The document summarizes the steps a browser takes when a user enters "google.com" in the address bar. It checks if it is a valid URL, determines if HTTPS is preferred, checks the browser and OS caches for the domain IP address, performs a DNS lookup if needed to obtain the IP address, sets up a secure TCP connection on port 443, and sends an HTTP GET request to fetch the requested resource.

1. Type“google.com”into
thebrowserandhitenter
Whathappensnext?
1 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

2. HowtheInternetworks
GraemeMathieson
Email me: mathie@woss.name
Tweet me: @mathie
2 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

3. google.com
3 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

4. IsitaURL?
· Yep. OK, cool, my work here is done.
· Kinda. Well, let’s turn it into a well formed URL.
· Nope. OK, I’m gonna assume you meant to
search for something. Let’s turn it into a well
formed URL for a web search.
4 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

5. HTTPStrictTransportSecurity
Does this site prefer HTTPS?
· Strict-Transport-Security header from
a previous request?
· In the browser’s list of HSTS preloads?
5 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

6. HTTPStrictTransportSecurity
Does this site prefer HTTPS?
· Yep OK, set the URL scheme to https.
· Nope Fine then. If you don’t care for security…
6 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

7. https://google.com/
7 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

8. Browsercache
Is the URL in the browser cache?
· Yep Let’s check it’s still valid.
· Nope Well, we’re going to have to fetch it.
8 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

9. Browsercache
Is the cached content still valid?
Expires
Cache-Control: max-age
· Yep Awesome. We might skip a network request!
· Nope OK, let’s check in with the server.
9 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

10. Browsercache
Should the cached content be revalidated?
Cache-Control: must-revalidate
· Yep OK, let’s check in with the server.
· Nope Awesome. Skip to rendering!
10 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

11. ParsetheURL
· Scheme: “https”
· Authority: “google.com”
· Path: “/“
11 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

12. DNSLookup:Browsercache
Is the hostname in the browser’s cache?
· Yep Awesome, let’s use that IP address.
· Nope OK, we’re going to have to do this the hard
way.
12 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

13. DNSLookup:OSresolver
Is the hostname in the operating system’s cache?
· Yep Job done. We’ll use that IP address.
· Nope OK, we’re really going to have to look it
up.
13 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

14. NameServiceSwitch
· Check /etc/hosts
· Try multicast DNS
· Perform a DNS lookup
14 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

15. DNSLookup
Get the IP address of a name server
· From DHCP
· Statically configured
15 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

16. DNSRecordTypes
· A and AAAA are address records: mappings from
name to IP address.
· PTR is a reverse mapping from IP address to
name
· NS is a pointer to a name server.
· Other record types: SOA, CNAME, MX, TXT.
16 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

17. SendtheDNSrequest
New is Apple iOS 9 & El Capitan
· Send out an AAAA request; and
· Send out an A request, in parallel.
17 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

18. RecursiveDNSrequest
Is the record in the name server’s cache?
· Yep Is it still valid? (TTL) If so, return the record.
Job done.
· Nope OK, we’ll need to look it up.
18 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

19. UpstreamDNSserver
Is our local DNS server configured to have one or
more upstream servers?
· Yep OK, let’s pass the request off to an upstream
and let it figure out the answer.
· Nope Damn. We’re going to have to do the hard
work ourselves.
19 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

20. RootDNSServers
· 13 well-known IP addresses of root servers.
· Really, they’re hundreds of machines
distributed globally.
· Authoritative for the root zone.
20 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

21. DNSAuthority
Root servers are authoritative for the root zone.
Know the canonical answer for who serves each
TLD: “.com”, “.net”, “.uk”, etc.
21 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

22. What’stheArecordfor
“google.com”?
22 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

23. Rootservers
What’s the A record for “google.com”?
· No idea, but here’s the list of name servers for
“.com”.
· Oh, and have the IP addresses of those name
servers, too.
23 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

24. Authoritativeserversfor“.com”
What’s the A record for “google.com”?
· No idea, but here’s a list of name servers for
“google.com”.
· Oh, and have the IP addresses of those name
servers, too.
24 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

25. Authoritativeserversfor“google.com”
What’s the A record for “google.com”?
· Hey, I know this! Here’s a list of IP addresses!
25 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

26. FiguringouttheTCPport
What TCP port should we connect to?
· Figure out from the URL scheme
· Ask the operating system: getservbyname()
· Name Service Switch
· grep '^https.*tcp' /etc/services
26 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

27. MakingaTCPconnection
We know the IP address and port. Now we can
connect!
27 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

28. TCP:Threewayhandshake
Open connection, and agree initial sequence
numbers.
· -> SYN
· <- SYN+ACK
· -> ACK
28 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

29. TransmissionControlProtocol(TCP)
· Ordered data transfer
· Reliable data transfer
· Flow control
· Congestion control
29 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

30. TransportLayerSecurity
Create a secure connection between the client
and server.
· Authentication of the server (and, optionally,
the client).
· Negotiate a session key.
· Encrypt data between client and server.
30 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

31. HTTP:GET
GET / HTTP/1.1
Host: google.com
31 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

32. FIN
FIN -> ACK -> FIN -> ACK
TCP/IP Illustrated by W. Richard Stevens
<https://woss.name/>
32 — © 2015 Graeme Mathieson. CC BY-SA 4.0.