Test failure: PingTest.SendPingWithLowTtl_RoundtripTimeIsNonZero flaky on macOS 26 ARM64 (sub-ms RTT truncated to 0) #132123

Test failure

System.Net.NetworkInformation.Tests.PingTest.SendPingWithLowTtl_RoundtripTimeIsNonZero is failing in outerloop on macOS 26 ARM64 (OSX.26.Arm64.Open), starting 2026-08-09.

Error message

Expected at least one TtlExpired reply with non-zero RoundtripTime across 3 attempts

Frequency

2 builds on main over the past 7 days, exclusively on the osx.26.arm64.open queue (runtime-libraries-coreclr outerloop-osx). Not seen on any other platform.

Root cause

Environment/platform-specific test fragility, not a product bug.

The test (src/libraries/System.Net.Ping/tests/FunctionalTests/PingTest.cs) pings with Ttl = 5, expects a TtlExpired/TimeExceeded reply, and asserts at least one of 3 attempts reports a non-zero RoundtripTime:

// On macOS, the raw socket implementation truncates sub-millisecond RTT to 0,
// and the first-hop router often responds in <1ms.
options.Ttl = 5;
bool gotNonZeroRtt = false;
for (int attempt = 0; attempt < 3; attempt++)
{
    PingReply pingReply = await ping.SendPingAsync(host, TestSettings.PingTimeout, payload, options);
    Assert.True(pingReply.Status == IPStatus.TimeExceeded || pingReply.Status == IPStatus.TtlExpired, ...);
    if (pingReply.RoundtripTime > 0) { gotNonZeroRtt = true; break; }
}
Assert.True(gotNonZeroRtt, "Expected at least one TtlExpired reply with non-zero RoundtripTime across 3 attempts");

The test comment already acknowledges the failure mode: macOS raw sockets truncate sub-millisecond RTT to 0. On the fast Azure OSX.26.Arm64 CI runners the first-hop router consistently replies in under 1ms, so all 3 attempts measure RoundtripTime == 0 and the assertion fails. This is inherently timing/environment dependent and will flake on fast, low-latency network paths.

Suggested fix

Make the test robust to sub-millisecond RTT on macOS, e.g.:

• Skip the non-zero-RTT assertion on macOS (the RTT-truncation platform), or
• Increase the number of attempts and/or accept RoundtripTime >= 0 on platforms known to truncate sub-ms RTT, keeping the strict > 0 assertion only where the raw-socket path preserves sub-ms timing.

Sample console log

https://helixr1107v0xdeko0k025g8.blob.core.windows.net/dotnet-runtime-refs-heads-main-c2d1c1347a4940c8a6/System.Net.Ping.Functional.Tests/3/console.ff3a2daa.log?helixlogtype=result

Known issue error message

Fill the error message using known issues.

{
  "ErrorMessage": "Expected at least one TtlExpired reply with non-zero RoundtripTime across 3 attempts",
  "BuildRetry": false,
  "ExcludeConsoleLog": false
}

Known issue validation

Build: 🔎 https://dev.azure.com/dnceng-public/public/_build/results?buildId=1546596
Error message validated: [Expected at least one TtlExpired reply with non-zero RoundtripTime across 3 attempts]
Result validation: ✅ Known issue matched with the provided build.
Validation performed at: 8/11/2026 9:19:16 AM UTC

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