begin prepping policy refactor manually

tcp/rto/timer.go

@@ -0,0 +1,259 @@
+package rto
+
+import (
+	"time"
+
+	"github.com/soypat/lneto/tcp"
+)
+
+// RFC 6298 retransmission-timeout (RTO) parameters. The algorithm keeps a
+// single retransmission timer per connection (RFC 6298 §5): the timer is
+// (re)started whenever new data is acknowledged while data remains in flight,
+// stopped when all data is acknowledged, and on expiry the oldest unacknowledged
+// segment is retransmitted and the RTO is doubled (exponential backoff, §5.5).
+const (
+	// rtoInitial is the RTO used before the first RTT measurement (RFC 6298 §2.1).
+	rtoInitial = time.Second
+	// rtoMin clamps the lower bound of the RTO. RFC 6298 §2.4 recommends a
+	// minimum of 1s, but that is punishing on the low-latency links lneto
+	// targets; like Linux we use a smaller floor so recovery on LAN/embedded
+	// links is timely.
+	rtoMin = 200 * time.Millisecond
+	// rtoMax clamps the upper bound across exponential backoff (RFC 6298 §5.5
+	// permits a maximum of at least 60s).
+	rtoMax = 60 * time.Second
+
+	// rttGainShift (alpha = 1/8) and rttvarGainShift (beta = 1/4) are the
+	// smoothing gains of RFC 6298 §2.3, applied as integer shifts.
+	rttGainShift    = 3 // alpha = 1/8
+	rttvarGainShift = 2 // beta  = 1/4
+	// rttvarK is the RTTVAR multiplier in RTO = SRTT + K*RTTVAR (RFC 6298 §2.3).
+	rttvarK = 4
+	// backoffMax caps the exponential-backoff doublings so RTO arithmetic cannot
+	// overflow and a wedged connection keeps probing at rtoMax.
+	backoffMax = 12
+)
+
+// Timer implements the RFC 6298 round-trip-time estimator and the single
+// retransmission timer as a [tcp.Policy]. Construct it with new(Timer) and hand
+// it to [tcp.ConnConfig.Policy]; the connection calls [Timer.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 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]
+// 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 {
+	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.
+
+	// 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.
+
+	// 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 when the timer is a peer in a
+	// [tcp.Composite] the controller never sees the timer's directive.
+	expirations uint32
+}
+
+var _ tcp.Policy = (*Timer)(nil)
+
+// Reset returns the estimator to its pre-connection state with the initial RTO.
+// It implements [tcp.Policy] and is called when the connection opens or aborts
+// so the estimator can be reused across connection reuse.
+func (r *Timer) Reset() { *r = Timer{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 }
+
+// CurrentRTO returns the timeout currently in effect, clamped to [rtoMin, rtoMax].
+func (r *Timer) CurrentRTO() time.Duration {
+	rto := r.rto
+	if rto < rtoMin {
+		rto = rtoMin
+	} else if rto > rtoMax {
+		rto = rtoMax
+	}
+	return rto
+}
+
+// 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 }
+
+// NextDeadline returns the monotonic-nanosecond instant at which the timer
+// expires, or 0 when it is not armed. It implements [tcp.Policy].
+func (r *Timer) 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(rx tcp.RxMeta) tcp.RxDirective {
+	return tcp.RxDirective{Keep: 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].
+//
+// A refused segment is ignored. Acting on 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(event tcp.RxEvent) {
+	incoming, now := event.Segment, event.Now
+	if !event.Accepted || !r.haveSeq || !incoming.Flags.HasAny(tcp.FlagACK) {
+		return
+	}
+	ack := incoming.ACK
+	if r.timing && !ack.LessThan(r.timedSeq) {
+		// ACK covers the timed segment: take the RTT sample (§4). A valid
+		// measurement collapses the backoff (§5.7).
