fix(quic): faster PTO with INITIAL_RTT=100ms and unified ptoBaseMs path

multiconnect handshakeloss / handshakecorruption tail-fail under the
runner's 30% packet drop / bit-flip scenarios because each PTO retransmit
chance gives ~49% one-side success (0.7² for both directions clear). With
INITIAL_RTT=333ms the first PTO fires at 999 ms and doubling tops out
at ~5 attempts in 30s — across 50 sequential connections, ~5% probability
some iteration runs out of retransmits before the per-iter budget.

Two coupled changes:

1. INITIAL_RTT_MS 333→100. RFC 9002 §6.2.2 spec-allowed (the standard
   default but explicitly configurable). Matches Chrome and
   Firefox/neqo. Pre-sample PTO is now 300 ms instead of 999 ms;
   doubling fits ~8 retransmit attempts in 30s instead of 5,
   pushing per-iter loss-recovery success past 99% under 30% drop.
   Spurious retransmits on slow paths are harmless (peer dedupes
   by packet number) and smoothed_rtt converges in one round-trip.

2. QuicConnectionDriver always uses lossDetection.ptoBaseMs() for the
   PTO timer, including before the first RTT sample. Pre-fix the
   driver hardcoded 1000ms as a "handshake-timeout safety floor"
   that ignored INITIAL_RTT_MS entirely — the PTO was always 1s
   pre-handshake regardless of the constant. Now both pre- and
   post-sample regimes go through the same calculation.

   max_ack_delay is gated to APPLICATION space (RFC 9002 §6.2.1) so
   pre-handshake PTOs aren't padded with the peer's quoted delay.

Two pre-existing tests (PtoTest, QuicLossDetectionTest) hard-coded
expected PTO durations derived from the old 333 ms constant; updated
them to express the relationships in terms of INITIAL_RTT_MS so future
tweaks don't desync.

Result: 21/21 against aioquic, picoquic, quic-go (handshake,
multiplexing, longrtt, transferloss, transfercorruption,
handshakeloss, handshakecorruption all pass on each peer).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

quic/src/commonMain/kotlin/com/vitorpamplona/quic/connection/QuicConnectionDriver.kt

@@ -139,13 +139,26 @@ class QuicConnectionDriver(
                     } ?: break
                 socket.send(out)
             }
-            val ptoBaseMs =
-                if (connection.lossDetection.hasFirstRttSample) {
-                    val maxAckDelayMs = connection.peerTransportParameters?.maxAckDelay ?: 0L
-                    connection.lossDetection.ptoBaseMs(maxAckDelayMs).coerceAtLeast(1L)
+            // Use the loss-detection's PTO calculation in BOTH the pre- and
+            // post-first-RTT-sample regimes. Pre-sample, smoothed_rtt =
+            // INITIAL_RTT_MS so ptoBaseMs returns
+            // INITIAL_RTT_MS * 3 + max_ack_delay (~300 ms with the 100 ms
+            // initial). max_ack_delay only applies to APPLICATION space per
+            // RFC 9002 §6.2.1; pre-handshake we pass 0. Earlier shape
+            // hardcoded 1000 ms here as a "handshake-timeout safety floor"
+            // — the cost was four PTO retransmits in ~30 s of loss
+            // recovery instead of the eight that 300 ms initial gives,
+            // pinching multiconnect handshake-loss tests at the tail.
+            val maxAckDelayMs =
+                if (connection.application.sendProtection != null) {
+                    connection.peerTransportParameters?.maxAckDelay ?: 0L
                 } else {
-                    1_000L
+                    0L
                 }
+            val ptoBaseMs =
+                connection.lossDetection
+                    .ptoBaseMs(maxAckDelayMs)
+                    .coerceAtLeast(1L)
             val backoff = (1L shl connection.consecutivePtoCount.coerceAtMost(6))
             val ptoMillis = (ptoBaseMs * backoff).coerceAtMost(60_000L)
             // Suspend until either: a wakeup arrives, or the PTO timer expires.

quic/src/commonMain/kotlin/com/vitorpamplona/quic/connection/recovery/QuicLossDetection.kt

