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[NPM-3586] Add RTT support to ebpf-less tracer (#31491)
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102 changes: 102 additions & 0 deletions
102
pkg/network/tracer/connection/ebpfless/tcp_processor_rtt.go
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,102 @@ | ||
| // Unless explicitly stated otherwise all files in this repository are licensed | ||
| // under the Apache License Version 2.0. | ||
| // This product includes software developed at Datadog (https://www.datadoghq.com/). | ||
| // Copyright 2024-present Datadog, Inc. | ||
|
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| //go:build linux_bpf | ||
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| package ebpfless | ||
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| import ( | ||
| "time" | ||
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| "github.com/DataDog/datadog-agent/pkg/util/log" | ||
| ) | ||
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| func absDiff(a uint64, b uint64) uint64 { | ||
| if a < b { | ||
| return b - a | ||
| } | ||
| return a - b | ||
| } | ||
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| // nanosToMicros converts nanoseconds to microseconds, rounding and converting to | ||
| // the uint32 type that ConnectionStats uses | ||
| func nanosToMicros(nanos uint64) uint32 { | ||
| micros := time.Duration(nanos).Round(time.Microsecond).Microseconds() | ||
| return uint32(micros) | ||
| } | ||
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| // rttTracker implements the RTT algorithm specified here: | ||
| // https://datatracker.ietf.org/doc/html/rfc6298#section-2 | ||
| type rttTracker struct { | ||
| // sampleSentTimeNs is the timestamp our current round trip began. | ||
| // If it is 0, there is nothing in flight or a retransmit cleared this | ||
| sampleSentTimeNs uint64 | ||
| // expectedAck is the ack needed to complete the round trip | ||
| expectedAck uint32 | ||
| // rttSmoothNs is the smoothed RTT in nanoseconds | ||
| rttSmoothNs uint64 | ||
| // rttVarNs is the variance of the RTT in nanoseconds | ||
| rttVarNs uint64 | ||
| } | ||
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| func (rt *rttTracker) isActive() bool { | ||
| return rt.sampleSentTimeNs > 0 | ||
| } | ||
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| // processOutgoing is called to (potentially) start a round trip. | ||
| // Records the time of the packet for later | ||
| func (rt *rttTracker) processOutgoing(timestampNs uint64, nextSeq uint32) { | ||
| if !rt.isActive() { | ||
| rt.sampleSentTimeNs = timestampNs | ||
| rt.expectedAck = nextSeq | ||
| } | ||
| } | ||
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| // clearTrip is called by a retransmit or when a round-trip completes | ||
| // Retransmits pollute RTT accuracy and cause a trip to be thrown out | ||
| func (rt *rttTracker) clearTrip() { | ||
| if rt.isActive() { | ||
| rt.sampleSentTimeNs = 0 | ||
| rt.expectedAck = 0 | ||
| } | ||
| } | ||
|
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| // processIncoming is called to (potentially) close out a round trip. | ||
| // Based off this https://github.com/DataDog/datadog-windows-filter/blob/d7560d83eb627117521d631a4c05cd654a01987e/ddfilter/flow/flow_tcp.c#L269 | ||
| // Returns whether the RTT stats were updated. | ||
| func (rt *rttTracker) processIncoming(timestampNs uint64, ack uint32) bool { | ||
| hasCompletedTrip := rt.isActive() && isSeqBeforeEq(rt.expectedAck, ack) | ||
| if !hasCompletedTrip { | ||
| return false | ||
| } | ||
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| elapsedNs := timestampNs - rt.sampleSentTimeNs | ||
| if timestampNs < rt.sampleSentTimeNs { | ||
| log.Warn("rttTracker encountered non-monotonic clock") | ||
| elapsedNs = 0 | ||
| } | ||
| rt.clearTrip() | ||
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| if rt.rttSmoothNs == 0 { | ||
| rt.rttSmoothNs = elapsedNs | ||
| rt.rttVarNs = elapsedNs / 2 | ||
| return true | ||
| } | ||
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| // update variables based on fixed point math. | ||
| // RFC 6298 says alpha=1/8 and beta=1/4 | ||
| const fixedBasis uint64 = 1000 | ||
| // SRTT < -(1 - alpha) * SRTT + alpha * R' | ||
| oneMinusAlpha := fixedBasis - (fixedBasis / 8) | ||
| alphaRPrime := elapsedNs / 8 | ||
| s := ((oneMinusAlpha * rt.rttSmoothNs) / fixedBasis) + alphaRPrime | ||
| rt.rttSmoothNs = s | ||
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| // RTTVAR <- (1 - beta) * RTTVAR + beta * |SRTT - R'| | ||
| oneMinusBeta := fixedBasis - fixedBasis/4 | ||
| rt.rttVarNs = (oneMinusBeta*rt.rttVarNs)/fixedBasis + absDiff(rt.rttSmoothNs, elapsedNs)/4 | ||
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| return true | ||
| } |
142 changes: 142 additions & 0 deletions
142
pkg/network/tracer/connection/ebpfless/tcp_processor_rtt_test.go
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,142 @@ | ||
| // Unless explicitly stated otherwise all files in this repository are licensed | ||
| // under the Apache License Version 2.0. | ||
| // This product includes software developed at Datadog (https://www.datadoghq.com/). | ||
| // Copyright 2024-present Datadog, Inc. | ||
|
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| //go:build linux_bpf | ||
|
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| package ebpfless | ||
|
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| import ( | ||
| "testing" | ||
