies the timer restarts (not
+// stops) when an ACK advances UNA but data remains in flight (RFC 6298 §5.3).
+func TestRTO_TimerRestartsWhilePartiallyAcked(t *testing.T) {
+	r := newRTO()
+	const iss = uint32(1000)
+	r.PostTx(rtoDataSeg(iss, 100), 0)
+	r.PostTx(rtoDataSeg(iss+100, 100), 0) // 200 octets outstanding, iss..iss+200.
+
+	dir := r.PreRx(rtoAckSeg(iss+100), 40*rtoMs) // acks first 100 only.
+	if !r.Running() {
+		t.Fatal("timer must remain armed while data is still in flight")
+	}
+	if r.NextDeadline() != 40*rtoMs+int64(r.CurrentRTO()) {
+		t.Errorf("deadline=%d, want %d", r.NextDeadline(), 40*rtoMs+int64(r.CurrentRTO()))
+	}
+	if !dir.Keep {
+		t.Error("PreRx must keep the segment")
+	}
+}
+
+// TestRTO_NoArmWithoutData verifies control-only segments neither arm the timer
+// nor start an RTT sample.
+func TestRTO_NoArmWithoutData(t *testing.T) {
+	r := newRTO()
+	r.PostTx(Segment{SEQ: 1000, Flags: FlagACK}, 0) // pure ACK, DATALEN==0.
+	if r.Running() || r.timing {
+		t.Error("pure control segment must not arm the timer or start a sample")
+	}
+}
+
+// TestRTO_BackoffCollapsesOnValidSample verifies a valid RTT measurement
+// collapses the exponential backoff counter (RFC 6298 §5.7).
+func TestRTO_BackoffCollapsesOnValidSample(t *testing.T) {
+	r := newRTO()
+	const iss = uint32(1000)
+	r.PostTx(rtoDataSeg(iss, 100), 0)
+	r.PreTx(int64(rtoInitial))                        // one timeout: backoff=1.
+	r.PostTx(rtoDataSeg(iss, 100), int64(rtoInitial)) // retransmit (no sample).
+	if r.backoff != 1 {
+		t.Fatalf("backoff=%d, want 1 after a timeout", r.backoff)
+	}
+	// New data sent and freshly sampled, then acked.
+	r.PostTx(rtoDataSeg(iss+100, 100), int64(rtoInitial)+rtoMs)
+	r.PreRx(rtoAckSeg(iss+200), int64(rtoInitial)+30*rtoMs)
+	if r.backoff != 0 {
+		t.Errorf("backoff=%d, want 0 after a valid RTT sample", r.backoff)
+	}
+}
+
+// TestRTO_Clamped verifies CurrentRTO is clamped to [rtoMin, rtoMax].
+func TestRTO_Clamped(t *testing.T) {
+	var r RTO
+	r.Reset()
+	r.rto = time.Nanosecond
+	if got := r.CurrentRTO(); got != rtoMin {
+		t.Errorf("CurrentRTO=%v, want floor %v", got, rtoMin)
+	}
+	r.rto = time.Hour
+	if got := r.CurrentRTO(); got != rtoMax {
+		t.Errorf("CurrentRTO=%v, want ceiling %v", got, rtoMax)
+	}
+}
+
+// TestRTO_UpdateRTTFirstSample verifies the first-measurement initialization of
+// SRTT/RTTVAR (RFC 6298 §2.2).
+func TestRTO_UpdateRTTFirstSample(t *testing.T) {
+	var r RTO
+	r.Reset()
+	r.updateRTT(100 * time.Millisecond)
+	if r.srtt != 100*time.Millisecond {
+		t.Errorf("srtt=%v, want 100ms", r.srtt)
+	}
+	if r.rttvar != 50*time.Millisecond {
+		t.Errorf("rttvar=%v, want 50ms", r.rttvar)
+	}
+	// RTO = SRTT + K*RTTVAR = 100 + 4*50 = 300ms.
+	if r.rto != 300*time.Millisecond {
+		t.Errorf("rto=%v, want 300ms", r.rto)
+	}
+}
+
+// TestRTO_ImplementsLossRecovery exercises RTO through the [LossRecovery]
+// interface: sending data arms a deadline and a full ACK disarms it.
+func TestRTO_ImplementsLossRecovery(t *testing.T) {
+	var lr LossRecovery = newRTO()
+	lr.Reset()
+	lr.PostTx(rtoDataSeg(1000, 100), 0)
+	if lr.NextDeadline() == 0 {
+		t.Error("expected an armed deadline after sending data")
+	}
+	if !lr.PreRx(rtoAckSeg(1100), 10*rtoMs).Keep {
+		t.Error("PreRx must keep")
+	}
+	if lr.NextDeadline() != 0 {
+		t.Error("expected disarmed timer after full ack")
+	}
+}

