What is HTTP

Client-server/Request-Response protocol. Default port 80, other can be used. Uses TCP, does not use UDP. Google’s QUIC uses HTTP over UDP
HTTP Clients: Web-browser, Any process can create socket and get information from server.

Features of HTTP

1. CONNECTIONLESS: HTTP-client(browser) initiates an HTTP request and after a request is made, the client disconnects from the server and waits for a response. server processes the request and re-establishes the connection with the client to send a response back.
2. MEDIA INDEPENDENT: Any type of data can be sent by HTTP as long as both the client and the server know how to handle the data content.
3. STATELESS: No session information is stored either by client or server.
4. Persistant & Parallel Connection: 2 access requests can be sent back-2-back without waiting for response to arrive. 

      Client                         Server
      <-----TCP-3-way-handshake---->
      --------GET Page-1----------->
      --------GET Page-2----------->
      <-------Page-1---------------
      <-------Page-2---------------

Advatanges, Disadvatanges/pitfalls of HTTP

Advatanges

1. Flexible. Whenever there are additional capabilities needed by an application, HTTP has the capability to extend. These can include Flash players and Acrobat reader.
2. Less connections. With HTTP2 and HTTP3 multiple requests can be sent in 1 packet.
3. Network BW: Less dues to less connections as HTTP2, HTTP3 sends multiple requests in 1 connection.
4. Error reporting: Reports errors without closing the TCP connection.

DisAdvatanges

1. Can only be used for point to point connection.
2. Does not have push capabilities as in [Long Pooling or server sent Events](/Networking/OSI-Layers/Layer5/WebServer_to_WebClient_Connection_Methods).
3. HTTPv2 does not offer reliable exchange (without retry logic).

HTTP Methods

Method / Operations Methods for requesting web page or content.
  Safe methods? Those which donot change state of server. Eg: GET, HEAD
  Read only methods? GET, OPTIONS, HEAD
  Idempotent methods? A operation that will produce the same results if executed once or multiple times. Eg: GET, HEAD, PUT, DELETE, TRACE, OPTIONS
  CRUD Operations: Protocol should support minimum these operations/methods.
Method Description
Create (POST) Send data to server, Server will create entry. Eg: Form submittion.
Read (GET) Reading data(page,object etc) from web server 

curl http://example.com
OR 
curl -X GET http://example.com
Update (PUT) Update existing data on web server.
Delete (DELETE) Delete content from server
HEAD Requests only the headers of a resource, not the body
Server responds with headers (including metadata, content-type, length, status codes, etc.), but no content.
Why its helpful? check what a GET request would return, without actually downloading the resource (useful for checking file size, last-modified, existence, etc.) 

curl -X HEAD http://example.com