Web 200: Anatomy of a Request

Web 200: Anatomy of a Request @johndbritton

How does this...

[enter]

...become this...

...on the internet?

Internet, how does it work?

• Driving a car • Shipping containers It’s all about
abstraction

Major players • HTTP • URI • Browser • Web
server • DNS • Operating system • Interface • Network • Router ! ! • ... among others

OSI Model • 7 Layers • We won’t cover everything
• Really boring • Required if you want some bogus certiﬁcates • Actually a useful concept

HTTP The language browsers speak

Client server model • Web page is a document •
User inputs http://example.com • The client (browser) makes a GET request • The server sends a response • The browser renders the page

URI • http://github.com/johndbritton • http - protocol • github.com -
domain • /johndbritton - resource

Request GET /johndbritton

Response <!DOCTYPE html> ...

More Requests GET /style.css GET /jquery.js GET /image.jpg GET /image2.jpg
...

More Responses (you get the idea)

Rendering • HTML - Structure • CSS - Style •
Javascript - Behavior

Telnet, Inspector, JSFiddle Let’s try them out

Browser - ONLY HTTP Doesn’t care about anything else

HTTP: methods / verbs • GET • POST • PUT
• DELETE • more: HEAD, PATCH, TRACE, OPTIONS, CONNECT

HTTP: responses • 1xx - informational • 2xx - success
• 3xx - redirect • 4xx - error • 5xx - server error

The webserver It speaks HTTP too

Two types of webapp • Static • Receive a request
• Find a ﬁle on disk • Respond with contents of the ﬁle • Dynamic • Receive a request • Run application logic • Return a dynamically generated response

DNS: name resolution Where do requests go?

github.com 207.97.227.239

Lookups are cached for improved performance

Possible cache hits • Local machine • Home router •
ISP • Upstream provider

No cache - worst case • 13 Root nameservers •
TLD nameserver • Authoritative nameserver • A record • IP address

dig github.com ; <<>> DiG 9.8.3-P1 <<>> github.com ... github.com.
2 IN A 207.97.227.239

Making a connection • Resolve name • Open a connection
• Speak HTTP

Network collection of nodes that can communicate directly

Interface connection from node to communication medium

IP address 0.0.0.0 - 255.255.255.255

Why 255? • 0 - 255 • binary, 8 bits
• 00000000 - 11111111 • 00000000.00000000.00000000.00000000 • 11111111.11111111.11111111.11111111

Network addresses • a.b.c.d/n (n = network mask / subnet)
• Private (non routable) networks • 10.0.0.0/8 • 172.16.0.0/12 • 192.168.0.0/16

Special addresses • 192.268.1.0/24 - network with subnet • 192.168.1.0
- network address • 192.168.1.255 - broadcast address

Network settings • Auto-conﬁgured via DHCP • IP: 192.168.1.101 •
Subnet Mask: 255.255.255.0 (/24) • Router: 192.168.1.1 • DNS Servers: 192.168.1.1

Network hardware • Hub - Dumb • Switch - Smarter
• Router - Smartest

Network protocols • Transport - chunks of data • TCP
• UDP • Routing - • RIP • OSPF

Transport • OS segments data, packages it up into packets
• TCP • Reliable - resend on transmission failure • UDP • Unreliable - send once

Routing • Routers connect networks, handle packets and don’t care
what’s inside • RIP • Distance vector (hop count) • OSPF • Open shortest path ﬁrst (link weight)

whatismyipaddress.com 166.137.88.161 github.com 207.97.227.239

NAT Network address translation

ifconﬁg 192.168.1.101 whatismyipaddress.com 166.137.88.161 github.com 207.97.227.239

traceroute, nmap, wireshark if we have time

Web 200: Anatomy of a Request

Web 200: Anatomy of a Request

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