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MTech(CSE)

M.TECH 1ST YEAR 1ST SEMESTER TEXT BOOKS AS PER JNTU-H(R13) SYLLABUS 

 

ADVANCED DATA STRUCTURES (2 TEXT BOOKS PDF)

ADVANCED OPERATING SYSTEM (TEXT BOOK PDF)

COMPUTER SYSTEM DESIGN (2 TEXT BOOKS PDF)

DISTRIBUTED SYSTEMS (TEXT BOOK PDF)

SOFTWARE ARCHITECTURE AND DESIGN PATTERNS (2 TEXT BOOKS))

SOFTWARE PROCESS AND PROJECT MANAGEMENT (TEXT BOOK))

Notes
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	MTECH CALENDER
.		.
MFCS NOTES		WEB TECHNOLOGIES
.		.
	MTECH SYLLABUS
	ADS LAB
JWT LAB		validation program
	Servlet program

 Database Access Notes
Distributed Databases Notes-All 8 units

1.Introduction: syllabus, administration and organisation of the course, general introduction in distributed DBMS

2. DDBMS Architecture: definition of DDBMS architecture, ANSI/SPARC standard, global, local, external, and internal schemas, DDBMS architectures, components of DDBMS

3.Distributed Database Design: conceptual design (what can be distributed, design patterns), top-down, bottom-up patterns, technical design (fragmentation, allocation and replication of fragments, optimality, heuristics)

4.Semantic Integrity Control: view management, security control, integrity control

5.Distributed Query Processing: overview of query processing and query optimization, query decomposition and data localization

6.Query decomposition and data localization: normalization, analysis, elimination of redundancy, rewriting, reduction for HF, reduction for VF

7.Optimization of Distributed Queries: basic concepts, distributed cost model, database statistics

8.Optimization of Distributed Queries: ordering of joins and semijoins, query optimization algorithms, INGRES, System R, hill climbing

9.Transactions: introduction to transactions, definition and examples, properties, classification, processing issues, execution

10.Concurrency Control: definition, execution schedules, examples, locking based algorithms, timestamp ordering algorithms, deadlock management

11.Reliability: definitions, basic concepts, local recovery management, distributed reliability protocols

