1 00:00:01,200 --> 00:00:04,786 Type democompany.com and press Enter. 2 00:00:04,927 --> 00:00:06,736 Nothing has been sent yet. 3 00:00:06,947 --> 00:00:10,649 The browser's first job is to guess what you meant. 4 00:00:10,849 --> 00:00:14,493 The address bar is not a URL field. 5 00:00:14,518 --> 00:00:15,770 It's a guesser. 6 00:00:16,073 --> 00:00:19,508 Is this a hostname, or words to search for? 7 00:00:19,812 --> 00:00:25,731 A dot and a known ending say hostname, so it builds a URL. 8 00:00:25,931 --> 00:00:29,644 You left out the scheme, so the browser supplies one. 9 00:00:29,855 --> 00:00:33,928 Most browsers now try https first. 10 00:00:33,953 --> 00:00:41,788 It lowercases the host, encodes anything illegal, and only then has a real address to work with. 11 00:00:41,988 --> 00:00:48,012 Where it fails: guess wrong, and your words go to a search engine instead. 12 00:00:48,408 --> 00:00:52,458 Every later step depends on that guess. 13 00:00:53,159 --> 00:00:55,340 A hostname is for people. 14 00:00:55,469 --> 00:01:05,986 The network routes to numbers, so the browser must turn www.democompany.com into an IP address. 15 00:01:06,116 --> 00:01:10,224 That's DNS, the Domain Name System. 16 00:01:10,423 --> 00:01:17,468 First it checks what it already knows: its own cache, then the operating system's. 17 00:01:17,610 --> 00:01:21,033 A recent answer costs nothing. 18 00:01:21,233 --> 00:01:28,069 Otherwise, a resolver, usually your internet provider's, walks the tree from the right. 19 00:01:28,373 --> 00:01:32,052 A root server points to the servers for .com. 20 00:01:32,262 --> 00:01:38,066 They point to Demo Company's own name servers, which give the answer. 21 00:01:38,266 --> 00:01:45,985 Fun fact: the whole internet's naming starts at thirteen root server names, lettered A to M. 22 00:01:46,381 --> 00:01:53,473 Behind those thirteen names stand well over a thousand machines around the world. 23 00:01:53,673 --> 00:01:59,267 Every answer comes with a time to live, so the next lookup can be skipped. 24 00:01:59,478 --> 00:02:02,286 Where it fails: no such name. 25 00:02:02,589 --> 00:02:10,598 Chrome calls it DNS_PROBE_FINISHED_NXDOMAIN, and nothing further happens. 26 00:02:11,298 --> 00:02:18,738 With an address in hand, packets set off across the internet, router to router, network to network. 27 00:02:18,949 --> 00:02:21,072 Nobody plans the whole route. 28 00:02:21,213 --> 00:02:26,041 Each router just passes the packet one hop closer. 29 00:02:26,241 --> 00:02:33,507 Every step from here is counted in round trips: a message out and an answer back. 30 00:02:33,707 --> 00:02:37,722 A round trip has a floor no upgrade can lower. 31 00:02:37,933 --> 00:02:43,074 Light in fiber covers about two hundred kilometres each millisecond. 32 00:02:43,285 --> 00:02:49,657 A server across an ocean is many milliseconds away, every single trip. 33 00:02:49,857 --> 00:02:54,534 Where it fails: packets go out and nothing comes back. 34 00:02:54,838 --> 00:03:00,234 The browser waits, then gives up with ERR_CONNECTION_TIMED_OUT. 35 00:03:00,935 --> 00:03:05,891 Next, TCP opens a reliable channel, with a handshake. 36 00:03:05,916 --> 00:03:13,309 The browser says "let's talk", the server says "yes, let's", and the browser confirms. 37 00:03:13,509 --> 00:03:19,033 That costs one full round trip before a single byte of the page can move. 38 00:03:19,430 --> 00:03:27,137 TCP then numbers every piece, resends what's lost, and puts it all back in order. 39 00:03:27,337 --> 00:03:32,920 Where it fails: the machine is there, but nothing is listening on that port. 40 00:03:33,061 --> 00:03:38,365 The server refuses, and Chrome shows ERR_CONNECTION_REFUSED. 41 00:03:39,065 --> 00:03:48,003 Over that channel, TLS adds encryption, and something just as important: proof of who is answering. 42 00:03:48,203 --> 00:03:55,237 In TLS 1.3, browser and server agree on keys in one more round trip. 43 00:03:55,355 --> 00:03:57,688 The server sends its certificate. 44 00:03:57,688 --> 00:04:05,036 The browser checks that it names this host, hasn't expired, and was signed by an authority it trusts. 45 00:04:05,235 --> 00:04:08,356 That check is what the padlock means. 46 00:04:08,381 --> 00:04:15,845 It's the difference between talking to Demo Company and talking to someone pretending to be Demo Company. 47 00:04:16,045 --> 00:04:20,699 Where it fails: an expired or mismatched certificate. 48 00:04:20,910 --> 00:04:28,466 Chrome stops cold with "Your connection is not private", and most people sensibly turn back. 49 00:04:29,166 --> 00:04:36,571 Only now does the browser send the request: a method, a path, and the host it wants. 50 00:04:36,771 --> 00:04:39,753 Then it waits while the server works. 51 00:04:39,836 --> 00:04:46,545 The time until the first byte of the answer comes back has a name: time to first byte. 52 00:04:46,745 --> 00:04:50,435 And Demo Company's first answer isn't the page. 53 00:04:50,460 --> 00:04:59,119 It's a redirect: 307, try www.democompany.com instead. 54 00:04:59,319 --> 00:05:05,946 A redirect is a whole new request, sometimes with a new lookup and a new connection. 55 00:05:06,064 --> 00:05:12,727 Where it fails: redirects that point at each other, which Chrome stops with ERR_TOO_MANY_REDIRECTS. 56 00:05:12,729 --> 00:05:17,870 Or a server that answers with a 500 error, or very slowly. 57 00:05:18,570 --> 00:05:19,510 Add it up. 58 00:05:19,510 --> 00:05:25,743 A lookup, a handshake, a key exchange, and the request itself. 59 00:05:25,943 --> 00:05:32,303 On a cold start that's about four round trips before the first byte, more with a redirect. 60 00:05:32,328 --> 00:05:38,364 At a hundred milliseconds a trip, nearly half a second, and the page hasn't begun. 61 00:05:38,564 --> 00:05:41,616 That's why the web works so hard to skip trips. 62 00:05:41,687 --> 00:05:45,400 Browsers cache names and reuse open connections. 63 00:05:45,472 --> 00:05:52,993 HTTP/3, over QUIC, folds the TCP and TLS handshakes into one. 64 00:05:53,181 --> 00:05:58,090 Preconnect starts this work early, for a host you know you'll need. 65 00:05:58,290 --> 00:06:03,489 And it's why you should link to the final address, not one that redirects. 66 00:06:03,631 --> 00:06:07,460 Every hop you add, the reader pays for. 67 00:06:08,160 --> 00:06:13,986 Open the Network panel, load Demo Company, and select the first request. 68 00:06:14,278 --> 00:06:24,435 The Timing tab names every step: DNS lookup, initial connection, SSL, request sent, and waiting for the server. 69 00:06:24,636 --> 00:06:28,280 Reload, and most of those steps vanish. 70 00:06:28,363 --> 00:06:32,831 The name is cached and the connection is still open. 71 00:06:33,030 --> 00:06:35,815 All of that, before the first byte. 72 00:06:36,211 --> 00:06:44,661 Next: what the browser does once the bytes arrive, and why one URL becomes seventy requests.