Computer Networks and Internet: Application Layer

1. Describe how Web caching can reduce the delay in receiving a requested object. Will Web caching reduce the delay for all objects requested by a user or for only some of the objects? Why?

Web caching

Web cache or otherwise called proxy server is a network entity used for satisfying the client request without involving origin web server. Hence we can reduce the delay in receiving a requested object across the internet, because it doesn't involve the origin server again and again.

Describe how Web caching can reduce the delay in receiving a requested object.

Web caching reduces the response time for client request. If there is a high speed connection between the client and the cache, and if the cache has the requested object, then the cache will be able to deliver the object rapidly to the client.
Web caching reduces the traffic on institution's access link to the internet. By reducing traffic, the institution does not have to up grade bandwidth as quickly; there by reducing costs.web cache reduces web traffic in the internet as a whole, there by improving performance for all applications.

Web cache has its own disk storage and keeps copies of recently requested objects in this storage.
When the user request for an object, then the user's HTTP requests are first directed to the web cache. If the requested object present in the web cache then sends the requested object to the client. If the requested object not in the web cache, then request message send to the origin server from the cache. Responses from the server are sending to the client and also a copy that requested object will store in the cache. If the user again request for this object, then the cache directly send the requested object to client without interacting with the server.

The steps include:

a. The browser establishes a TCP connection to the web cache and sends an HTTP request for the object to the web cache.

b. The web cache checks to see if it has a copy of the object stored locally. If it does, the web cache forwards the object within an HTTP response message to the client browser.

c. If the web cache doesn't have the object, the web cache opens a TCP connection to the origin server. The web cache then sends an HTTP request for the object in to the TCP connection. After receiving this request, the origin server sends the object within an HTTP response to the web cache.

d. When the web cache receives the object, it stores a copy in its local storage and forwards a copy, within an HTTP response message, to the client browser.

2. Telnet into a Web server and send a multiline request message. Include in the request message the If-modified-since: header line to force a response message with the 304 Not Modified status code.

A web server, responding to a HTTP GET request sends the required content to the client. Normally we don't see what is sent between the web client and the server. However we can easily see what is sent by the server by telneting to that web server.

“telnet” command:

It is a command used in windows (also in linux) to connect to the web server. The server responds to the HTTP GET requests. It reply to the client with the requested information.

Command to open the connection with web server:

telnet <domain name of web server> <port number>

example: telnet 23

After establishing the connection with the web server, it is possible to request a specified page from the web server.

Command to request the page web server:

GET <page name> HTTP/1.0

If the page is available then the server sends the page details. It include the “If-modified-since” message with the 304 Not Modified status code in the response.

3. Why is it said that FTP sends control information "out-of-band"?

FTP uses two parallel TCP connections, one connection for sending control information (such as a request to transfer a file) and another connection for actually transferring the file. Because the control information is not sent over the same connection that the file is sent over, FTP sends control information out of band.

4. Suppose Alice, with a Web-based e-mail account (such as Hotmail or gmail) sends a message to Bob, who accesses his mail from his mail server using POP3. Discuss how the message gets from Alice's host to Bob's host. Be sure to list the series of application-layer protocols that are used to move the message between the two hosts.

Here is the example header:

From: Media Temple user (
Subject: article: How to Trace a Email
Date: January 25, 2011 3:30:58 PM PDT
Return-Path: <>
Delivery-Date: Tue, 25 Jan 2011 15:31:01 -0700
Received: from ([]:54907) by with esmtp (Exim 4.63) (envelope-from <>) id 1KDoNH-0000f0-RL for; Tue, 25 Jan 2011 15:31:01 -0700
Received: by with SMTP id y22so795146pof.4 for <>; Tue, 25 Jan 2011 15:30:58 -0700 (PDT)
Received: by with SMTP id t17mr3929916rvm.251.1214951458741; Tue, 25 Jan 2011 15:30:58 -0700 (PDT)
Received: by with HTTP; Tue, 25 Jan 2011 15:30:58 -0700 (PDT)
Dkim-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=gamma; h=domainkey-signature:received:received:message-id:date:from:to :subject:mime-version:content-type; bh=+JqkmVt+sHDFIGX5jKp3oP18LQf10VQjAmZAKl1lspY=; b=F87jySDZnMayyitVxLdHcQNL073DytKRyrRh84GNsI24IRNakn0oOfrC2luliNvdea LGTk3adIrzt+N96GyMseWz8T9xE6O/sAI16db48q4Iqkd7uOiDvFsvS3CUQlNhybNw8m CH/o8eELTN0zbSbn5Trp0dkRYXhMX8FTAwrH0=
Domainkey-Signature: a=rsa-sha1; c=nofws;; s=gamma; h=message-id:date:from:to:subject:mime-version:content-type; b=wkbBj0M8NCUlboI6idKooejg0sL2ms7fDPe1tHUkR9Ht0qr5lAJX4q9PMVJeyjWalH 36n4qGLtC2euBJY070bVra8IBB9FeDEW9C35BC1vuPT5XyucCm0hulbE86+uiUTXCkaB 6ykquzQGCer7xPAcMJqVfXDkHo3H61HM9oCQM=
Message-Id: <>
Mime-Version: 1.0
Content-Type: multipart/alternative; boundary="----=_Part_3927_12044027.1214951458678"
X-Spam-Status: score=3.7 tests=DNS_FROM_RFC_POST, HTML_00_10, HTML_MESSAGE, HTML_SHORT_LENGTH version=3.1.7
X-Spam-Level: ***
Message Body: This is a Knowledge Base article that provides information on how to find email headers and use the data to trace an email.

5. From a user's perspective, what is the difference between the download-and-delete mode and the download-and-keep mode in POP3?

yes it is possible,
Mx record allows a company to have the same aliased name for its mail server and for one of its other servers (such as its web server).

6. Look over your received emails and examine the header of a message sent from a user with an .edu email address. Is it possible to determine from the header the IP address of the host from which the message was sent? Do the same for a message sent from a gmail account.

This can potentially be done through some sort of social engineering, if you can send an email back to the original sender and get them to click on a link in the email.
You need to set up a website which logs the visitor's IP address, then somehow make the person who sent you that email visit your site. This probably involves sending them an email and hoping they read the email and click a link in it.
A related approach is to send them a HTML email that contains an inline image hosted on your website. However, most email clients do not load linked images by default, precisely to protect the privacy of their users. Therefore, in most cases this will not work (unless the person just happens to have overridden the defaults in their email client)

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