Overlapping Functions of OSI Layers

We all know that there are 7 layers in OSI Model. All 7 layers of OSI Model forms the basis of modern communication among LANs. The basic functionality of all layers is not very difficult to understand. But due to overlapping of some functions among layers, many students become confused. For example, error handling is done both at DLL and Transport Layer.

Lets discuss functions and services of OSI layers to get a clear idea about what each layer does.

First, Transport Layer vs Network Layer->

• Transport Layer does delivery of ENTIRE message but Network Layer delivers INDIVIDUAL packets.
• Transport Layer treats entire message as a single unit and ensures the whole message arrives intact and the destination. But Network layer sees no relation b/w two data packets and treats them independent entities.
• Transport Layer does delivery in a Process-to-Process manner. (process refers to an application. e.g. email message must be delivered via source email process to destination email process). Network layer is responsible to take the message to the final system only. It is Transport Layer that takes the message to the right process. An analogy for this can be: You go to meet one of your friends at his/her office. You reached at the right office address (Network Layer). From the reception, you are directed towards the deptt. where your friend works for (Transport Layer).
• The major protocols of Transport Layer: TCP, UDP and SCTP. The major protocols of Network Layer: ARP, RARP, IGP, BGP IPv4, and IPv6.
• There are different words for same term/unit information on different layers. For example, in OSI model, we use a term-PDU (Protocol Data Unit) which is referenced differently at layers:
  
  Layer Name         PDU

          Data Link Layer    Frame
          Network Layer      Packet
          Transport Layer    Segment
         Application Layer  Message

• Sliding Window Protocol works both at DLL and TL. The difference is that at DLL, individual bits are processed but at TL, processing is done at BYTE level.
• There are many devices used in LANs which work at different layers. Many students find it hard to understand which device used at which layer. Let's understand the purpose of using a device which will help you to remember their use at right layer
  Repeater: It is installed at specified distance and amplifies the "signal". As we know signals are handled at layer 1, hence, repeater works at layer 1.
  Hub: 2 types-passive hub and active hub. Passive hub simply passes the original signal and works at layer 1. Active hub is nothing but a switch; it works at layer 2. It is also called switching hub.
  Modem: A modem deals with transmission at signal level and works at layer 1.
  Bridge and Switch: Both handle MAC addresses. Since MAC addresses are handled at layer 2. So these devices work at Layer 2.
  Router: I believe many among you know it well that routers deal with IP address and work at Layer 3.
  Gateway: They are very similar to bridge but deal with inter-protocol conversion. Therefore, gateways work at all layers, i.e. 1-7, to handle conversion at all compatible layers.

I hope this post will help to clarify some doubts about OSI model. Please share your views to let me know of the topics which should be discussed here.

Important Links for UGC NET Subjects

I have been in the habit of searching web links on topics I couldn't find much information in books or were too theoretical. In this process, I made a list of URLs which contain useful stuff in a good language. I am posting these URLs for all my friends who are aiming to crack the NUT errrrrrr NET :-)
I am not boasting of providing best of links but just want to say I got good help from these URLs.

All the best for December' 15 Exam!

NETWORKING:
http://www.perihel.at/2/basics/  (Very good site to learn at deep level)
http://ckp.made-it.com/ (Good link to learn OSI layer 1 & 2)
http://flylib.com/books/en/2.567.1.44/1/
http://www.differencebetween.com/category/technology/it/

Database:
http://djitz.com/guides/how-to-check-if-a-relation-is-in-bcnf-3nf-or-both/
http://www.youtube.com/watch?v=IiEAfRoHIH8&feature=endscreen&NR=1
http://www.differencebetween.com/difference-between-deferred-update-and-vs-immediate-update/
www.eecs.umich.edu/~teorey/lec.notes.pdf

