HTTP

Junghoo Cho

Web Interaction Example

How Does It Work?

Internet

Core Internet Standards

  • DNS (domain name service)
    • Internet protocol to map domain names to IPs
    • ICANN manages TLD (top-level domains)
  • TCP/IP (transmission control protocol and internet protocol)
    • Internet routing and transportation protocol

Core Internet Standards

  • HTTP (hypertext transportation protocol)
    • Communication protocol between web servers and web clients
  • Encoding standards
    • Text: ASCII, Unicode (UTF-8)
    • Multimedia: JPEG, MP3, H.264, …
  • MIME (multipurpose internet mail extensions)
    • Data-type-specification standard

Core Internet Standards

  • HTML (hypertext markup language)
    • Markup standard
  • CSS (Cascading style sheet)
    • Styling and formatting standard
  • Javascript
    • De facto Web programming language

HTTP

  • HTTP/2 (2015) is most recent
  • HTTP/1.1 (1996) is still extremely popular
    • Major browsers support HTTP/2 only over HTTPS
  • Learn HTTP/1.1 first
    • Then talk about HTTP/2
  • Two key properties of HTTP
    • Request & response paradigm
    • Stateless protocol

HTTP: Request & Response

  • All interactions start with a client’s request

HTTP: Stateless Protocol

  • Every request is handled independently of others
    • The server is not required to remember “history” of past requests
  • Questions
    • Q: What are pros/cons of stateless protocol?
    • Q: what are the implications?

HTTP Example

HTTP Message

  • A message is either a request or response
  • Message structure
    • request/status line
    • header
    • [empty line]
    • body
  • Bare minimum HTTP request:
    GET / HTTP/1.0

HTTP Request Example

GET /apps/echo.py HTTP/1.1
Host: oak.cs.ucla.edu
Connection: keep-alive
Cache-Control: max-age=0
Dnt: 1
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/80.0.3987.149 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9,ko;q=0.8
Cookie: _xsrf=2|11b01aa1

HTTP Response Example

HTTP/1.1 200 OK
Date: Wed, 04 Apr 2019 03:20:33 GMT
Cache-Control: max-age=0, no-cache, s-maxage=10
Connection: Keep-Alive
Content-Encoding: gzip
Content-Length: 997
Content-Type: text/html; charset=UTF-8
Server: Apache/2.4.29 (Ubuntu)
Vary: Accept-Encoding

<html>
<head><title>Example page</title></head>
...

HTTP Request Line

  • METHOD PATH PROTOCOL_VERSION

    GET /apps/echo.py HTTP/1.1

HTTP Methods (1)

  • GET: “retrieve” data
    • IMPORTANT: NO significant side effect at server
  • POST: “post” data at the specified URL
    • May leave a side-effect on the server
  • PUT: “place” the data at the URL (~ replace the data)
  • DELETE: “delete” the data at the URL

HTTP Methods (2)

  • Less common:
    • HEAD: same as GET but requests header only
    • OPTIONS: requests information on options available at the server
    • TRACE: the final recipient returns the whole request message in the response body

HTTP Header

GET /apps/echo.py HTTP/1.1
Host: oak.cs.ucla.edu
Connection: keep-alive
Cache-Control: max-age=0
Dnt: 1
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/80.0.3987.149 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9,ko;q=0.8
Cookie: _xsrf=2|11b01aa1

HTTP Response

HTTP/1.1 200 OK
Date: Wed, 04 Apr 2019 03:20:33 GMT
Cache-Control: max-age=0, no-cache, s-maxage=10
Connection: Keep-Alive
Content-Encoding: gzip
Content-Length: 997
Content-Type: text/html; charset=UTF-8
Server: Apache/2.4.29 (Ubuntu)
Vary: Accept-Encoding

<html>
<head><title>Example page</title></head>
...

HTTP Response

  • Structure
    • Status line
    • Header
    • [empty line]
    • body
  • Status line
    • VERSION STATUS_CODE REASON_PHRASE
    • Example: HTTP/1.1 200 OK

Status Code

  • 2xx: Success
  • 3xx: Redirection
  • 4xx: Client Error
  • 5xx: Server Error

HTTP Response Header

HTTP/1.1 200 OK
Date: Wed, 04 Apr 2019 03:20:33 GMT
Cache-Control: max-age=0, no-cache
Connection: Keep-Alive
Content-Encoding: gzip
Content-Length: 997
Content-Type: text/html; charset=UTF-8
Server: Apache/2.4.29 (Ubuntu)
Vary: Accept-Encoding

<html>
<head><title>Example page</title></head>
...

Questions?

HTTP/2: Background

  • Many high-latency limited-bandwidth mobile devices
  • Many objects are needed to display a single page
    • HTML, image, CSS, JavaScript, …
    • ~ 100 objects, ~ 2MB

HTTP/2

  • Key assumptions have been relaxed for speed and efficiency
    • Server push
    • Stateful header compression
    • Binary encoding
    • …

HTTP/2 New Features (1)

  • “Multiplexed streams”
    • Multiple outstanding requests through a single connection
    • Split messages into small frames
    • Priority specification
  • HPACK
    • Stateful HTTP header compression

HTTP/2 New Features (2)

References