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core(network-analyzer): fix num of roundtrips for h3 estimates #15102

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May 24, 2023
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core(network-analyzer): fix num of roundtrips for h3 estimates #15102

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core(network-analyzer): fix number roundtrips in h3 fallback rtt esti…
connorjclark May 22, 2023
22d95e7
ttfb
connorjclark May 23, 2023
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3 changes: 2 additions & 1 deletion core/computed/metrics/time-to-first-byte.js
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Expand Up @@ -26,7 +26,8 @@ class TimeToFirstByte extends NavigationMetric {

// Estimate when the connection is not warm.
// TTFB = DNS + (SSL)? + TCP handshake + 1 RT for request + server response time
let roundTrips = 3;
let roundTrips = 2;
if (!mainResource.protocol.startsWith('h3')) roundTrips += 1; // TCP
if (mainResource.parsedURL.scheme === 'https') roundTrips += 1;
const estimatedTTFB = data.settings.throttling.rttMs * roundTrips + observedResponseTime;

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5 changes: 3 additions & 2 deletions core/lib/dependency-graph/simulator/network-analyzer.js
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Expand Up @@ -194,7 +194,8 @@ class NetworkAnalyzer {

// Assume everything before sendStart was just DNS + (SSL)? + TCP handshake
// 1 RT for DNS, 1 RT (maybe) for SSL, 1 RT for TCP
let roundTrips = 2;
let roundTrips = 1;
if (!record.protocol.startsWith('h3')) roundTrips += 1; // TCP
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We should add this logic to TTFB as well.

WDYT about adding some kind of getRoundTripCount helper function to NetworkRequest so that we don't have to maintain this in 3 different places?

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It's a bit more complicated than first glance to share this code, since the analyzer has its own concept of "is this connection reused?". I can try in a followup.

if (record.parsedURL.scheme === 'https') roundTrips += 1;
return timing.sendStart / roundTrips;
});
Expand Down Expand Up @@ -227,8 +228,8 @@ class NetworkAnalyzer {
// TTFB = DNS + (SSL)? + TCP handshake + 1 RT for request + server response time
if (!connectionReused) {
roundTrips += 1; // DNS
if (!record.protocol.startsWith('h3')) roundTrips += 1; // TCP
if (record.parsedURL.scheme === 'https') roundTrips += 1; // SSL
roundTrips += 1; // TCP handshake
}

// subtract out our estimated server response time
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