OS:
http://www.personal.kent.edu/~rmuhamma/OpSystems/os.html
http://www.computerhope.com/unix/utr.htm
http://www.indiabix.com/computer-science/operating-systems-concepts/015001
http://www.chibios.org/dokuwiki/doku.php?id=chibios:articles:semaphores_mutexes
http://www1bpt.bridgeport.edu/sed/projects/cs503/Spring_2001/kode/os/deadlock.htm#
http://all-operatingsystems.blogspot.in/2009/10/multiple-choice-questions-on-operating_8948.html
http://www-ist.massey.ac.nz/csnotes/355/lectures/monitors.pdf
http://siber.cankaya.edu.tr/ozdogan/OperatingSystems/ceng328/node197.html

Automata Notes:
http://cs.union.edu/~striegnk/learn-prolog-now/html/node55.html#sec.l7.cfg
http://www.cs.uky.edu/~lewis/texts/theory/languages/cfl-prop.pdf
http://www.cs.uky.edu/~lewis/texts/theory/automata/lb-auto.pdf

http://courses.cs.vt.edu/~cs4114/
http://www.cs.cmu.edu/~eugene/teach/auto01/notes/
http://www.tenouk.com/visualcplusmfc/visualcplusmfc16.html  (Great site for learning Windows Programming)
http://www.youtube.com/watch?v=IhyEGNn-7Uo&feature=related  (Video on Turing Machine)
http://www.youtube.com/watch?v=id7k6gUkZ8Y&feature=relmfu  (Video on NP-Complete Problem)
http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/compiler-desing/ui/Course_home-4.htm
http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-%20Guwahati/afl/index.htm
http://www.bukisa.com/articles/393888_basics-of-message-handling-in-windows#ixzz1eRNPTl6O
http://www.di.uevora.pt/~lmr/syscalls.html
http://www.psexam.com/Database-Management-System-and-Design/dbms-and-design-mcq-set-3.html

Computer Organization:
http://blogs-static.maktoob.com/wp-content/blogs.dir//67186/files//2010/02/digital-electronics-introduction.pdf

Software Engg.
http://www.smartdraw.com/uml-diagram/
http://users.csc.calpoly.edu/~jdalbey/SWE/Design/STDexamples.html
http://www2.cs.unibw.de/tools/DiaGenDiaMeta/img/nsd_t.png   (It has Nassi-Shneiderman Diagrams)

Computer Graphics:
http://marintellect.com/help_topics/graphics/index.html
http://www.slideshare.net/askme/chapter-10-280485?src=related_normal&rel=2953874

Computer Organization & Microprocessor:
http://www.allaboutcircuits.com/
http://ptucse.loremate.com/sp/node/6    (Imp Q & A)
http://www.ustudy.in/node/3036 ( Learning gates & circuits)

AI/ADA:
http://www.personal.kent.edu/~rmuhamma/Algorithms/algorithm.html

C/C++/Data Structure:
https://www.securecoding.cert.org/confluence/display/seccode/STR36-C.+Do+not+specify+the+bound+of+a+character+array+initialized+with+a+string+literalhttp://www.cs.umd.edu/class/fall2006/cmsc351/notes/
http://www.youtube.com/watch?v=eWeqqVpgNPg   (Video on Trees traversals)
http://faculty.cs.niu.edu/~freedman/340/340notes/340hash.htm  (Hashing Technique)
http://www.geeksforgeeks.org/g-fact-37/ (Virtual destructor)
http://www.geeksforgeeks.org/pure-virtual-destructor-c/

Probability Theory, Permutation Combination:
http://iim-cat-questions-answers.2iim.com/quant_math_sample_problems/permutation_combination_probability/
http://homes.ieu.edu.tr/uufuktepe/m144/logic/1.htm (Propositions)
http://mathworld.wolfram.com

Option-V Unix/Windows:
http://pravin.insanitybegins.com/win32/lesson2  (6 functions for window creation)

Online Objective Tests:

http://www.careerride.com/Online-practice-test.aspx

UGC NET/SET Exam: Some Ideas for success

Friends,

I want to share some of my ideas on NET/SET preparation. I have done MCA and qualified UGC NET exam twice and SET(H.P.)-2013 exam.