12.Reliability: distributed reliability protocols, 2PC protocol

Notes

Sub: SOFTWARE QUALITY AND ASSURANCE TESTING

Notes 1

Notes 2

Notes 3

NPTEL

Course Name	Type	Coordinators	Institute	Available
Artificial Intelligence(Prof.P.Dasgupta)	Video	Prof. P. Dasgupta	IIT Kharagpur	Course contents
CAD for VLSI Design II	Web	Prof. V. Kamakoti Prof. Shankar Balachandran	IIT Madras	Course contents
Computational Geometry	Video	Prof. Sandeep Sen	IIT Delhi	Course contents
Computer Architecture	Video	Prof. Anshul Kumar	IIT Delhi	Course contents
Computer Networks	Web	Prof. Hema A Murthy	IIT Madras	Course contents
Computer Organization	Video	Prof. S. Raman	IIT Madras	Course contents
Design and Analysis of Algorithms	Video	Prof. Sundar Viswanathan Prof. Ajit A Diwan Prof. Abhiram G Ranade	IIT Bombay	Course contents
Electronic Design Automation	Video	Prof. Indranil Sengupta	IIT Kharagpur	Course contents
Logic for CS	Video	Prof. S. Arun Kumar	IIT Delhi	Course contents
Principles of Communication	Web	Prof. Hema A Murthy	IIT Madras	Course contents
Programming and Data Structure	Video	Dr. P.P.Chakraborty	IIT Kharagpur	Course contents
Artificial Intelligence	Video	Prof. S. Sarkar Prof. Anupam Basu	IIT Kharagpur	Syllabus Course contents
Artificial Intelligence	Web	Prof. P. Mitra Prof. S. Sarkar	IIT Kharagpur	Syllabus Course contents
CAD for VLSI Design I	Web	Prof. V. Kamakoti Prof. Shankar Balachandran	IIT Madras	Syllabus Course contents
Compiler Design	Web	Prof. Sanjeev K Aggarwal	IIT Kanpur	Syllabus Course contents
Compiler Design	Video	Prof. Y.N. Srikanth	IISc Bangalore	Syllabus Course contents
Computational Number Theory & Cryptography	Web	Dr. Pinaki Mitra	IIT Guwahati	Syllabus Course contents
Computer Graphics	Web	Prof. Prem K Kalra	IIT Delhi	Syllabus Course contents
Computer Graphics	Video	Prof. Sukhendu Das	IIT Madras	Syllabus Course contents
Computer Networks	Web	Prof. Ajit Pal	IIT Kharagpur	Syllabus Course contents
Computer Networks	Video	Prof. Sujoy Ghosh	IIT Kharagpur	Syllabus Course contents
Computer Organisation and Architecture	Web	Prof. Bhaskaran Raman	IIT Bombay	Syllabus Course contents
Computer Organization and Architecture	Web	Prof. Jatindra Kumar Deka	IIT Guwahati	Syllabus Course contents
Cryptography and Network Security	Video	Dr. Debdeep Mukhopadhyay	IIT Kharagpur	Syllabus Course contents
Data Communication	Video	Prof. Ajit Pal	IIT Kharagpur	Syllabus Course contents
Data Communications	Web	Prof. H.S. Jamadagni	IISc Bangalore	Syllabus Course contents
Data Structures And Algorithms	Video	Prof. Naveen Garg	IIT Delhi	Syllabus Course contents
Data Structures and Program Methodology	Web	Dr. Pradip K Das Dr. S.V. Rao	IIT Guwahati	Syllabus Course contents
Database Design	Video	Dr. S. Srinath Prof. D. Janaki Ram	IIIT Bangalore IIT Madras	Syllabus Course contents
Design and Analysis of Algorithms	Web	Prof. Sundar Viswanathan Prof. Ajit A Diwan Prof. Abhiram G Ranade	IIT Bombay	Syllabus Course contents
Digital Image Processing	Web	Dr. G. Harit	IIT Kharagpur	Syllabus Course contents
Digital Systems	Web	Prof. N.J. Rao	IISc Bangalore	Syllabus Course contents
Discrete Mathematical Structures	Video	Prof. Kamala Krithivasan	IIT Madras	Syllabus Course contents
Graph Theory	Video	Dr. L. Sunil Chandran	IISc Bangalore	Syllabus Course contents
High Performance Computer Architecture	Video	Prof. Ajit Pal	IIT Kharagpur	Syllabus Course contents
High Performance Computing	Video	Prof. Mathew Jacob	IISc Bangalore	Syllabus Course contents
Indexing and Searching Techniques in Databases	Web	Dr. Arnab Bhattacharya	IIT Kanpur	Syllabus Course contents
Internet Technology	Video	Prof. Indranil Sengupta	IIT Kharagpur	Syllabus Course contents
Introduction to Computer Graphics	Video	Prof. Prem K Kalra	IIT Delhi	Syllabus Course contents
Introduction to Database Systems and Design	Web	Prof. P.Sreenivasa Kumar	IIT Madras	Syllabus Course contents
Introduction to Problem Solving and Programming	Web	Prof. S. Arun Kumar	IIT Delhi	Syllabus Course contents
Introduction to Problem Solving and Programming	Video	Prof. D. Gupta	IIT Kanpur	Syllabus Course contents
