The Hypertext Transfer Protocol (HTTP) is an application layer protocol for distributed, collaborative, hypermedia information systems and is the foundation for data communication on the World Wide Web. HTTP defines methods like GET and POST to indicate the desired action on a resource and uses status codes in responses. It also uses headers in requests and responses to define parameters of the transaction. HTTP typically uses TCP port 80 to allow a client to open a connection and send requests to a server, which responds with messages that may contain requested resources or error information.

1. Hypertext Transport Protocol
Keerthana Krishnan
C6A, Rl : 35

2. What is a protocol?
•In diplomatic circles, a protocol is the set of rules
governing a conversation between people
•We have seen that the client and server carry on a
machine-to-machine conversation
•A network protocol is the set of rules governing a
conversation between a client and a server
•There are many protocols, HTTP is just one

3. Development
The standards development of HTTP was coordinated by
the Internet Engineering Task Force (IETF) and the World
Wide Web Consortium (W3C), culminating in the
publication of a series of Requests for Comments
(RFCs)
RFC 2616 (June 1999) defined HTTP/1.1, the version of
HTTP most commonly used today.
In June 2014, RFC 2616 was retired and HTTP/1.1 was
redefined by new RFCs 7230-7235.
HTTP/2 is currently in draft form.

4. The TCP/IP protocol layers
Application
Transport
Internet
Data link
Physical
Get useful work done – retrieve Web pages, copy
files, send and receive email, etc.
Make client-server connections and optionally
control transmission speed, check for errors, etc.
Route packets between networks
Route data packets within the local area network
Specify what medium connects two nodes, how
binary ones and zeros are differentiated, etc,
The application program is king – it gets work done using the lower level
layers for communication between the client and server.

5. The application layer is boss – the top layer
•Your boss says: Send this package to Miami -- I don't care if you use Federal
Express, UPS, or any other means. Also, let me know when it arrives or if it
cannot be delivered for some reason.
•The application program says: Send this request to the server -- I don't care
how you do it or whether it goes over phone lines, radio, or anything else about
the details. Just send the message, and let me know when it arrives or if it
cannot be delivered for some reason.
Layer Function
Application Do useful work like Web browsing, email, and
file transfer
Lower layers Handle communication between the client and
server
There are five TCP/IP layers, the application layer and four lower layers.

6. HTTP is an application layer protocol
•The Web client and the Web server are application programs
•Application layer programs do useful work like retrieving Web
pages, sending and receiving email or transferring files
•Lower layers take care of the communication details
•The client and server send messages and data without knowing
anything about the communication network

7. Computer Port
l A port is an application-specific or process-specific
software construct serving as a communications endpoint
in a computer's host operating system.
lThe purpose of ports is to uniquely identify different
applications or processes running on a single computer
and thereby enable them to share a single physical
connection to a packet-switched network like the Internet.
lA port is associated with an IP address of the host, as
well as the type of protocol used for communication.
l A port is identified for each address and protocol by a
16-bit number, commonly known as the port number.

8. HTTP : Hypertext Transfer Protocol
The Hypertext Transfer Protocol (HTTP) is an
application protocol for distributed, collaborative,
hypermedia information systems.
HTTP is the foundation of data communication for
the World Wide Web.
Hypertext is structured text that uses logical links
(hyperlinks) between nodes containing text. HTTP
is the protocol to exchange or transfer hypertext

9. •I would like to open a
connection
•GET <file location>
•Display response
•Close connection
•OK
•Send page or error message
•OK
Client Server
HTTP is the set of rules governing the format and content of the
conversation between a Web client and server
An HTTP conversation

10. Persistant Connection
lHTTP persistent connection, also called HTTP
keep-alive, or HTTP connection reuse, is the idea
of using a single TCP connection to send and
receive multiple HTTP requests/responses, as
opposed to opening a new connection for every
single request/response pair.
lThe newer SPDY protocol uses the same idea
and takes it further to allow multiple concurrent
requests/responses to be multiplexed over a
single connection.
lIt is a feature for HTTP 1.1, allowing to pipeline
requests