tcp/rtointegration_test.go

@@ -0,0 +1,120 @@
+package tcp
+
+import (
+	"math/rand"
+	"testing"
+	"time"
+
+	"github.com/soypat/lneto/ethernet"
+)
+
+// TestHandlerRetransmitsAfterRTO covers the seam between a Handler and its
+// LossRecovery, which the RTO unit tests do not: a lost data segment must be
+// resent once the timer expires, with nothing arriving to prompt it.
+func TestHandlerRetransmitsAfterRTO(t *testing.T) {
+	const mtu = ethernet.MaxMTU
+	const maxpackets = 4
+	rng := rand.New(rand.NewSource(5))
+	client, server := newHandler(t, mtu, maxpackets), newHandler(t, mtu, maxpackets)
+
+	var now int64 // injected monotonic clock, in nanoseconds
+	client.SetLossRecovery(new(RTO), func() int64 { return now })
+
+	setupClientServer(t, rng, client, server)
+	var rawbuf [mtu]byte
+	establish(t, client, server, rawbuf[:])
+
+	data := []byte("hello")
+	if n, err := client.Write(data); err != nil || n != len(data) {
+		t.Fatal("client write:", n, err)
+	}
+	clear(rawbuf[:])
+	n, err := client.Send(rawbuf[:])
+	if err != nil || n == 0 {
+		t.Fatal("client send:", n, err)
+	}
+	// That frame is lost: it is never handed to the server.
+
+	// Nothing may come back before the timer expires.
+	var probe [mtu]byte
+	if n, err := client.Send(probe[:]); err != nil || n != 0 {
+		t.Fatalf("client sent %d bytes before the RTO expired (err %v)", n, err)
+	}
+
+	now += int64(3 * time.Second) // past the initial RTO and one backoff
+
+	clear(probe[:])
+	n, err = client.Send(probe[:])
+	if err != nil {
+		t.Fatal("client send after RTO:", err)
+	}
+	if n == 0 {
+		t.Fatal("no retransmission after the RTO expired: the loss-recovery directive is never applied")
+	}
+	if err := server.Recv(probe[:n]); err != nil {
+		t.Fatal("server refused the retransmission:", err)
+	}
+	got := make([]byte, 16)
+	nr, err := server.Read(got)
+	if err != nil || string(got[:nr]) != string(data) {
+		t.Fatalf("server read %q (%v), want %q", got[:nr], err, data)
+	}
+}
+
+// TestHandlerRetransmitsAfterCloseWithUnackedData is the write-then-close case
+// every server performs. With the last data segment lost, the FIN behind it sits
+// above a gap the peer cannot cross, so FIN-WAIT-1 must still retransmit that
+// data or both sides wait forever.
+func TestHandlerRetransmitsAfterCloseWithUnackedData(t *testing.T) {
+	const mtu = ethernet.MaxMTU
+	const maxpackets = 4
+	rng := rand.New(rand.NewSource(9))
+	client, server := newHandler(t, mtu, maxpackets), newHandler(t, mtu, maxpackets)
+
+	var now int64
+	client.SetLossRecovery(new(RTO), func() int64 { return now })
+
+	setupClientServer(t, rng, client, server)
+	var rawbuf [mtu]byte
+	establish(t, client, server, rawbuf[:])
+
+	data := []byte("last response bytes")
+	if n, err := client.Write(data); err != nil || n != len(data) {
+		t.Fatal("client write:", n, err)
+	}
+	clear(rawbuf[:])
+	n, err := client.Send(rawbuf[:]) // this frame is lost in transit
+	if err != nil || n == 0 {
+		t.Fatal("client send:", n, err)
+	}
+
+	// The application closes right after writing.
+	if err := client.Close(); err != nil {
+		t.Fatal("client close:", err)
+	}
+	var finbuf [mtu]byte
+	nfin, err := client.Send(finbuf[:]) // FIN (also lost, or simply unacked)
+	if err != nil {
+		t.Fatal("client send FIN:", err)
+	}
+	t.Logf("state after close: %s (FIN frame %d bytes)", client.State(), nfin)
+
+	now += int64(3 * time.Second) // past the RTO
+
+	var probe [mtu]byte
+	n, err = client.Send(probe[:])
+	if err != nil {
+		t.Fatal("client send after RTO:", err)
+	}
+	if n == 0 {
+		t.Fatalf("no retransmission in %s: unacknowledged data is stranded by the close", client.State())
+	}
+	if err := server.Recv(probe[:n]); err != nil {
+		t.Fatal("server refused the retransmission:", err)
+	}
+	got := make([]byte, 32)
+	nr, err := server.Read(got)
+	if err != nil || string(got[:nr]) != string(data) {
+		t.Fatalf("server read %q (%v), want %q", got[:nr], err, data)
+	}
+}

