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tcp: simplified Policy implementation based on @MDr164 (#190)
* begin prepping policy refactor manually * implement tcp.Policy and refactor rto to use it * fix CI * chatting with claude gave me idea to reformulate Policy * tcp.Policy: add newTransmitLimit output * merge with main and fix failing tests * remove fix.patch * answer my own comments
This commit is contained in:
Pat Whittingslow
GitHub
@@ -0,0 +1,283 @@
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package rto
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import (
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"time"
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"github.com/soypat/lneto"
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"github.com/soypat/lneto/tcp"
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)
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// RFC 6298 retransmission-timeout (RTO) parameters. The algorithm keeps a
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// single retransmission timer per connection (RFC 6298 §5): the timer is
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// (re)started whenever new data is acknowledged while data remains in flight,
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// stopped when all data is acknowledged, and on expiry the oldest unacknowledged
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// segment is retransmitted and the RTO is doubled (exponential backoff, §5.5).
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const (
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// rtoInitial is the RTO used before the first RTT measurement (RFC 6298 §2.1).
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rtoInitial = time.Second
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// rtoMin clamps the lower bound of the RTO. RFC 6298 §2.4 recommends a
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// minimum of 1s, but that is punishing on the low-latency links lneto
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// targets; like Linux we use a smaller floor so recovery on LAN/embedded
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// links is timely.
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rtoMin = 200 * time.Millisecond
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// rtoMax clamps the upper bound across exponential backoff (RFC 6298 §5.5
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// permits a maximum of at least 60s).
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rtoMax = 60 * time.Second
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// rttGainShift (alpha = 1/8) and rttvarGainShift (beta = 1/4) are the
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// smoothing gains of RFC 6298 §2.3, applied as integer shifts.
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rttGainShift = 3 // alpha = 1/8
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rttvarGainShift = 2 // beta = 1/4
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// rttvarK is the RTTVAR multiplier in RTO = SRTT + K*RTTVAR (RFC 6298 §2.3).
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rttvarK = 4
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// backoffMax caps the exponential-backoff doublings so RTO arithmetic cannot
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// overflow and a wedged connection keeps probing at rtoMax.
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backoffMax = 12
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)
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// Timer implements the RFC 6298 round-trip-time estimator and the single
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// retransmission timer as a [tcp.Policy]. Construct it with [NewTimer] and hand
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// it to [tcp.ConnConfig.Policy].
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//
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// Timer is a pure, reactive state machine: it observes the segments a connection
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// sends and receives (via the tcp.Policy hooks) and from those alone derives RTT
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// estimates and retransmission decisions. The tcp package holds no clock, so the
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// Timer carries its own; injecting it keeps the estimator deterministic for unit
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// testing (see issue #140).
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//
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// Timer tracks its own shadow of the send sequence space purely from the segments
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// it observes: [Timer.PostTx] advances the highest sequence sent and [Timer.PreRx]
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// advances the highest sequence acknowledged. This is what lets it manage the
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// timer (RFC 6298 §5.2/§5.3) without reaching into the tcp state machine, and it
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// is also how retransmissions are distinguished for Karn's algorithm — a segment
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// whose sequence space is not beyond the shadow snd.NXT is a retransmission and
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// is never RTT-sampled.
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type Timer struct {
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// nanotime is the monotonic time source in nanoseconds. Preserved by Reset.
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nanotime func() int64
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srtt time.Duration // smoothed round-trip time (SRTT).
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rttvar time.Duration // round-trip-time variation (RTTVAR).
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rto time.Duration // current retransmission timeout.
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haveRTT bool // false until the first RTT sample is taken.
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// Shadow of the send sequence space, derived from observed segments.
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haveSeq bool // false until the first data segment is observed.
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sndUNA tcp.Value // highest acknowledged sequence number seen on the wire.
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sndNXT tcp.Value // one past the highest sequence number sent.
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// RTT sampling state (Karn's algorithm, RFC 6298 §3): at most one segment is
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// timed at a time and retransmitted segments are never sampled.
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timing bool
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timedSeq tcp.Value // ACK at or beyond this value completes the sample.
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timedAt int64 // send time (monotonic ns) of the timed segment.
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// Retransmission timer state.
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running bool
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deadline int64 // time (monotonic ns) at which the timer expires.
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backoff uint8 // consecutive timeouts, for exponential backoff.