@@ -180,8 +180,30 @@ class QuicLossDetection {
     }
 
     companion object {
-        /** RFC 9002 §6.2.2 default initial RTT before a sample arrives. */
-        const val INITIAL_RTT_MS: Long = 333L
+        /**
+         * Initial RTT before the first sample arrives. RFC 9002 §6.2.2
+         * specifies a 333ms default but explicitly says "designs SHOULD
+         * allow it to be configurable". 100ms matches Chrome and
+         * Firefox/neqo's defaults — typical paths are 30–80ms, and the
+         * lower estimate gives faster PTO retransmits during the first
+         * round-trip when no real RTT sample exists yet.
+         *
+         * Why this matters for interop: PTO duration is
+         * `smoothed_rtt + max(4*rttvar, 1ms) + max_ack_delay`, doubled
+         * per consecutive PTO. With INITIAL_RTT=333 the very first
+         * retransmit is at +999ms, then 2s, 4s, 8s — only ~5 attempts
+         * fit in a 30s loss-recovery budget. With 100ms the first
+         * retransmit is at +300ms, then 600ms, 1.2s, 2.4s — twice as
+         * many attempts before the budget runs out. The handshakeloss /
+         * handshakecorruption multiconnect tests at 30% drop need every
+         * PTO opportunity to land 50 successful handshakes within the
+         * runner's 300s testcase budget.
+         *
+         * Spurious retransmits on slower paths are the trade-off, but
+         * they're harmless (peer dedupes via packet number) and the
+         * smoothed RTT updates within one round-trip on first ACK.
+         */
+        const val INITIAL_RTT_MS: Long = 100L
 
         /** RFC 9002 §6.1.1 packet-reordering threshold (number of PNs). */
         const val PACKET_THRESHOLD: Long = 3L

quic/src/commonTest/kotlin/com/vitorpamplona/quic/connection/recovery/PtoTest.kt

@@ -35,15 +35,19 @@ class PtoTest {
     @Test
     fun ptoBeforeFirstRttSample_usesInitialDefault() {
         val ld = QuicLossDetection()
-        // Before any sample: smoothed_rtt = 333, rttvar = 333/2 = 166.
-        // PTO = 333 + max(4*166, 1) + 0 = 333 + 664 = 997.
-        assertEquals(997L, ld.ptoBaseMs(maxAckDelayMs = 0L))
+        // Before any sample: smoothed_rtt = INITIAL_RTT_MS, rttvar = INITIAL_RTT_MS/2.
+        // PTO = smoothed_rtt + max(4*rttvar, 1) + 0
+        //     = INITIAL_RTT_MS + 4*(INITIAL_RTT_MS/2)
+        //     = INITIAL_RTT_MS * 3.
+        val initRtt = QuicLossDetection.INITIAL_RTT_MS
+        assertEquals(initRtt * 3L, ld.ptoBaseMs(maxAckDelayMs = 0L))
     }
 
     @Test
     fun ptoIncludesMaxAckDelay() {
         val ld = QuicLossDetection()
-        assertEquals(997L + 25L, ld.ptoBaseMs(maxAckDelayMs = 25L))
+        val initRtt = QuicLossDetection.INITIAL_RTT_MS
+        assertEquals(initRtt * 3L + 25L, ld.ptoBaseMs(maxAckDelayMs = 25L))
     }
 
     @Test

quic/src/commonTest/kotlin/com/vitorpamplona/quic/connection/recovery/QuicLossDetectionTest.kt

@@ -97,8 +97,10 @@ class QuicLossDetectionTest {
     @Test
     fun lossDelay_floor() {
         val ld = QuicLossDetection()
-        // Initial: smoothed=333, latest=333. Loss delay = 333*9/8 = 374.
-        assertEquals(374L, ld.lossDelayMs())
+        // Initial: smoothed=INITIAL_RTT_MS, latest=INITIAL_RTT_MS.
+        // Loss delay = max_rtt * 9/8 = INITIAL_RTT_MS * 9 / 8.
+        val initRtt = QuicLossDetection.INITIAL_RTT_MS
+        assertEquals(initRtt * 9L / 8L, ld.lossDelayMs())
     }
 
     @Test