Parse network monitor cap file

gistfile1.js

var sourceAddress = [192,168,1,2];
var targetAddress = [192,168,1,1];

function readCapFile() {
  var xhr = new XMLHttpRequest();
  xhr.open('GET', 'test.cap');
  xhr.responseType = 'arraybuffer';
  xhr.onload = function () {
    var file = new Uint8Array(xhr.response);
    if (file[0] !== 0x47 || file[1] !== 0x4D) throw new Error('invalid cap file');
    findCapFrames(file);
  };
  xhr.send();
}

function findCapFrames(file) {
  // http://msdn.microsoft.com/en-us/library/windows/desktop/ee817717%28v=vs.85%29.aspx
  var frameTableOffset = file[0x18] | (file[0x19] << 8) | (file[0x1A] << 16) | (file[0x1B] << 24); 
  var frameTableLength = file[0x1C] | (file[0x1D] << 8) | (file[0x1E] << 16) | (file[0x1F] << 24);

  var pos = frameTableOffset, count = frameTableLength >> 2;
  var framesOffsets = [];
  for (var i = 0; i < count; i++, pos += 4) {
    framesOffsets.push(file[pos] | (file[pos + 1] << 8) | (file[pos + 2] << 16) | (file[pos + 3] << 24));
  }
  // http://msdn.microsoft.com/en-us/library/windows/desktop/ee831821%28v=vs.85%29.aspx
  var frames = [];
  for (var i = 0; i < count; i++) {
    pos = framesOffsets[i];
    var timestamp = (file[pos] | (file[pos + 1] << 8) | (file[pos + 2] << 16)) +
      ((file[pos + 3] | (file[pos + 4] << 8) | (file[pos + 5] << 16)) * 16777216) +
      (file[pos + 6] | (file[pos + 7] << 8)) * 281474976710656;
    var length = file[pos + 8] | (file[pos + 9] << 8) | (file[pos + 10] << 16) | (file[pos + 11] << 24);
    var available = file[pos + 12] | (file[pos + 13] << 8) | (file[pos + 14] << 16) | (file[pos + 15] << 24);
    pos += 16;
    frames.push({timestamp: timestamp * 1e-6, available: available, macFrame: file.subarray(pos, pos + length)});
  }
  // parse
  var lastSeq = {'in': 0, 'out': 0};
  for (var i = 0; i < count; i++) {
    // guessing ip, http://en.wikipedia.org/wiki/IPv4
    var frame = frames[i];
    var macFrame = frame.macFrame;
    var tcpPos, direction = null;
    for (var j = 0; j < macFrame.length; j++) {
      if (macFrame[j] === 0x45 && // Ipv4 with IHL=5
          macFrame[j + 6] === 0x40 && macFrame[j + 7] === 0x00 && // don't frag
          macFrame[j + 9] === 0x06) { // TCP
        if (macFrame[j + 12] === sourceAddress[0] && macFrame[j + 13] === sourceAddress[1] &&
            macFrame[j + 14] === sourceAddress[2] && macFrame[j + 15] === sourceAddress[3] &&
            macFrame[j + 16] === targetAddress[0] && macFrame[j + 17] === targetAddress[1] &&
            macFrame[j + 18] === targetAddress[2] && macFrame[j + 19] === targetAddress[3]) {
          direction = 'out'; tcpPos = j + 20; break;
        }
        if (macFrame[j + 16] === sourceAddress[0] && macFrame[j + 17] === sourceAddress[1] &&
            macFrame[j + 18] === sourceAddress[2] && macFrame[j + 19] === sourceAddress[3] &&
            macFrame[j + 12] === targetAddress[0] && macFrame[j + 13] === targetAddress[1] &&
            macFrame[j + 14] === targetAddress[2] && macFrame[j + 15] === targetAddress[3]) {
          direction = 'in'; tcpPos = j + 20; break;
        }
      }
    }
    if (!direction) continue;
    frame.direction = direction;
    frame.srcPort = (macFrame[tcpPos] << 8) | macFrame[tcpPos + 1];
    frame.destPort = (macFrame[tcpPos + 2] << 8) | macFrame[tcpPos + 3];
    var seq = ((macFrame[tcpPos + 4] << 24) | (macFrame[tcpPos + 5] << 16) | (macFrame[tcpPos + 6] << 8) | macFrame[tcpPos + 7]) >>> 0;
    var ack = ((macFrame[tcpPos + 8] << 24) | (macFrame[tcpPos + 9] << 16) | (macFrame[tcpPos + 10] << 8) | macFrame[tcpPos + 11]) >>> 0;
    if (ack === 0) {
      lastSeq = {'in': 0, 'out': 0};
    }
    if (lastSeq[direction] > seq) {
//      console.log('retransmit');
      continue;
    }
    lastSeq[direction] = seq;

    var dataPos = tcpPos + 4 * (macFrame[tcpPos + 12] >> 4);
    if (dataPos < macFrame.length) {
      frame.data = macFrame.subarray(dataPos);
    }
  }

  frames = frames.filter(function (f) { return f.data; });
  processFrames(frames);
}

function processFrames0(frames) {
  frames.forEach(function (f) {
    var el = document.createElement('div');
    el.textContent = f.direction + '(' + f.srcPort + '->' + f.destPort + '): ' +
      f.timestamp.toFixed(3) + ' '  + f.data.length;
/*
       Array.prototype.map.call(f.data,
          function (i) { return (i + 256).toString(16).substr(1); }).join(' ') + ' :: ' +
       String.fromCharCode.apply(String, f.data);*/
    document.body.appendChild(el);
  });
}