+		r.updateRTT(time.Duration(now - r.timedAt))
+		r.timing = false
+		r.backoff = 0
+	}
+	if r.sndUNA.LessThan(ack) && !r.sndNXT.LessThan(ack) {
+		// ACK advances snd.UNA and does not exceed what we have sent.
+		r.sndUNA = ack
+	}
+	if r.sndUNA == r.sndNXT {
+		r.running = false // §5.3: all outstanding data acknowledged.
+	} else {
+		// §5.3: new (but not all) data acknowledged — restart the timer.
+		r.running = true
+		r.deadline = now + int64(r.CurrentRTO())
+	}
+}
+
+// WriteOptions adds no TCP options: retransmission timing needs none of its
+// own. It implements [tcp.Policy].
+func (r *Timer) WriteOptions(plan tcp.TxPlan, opts []byte) uint8 { return 0 }
+
+// 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 — returning a
+// directive that asks the connection to retransmit from snd.UNA (go-back-N). It
+// implements [tcp.Policy].
+func (r *Timer) PreTx(intent tcp.TxIntent) tcp.TxDirective {
+	now := intent.Now
+	if !r.running || now < r.deadline || r.sndUNA == r.sndNXT {
+		return tcp.TxDirective{}
+	}
+	r.expirations++
+	r.timing = false // §5.4: do not sample a retransmitted segment.
+	if r.backoff < backoffMax {
+		r.backoff++
+		r.rto = min(r.CurrentRTO()*2, rtoMax) // §5.5: RTO = RTO * 2.
+	}
+	r.running = true
+	r.deadline = now + int64(r.CurrentRTO())
+	return tcp.TxDirective{Retransmit: true, RetransmitFrom: intent.UNA}
+}
+
+// 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(outgoing tcp.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())
+	if !r.haveSeq {
+		r.haveSeq = true
+		r.sndUNA = segStart
+		r.sndNXT = segStart
+	}
+	if !r.sndNXT.LessThan(segEnd) {
+		// tcp.Segment does not extend the send sequence: it is a retransmission.
+		// Discard any outstanding RTT sample per Karn's algorithm. The timer was
+		// already (re)armed by PreTx on the timeout that triggered this resend.
+		r.timing = false
+		return
+	}
+	r.sndNXT = segEnd
+	if !r.timing {
+		r.timing = true
+		r.timedSeq = segEnd
+		r.timedAt = now
+	}
+	if !r.running {
+		r.running = true
+		r.deadline = now + int64(r.CurrentRTO())
+	}
+}
+
+// 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) {
+	if sample <= 0 {
+		return
+	}
+	if !r.haveRTT {
+		// First measurement (RFC 6298 §2.2).
+		r.srtt = sample
+		r.rttvar = sample / 2
+		r.haveRTT = true
+	} else {
+		// Subsequent measurements (RFC 6298 §2.3):
+		//   RTTVAR = (1-beta)*RTTVAR + beta*|SRTT-R|
+		//   SRTT   = (1-alpha)*SRTT  + alpha*R
+		diff := r.srtt - sample
+		if diff < 0 {
+			diff = -diff
+		}
+		r.rttvar += (diff - r.rttvar) >> rttvarGainShift
+		r.srtt += (sample - r.srtt) >> rttGainShift
+	}
+	r.rto = r.srtt + rttvarK*r.rttvar
+}

tcp/rto/timer_test.go

@@ -0,0 +1,286 @@
+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
+	r.Reset()
+	return &r
+}
+
+// rxAt builds the minimal tcp.RxMeta for driving PreRx directly.
+func rxAt(seg tcp.Segment, now int64) tcp.RxMeta { return tcp.RxMeta{Segment: seg, Now: now} }
+
+// acceptedAt builds the event for a segment the connection accepted, which is what
+// drives the estimator. Timing state is only allowed to move for those.
+func acceptedAt(seg tcp.Segment, now int64) tcp.RxEvent {
+	return tcp.RxEvent{Segment: seg, Now: now, Accepted: true}
+}
+
+// txAt builds the minimal tcp.TxIntent for driving Timer.PreTx directly: the timer
+// tracks the send sequence itself via PostTx and only reads the clock.
