What is a Web Server

1
What is a Web Server?

• Program that understands the HTTP protocol and
  generates appropriate responses
• Clients connect to the machine
• Clients send a request
• Server reads request, generates response
• Client interprets response appropriately

2
A Simplified Web Server

• Client asks for file
• Server finds appropriate file
• Server sends back a response header followed by
  the files data
• Server closes connection

3
What Does Connect Mean?

• For all practical purposes, it looks like theres
  data available via a file descriptor
• Stream of bytes
• Can be treated like any other file descriptor
• Not a FILE (like stdio, stderr)
• Must use read() and write() system calls

4
How Do You Identify Machines

• Names or numbers and ports
• http//www.domain.com implies a machine named
  www.domain.com and a default port of 80
• http//127.0.0.131415/index.html
• Refers to current box (127.0.0.1 is me)
• Port is 31415 (used for this project)
• File is named index.html

5
How Do You Identify Files?

• File name is specified in request
• Server maps that name to a real file
• Mapping can be whatever server wants
• For example, /vivek/index.html is really
  /n/fs/fac/vivek/public_html/index.html

6
How Does a Request Look?

• GET /index.html HTTP/1.0\r\n
• Connection Keep-Alive\r\n
• User-Agent Mozilla/4.72 en (X11..)\r\n
• Host 127.0.0.131415\r\n
• Accept image/gif, image/jpeg, /\r\n
• Accept-Encoding gzip\r\n
• Accept-Language en\r\n
• Accept-Charset iso-8859-1,,utf-8\r\n
• \r\n

7
What Do You Care About?

• GET /index.html HTTP/1.0
• In particular, just index.html
• Assume / means /index.html

8
How Does a Response Look?

• HTTP/1.0 200 OK\r\n
• Date blah-blah-blah\r\n
• Server blah-blah-blah\r\n
• Content-Type important\r\n
• Last-Modified blah-blah-blah\r\n
• \r\n
• Raw data

9
Whats a Minimal Response?

• HTTP/1.0 200 OK
• Content-Type important\r\n
• \r\n
• Data

10
What is Content-Type?

• text/html
• image/gif
• image/jpeg
• Others not needed

11
So Whats So Difficult?

• Threads
• Blocking
• Select