tcp/txqueue.go

@@ -227,39 +227,18 @@ func (rtx *ringTx) RetransmitFromUNA() {
 	if oldest == nil {
 		return // Nothing in the retransmission queue.
 	}
-	rtx.RetransmitFrom(oldest.seq)
-}
-
-// RetransmitFrom rewinds the transmit queue so sent-but-unacked data at and
-// after seq becomes unsent again; the next MakePacket calls re-send it. seq is
-// snapped down to the start of the packet containing it — the retransmission
-// queue tracks whole packets, so sub-packet rewind is not representable. It is
-// a no-op when seq is not covered by any queued packet (nothing to resend).
-//
-// Callers must pair this with [ControlBlock.RetransmitFrom] using the same seq
-// so the send sequence space and the transmit buffer rewind together.
-func (rtx *ringTx) RetransmitFrom(seq Value) {
-	pkt := rtx.slist.packetContaining(seq)
-	if pkt == nil {
-		return // seq not in the retransmission queue.
-	}
-	rewindOff, rewindSeq := pkt.off, pkt.seq
-	// The write position is unsentend, except when the unsent region is empty
-	// (unsentend==0) in which case data ends where the sent region ends. Capture
-	// it before reopening the unsent region over the rewound packets.
-	writeEnd := rtx.unsentend
-	if writeEnd == 0 {
-		writeEnd = rtx.sentend
-	}
-	if rewindOff == rtx.sentoff {
-		rtx.sentoff = 0 // Whole queue rewound: sent region becomes empty.
+	unaSeq := oldest.seq
+	if rtx.sentend != 0 {
+		// Merge sent region [sentoff, sentend) back into unsent.
+		rtx.unsentoff = rtx.sentoff
+		if rtx.unsentend == 0 {
+			rtx.unsentend = rtx.sentend
+		}
+		rtx.sentoff = 0
 		rtx.sentend = 0
-	} else {
-		rtx.sentend = rewindOff
 	}
-	rtx.unsentoff = rewindOff
-	rtx.unsentend = writeEnd
-	rtx.slist.truncateFrom(rewindSeq)
+	// Clear packet metadata; sequence tracking restarts from UNA.
+	rtx.slist.Reset(cap(rtx.slist.pkts), unaSeq)
 }
 
 func (rtx *ringTx) consolidateBufs() {
@@ -352,36 +331,6 @@ func (sl *sentlist) Free() int {
 	return cap(sl.pkts) - len(sl.pkts)
 }
 
