tcp-connection.js

/**
   * Simulates a network download of a particular number of bytes over an optional maximum amount of time
   * and returns information about the ending state.
   *
   * See https://hpbn.co/building-blocks-of-tcp/#three-way-handshake and
   *  https://hpbn.co/transport-layer-security-tls/#tls-handshake for details.
   *
   * @param {number} bytesToDownload
   * @param {DownloadOptions} [options]
   * @return {DownloadResults}
   */
  simulateDownloadUntil(bytesToDownload, options) {
    const {timeAlreadyElapsed = 0, maximumTimeToElapse = Infinity, dnsResolutionTime = 0} =
      options || {};

    if (this._warmed && this._h2) {
      bytesToDownload -= this._h2OverflowBytesDownloaded;
    }
    const twoWayLatency = this._rtt;  // 150 ms
    const oneWayLatency = twoWayLatency / 2;  // 75 ms
    const maximumCongestionWindow = this._computeMaximumCongestionWindowInSegments(); // 20

    let handshakeAndRequest = oneWayLatency;  // 75ms
    if (!this._warmed) {
      handshakeAndRequest =
        // DNS lookup
        dnsResolutionTime + // 169
        // SYN
        oneWayLatency +
        // SYN ACK
        oneWayLatency +
        // ACK + initial request
        oneWayLatency +
        // ClientHello/ServerHello assuming TLS False Start is enabled (https://istlsfastyet.com/#server-performance).
        (this._ssl ? twoWayLatency : 0);  // 150
    }

    let roundTrips = Math.ceil(handshakeAndRequest / twoWayLatency);  // 3
    let timeToFirstByte = handshakeAndRequest + this._serverLatency + oneWayLatency;  // 544 ms + Server response time
    if (this._warmed && this._h2) timeToFirstByte = 0;

    const timeElapsedForTTFB = Math.max(timeToFirstByte - timeAlreadyElapsed, 0);
    const maximumDownloadTimeToElapse = maximumTimeToElapse - timeElapsedForTTFB;

    let congestionWindow = Math.min(this._congestionWindow, maximumCongestionWindow); // 10
    let totalBytesDownloaded = 0;
    if (timeElapsedForTTFB > 0) {
      totalBytesDownloaded = congestionWindow * TCP_SEGMENT_SIZE; // 1460 * 10
    } else {
      roundTrips = 0;
    }

    let downloadTimeElapsed = 0;
    let bytesRemaining = bytesToDownload - totalBytesDownloaded;
    while (bytesRemaining > 0 && downloadTimeElapsed <= maximumDownloadTimeToElapse) {
      roundTrips++;
      downloadTimeElapsed += twoWayLatency;
      congestionWindow = Math.max(Math.min(maximumCongestionWindow, congestionWindow * 2), 1);

      const bytesDownloadedInWindow = congestionWindow * TCP_SEGMENT_SIZE;  // 1460 * 20
      totalBytesDownloaded += bytesDownloadedInWindow;
      bytesRemaining -= bytesDownloadedInWindow;
    }

    const timeElapsed = timeElapsedForTTFB + downloadTimeElapsed;
    const extraBytesDownloaded = this._h2 ? Math.max(totalBytesDownloaded - bytesToDownload, 0) : 0;
    const bytesDownloaded = Math.max(Math.min(totalBytesDownloaded, bytesToDownload), 0);

    return {
      roundTrips,
      timeElapsed,
      bytesDownloaded,
      extraBytesDownloaded,
      congestionWindow,
    };
  }
}