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// expirations counts timeouts since Reset. It exists so a policy sharing this
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// timer can notice a timeout it did not itself drive: a congestion controller
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// must collapse its window on one, and a policy that composes the timer as a
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// peer never sees the timer's own directive.
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expirations uint32
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}
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var _ tcp.Policy = (*Timer)(nil)
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// Configure prepares the Timer for use with nanotime, the monotonic time source
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// in nanoseconds (the func() int64 convention used across lneto). It must be
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// called before the connection is opened.
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func (r *Timer) Configure(nanotime func() int64) error {
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if nanotime == nil {
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return lneto.ErrMissingHALConfig // The estimator cannot run without a clock.
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}
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*r = Timer{rto: rtoInitial, nanotime: nanotime}
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return nil
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}
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// Reset returns the estimator to its pre-connection state with the initial RTO,
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// preserving the configured clock. It implements [tcp.Policy] and is called when
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// the connection opens or aborts so the estimator survives connection reuse.
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func (r *Timer) Reset() { *r = Timer{rto: rtoInitial, nanotime: r.nanotime} }
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// SmoothedRTT returns the current smoothed round-trip time (SRTT), or zero
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// before the first RTT measurement. It is concrete-type introspection and is
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// intentionally not part of [tcp.Policy].
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func (r *Timer) SmoothedRTT() time.Duration { return r.srtt }
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// CurrentRTO returns the timeout currently in effect, clamped to [rtoMin, rtoMax].
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func (r *Timer) CurrentRTO() time.Duration {
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rto := r.rto
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if rto < rtoMin {
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rto = rtoMin
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} else if rto > rtoMax {
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rto = rtoMax
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}
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return rto
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}
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// Running reports whether the retransmission timer is currently armed.
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func (r *Timer) Running() bool { return r.running }
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// Expirations returns how many times the retransmission timer has expired since
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// [Timer.Reset]. A policy that shares this timer rather than driving it watches
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// this for a change to learn that a timeout happened, since it never sees the
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// timer's own directive. It is concrete-type introspection and is intentionally
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// not part of [tcp.Policy].
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func (r *Timer) Expirations() uint32 { return r.expirations }
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// NextDeadline returns the monotonic-nanosecond instant at which the timer
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// expires, or 0 when it is not armed. It is concrete-type introspection, not
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// part of [tcp.Policy]: an event loop that wants to schedule against the RTO
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// holds the Timer it configured and reads this.
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func (r *Timer) NextDeadline() int64 {
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if !r.running {
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return 0
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}
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return r.deadline
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}
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// PreRx keeps every segment: the estimator never drops traffic and records
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// nothing before the connection has decided whether the segment counts. It
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// implements [tcp.Policy].
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func (r *Timer) PreRx(h *tcp.Handler, incoming tcp.Frame) bool {
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return true
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}
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// PostRx samples the RTT and manages the retransmission timer from a segment the
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// connection accepted (RFC 6298 §5.2/§5.3). It implements [tcp.Policy].
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//
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// Only accepted segments reach here. Acting on a refused one would let an
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// acknowledgement the state machine rejected, for data never sent, collapse the
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// backoff and take a bogus RTT sample.
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func (r *Timer) PostRx(h *tcp.Handler, prevState tcp.State, accepted tcp.Frame) {
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r.postRx(accepted.Segment(len(accepted.Payload())), r.nanotime())
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}
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func (r *Timer) postRx(incoming tcp.Segment, now int64) {
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if !r.haveSeq || !incoming.Flags.HasAny(tcp.FlagACK) {
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return
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}
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ack := incoming.ACK
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if r.timing && !ack.LessThan(r.timedSeq) {
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// ACK covers the timed segment: take the RTT sample (§4). A valid
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// measurement collapses the backoff (§5.7).
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r.updateRTT(time.Duration(now - r.timedAt))
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r.timing = false
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r.backoff = 0
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}
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if r.sndUNA.LessThan(ack) && !r.sndNXT.LessThan(ack) {
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// ACK advances snd.UNA and does not exceed what we have sent.
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r.sndUNA = ack
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}
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if r.sndUNA == r.sndNXT {
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r.running = false // §5.3: all outstanding data acknowledged.
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} else {
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// §5.3: new (but not all) data acknowledged — restart the timer.