Low Power VLSI Circuits & Systems	Video	Prof. Ajit Pal	IIT Kharagpur	Syllabus Course contents
Microprocessors and Microcontrollers	Web	Prof. Krishna Kumar	IISc Bangalore	Syllabus Course contents
Natural Language Processing	Video	Prof. Pushpak Bhattacharyya	IIT Bombay	Syllabus Course contents
Numerical Optimization	Video	Dr. Shirish K. Shevade	IISc Bangalore	Syllabus Course contents
Operating Systems	Web	Prof. P.C.P. Bhatt	IISc Bangalore	Syllabus Course contents
Parallel Computer Architecture	Web	Dr. Mainak Chaudhuri	IIT Kanpur	Syllabus Course contents
Pattern Recognition	Web	Prof. V. Susheela Devi Prof. M. Narasimha Murty	IISc Bangalore	Syllabus Course contents
Performance Evaluation of Computer Systems	Video	Prof. Krishna Moorthy Sivalingam	IIT Madras	Syllabus Course contents
Principles of Programming Languages	Video	Prof. S. Arun Kumar	IIT Delhi	Syllabus Course contents
Program Optimization for Multi-core Architectures	Web	Prof. Rajat Moona Dr. Mainak Chaudhuri Prof. Sanjeev K Aggarwal	IIT Kanpur	Syllabus Course contents
Real Time Systems	Video	Prof. Rajib Mall	IIT Kharagpur	Syllabus Course contents
Real Time Systems	Web	Prof. Rajib Mall	IIT Kharagpur	Syllabus Course contents
Software Engineering	Video	Prof. N.L. Sarda Prof. Umesh Bellur Prof. Rushikesh K Joshi	IIT Bombay	Syllabus Course contents
Software Engineering	Web	Prof. Rajib Mall	IIT Kharagpur	Syllabus Course contents
System Analysis and Design	Video	Prof. V. Rajaraman	IISc Bangalore	Syllabus Course contents
System Analysis and Design	Web	Prof. V. Rajaraman	IISc Bangalore	Syllabus Course contents
Theory of Automata and Formal Languages	Web	Dr. Diganta Goswami	IIT Guwahati	Syllabus Course contents
Theory of Automata, Formal Languages and Computation	Video	Prof. Kamala Krithivasan	IIT Madras	Syllabus Course contents
Theory of Computation	Video	Prof. Somenath Biswas	IIT Kanpur	Syllabus Course contents
VLSI Design Verification and Test	Web	Dr. Santosh Biswas Prof. Jatindra Kumar Deka	IIT Guwahati	Syllabus Course contents
Advanced Computer Graphics	Video	Prof. C. Sharat	IIT Bombay	Syllabus only
Bioinformatics	Web	Prof. Harish Karnick Dr. Arnab Bhattacharya Prof. Somenath Biswas	IIT Kanpur	Syllabus only
Combinatorics	Video	Dr. L. Sunil Chandran	IISc Bangalore	Syllabus only
Computer Security and Cryptography - I	Video	Prof. Bernard Menezes	IIT Bombay	Syllabus only
Design Verification and Test of Digital VLSI Circuits	Video	Dr. Santosh Biswas Prof. Jatindra Kumar Deka	IIT Guwahati	Syllabus only
Distributed Computing Systems	Web	Prof. Ananthanarayana V.S	NITK	Syllabus only
Game Theory	Video	Prof. Y. Narahari	IISc Bangalore	Syllabus only
Grid Computing	Web	Prof. D. Janaki Ram	IIT Madras	Syllabus only
Human-Computer Interaction	Web	Pradeep P Yammiyavar Dr.Samit Bhattacharya	IIT Guwahati	Syllabus only
Introduction to Cognitive Science	Video	Prof. Amitabha Mukerjee	IIT Kanpur	Syllabus only
Introduction To Optimization	Web	Prof. Keshava Prasad Halemane	NITK	Syllabus only
Introduction to Programming	Video	Prof. C. Sharat	IIT Bombay	Syllabus only
Mobile Computing	Web	Prof. R.K. Ghosh	IIT Kanpur	Syllabus only
Object Computing	Web	Dr. R. Nadarajan	PSG College of Technology	Syllabus only
Object Computing	Video	Dr. R. Nadarajan	PSG College of Technology	Syllabus only
Operating System	Video	Prof. Kumkum Garg	IIT Roorkee	Syllabus only
Parallel Algorithms	Video	Prof. R. Seethalakshmi Dr. N. Sairam	SASTRA University	Syllabus only
Pattern Recognition	Video	Prof. Sukhendu Das Prof. C.A. Murthy	IIT Madras Indian Statistical Institute	Syllabus only
Pattern Recognition	Web	Dr. C. Chandra Sekhar	IIT Madras	Syllabus only
Principles of Compiler Design	Video	Prof. Y.N. Srikanth	IISc Bangalore	Syllabus only
Riemann Hypothesis and its Applications	Video	Prof. Manindra Agrawal	IIT Kanpur	Syllabus only
Self-Stabilizing Distributed Algorithms	Web	Dr. Sushanta Karmakar	IIT Guwahati	Syllabus only
Software Architecture and Design	Web	Prof. T.V. Prabhakar	IIT Kanpur	Syllabus only
Storage Systems	Video	Dr. K. Gopinath	IISc Bangalore	Syllabus only
Biometrics	Video	Prof. Phalguni Gupta	IIT Kanpur	Under development
Parallel Computing	Video	Dr. Subodh Kumar	IIT Delhi	Under development

