Introduction of HTTP
HTTP is a standard web transfer protocol for fetching resources like HTML documents, images, videos. HTTP stands for Hyper Text Transfer Protocol. HTTP is an application layer protocol. HTTP is standard. HTTP uses port no 80. HTTP is a client-server protocol. HTTP behaviour is request-response.
Figure: HTTP Request - Response
HTTP Transaction - Click here for video
An HTTP
transaction has the following components:
Connection: The client makes a
connection to the server.
Request: The client requests
information from the server.
Response: The server either provides
the information to the client or refuses to provide the information.
Close: Either or both parties terminate
the transaction.
Let us discuss all the components of HTTP Transaction one by one in this post.
Figure: Components of HTTP Transaction
Connection
The first part of an HTTP transaction is the connection. The translation of the name address into IP address is done by DNS. The client program connects to the server by a TCP connection at the address specified in the URL. Example: http://www.facebook.com
Request
There are
two different types of requests: Simple Request, Full Request
HTTP Simple Request - Click here for video
A simple request is just a single GET line naming the page desired, without the protocol version. The response is just the raw page with no headers, no MIME, and no encoding.
HTTP Full Request - Click here for video
Full request is indicated by the presence of the protocol version on the GET request line. The first word on the full request line is simply the name of the method to be executed on the web page. Full request contains the command, the page desired, and the protocol/version. The names are case sensitive, so, GET is a valid method but get is invalid.
Request Line
Below figure shows the format of request message.
Figure: Format of Request message
Request line contains following things: Request type, Resources, HTTP version. Example of request line is as below:
Request
type: It categorizes the request message into several methods for
HTTP version 1.1. i.e., GET, POST, DELETE, PUT, COPY, MOVE, LINK, UNLINK
Resource: URL is
used access information from internet. The URL defines four things 1. Method 2.
Host 3. Port 4. Path.
Example, Protocol://Host:Port/Path
⇒ http://192.168.1.1:80/home.html
HTTP
Version: HTTP Version 1.1 is used here
because of full request.
Realtime example of http request message
Response
Below figure shows the format of response message.
Figure: Format of Response Message
The
status line is the first line in the response message. It consists of three items: The HTTP version number,
showing the HTTP specification to which, the server has tried to make the
message comply. A status code, which is a three-digit number indicating the
result of the request.
Example of status line is as below,
HTTP
Version: HTTP Version 1.1 is used here
because of full request.
Status
Code & Phrase: There are some of the status codes meaning:
200 OK: Request succeeded.
301 Moved Permanently: Requested object moved.
400 Bad Request: Request message not understood by server.
404 Not Found: Requested document not found on this server.
505 HTTP Version Not Supported: Requested http version not
support.
Realtime example of http response message
Close
After the client and server have exchanged
information by sending requests and responses, both parties may end the
transaction by closing the connection. This is done by terminating the TCP
connection (usually to port 80) that was set up in the connection stage.
HTTP Header - Click Here for Video
Header can be one or more header lines. Each header line is made of a header name, a colon, a space and a header value. The header exchange additional information between the client and the server. Below figure shows the HTTP header structure.
Figure: HTTP Header Structure
A four different header are used: general header, response header (in response message), entity header, request header (in request message). Below figure shows the difference between response and request header.
Figure: Format of Request and Response message
General header: General header includes general information
about the message. Request and a response both contains general header.
Response header: Response header can be present only in a
response message. It specifies the server’s configuration and special
information about the request.
Request header: Request header can be present only in a
request message. It specifies the client’s configuration and the client preferred
document format.
Entity header: Entity header gives information about the
body of the document. It is mostly present in response messages, some request
message, such as POST and PUT methods, that contain a body also use this type
of header.
Realtime example of http header
Date: It indicates the time and date when the HTTP response was created and
sent by the server.
Server: It indicates the message was generated by an Apache Web server.
Last-Modified: It indicates the time and date when the
object was created or last modified.
Content-Length: It indicates the number of bytes in the
object being sent.
Content-Type: It indicates object in the entity body is
HTML text.
HTTP Connection - Click here for video
The HTTP CONNECT method starts
two-way communications with the requested resource. It can be used to
open a tunnel.
There are two types of HTTP Connection: Non-Persistent HTTP and Persistent HTTP. Let’s discuss HTTP response time and RTT (round trip time). Below figure shows the HTTP response time and RTT.
Figure: HTTP Connection
RTT (round-trip time): A time for a small
packet to travel from client to server and server to client.
HTTP response time: 1-RTT to initiate TCP connection, 1-RTT for
HTTP request-response and File transmission time.
HTTP Response Time = No. of RTT + file
transmit time
Non-persistent HTTP
Non-persistent connections are the default
mode for HTTP/1.0. A non-persistent connection is closed after the server sends
the requested object to the client. The connection is used exactly for one
request and one response. For downloading multiple objects, it required
multiple connections.
Non-Persistent HTTP = 2 RTT + 1 file-transmit time
Figure: Non-persistent HTTP Connection
Example: Transferring a
webpage from server to client, webpage consists of a base HTML file and 10 JPEG
images. Total 11 object are resided on server.
11 objects = 22 RTT + 11 file-transmit time
Persistent HTTP
HTTP 1.1 made persistent connections the
default mode. The server now keeps the TCP connection open for a certain period
of time after sending a response. This enables the client to make multiple
requests over the same TCP.
There are two types of Persistent HTTP
Connection: Persistent HTTP without pipelining and Persistent HTTP with
pipelining.
Persistent HTTP without pipelining
The client issues a new request only when the previous response has been received. The client experiences one RTT in order to request and receive each of the referenced objects. TCP connection is idle. i.e., does nothing while it waits for another request to arrive. This idle situation wastes server resources.
Figure: Persistent HTTP Connection without pipelining
In above figure, new object request sends after
receive previous object.
Persistent HTTP with Pipelining
Default mode of HTTP 1.1 uses persistent
connections with pipelining. Client issues a request as soon as it encounter a reference.
The HTTP client can make back-to-back requests for the referenced objects. It
can make a new request before the back-to-back requests; it sends the objects
back-to-back. Pipelined TCP connection remains idle for a smaller fraction of
time.
Figure: Persistent HTTP Connection with pipelining
In above figure, new object request sends receive previous object, but all the objects are received in a sequence because this method uses pipelining concepts.
Persistent HTTP Vs Non-Persistent HTTP - Click here for video
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