-// packetContaining returns the queued packet whose sequence range covers seq, or
-// nil when no packet does. It is the floor lookup a retransmission rewind needs:
-// seq lands inside a packet and the whole packet is resent.
-func (sl *sentlist) packetContaining(seq Value) *ringidx {
-	for i := range sl.pkts {
-		pkt := &sl.pkts[i]
-		if pkt.seq.LessThanEq(seq) && seq.LessThan(pkt.endSeq()) {
-			return pkt
-		}
-	}
-	return nil
-}
-
-// truncateFrom drops the packet starting at seq and every packet sent after it,
-// so their data can be re-queued as unsent. seq must be a packet start sequence
-// (see [sentlist.packetContaining]). When no packet survives, the auxiliary
-// sequence counter is rewound to seq so [sentlist.EndSeq] keeps reporting where
-// the next packet begins.
-func (sl *sentlist) truncateFrom(seq Value) {
-	for i := range sl.pkts {
-		if sl.pkts[i].seq == seq {
-			sl.pkts = sl.pkts[:i]
-			if i == 0 {
-				sl.ssn = seq
-			}
-			return
-		}
-	}
-}
-
 func (sl *sentlist) AddPacket(datalen, off, bufsize int, seq Value) *ringidx {
 	free := sl.Free()
 	if free == 0 {

