Trace Capture & Retrace

QUIC connections can record a deterministic-replay trace — every datagram, state transition, error, path-stats snapshot, and (optionally) connectivity change, stamped against one clock. Capture is opt-in and zero-cost when off (the default): a null trace is byte-identical to the pre-capture path.

A trace is useful for three things:

Field diagnostics — attach the trace to a bug report; it's a packet-level record of what the connection actually did.
Deterministic replay — feed the input events back through the simulation harness to reproduce a failure offline (see RFC_DETERMINISTIC_SIMULATION.md).
Post-hoc retrace — on an obfuscated release build, class names in the trace are renamed by R8/ProGuard; a companion tool maps them back using your app's mapping.txt.

The sink is typed

Capture is driven by a TraceSink — a functional interface that receives a typed TraceEvent, not a string:

fun interface TraceSink {
    fun emit(event: TraceEvent)
}

TraceEvent.toString() renders the canonical v1 line (one event per line, space-delimited), so a file/network sink is just event.toString(). But because the event is typed, a sink can also match on it directly — filter, count, or react without parsing. TraceEvent.parse(line) / parseAll(lines) decode a persisted trace back into events, and parse(e.toString()) == e holds for every event.

Enabling capture on a client

Set QuicOptions.trace to a QuicTraceCapture. The engine then wraps the connection's UDP channels in a recording decorator and mirrors state/error/stats transitions onto your sink.

import com.ditchoom.socket.NetworkMonitor
import com.ditchoom.socket.quic.QuicOptions
import com.ditchoom.socket.quic.trace.QuicTraceCapture
import com.ditchoom.socket.quic.trace.TraceEvent
import com.ditchoom.socket.quic.trace.TraceSink
import java.io.File
import kotlin.time.Duration.Companion.seconds

// A sink may be invoked from several coroutines (the driver loop, per-path readers, the monitor
// collectors) — treat it like a log sink and guard shared state.
val out = File("captured.trace").bufferedWriter()
val capture = QuicTraceCapture(
    sink = TraceSink { event -> synchronized(out) { out.appendLine(event.toString()) } },
    // Optional: also fold connectivity state (NET_AVAIL / NET_ID) into the SAME trace — the
    // airplane-mode toggle / Wi-Fi↔cellular handoff, not just QUIC traffic. Client-only.
    networkMonitor = NetworkMonitor.default(),
)

withQuicConnection(
    hostname = "example.com",
    port = 443,
    quicOptions = QuicOptions(alpnProtocols = listOf("h3"), trace = capture),
    timeout = 15.seconds,
) {
    val stream = openStream()
    // … normal QUIC usage; every datagram, state change, error, and stats snapshot is recorded …
    stream.close()
}
out.flush()

Consuming typed events instead of lines

TraceSink { event ->
    when (event) {
        is TraceEvent.State -> println("state → ${event.name} ${event.detail ?: ""}")
        is TraceEvent.Error -> println("error → ${event.type}: ${event.message}")
        else -> out.appendLine(event.toString())
    }
}

Enabling capture on a server

A server handles many connections; the v1 grammar carries no connection identifier and each connection records against its own clock origin. So for replayable per-connection traces, use the sinkFor factory constructor — it's invoked once per accepted connection, and returning a fresh sink each time keeps the connections isolated:

val capture = QuicTraceCapture(
    sinkFor = {
        val writer = newTraceFileForThisConnection().bufferedWriter()
        TraceSink { event -> synchronized(writer) { writer.appendLine(event.toString()) } }
    },
)

withQuicServer(port = 4433, tlsConfig = tlsConfig, quicOptions = quicOptions.copy(trace = capture)) {
    connections { /* … */ }
}

The single-TraceSink convenience constructor (QuicTraceCapture(sink = …)) is the opposite choice on purpose: it hands the same sink to every connection (log-sink semantics) — fine for a single connection or aggregate diagnostics, but it interleaves concurrent connections onto one stream.

Event types

Each TraceEvent is either a replayable input or an observed observation (RFC §2):

Event	`v1` tag	Role	Carries
`DgramOut`	`DGRAM_OUT`	observation	datagram sent (len, path, hex)
`DgramIn`	`DGRAM_IN`	input	datagram received (len, path, hex)
`State`	`STATE`	observation	`QuicConnectionState` transition (qualified class name + detail)
`PathState`	`PATH_STATE`	observation	migration phase + local host/port
`Error`	`ERROR`	input	typed error (qualified class name + message)
`Stats`	`STATS`	observation	quiche path-stats snapshot
`NetAvail`	`NET_AVAIL`	input	`NetworkMonitor.availability` emission
`Net`	`NET_ID`	input	`NetworkMonitor.networkId` emission
`Liveness`	`LIVENESS`	input	liveness probe outcome

State.name and Error.type are captured as qualified class names (::class.qualifiedName), never simpleName. That's what makes the next section work.

Retracing an obfuscated trace

On a minified release build (R8 on Android, or ProGuard/R8 over a JVM app), the class names in STATE and ERROR are renamed — a trace line reads STATE a.b.c instead of STATE …QuicConnectionState.Established. There are no keep-rules to add: capture uses qualified names precisely so the rename is reversible after the fact, the way a crash reporter symbolicates a stack trace. You just need to keep the build's mapping.txt.

The retrace tool is a separate, JVM-only artifact (it pulls com.android.tools:r8, so add Google's Maven repo):

repositories {
    google()        // com.android.tools:r8 (runtime-transitive)
    mavenCentral()
}
dependencies {
    implementation("com.ditchoom:socket-quic-trace-tools:<latest-version>")
}

import com.ditchoom.socket.quic.trace.tools.TraceDeobfuscator
import java.io.File

// mapping.txt from the release build (e.g. app/build/outputs/mapping/release/mapping.txt).
val deobfuscator = TraceDeobfuscator.fromMapping(File("mapping.txt").readText())

val readable = deobfuscator.deobfuscateAll(File("captured.trace").readLines())
File("captured.deobf.trace").writeText(readable.joinToString("\n"))

Only the STATE/ERROR class-name tokens are rewritten; every other event and field passes through untouched, and a name absent from the mapping (a non-obfuscated trace, or a Kotlin/Native or JS trace — those aren't obfuscated) is a clean identity pass-through. It handles both R8 and plain ProGuard mappings (same grammar).

From Gradle

If you build from this repository, the same tool is wired as a Gradle task:

./gradlew :socket-quic-trace-tools:retraceQuicTrace \
    -PtraceIn=captured.trace -Pmapping=mapping.txt -PtraceOut=captured.deobf.trace

End-to-end

Capture — set QuicOptions.trace = QuicTraceCapture(…) and persist each event.toString().
Ship the app minified (R8/ProGuard) and archive its mapping.txt. STATE/ERROR names land obfuscated in traces from the field.
Retrace offline — feed the captured trace + mapping.txt to TraceDeobfuscator (or the Gradle task) to get readable class names back.

Next Steps

Connections & Streams — the stream model you're tracing
Typed Stream Multiplexing — codec-typed messages over the same connection

The sink is typed​

Enabling capture on a client​

Consuming typed events instead of lines​

Enabling capture on a server​

Event types​

Retracing an obfuscated trace​

From Gradle​

End-to-end​

Next Steps​