core(network-analyzer): include 0 start times in rtt estimate

[connorjclark]

There are many requests in the lantern database with a connectStart of zero. For example, in lantern-data/https---www-cnet-com--mobile-unthrottled-5-devtoolsLog.json. All of these requests also have a dnsStart of 0 (by necessity), though not all requests with a dnsStart of 0 also have a connectStart of 0.

From cnet.com:

• request 7921.127 was a redirect - the original request has a non-zero dns/connection start time. The second request is the 0 one. The second request is ignore in the rtt estimation because the connection is marked as re-used.
• request 7921.105 tracking pixel, not a redirect. I can only guess that Chrome just happened to already connect to this origin recently (maybe from a subframe?), though it is not reflected in the logs.

From https://en-maktoob.yahoo.com/?p=xa:

• request 19743.2 connectStart AND sslStart are both zero (connectStart: 0, connectEnd: 10.374, sslStart: 0, sslEnd: 10.362). It's a h2 request, so it's unclear what this actually means. Surely doing a TCP or a SSL handshake takes more than <1ms, so I think we can assume this represents just a single roundtrip of connection time.

(btw, this is a good link)

[brendankenny]

These seem like good assumptions, but can you capture a netlog of one of these cases so we can resolve some of these questions? Ideally future investigations of RTT estimating can terminate here rather than continuing up blame.

[connorjclark]

Here's a netlog and dt log of cnet.com https://gist.github.com/connorjclark/3b85c1e06667beed53fb0ef27123c987

url https://pixel.tapad.com/idsync/ex/receive?partner_id=APPNEXUS&partner_device_id=1400146762626694345&pt=c0b0432c-64e8-4f10-b286-5ed2d7f9ec9d%2C%2C req id 31618.5 has timing of "dnsStart":0,"dnsEnd":0,"connectStart":0,"connectEnd":18.122,"sslStart":0,"sslEnd":18.119

Search for tapad in the netlog, and you see there are two connections open for that domain (same site and cross site).

Every request to that domain:

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[connorjclark]

I thought it could be related to subframes opening a connection before the top frame makes its first request to the same origin. And it could play a part. But I can't get a consistent connect start of 0 (just very close). There must be too much out of our control, this can just happen rarely.

My test fixture:

<!DOCTYPE html>
<html>
<head>
  <link rel='dns-prefetch' href='https://fonts.gstatic.com' crossorigin />
  <link rel='preconnect' href='https://fonts.gstatic.com' crossorigin />
  <style>
    div {
      font-family: 'Droid Sans';
    }
  </style>
</head>
<body>

<div>preconnect</div>
<script>
  let timeout = 0;
  if (document.location.search !== '?noframe') {
    const el = document.createElement('iframe');
    el.src = '?noframe';
    document.body.append(el);
    timeout = 10 * 1000;
  }

  setTimeout(() => {
    const el = document.createElement('link');
    el.href = 'https://fonts.googleapis.com/css?family=PT+Serif:regular,italic,bold|PT+Sans:regular,italic,bold|Droid+Sans:400,700|Lato:700,900';
    el.rel = 'stylesheet';
    document.head.append(el);

    const el2 = document.createElement('script');
    el2.src = './hello.js?redirect=./hello.js';
    document.head.append(el2);
  }, timeout);
</script>

</body>
</html>