tcp/txqueue_retransmit_test.go

@@ -1,215 +0,0 @@
-package tcp
-
-import (
-	"bytes"
-	"testing"
-)
-
-// newRetransmitQueue builds a queue holding npkt sent packets of pktlen octets
-// each, starting at iss, plus any leftover unsent data. It returns the queue and
-// the full byte stream that was written.
-func newRetransmitQueue(t *testing.T, bufsize, maxPkts, npkt, pktlen, unsent int, iss Value) (*ringTx, []byte) {
-	t.Helper()
-	var rtx ringTx
-	if err := rtx.Reset(make([]byte, bufsize), maxPkts, iss); err != nil {
-		t.Fatal(err)
-	}
-	stream := make([]byte, npkt*pktlen+unsent)
-	for i := range stream {
-		stream[i] = byte(i + 1) // Non-zero so a stale ring shows up as a mismatch.
-	}
-	if n, err := rtx.Write(stream); err != nil || n != len(stream) {
-		t.Fatalf("write n=%d err=%v", n, err)
-	}
-	seq := iss
-	scratch := make([]byte, pktlen)
-	for i := range npkt {
-		n, err := rtx.MakePacket(scratch, seq)
-		if err != nil {
-			t.Fatalf("packet %d: %v", i, err)
-		}
-		if n != pktlen {
-			t.Fatalf("packet %d: n=%d, want %d", i, n, pktlen)
-		}
-		seq += Value(n)
-	}
-	testQueueSanity(t, &rtx)
-	return &rtx, stream
-}
-
-// mustRemake asserts the queue re-emits datalen octets at seq matching want.
-func mustRemake(t *testing.T, rtx *ringTx, seq Value, want []byte) {
-	t.Helper()
-	got := make([]byte, len(want))
-	n, err := rtx.MakePacket(got, seq)
-	if err != nil {
-		t.Fatalf("MakePacket at seq %d: %v", seq, err)
-	}
-	if n != len(want) {
-		t.Fatalf("MakePacket at seq %d: n=%d, want %d", seq, n, len(want))
-	}
-	if !bytes.Equal(got, want) {
-		t.Fatalf("MakePacket at seq %d: got %v, want %v", seq, got, want)
-	}
-}
-
-// TestRingTx_RetransmitFromBoundary rewinds to the start of the second of three
-// sent packets: the first stays sent, the rest become unsent and re-emit their
-// original bytes.
-func TestRingTx_RetransmitFromBoundary(t *testing.T) {
-	const iss, pktlen = Value(100), 4
-	rtx, stream := newRetransmitQueue(t, 64, 4, 3, pktlen, 0, iss)
-
-	sentBefore := rtx.BufferedSent()
-	rtx.RetransmitFrom(iss + pktlen) // Start of packet 2.
-	testQueueSanity(t, rtx)
-
-	if got := rtx.BufferedSent(); got != pktlen {
-		t.Fatalf("sent=%d, want %d (only packet 1 remains sent)", got, pktlen)
-	}
-	if got := rtx.BufferedUnsent(); got != sentBefore-pktlen {
-		t.Fatalf("unsent=%d, want %d", got, sentBefore-pktlen)
-	}
-	mustRemake(t, rtx, iss+pktlen, stream[pktlen:2*pktlen])
-	testQueueSanity(t, rtx)
-	mustRemake(t, rtx, iss+2*pktlen, stream[2*pktlen:3*pktlen])
-	testQueueSanity(t, rtx)
-}
-
-// TestRingTx_RetransmitFromMidPacket verifies a sequence inside a packet is
-// snapped down to that packet's start: the queue tracks whole packets.
-func TestRingTx_RetransmitFromMidPacket(t *testing.T) {
-	const iss, pktlen = Value(100), 4
-	rtx, stream := newRetransmitQueue(t, 64, 4, 3, pktlen, 0, iss)
-
-	rtx.RetransmitFrom(iss + pktlen + 2) // Two octets into packet 2.
-	testQueueSanity(t, rtx)
-
-	if got := rtx.BufferedSent(); got != pktlen {
-		t.Fatalf("sent=%d, want %d: rewind must floor to the packet start", got, pktlen)
-	}
-	mustRemake(t, rtx, iss+pktlen, stream[pktlen:2*pktlen])
-}
-
-// TestRingTx_RetransmitFromOldest rewinds the whole queue, which must match
-// RetransmitFromUNA.
-func TestRingTx_RetransmitFromOldest(t *testing.T) {
-	const iss, pktlen, npkt = Value(100), 4, 3
-	rtx, stream := newRetransmitQueue(t, 64, 4, npkt, pktlen, 0, iss)
-	rtx.RetransmitFrom(iss)
-	testQueueSanity(t, rtx)
-
-	viaUNA, _ := newRetransmitQueue(t, 64, 4, npkt, pktlen, 0, iss)
-	viaUNA.RetransmitFromUNA()
-	testQueueSanity(t, viaUNA)
-
-	if rtx.BufferedSent() != 0 {
-		t.Fatalf("sent=%d, want 0 after a full rewind", rtx.BufferedSent())
-	}
-	if rtx.BufferedUnsent() != npkt*pktlen {
-		t.Fatalf("unsent=%d, want %d", rtx.BufferedUnsent(), npkt*pktlen)
-	}