How Internet TCP IP Works

Crypt Key

"Crypt" means "encrypt" and "graphy" means "writing".  Cryptography is the study of writing the code in an encrypted form. 

Security in Computer Networking

Consider the today's scenario of Internet where , there are more than 2 billion Internet users throughout the Globe. People interacting with each other, exchanging mails, purchasing items online and many other activities are being done over Internet online.

Thus there is a great need of securing the data of the users from the bad commodity or person, or we can say this bad person as an Intruder.

Suppose there are 2 persons , James and Steve. They want to communicate over Internet. Lets consider a scenario, what can be there basic needs when they communicate. First of all, they both surely wanted that the data they are exchanging, must not be read by any third person, it should be between them only. Secondly, while transmitting the data, the message should not be deleted, or modified. It should reach the destination in its original form. Thirdly, they both wants to verify that the person they are communicating , is the legal person, with whom they wants to communicate. Steve wants to make sure that the person on other side is James only, and James wants to make sure, that he is communicating with Steve only.

Taking these 3 scenarios, we can say that there are some basic needs of the communicating parties. These are :

1. Confidentiality :

The term confidentiality means that the meaning of the data must be understood by the sender and the intended receiver only. As an Intruder can steal the data during its transmission. Thus the message must be encrypted, so the Intruder is unable to understand the message.

2. Integrity :

James and Steve wants to ensure that the data is not altered during its transmission  neither by an Intruder, nor accidentally. In Transport Layer and Network Layer, we have checksum for error in the message. But prior to that, Data Integrity is used for this purpose.

3. Authentication :

In real world, when you meet a person and talks to him, you both just tell your name to each other, in order to verify each other identity. But when you are communicating over Internet, and you cannot see the other party, its very difficult to authenticate the identity of that person. Thus Authentication includes verifying the identity of the person to each other over Communication Medium.

4. Organisation Security :

As you must be aware, that now a days , almost every organisation's network is connected to the outer world Internet. Thus an Intruder can get access of the Organisation Network, he can deposit harmful malwares or worms in the end systems of the organisation network, can access secret information of the organisation. To secure organisation from such attacks, we have Firewalls and Intrusion detection System in Computer Networks which we will cover in our later posts. 

Now lets have a look at what an Intruder can do and how it can harm the network. Suppose James and Steve are communicating and they want to ensure confidentiality, Data Integrity and Authentication.  Now what different things an Intruder can do.

i)   Record the messages on the channel.

ii)   Delete or Modify the original message during its transmission.

iii)   Impersonating himself as someone else.

If proper measures are not taken, then an Intruder can attack in a numerous number of ways. For example, if not properly encrypted, an Intruder can steal your username and Password. He can do Denial of Service ( DoS ) attack by overloading the network resources and disabling other Network users to communicate. There are various other attacks also. We will discuss each and there measure in detail , later in this post and in the coming post.

Till now you must have understsood that there is a great need of securing our data while transferring it over Communication medium. There are various techniques to safeguard our data, these are known as Cryptography Techniques.

Cryptographic techniques are so much much developed in past 30-40 years, that they themselves include Confidentiality, Integrity and Authentication. So there is no need of applying for different security algorithms to provide all these services individually. 

Cryptography allows a sender to be-fool an Intruder by encrypting the message in some other format using certain technique or algorithm. The Intruder can be disguised, that he cannot get any information from the data, if he is able to intercept it. Yes , the authenticated receiver will be able to receive the original data from the disguised data. 

Let us suppose , James and Steve wants to communicate. James wants to send a message to Steve.  For example: James wants to ask "How are you Steve". Thus James message in original form is known as Plain text. James will use an encryption algorithm to encrypt his original message, to save it from the intruder attack. The encrypted message is known as Cipher Text. Cipher Text is not understandable by the intruder. 

But as you know, in today's global world, almost all the encryption and decryption algorithms are open to every person on the Internet. Even intruder knows these algorithms.So if intruder knows the encryption method, he could have easily decrypt the message. But still something is there, that is preventing the intruder to decrypt and extract the transmitted message, that is known as key.

A key can be anything like a string of characters or numbers etc. Say in this case, the encryption algorithm takes key A, message m as input and produces the cipher text as output. The cipher text here will be denoted as A(m).

Cipher Text ( C ) = A(m)

This means that the plain text, m is encrypted using a key A.  On the receiving side, Steve will provide a key B and the cipher text to the decryption algorithm, that will generate the plain text.

Plain Text (m) = B (C)

or

we can also write it as   , Plain Text (m) = B(A(m))

There are two types of encryption Algorithms.

i) Symmetric Key Algorithm :

In these algorithms, both sender and receiver have the identical keys. They share it with each other secretly.

ii) Public Key Algorithm :

In these types of algorithms, sender and receiver make use of a pair of keys. One of the keys is known to both sender and receiver and infact to the rest of the world, and the other key is known only to one , either sender or receiver, but not to both.