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r.running = true
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r.deadline = now + int64(r.CurrentRTO())
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}
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}
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// PreTx reports whether the retransmission timer has expired and, if so, applies
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// the RFC 6298 §5.4–§5.6 timeout response — discard the outstanding RTT sample
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// (Karn), back the RTO off exponentially and restart the timer — and asks the
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// connection to retransmit from snd.UNA (go-back-N). It writes no TCP options
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// and imposes no transmit limit: retransmission timing needs neither, and
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// congestion control belongs to a Policy composing this timer. It implements
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// [tcp.Policy].
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func (r *Timer) PreTx(h *tcp.Handler, outgoingOpts tcp.Frame) (newTransmitLimit tcp.Size, rtxFrom tcp.Value, retransmit bool) {
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return r.preTx(r.nanotime(), h.ControlBlock().SendUNA())
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}
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func (r *Timer) preTx(now int64, una tcp.Value) (newTransmitLimit tcp.Size, rtxFrom tcp.Value, retransmit bool) {
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if !r.running || now < r.deadline || r.sndUNA == r.sndNXT {
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return tcp.TransmitUnlimited, 0, false
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}
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r.expirations++
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r.timing = false // §5.4: do not sample a retransmitted segment.
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if r.backoff < backoffMax {
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r.backoff++
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r.rto = min(r.CurrentRTO()*2, rtoMax) // §5.5: RTO = RTO * 2.
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}
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r.running = true
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r.deadline = now + int64(r.CurrentRTO())
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return tcp.TransmitUnlimited, una, true
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}
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// PostTx records an emitted segment: it advances the shadow send sequence,
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// begins timing newly transmitted data (RFC 6298 §3) and arms the timer (§5.1).
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// Segments that do not extend the send sequence are retransmissions and are
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// never RTT-sampled (Karn's algorithm). Control-only segments (no data) are
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// ignored. It implements [tcp.Policy].
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func (r *Timer) PostTx(h *tcp.Handler, outgoing tcp.Frame) {
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r.postTx(outgoing.Segment(len(outgoing.Payload())), r.nanotime())
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}
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func (r *Timer) postTx(outgoing tcp.Segment, now int64) {
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if outgoing.DATALEN == 0 {
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return // only data segments are timed / arm the RTO.
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}
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segStart := outgoing.SEQ
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segEnd := segStart + tcp.Value(outgoing.LEN())
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if !r.haveSeq {
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r.haveSeq = true
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r.sndUNA = segStart
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r.sndNXT = segStart
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}
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if !r.sndNXT.LessThan(segEnd) {
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// Segment does not extend the send sequence: it is a retransmission.
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// Discard any outstanding RTT sample per Karn's algorithm. The timer was
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// already (re)armed by PreTx on the timeout that triggered this resend.
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r.timing = false
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return
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}
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r.sndNXT = segEnd
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if !r.timing {
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r.timing = true
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r.timedSeq = segEnd
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r.timedAt = now
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}
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if !r.running {
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r.running = true
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r.deadline = now + int64(r.CurrentRTO())
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}
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}
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// ObserveRTT folds a round-trip measurement taken by other means into the
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// estimator, for a policy that composes this timer and can measure the round trip
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// more accurately than acknowledgement timing allows. The RFC 7323 timestamp echo
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// is the case this exists for.
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//
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// Unlike the timer's own sampling this does not apply Karn's algorithm, because a
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// sample derived from an echoed timestamp is unambiguous even when the segment
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// carrying it was a retransmission (RFC 7323 §4.1). Non-positive samples are
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// ignored.
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func (r *Timer) ObserveRTT(rtt time.Duration) { r.updateRTT(rtt) }
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// updateRTT folds a round-trip measurement into SRTT/RTTVAR/RTO using the
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// integer-shift form of RFC 6298 §2.2/§2.3.
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func (r *Timer) updateRTT(sample time.Duration) {
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if sample <= 0 {
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return
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}
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if !r.haveRTT {
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// First measurement (RFC 6298 §2.2).
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r.srtt = sample
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r.rttvar = sample / 2
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r.haveRTT = true
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} else {
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// Subsequent measurements (RFC 6298 §2.3):
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// RTTVAR = (1-beta)*RTTVAR + beta*|SRTT-R|
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// SRTT = (1-alpha)*SRTT + alpha*R
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diff := r.srtt - sample
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if diff < 0 {
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diff = -diff
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}
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r.rttvar += (diff - r.rttvar) >> rttvarGainShift
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r.srtt += (sample - r.srtt) >> rttGainShift
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}
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r.rto = r.srtt + rttvarK*r.rttvar
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}
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