-	if rtx.BufferedSent() != viaUNA.BufferedSent() || rtx.BufferedUnsent() != viaUNA.BufferedUnsent() {
-		t.Fatal("RetransmitFrom(oldest) must match RetransmitFromUNA")
-	}
-	mustRemake(t, rtx, iss, stream[:pktlen])
-}
-
-// TestRingTx_RetransmitFromUnknownSeq verifies a sequence covered by no queued
-// packet leaves the queue untouched.
-func TestRingTx_RetransmitFromUnknownSeq(t *testing.T) {
-	const iss, pktlen, npkt = Value(100), 4, 3
-	rtx, _ := newRetransmitQueue(t, 64, 4, npkt, pktlen, 0, iss)
-	sent, unsent := rtx.BufferedSent(), rtx.BufferedUnsent()
-
-	rtx.RetransmitFrom(iss - 1)           // Before the queue.
-	rtx.RetransmitFrom(iss + npkt*pktlen) // One past the last octet sent.
-	rtx.RetransmitFrom(iss + 1000)        // Far beyond.
-	testQueueSanity(t, rtx)
-
-	if rtx.BufferedSent() != sent || rtx.BufferedUnsent() != unsent {
-		t.Fatalf("queue moved: sent %d→%d, unsent %d→%d", sent, rtx.BufferedSent(), unsent, rtx.BufferedUnsent())
-	}
-}
-
-// TestRingTx_RetransmitWithUnsentTail verifies a rewind reopens the unsent region
-// over the rewound packets without losing the unsent tail behind them.
-func TestRingTx_RetransmitWithUnsentTail(t *testing.T) {
-	const iss, pktlen, npkt, tail = Value(100), 4, 2, 5
-	rtx, stream := newRetransmitQueue(t, 64, 4, npkt, pktlen, tail, iss)
-	if got := rtx.BufferedUnsent(); got != tail {
-		t.Fatalf("unsent tail=%d, want %d", got, tail)
-	}
-
-	rtx.RetransmitFrom(iss + pktlen) // Rewind the second packet only.
-	testQueueSanity(t, rtx)
-
-	if got := rtx.BufferedUnsent(); got != pktlen+tail {
-		t.Fatalf("unsent=%d, want %d (rewound packet plus the tail)", got, pktlen+tail)
-	}
-	// The rewound packet re-emits first, then the tail follows in order.
-	mustRemake(t, rtx, iss+pktlen, stream[pktlen:2*pktlen])
-	testQueueSanity(t, rtx)
-	mustRemake(t, rtx, iss+2*pktlen, stream[2*pktlen:])
-}
-
-// TestRingTx_RetransmitAfterDrainedUnsent pins the write-position recovery: when
-// every octet written has been packetized the unsent region is empty, so the
-// rewind must reconstruct where data ends from the sent region.
-func TestRingTx_RetransmitAfterDrainedUnsent(t *testing.T) {
-	const iss, pktlen, npkt = Value(100), 4, 3
-	rtx, stream := newRetransmitQueue(t, 64, 4, npkt, pktlen, 0, iss)
-	if got := rtx.BufferedUnsent(); got != 0 {
-		t.Fatalf("unsent=%d, want 0: all written data was packetized", got)
-	}
-
-	rtx.RetransmitFrom(iss + pktlen)
-	testQueueSanity(t, rtx)
-
-	if got := rtx.BufferedUnsent(); got != 2*pktlen {
-		t.Fatalf("unsent=%d, want %d: rewind lost the end of the data", got, 2*pktlen)
-	}
-	mustRemake(t, rtx, iss+pktlen, stream[pktlen:2*pktlen])
-	testQueueSanity(t, rtx)
-	mustRemake(t, rtx, iss+2*pktlen, stream[2*pktlen:3*pktlen])
-}
-
-// TestRingTx_RetransmitWrapped exercises a rewind on a queue whose regions wrap
-// the end of the ring buffer.
-func TestRingTx_RetransmitWrapped(t *testing.T) {
-	const bufsize, pktlen = 16, 4
-	const iss = Value(100)
-	var rtx ringTx
-	if err := rtx.Reset(make([]byte, bufsize), 4, iss); err != nil {
-		t.Fatal(err)
-	}
-	// Push the queue most of the way around the ring, acking as we go.
-	seq := iss
-	scratch := make([]byte, pktlen)
-	for round := range 3 {
-		chunk := make([]byte, pktlen)
-		for i := range chunk {
-			chunk[i] = byte(round*pktlen + i + 1)
-		}
-		if _, err := rtx.Write(chunk); err != nil {
-			t.Fatal(err)
-		}
-		if _, err := rtx.MakePacket(scratch, seq); err != nil {
-			t.Fatal(err)
-		}
-		seq += Value(pktlen)
-		if round < 2 {
-			if err := rtx.RecvACK(seq); err != nil {
-				t.Fatal(err)
-			}
-		}
-		testQueueSanity(t, &rtx)
-	}
-	// Two packets outstanding, straddling the wrap. Rewind the newest.
-	rewindSeq := seq - Value(pktlen)
-	want := append([]byte(nil), scratch...)
-	rtx.RetransmitFrom(rewindSeq)
-	testQueueSanity(t, &rtx)
-	mustRemake(t, &rtx, rewindSeq, want)
-	testQueueSanity(t, &rtx)
-}