Let us start with our discussion over different encryption Algorithm. First we will be going through Symmetric Key Algorithms and then moving on to various public key encryption techniques.

Symmetric Key Algorithm in Cryptography

Till now what have you understand about Cryptography. It must be that Cryptography is just putting one thing in place of the other using certain techniques so that it should not be understood by any wrong person. So we shall now look at various symmetric algorithms that are almost 500 years old or more. For Symmetric Key cryptography algorithms , we will use key as K. 

1. Caesar Cipher :

For example : James wants to send a message to Steve, 

       "James, Meet me at University, Steve". 

Caesar Cipher will replace the every alphabet by its K letter later in the alphabet. Say if key K=5.  Then every alphabet in the plain message will be replaced by its 5 letter later alphabet. Therefore in this case, the cipher text will be as follows.

m= James, Meet me at University, Steve

K=5

Cipher Text ( C) = K(m) = K(5) 

'J' will be replaced by 'O', 'a' will be replaced by 'f' and similarly all the letters will be replaced. So our generated cipher text would be,

Cipher Text = Ofrjx, Rjjy rj fy Zsnajwxnyd, Xyjaj

Disadvantage of Caesar Cipher

Might be , the above Cipher text looks very difficult to read or unintelligible. But as you know there are only 26 alphabets in English. Thus if the intruder came to know that Caesar Cipher is used, then its very easy for him to break the code, as there are only 25 key value possible. So he can use hit and try method for 25 times and will surely obtain the original message in one of its try. 

2. Mono-Alphabetic Cipher :

Mono-alphabetic Cipher is an improvement over Caesar cipher. In Caesar Cipher, we were only able to replace the letters according to a pattern ( substituting according to K ) . But in Mono-alphabetic Cipher, we can replace the letters randomly. This means any letter can be substituted for any letter, as long as for a single letter throughout the message, same susbstitute must be used. 

Lets take an Example of a mono-alphabetic Cipher. 

The plain message, "James, Meet me at University, Steve" will be encrypted as ,

Cipher Text = pqdtl, Dttz dt qz Xfoctklozn, Lztct

Mono-alphabetic cipher can be extended to 26! ways to encrypt your text. It is almost equal 10^26. Thus, even if the intruder knows , that you have used Mono-alphabetic cipher algorithm, then using a Brute Force Approach ( Hit and Trial Approach ) also, it will be very brainstorming task for him to crack the code.

3. Poly-alphabetic Cipher :

One more improvement of Caesar Cipher was Poly-alphabetic Cipher. In poly-alphabetic , we use 2 Caesar ciphers together. There are 2 keys such as ( with K=3 and K=5). Now we can chose that how these 2 keys should be used in a pattern. For Example: C1  , C1 , C2 , C2 , C1. The 1st letter will be encrypted using 1st encryption key, the second will also be using 1st encryption key, 3rd will be using 2nd encryption key, 4th will use 2nd key also and the 5th letter will be encrypted using 1st encryption key. And this pattern will be followed for the coming letters also. Lets take our Example :

Plain Text (m) = "James, Meet me at the University, Steve".

Keys, K=3, K=5

                           

Cipher Text (C) = Mdrjv, Phjy ph dy ykh  Xsnyhuxnwb, Vyjyh

In Poly-alphabetic Cipher, the Encryption and the Decryption key is the knowledge of the two ciphers i.e.(K=3 and K=5) and the pattern C1  , C1 , C2 , C2 , C1. 

• Since now a days , technology and the communication over Internet has grown to that extent, that these Symmetric Key Cryptographic Techniques also don't work very effectively. As the 2 commodities communicating needs to share the key. Which is not feasible. Thus these techniques are hardly in use in today's world. The techniques which are currently in use are PGP or Public Key Encryption.

Public Key Cryptography

Securing Computer Networks using Public Key

Symmetric Key Cryptographic Techniques getting obsolete with the increasing number of user over the Internet, public key cryptographic techniques came into existence and are playing a vital role in today's computer network.

In Public Key Cryptography, inspite of having a shared key between sender and receiver, there are 2 keys.
Let us Suppose, James is the sender and Steve is the receiver.
Steve will have 2 keys , one is Public Key- that is available to the whole world and the second is Private
Key- that is only known to Steve. We will use Us   for the public key of Steve and Ks   for the private key of Steve.