11. An HTTP Session
l An HTTP client initiates a request by establishing
a Transmission Control Protocol (TCP) connection
to a particular port on a server (typically port 80,
occasionally port 8080
l An HTTP server listening on that port waits for a
client's request message.
lUpon receiving the request, the server sends
back a status line, such as "HTTP/1.1 200 OK",
and a message of its own.
lThe body of this message is typically the
requested resource, although an error message or
other information may also be returned

12. HTTP Methods
HTTP defines methods to indicate the desired action to
be performed on the identified resource.

13. Main Methods
GET
lretrieve a URL from the server
lsimple page request
lrun a CGI program
lrun a CGI with arguments attached to the URL
POST
lpreferred method for forms processing
lrun a CGI program
lparameterized data in system
lmore secure and private
PUT
lUsed to transfer a file from the client to the server

14. HTTP Status codes Part I

15. HTTP Status codes (continued)

16. HTTP Headers
lHTTP header fields are components of the header
section of request and response messages in the
Hypertext Transfer Protocol (HTTP).
lThey define the operating parameters of an HTTP
transaction.
lThe header fields are transmitted after the request or
response line, which is the first line of a message.
lHeader fields are colon-separated name-value pairs in
clear-text string format, terminated by a carriage return
(CR) and line feed (LF) character sequence.
lThe end of the header section is indicated by an empty
field, resulting in the transmission of two consecutive CR-
LF pairs.

17. Table 27.3 General headers

18. Table 27.4 Request headers

19. Table 27.5 Response headers

20. Table 27.6 Entity headers

21. Figure 27.15 Header format

22. This example retrieves a document. We use the GET
method to retrieve an image with the path
/usr/bin/image1. The request line shows the method
(GET), the URL, and the HTTP version (1.1). The header
has two lines that show that the client can accept
images in the GIF or JPEG format. The request does not
have a body. The response message contains the
status line and four lines of header. The header lines
define the date, server, MIME version, and length of the
document. The body of the document follows the
header (see Figure 27.16).
Example 27.1

23. Figure 27.16 Example 27.1

24. In this example, the client wants to send data to the
server. We use the POST method. The request line
shows the method (POST), URL, and HTTP version
(1.1). There are four lines of headers. The request body
contains the input information. The response message
contains the status line and four lines of headers. The
created document, which is a CGI document, is
included as the body (see Figure 27.17).
Example 27.2

25. Figure 27.17 Example 27.2

26. Telnet
lTelnet is a network protocol used on the Internet or local
area networks to provide a bidirectional interactive text-
oriented communication facility using a virtual terminal
connection.
lUser data is interspersed in-band with Telnet control
information in an 8-bit byte oriented data connection
over the Transmission Control Protocol (TCP).

27. lTelnet provided access to a command-line
interface (usually, of an operating system) on a
remote host.
lMost network equipment and operating systems
with a TCP/IP stack support a Telnet service for
remote configuration (including systems based on
Windows NT).
lHowever, because of serious security issues when
using Telnet over an open network such as the
Internet, its use for this purpose has waned
significantly in favor of SSH.

28. HTTP uses ASCII characters. A client can directly
connect to a server using TELNET, which logs into port
80 (see next slide). The next three lines show that the
connection is successful. We then type three lines. The
first shows the request line (GET method), the second
is the header (defining the host), the third is a blank,
terminating the request. The server response is seven
lines starting with the status line. The blank line at the
end terminates the server response. The file of 14,230
lines is received after the blank line (not shown here).
The last line is the output by the client.
Example 27.3

29. Example 27.3 (continued)

30. More about Telnet
lConnect to other servers through telnet to view
their animated ASCII art, games, etc they offer
the public.
lMore details at :
http://mewbies.com/acute_terminal_fun_telnet_p
ublic_servers_watch_star_wars_play_games_etc.h
tm

31. References
PPT on HTTP available at :
bpastudio.csudh.edu/fac/lpress/presentations/http/
http.ppt
Chapter 27, WWW and HTTP from the Fourth
Edition of "Data Communications and
Networking", by A. Behrouz Forouzan, Tata
McGraw-Hill Education, 2006