Application layer

HTTP –
HyperText
Transfer
Protocol
○ Many applications are running concurrently over
the Web, such as web browsing/surfing, e-mail,
file transfer, audio & video streaming, and so
on. In order for proper communication to take
place between the client and the server, these
applications must agree on a specific application-
level protocol such as HTTP, FTP, SMTP, POP, and
etc.
3

HyperText Transfer Protocol (HTTP)
HTTP is an asymmetric request-response client-server protocol as
illustrated. An HTTP client sends a request message to an HTTP
server. The server, in turn, returns a response message. In other
words, HTTP is a pull protocol, the client pulls information from the
server (instead of server pushes information down to the client).
4

“ 5
Whenever you issues a URL from your browser to get a web resource using HTTP,
e.g. http://www.nowhere123.com/index.html, the browser turns the URL into a request message and sends it to the
HTTP server. The HTTP server interprets the request message, and returns you an appropriate response message
which is either the resource you requested or an error message. This process is illustrated below:

HTTP – Request Message
As mentioned, whenever you enter a URL in the address box of the browser, the browser
translates the URL into a request message according to the specified protocol; and sends
the request message to the server. For example, the browser translated the URL
http://www.test101.com/doc/index.html into the following request message:
6

“Request Message Format-
bytecode
for carriage
return
bytes that
are placed as
end-of-line
markers.

HTTP – Response Message
When this request message reaches the server, the server can take either one of these
actions:
1)The server interprets the request received, maps the request into a file under the
server's document directory, and returns the file requested to the client.
2)The server interprets the request received, maps the request into a program kept in the
server, executes the program, and returns the output of the program to the client.
3)The request cannot be satisfied, the server returns an error message.

“○ HTTP Request Methods
HTTP protocol defines a set of request methods. The methods are:
GET: A client can use the GET request to get a web resource from the server.
HEAD: A client can use the HEAD request to get the header that a GET request would have
obtained. Since the header contains the last-modified date of the data, this can be used to
check against the local cache copy.
POST: Used to post data up to the web server.
PUT: Ask the server to store the data.
DELETE: Ask the server to delete the data.
TRACE: Ask the server to return a diagnostic trace of the actions it takes.
OPTIONS: Ask the server to return the list of request methods it supports.
CONNECT: Used to tell a proxy to make a connection to another host and simply reply the
content, without attempting to parse or cache it. This is often used to make SSL connection
through the proxy.

GET - Request method
GET is the most common HTTP request method. A client can use the GET request method to request (or
"get") for a piece of resource from an HTTP server. A GET request message takes the following syntax:
GET request-URI HTTP-version
(optional request headers)
(blank line)
(optional request body)
 The keyword GET is case sensitive and must be in uppercase.
 request-URI: specifies the path of resource requested, which must begin from the root
"/" of the document base directory.
 HTTP-version: Either HTTP/1.0 or HTTP/1.1. This client negotiates the protocol to be used
for the current session. For example, the client may request to use HTTP/1.1. If the server
does not support HTTP/1.1, it may inform the client in the response to use HTTP/1.0.
 The client uses the optional request headers (such as Accept, Accept-Language, and etc)
to negotiate with the server and ask the server to deliver the preferred contents (e.g., in
the language that the client preferred).
 GET request message has an optional request body which contains the query string

“○ For Deep learning of HTTP protocol-
Testing HTTP Requests
Network Program
Response Status Code
Misspelt Request Method
404 File Not Found
Wrong HTTP Version Number
Wrong Request-URI
Keep-Alive Connection
https://www.ntu.edu.sg/home/ehchua/programming/webprog
ramming/HTTP_Basics.html

Simple Mail Transfer Protocol (SMTP)
SMTP is a push protocol and is used to send the mail whereas POP (post office protocol)
or IMAP (internet message access protocol) are used to retrieve those mails at the
receiver’s side.

Connection Establishment-
MTA Client MTA Server
220 Service Ready
Helo deanza.edu
250 OK

“○ Post Office Protocol-
Post Office Protocol (POP) is a type of computer networking and Internet standard protocol that extracts
and retrieves email from a remote mail server for access by the host machine.
Techopedia explains Post Office Protocol (POP)
Post Office Protocol is the primary protocol behind email communication. POP works through a
supporting email software client that integrates POP for connecting to the remote email server and
downloading email messages to the recipient’s computer machine.
POP uses the TCP/IP protocol stack for network connection and works with Simple Mail Transfer Protocol
(SMTP) for end-to-end email communication, where POP pulls messages and SMTP pushes them to the
server. As of 2012, Post Office Protocol is in its third version known as POP 3 and is commonly used in
most email client/server communication architecture.

“○ Disadvantages of POP3
1)Opening attachments is a quick and painless process, unless the attachment has a virus
payload in it.
If you have JavaScript enabled in your email reader, you might be target of rogue JavaScript
embedded in an email.
2)All messages are stored on your hard drive eating up what is sometimes very valuable
space.
Since all attachments are stored (downloaded) on your PC there is a potential danger of
virus attack if they are not properly scanned by virus scanners. Then sirus scans can only
address 60% of attacks effectively leaving your PC to a great danger.
3)Email folders can become corrupted and sometimes lost forever. Recovering is often a
painful exercise.
4)All messages are stored on your system, and privacy disappears when someone sits down
at your machine. Even if your email reader is password protected, it is often possible for
someone who knows what they are doing to read your email by using another application
to open your mail folders.

Thanks !
Any questions?
You can find detail info at
https://en.wikibooks.org/wiki/Communication_Networks/HTTP_Protocol
😉
https://technet.microsoft.com/en-us/library/cc737236(v=ws.10).aspx

Application layer

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