The very first time I appeared in NET exam simply to know the pattern of the exam and I didn't prepare much. At that time, exam had subjective pattern. I was able to clear Paper I and Paper II but I could not clear paper III. Still, I was very happy that I cleared first 2 papers. Then, I appeared again, then again. In total 5 of my efforts, I was not able to clear Paper III. In my 4th and 5th attempt, pattern of the paper had become totally objective. Still, I was not able to clear paper III.

I had started losing my faith in myself and I decided to opt out of Dec' 13 NET Exam and preferred to sit in another exam on the same date. I believed that the same story will get repeated. But one good thing I did was that before Dec' 13 exam, Himachal Pradesh Public Service commission had announced SET/SLET exam for the state universities/colleges. I applied for the exam and had appeared in the exam on November 17, 2013. The result of the exam was announced on  March 14, 2014 and I was declared qualified! WOW!!

The success in SET gave me much needed high and faith in myself. The point here is, we throw seeds on the land without bothering about which seed will bore fruits and when. Our efforts in one direction may show result somewhere else. The time I devoted in preparation of NET helped me to crack SET exam. Further, the success in SET exam enthused me and started preparing in a more determined manner. SET is limited in scope and a bit easier than NET. But success in this exam rekindled my faith in myself that NET too is not far and can be cracked with some more planning.

And the result was that I was able to qualify December 2014 exam. For the first time, I Cracked the "hard NUT" called UGC NET.

If one is failing 3 or more times in NET, it is quite possible that one will get disappointed and will lose one's confidence. To uplift one's spirits, focusing on some other exam/objective (for time being) of your chosen career will help you to forget the stress you might have been facing due to rejection in NET. You may choose to focus on some other competition exams for computer teacher/lecturers like CTET, SET, KV schools exams etc. whichever is announced in the time frame. Believe me, these exams can be cracked more easily than NET.

Your sole objective may be NET only, but if you succeed in other exams I have mentioned, it will lift your morale and will give birth to an enthusiastic mindset which is very crucial for cracking exams like NET.

PLEASE post your ideas (If whatever I have mentioned is right or wrong)..

Some useful stuff on UNIX and Network Security

1) Secure Shell (SSH) is a UNIX-based command interface and protocol for securely getting access to a remote computer. It is a suite of three utilities - slogin, ssh, and scp - that are secure versions of the earlier UNIX utilities, rlogin, rsh, and rcp. SSH uses RSA public key cryptography for both connection and authentication. Its default encryption algorithm is IDEA (International Data Encryption Algorithm).

2) IDEA encryption algorithm was developed at ETH in Zurich, Switzerland. It uses a block cipher with a 128-bit key, and is considered to be very secure and among the best publicly known algorithms.

3) A spam cocktail (or anti-spam cocktail) is the use of several different technologies in combination to successfully identify and minimize spam. It puts each e-mail message through a series of tests that provides a numeric score showing how likely the message is to be spam. Scores are computed and the message is assigned a probability rating.

4) Non-repudiation is the assurance that someone cannot deny something. Typically, non-repudiation refers to the ability to ensure that a party to a contract or a communication cannot deny the authenticity of their signature on a document or the sending of a message that they originated.

5) Echelon is an officially unacknowledged U.S.-led global spy network that operates an automated system for the interception and relay of electronic communications. Monitored transmissions are said to include up to 3 billion communications daily, including all the telephone calls, e-mail messages, faxes, satellite transmissions, and Internet downloads of both public and private organizations and citizens worldwide. Led by the U.S. National Security Agency (NSA).

6) On UNIX systems, snort is an open source network intrusion detection system (NIDS). Snort is a packet sniffer that monitors network traffic in real time, scrutinizing each packet closely to detect a dangerous payload or suspicious anomalies. Through protocol analysis and content searching and matching, Snort detects attack methods, including denial of service, buffer overflow, CGI attacks, stealth port scans, and SMB probes.

7) Server Message Block (SMB) is a network protocol mostly used by Windows-based computers that allows systems within the same network to share files. It allows computers connected to the same network or domain to access files from other local computers as easily as if they were on the computer's local hard drive.