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| "github.com/stretchr/testify/require" | ||
| ) | ||
|
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| func TestNanosToMicros(t *testing.T) { | ||
| require.Equal(t, uint32(0), nanosToMicros(0)) | ||
| require.Equal(t, uint32(0), nanosToMicros(200)) | ||
| require.Equal(t, uint32(1), nanosToMicros(500)) | ||
| require.Equal(t, uint32(1), nanosToMicros(1000)) | ||
| require.Equal(t, uint32(1), nanosToMicros(1200)) | ||
| require.Equal(t, uint32(123), nanosToMicros(123*1000)) | ||
| } | ||
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| func TestSingleSampleRTT(t *testing.T) { | ||
| var rt rttTracker | ||
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| require.False(t, rt.isActive()) | ||
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| rt.processOutgoing(1000, 123) | ||
| require.True(t, rt.isActive()) | ||
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| hasUpdated := rt.processIncoming(2000, 42) | ||
| // ack is too low, not a round trip | ||
| require.False(t, hasUpdated) | ||
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| // ack is high enough to complete a round trip | ||
| hasUpdated = rt.processIncoming(3000, 123) | ||
| require.True(t, hasUpdated) | ||
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| require.Equal(t, uint64(2000), rt.rttSmoothNs) | ||
| require.Equal(t, uint64(1000), rt.rttVarNs) | ||
| } | ||
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| func TestLowVarianceRtt(t *testing.T) { | ||
| var rt rttTracker | ||
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| for i := range 10 { | ||
| ts := uint64(i + 1) | ||
| seq := uint32(123 + i) | ||
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| startNs := (2 * ts) * 1000 | ||
| endNs := startNs + 1000 | ||
| // round trip time always the 1000, so variance goes to 0 | ||
| rt.processOutgoing(startNs, seq) | ||
| hasUpdated := rt.processIncoming(endNs, seq) | ||
| require.True(t, hasUpdated) | ||
| require.Equal(t, rt.rttSmoothNs, uint64(1000)) | ||
| } | ||
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| // after 10 iterations, the variance should have mostly converged to zero | ||
| require.Less(t, rt.rttVarNs, uint64(100)) | ||
| } | ||
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| func TestConstantVarianceRtt(t *testing.T) { | ||
| var rt rttTracker | ||
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| for i := range 10 { | ||
| ts := uint64(i + 1) | ||
| seq := uint32(123 + i) | ||
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| startNs := (2 * ts) * 1000 | ||
| endNs := startNs + 500 | ||
| if i%2 == 0 { | ||
| endNs = startNs + 1000 | ||
| } | ||
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| // round trip time alternates between 500 and 100 | ||
| rt.processOutgoing(startNs, seq) | ||
| hasUpdated := rt.processIncoming(endNs, seq) | ||
| require.True(t, hasUpdated) | ||
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| require.LessOrEqual(t, uint64(500), rt.rttSmoothNs) | ||
| require.LessOrEqual(t, rt.rttSmoothNs, uint64(1000)) | ||
| } | ||
|
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| // This is not exact since it uses an exponential rolling sum | ||
| // In this test, the time delta alternates between 500 and 1000, | ||
| // so rttSmoothNs is 750, for an average difference of ~250. | ||
| const epsilon = 20 | ||
| require.Less(t, uint64(250-epsilon), rt.rttVarNs) | ||
| require.Less(t, rt.rttVarNs, uint64(250+epsilon)) | ||
| } | ||
|
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| func TestTcpProcessorRtt(t *testing.T) { | ||
| pb := newPacketBuilder(lowerSeq, higherSeq) | ||
| syn := pb.outgoing(0, 0, 0, SYN) | ||
| // t=200 us | ||
| syn.timestampNs = 200 * 1000 | ||
| synack := pb.incoming(0, 0, 1, SYN|ACK) | ||
| // t=300 us, for a round trip of 100us | ||
| synack.timestampNs = 300 * 1000 | ||
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| f := newTcpTestFixture(t) | ||
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| f.runPkt(syn) | ||
| // round trip has not completed yet | ||
| require.Zero(t, f.conn.RTT) | ||
| require.Zero(t, f.conn.RTTVar) | ||
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| f.runPkt(synack) | ||
| // round trip has completed in 100us | ||
| require.Equal(t, uint32(100), f.conn.RTT) | ||
| require.Equal(t, uint32(50), f.conn.RTTVar) | ||
| } | ||
|
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| func TestTcpProcessorRttRetransmit(t *testing.T) { | ||
| pb := newPacketBuilder(lowerSeq, higherSeq) | ||
| syn := pb.outgoing(0, 0, 0, SYN) | ||
| // t=200 us | ||
| syn.timestampNs = 200 * 1000 | ||
| synack := pb.incoming(0, 0, 1, SYN|ACK) | ||
| // t=300 us, for a round trip of 100us | ||
| synack.timestampNs = 300 * 1000 | ||
|
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| f := newTcpTestFixture(t) | ||
|
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| f.runPkt(syn) | ||
| // round trip has not completed yet | ||
| require.Zero(t, f.conn.RTT) | ||
| require.Zero(t, f.conn.RTTVar) | ||
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| f.runPkt(syn) | ||
| // this is a retransmit, should reset the round trip | ||
| require.Zero(t, f.conn.RTT) | ||
| require.Zero(t, f.conn.RTTVar) | ||
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| f.runPkt(synack) | ||
| // should STILL not have a round trip because the retransmit contaminated the results | ||
| require.Zero(t, f.conn.RTT) | ||
| require.Zero(t, f.conn.RTTVar) | ||
| } |
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