x/xnet/stack-async.go

@@ -53,6 +53,10 @@ type StackAsync struct {
 	lookup  dns.Message
 	dnssv   netip.Addr
 
+	// ephPort drives sequential ephemeral-port allocation (see
+	// [StackAsync.ephemeralPort]); zero means not yet seeded.
+	ephPort uint32
+
 	ntpUDP internet.StackUDPPort
 	ntp    ntp.Client
 
@@ -352,6 +356,23 @@ func (s *StackAsync) Prand32() (randval uint32) {
 	return randval
 }
 
+// ephemeralPort returns the next port of the IANA dynamic range (49152-65535,
+// RFC 6335 §6), allocated sequentially from a random per-stack start so a port is
+// revisited only after the full 16384-port cycle. Random selection instead reuses
+// a recent port at birthday-paradox rates, and a reused 4-tuple can collide with
+// state the previous conversation left behind (a TIME-WAIT, a NAT flow entry)
+// which swallows the new SYN.
+func (s *StackAsync) ephemeralPort() uint16 {
+	s.mu.Lock()
+	if s.ephPort == 0 {
+		s.ephPort = s.prand32()%16384 | 1
+	}
+	port := 49152 + s.ephPort%16384
+	s.ephPort++
+	s.mu.Unlock()
+	return uint16(port)
+}
+
 func (s *StackAsync) prand32() uint32 {
 	/* Algorithm "xor" from p. 4 of Marsaglia, "Xorshift RNGs" */
 	seed := internal.Prand32(s.prng)

x/xnet/stack-go.go

@@ -102,8 +102,9 @@ func (s StackGo) SocketNetip(ctx context.Context, network string, family, sotype
 	isDial := raddr.IsValid() && !raddr.Addr().IsUnspecified()
 	if laddr.Port() == 0 {
 		// Auto-assign an ephemeral port for both outbound dials and for listeners
-		// that did not request a fixed port.
-		laddr = netip.AddrPortFrom(laddr.Addr(), uint16(49152+s.blk.async.Prand32()%16384))
+		// that did not request a fixed port. Sequential, not random: see
+		// [StackAsync.ephemeralPort] for why random selection breaks dial churn.
+		laddr = netip.AddrPortFrom(laddr.Addr(), s.blk.async.ephemeralPort())
 	}
 	if laddr.Addr().IsUnspecified() {
 		// Fill in the stack's configured address for the requested family.

x/xnet/xnet_test.go

@@ -1196,3 +1196,39 @@ func TestEgressIP_TCPMSSAdvertisesMTU(t *testing.T) {
 		t.Errorf("advertised MSS = %d, want %d (MTU %d - 40)", gotMSS, wantMSS, mtu)
 	}
 }
+
+// TestEphemeralPortSequence checks no ephemeral port is reused before the whole
+// 16384-port dynamic range has cycled, which is what keeps a redial off the
+// teardown state (TIME-WAIT, NAT flow entries) of the conversation before it.
+func TestEphemeralPortSequence(t *testing.T) {
+	s := new(StackAsync)
+	err := s.Reset(StackConfig{
+		Hostname:          "eph",
+		RandSeed:          42,
+		StaticAddress4:    [4]byte{10, 0, 0, 50},
+		MaxActiveTCPPorts: 1,
+		HardwareAddress:   [6]byte{0xbe, 0xef, 0, 0, 0, 50},
+		MTU:               ethernet.MaxMTU,
+		ICMPQueueLimit:    2,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	const cycle = 16384
+	var seen [cycle]bool
+	for i := range cycle {
+		port := s.ephemeralPort()
+		if port < 49152 {
+			t.Fatalf("port %d below dynamic range (RFC 6335)", port)
+		}
+		idx := port - 49152
+		if seen[idx] {
+			t.Fatalf("port %d reused after only %d allocations (want full %d cycle)", port, i, cycle)
+		}
+		seen[idx] = true
+	}
+	// The cycle is exhausted: the next allocation may legitimately reuse.
+	if got := s.ephemeralPort(); got < 49152 {
+		t.Fatalf("post-cycle port %d below dynamic range", got)
+	}
+}