8 ) Greynet (or graynet)) is a term for the use of unauthorized applications on a corporate network. A greynet application is a network-based program that corporate network users download and install without permission from their company's IT department. They introduce security risks, including client code vulnerabilities and new avenues for attack, and can lead to data loss and property or identity theft.

9) Munging is the deliberate alteration of an e-mail address on a Web page to hide the address from spambot programs that examine the Internet for e-mail addresses. Such addresses are easily recognized because they contain the @ symbol. Address munges should allow a real person reading the content (as opposed to a program scanning it) to easily deduce the true e-mail address.

Here is an example of a munged address: editor at xyz dot com.

10) Traceroute is a utility that records the route (the specific gateway computers at each hop) through the Internet between your computer and a specified destination computer. It also calculates and displays the amount of time each hop took. This utility initiates the sending of a packet using Internet Control Message Protocol (ICMP), including in the packet a time limit value (known as the "time to live" (TTL) that is designed to be exceeded by the first router that receives it, which will return a Time Exceeded message. This enables traceroute to determine the time required for the hop to the first router. Increasing the time limit value, it resends the packet so that it will reach the second router in the path to the destination, which returns another Time Exceeded message, and so forth. Finally, traceroute determines when the packet has reached the destination.

11) Intrusion Detection Systems (IDS) is a passive way to monitor a network and its computers. The program keep a watch on all inbound and outbound traffic and looks for attacks. It sends an alert when it finds something unusual. It can only act on known intrusion signatures. Intrusion Detection Systems (IPS) tools perform active, in-line monitoring and can prevent attacks by known and unknown sources.

12) An IDS is not a substitute for a firewall. A firewall can stop intrusions like an IDS but is not able to monitor activity from inside the network.

13) Honey Pot Systems are decoy servers or systems setup to gather information regarding an attacker or intruder into your system. It can be setup inside, outside or in the DMZ of a firewall design or even in all of the location.

14) Port Scanning identifies open doors to a computer. There is no way to stop someone from port scanning your computer while you are on the Internet because accessing an Internet server opens a port, which opens a door to your computer. Types of port scans:
vanilla: the scanner attempts to connect to all 65,535 ports
strobe: a more focused scan looking only for known services to exploit
fragmented packets: the scanner sends packet fragments that get through simple packet filters in a firewall
UDP: the scanner looks for open UDP ports
sweep: the scanner connects to the same port on more than one machine
FTP bounce: the scanner goes through an FTP server in order to disguise the source of the scan.
stealth scan: the scanner blocks the scanned computer from recording the port scan activities.

15) Types of firewall techniques:
Packet filter: Looks at each packet entering or leaving the network and accepts or rejects it based on user-defined rules. Packet filtering is fairly effective and transparent to users, but it is difficult to configure. In addition, it is susceptible to IP spoofing.
Application gateway: Applies security mechanisms to specific applications, such as FTP and Telnet servers. This is very effective, but can impose a performance degradation.
Circuit-level gateway: Applies security mechanisms when a TCP or UDP connection is established. Once the connection has been made, packets can flow between the hosts without further checking.
Proxy server: Intercepts all messages entering and leaving the network. The proxy server effectively hides the true network addresses.

Answer Keys for June 2013 - Computer Science

After researching different resources, I am able to compile Answer keys for June 2013 paper of Computer Science. First, I am posting keys for Paper III as this paper is tougher to clear than othet two papers. I am sure 90% of the answers I have given are correct.
1. A
2. D
3. C
4. B
5. B
6. B
7. C
8. D
9. A or B
10. D
11. C
12. --
13. D
14. D
15. --
16. A
17. A
18. B
19. C
20. D
21. D
22. B
23. C
24. A
25. --
26. A
27. B
28. C
29. D
30. A
31. B
32. A
33. C
34. D
35. D
36. --
37. --
38. B
39. C
40. B
41. C
42. D
43. --
44. B
45. A
46. C
47. B
48. A
49. B
50. A
51. C
52. A
53. A
54. D
55. A
56. --
57. A
58. B
59. --
60. D
61. B
62. A
63. B
64. C
65. B
66. D
67. --
68. D
69. C
70. C
71. B
72. C
73. C
74. --
75. --