07: HTTP Server

Learning Objectives

• Students will learn to read and write HTTP Request and Response packets
• Students will learn to write HTTP servers for handling HTTP requests

Resources

• Skim Official W3 HTTP Protocol
• Skim wiki list of header fields
• Skim wiki list of status codes

HTTP

The Hyper Text Transfer Protocol (HTTP) is a stateless request-response application layer protocol. HTTP is used to build distributed, collaborative, hypermedia information systems. HTTP is the foundation for the world wide web. Applications built using HTTP subscribe to the client-server computing model. In the client-server computing model a host designed to provide a service is called a server and clients are hosts that make requests to the service. The HTTP specification defines how requests and responses should be formatted, but not what a service should represent. HTTP is often associated with serving .html files but is also used to transfer images, videos, JSON, XML, binary executables, and much more.

HTTP Requests

A HTTP/1.1 request is formatted in text and transferred using TCP. The first line of the request contains the METHOD, URL, and HTTP VERSION. The following lines are the request HEADERS. Each header is separated by a newline character. A header is a key value pair separated using a colon. Headers containing more than one value separate each value using a semicolon. The header section of the request is terminated with an empty line. An optional body follows the header section.

HTTP Method	Request Has Body	Response Has Body	Safe	Idempotent	Cacheable	Function
GET	No	Yes	Yes	Yes	Yes	Retrieve a resource
HEAD	No	No	Yes	Yes	Yes	Like GET but headers only
POST	Yes	Yes	No	No	Yes	Create a resource
PUT	Yes	Yes	No	Yes	No	Update a resource
DELETE	No	Yes	No	Yes	No	Delete a resource
CONNECT	Yes	Yes	No	No	No	Create TCP/IP tunnel
OPTIONS	Optional	Yes	Yes	Yes	No	Returns supported methods for a URL
TRACE	No	Yes	Yes	Yes	No	Echos retrieved request
PATCH	Yes	Yes	No	No	No	Partial modification of resource

Safe methods should only be used for information retrieval and should not change the server state.
Idempotent methods means if two identical requests are made they should get an identical response.
Cacheable means the client should be able to cache the response.

Example HTTP Request

POST /api/note HTTP/1.1
Host: api.example.com
Origin: www.example.com
Authorization: Beader bHVsIHRoaXMgaXMgYSBmYWtlIHNlY3JldCB0b2tlbg==
Accept: application/json
Content-Type: application/json; charset=UTF-8
Content-Length: 58

{"title":"kata","content":"get 100 points on hacker rank"}

HTTP Response

A HTTP/1.1 response is also formatted in text and transferred using TCP. The first line of the response contains the HTTP VEERSION, STATUS CODE, and STATUS MESSAGE. The following lines are the request headers and are formatted exactly the same way as the request headers. The header section of the request is terminated with an empty line. An optional body follows the header section.

Example HTTP Response

HTTP/1.1 200 OK
Date: Tue, 22 Aug 2017 06:34:16 GMT
Content-Type: application/json; charset=UTF-8
Content-Encoding: UTF-8
Content-Length: 82
Last-Modified: Mon, 21 Aug 2017 12:10:38 GMT
Server: Apache/1.3.3.7 (Unix) (Red-Hat/Linux)
ETag: "3f80f-1b6-3e1cb03b"
Connection: close

{"id":"1234123412341324","title":"kata","content":"get 100 points on hacker rank"}