+func txAt(now int64) tcp.TxIntent { return tcp.TxIntent{Now: now} }
+
+func TestRTO_Reset(t *testing.T) {
+	var r Timer
+	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(rxAt(ackSeg(iss+100), 40*rtoMs)).Keep {
+		t.Error("PreRx must keep the segment")
+	}
+	r.PostRx(acceptedAt(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 r.PreTx(txAt(int64(rtoInitial) - 1)).Retransmit {
+		t.Fatal("must not retransmit before the deadline")
+	}
+	dir := r.PreTx(tcp.TxIntent{Now: int64(rtoInitial), UNA: tcp.Value(iss), NXT: tcp.Value(iss + 100)})
+	if !dir.Retransmit {
+		t.Fatal("RTO must fire at the deadline with data outstanding")
+	}
+	if dir.RetransmitFrom != tcp.Value(iss) {
+		t.Errorf("retransmit from %d, want snd.UNA=%d", dir.RetransmitFrom, 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(txAt(int64(rtoInitial)))
+	r.PostTx(dataSeg(iss, 100), int64(rtoInitial))
+	// ACK now arrives; no sample should be taken since timing was discarded.
+	r.PostRx(acceptedAt(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(acceptedAt(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(txAt(int64(rtoInitial)))               // 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(acceptedAt(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) {
+	var r Timer
+	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 Timer
+	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_ImplementsPolicy exercises Timer through the [tcp.Policy]
+// interface: sending data arms a deadline and a full ACK disarms it.
+func TestRTO_ImplementsPolicy(t *testing.T) {
+	var lr tcp.Policy = newRTO()
+	lr.Reset()
+	lr.PostTx(dataSeg(1000, 100), 0)
+	if lr.NextDeadline() == 0 {
+		t.Error("expected an armed deadline after sending data")
+	}
+	if !lr.PreRx(rxAt(ackSeg(1100), 10*rtoMs)).Keep {
+		t.Error("PreRx must keep")
+	}
+	lr.PostRx(acceptedAt(ackSeg(1100), 10*rtoMs))
+	if lr.NextDeadline() != 0 {
+		t.Error("expected disarmed timer after full ack")
+	}
+}
+
+// TestRTO_IgnoresRejectedSegment verifies the estimator does not act on a segment
+// the connection refused. PreRx runs before the state machine has judged the
+// segment, so an acknowledgement for data never sent would otherwise collapse the
+// backoff and take a bogus round-trip sample.
+func TestRTO_IgnoresRejectedSegment(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, refused by the connection.
+	bogus := ackSeg(iss + 100000)
+	if !r.PreRx(rxAt(bogus, 40*rtoMs)).Keep {
+		t.Error("PreRx must keep: dropping is not the estimator's business")
+	}
+	r.PostRx(tcp.RxEvent{Segment: bogus, Now: 40 * rtoMs, Accepted: false})
+
+	if r.NextDeadline() != armed {
+		t.Errorf("deadline moved to %d on a refused segment, want it left at %d",
+			r.NextDeadline(), armed)
+	}
+	if r.SmoothedRTT() != 0 {
+		t.Errorf("took an RTT sample of %v from a refused segment", r.SmoothedRTT())
+	}
+	if !r.Running() {
+		t.Error("timer disarmed by a refused acknowledgement")
+	}
+}
+
+// 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++ {
+		dir := r.PreTx(tcp.TxIntent{Now: now, UNA: tcp.Value(iss), NXT: tcp.Value(iss + 1)})
+		if !dir.Retransmit {
+			t.Fatalf("attempt %d: timer did not fire; the probe would never be resent", attempt)
+		}
+		if dir.RetransmitFrom != tcp.Value(iss) {
+			t.Errorf("attempt %d: retransmit from %d, want the probe octet at %d",
+				attempt, dir.RetransmitFrom, 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)
+	}
+}