James in order to to send message to Steve securely, will 1st fetch the public key of Steve (that is known to everyone). Now James will encrypt message, m , using an encryption algorithm and the public key if Steve. That means, James will perform, Us (m). On receiving the encrypted message from James, Steve will use a decryption algorithm and his private key, to get the plain text. Steve will compute Ks(Us (m)). You can clearly see that, James and Steve can securely send message to each other, without sharing or distribution of any key ( that has to be done in Symmetric Key Cryptography).

But a problem arises in such cases. As the public key of Steve is well known to everyone around the globe, thus anyone can encrypt a message using using his public and send it to Steve, impersonating himself as James. In Symmetric Key cryptography, as sender and receiver share the key, they both are verified of each other identity. But this is no longer available in Public Key Cryptography, as the public key is known to everyone. Therefore, to overcome this problem of authentication, we have a phenomenon , known as Digital Signature, that I will discuss with you later.


Let us now start with some Public Key Encryption Algorithms.


RSA Encryption Algorithm


RSA was named after the scientists who developed it i.e. Ron Rivest , Adi Shamir , Leonard Adleman . Lets now see the working of of RSA. But before directly coming to the implementation of RSA, lets have a look at some mathematical calculations, as RSA makes a large use of modulo-n for computing and encrypting a data. In modulo,  if you compute A modulo n, then the result will be the remainder lest after dividing A by n. For example : If you compute 24 mod 5, the result will be 4. There are various operations associated with modulo. These are as follows.

• [( (a mod n) + ( b mod n )] mod n = (a + b) mod n

• [( (a mod n) - ( b mod n )] mod n = (a - b) mod n

• [( (a mod n) * ( b mod n )] mod n = (a * b) mod n

• ( ( a mod n)^d ) mod n = ( a^d) mod n

While you send a message to someone, it is nothing but a stream of bytes , that is following a bit pattern. And a bit pattern can be easily represented by an Integer. For Example : The bit pattern 0110 can be written as 6 , 1001 can be written as 9 and so on every bit pattern. Therefore, when you are encrypting a message using RSA, you are actually encrypting an Integer.

Now to generate a public key and a private key in RSA algorithm, Steve will perform the following steps :

1. Choose two large prime numbers  a and b.

Now a question arises, how large the numbers must be ? The answer is, as mush the numbers are large, the more harder is to break the RSA algorithm. But also, it will takes more time to compute encryption and decryption. Thus it is recommended that you use prime numbers of the order 1024 bits.

2. Calculate n = a * b and  y = ( a-1) * ( b-1 )

3. Select a number e , less than n ( e < n ) such that e and n doesn't have any common factor other than 1. Hence m and n are said to be as relatively prime.

e will be used in encrypting the message.

• 5 and 11 are relatively prime , as they have only 1 as common factor. 

4. Find a number z such that ez-1 is exactly divisible by y. We can also write it as that select z such that 

                                                         ez mod y =1.

z will be used decrypting the message.

5.  Now the public and the private key of Steve are ready. The public key , Us that will be available to the whole world is  ( n, e) and his private key , Ks is  ( n, z ).

\

James is sending a message m to Steve. Message will be represented by some integer that can be mathematically computed. Thus James will encrypt the message in this way.

Cipher text or Encrypted Message ( c ) = ( m^e ) mod n

The bit pattern corresponding to c, will be sent to Steve.

On the receiving side, to get the original message, Steve will decrypt it in the following way.

Plain text ( m ) = ( c^z ) mod n.

Let us take an Example that will make clear , how the RSA algorithm works.

Suppose James has a message , m=5 that he wants to send to Steve. So he start with RSA algorithm.

1. He chooses two prime numbers, say a = 17 and b = 7 .

2. Calculate n= a * b= 17 * 7 = 119 and y = ( 17-1 ) * ( 7-1 ) = 16 * 6 = 96.

3. Select e=5. Thus 5 and 119 are relatively prime.

4. To calculate in order to fulfill ez mod y=1. We will get z as 77. 

5. Therefore Steve public key = ( 119, 5) and private key = ( 119, 77). 

Thus while encrypting the message before sending it, James will do the following computation. 

Cipher Text ( c ) = ( m^e) mod n  = ( 5^5) mod 119 = 3125 mod 119 = 31

On the receiving side, while decrypting the cipher text, Steve will do the following computation.

Plain Text ( m ) = (c^z) mod n = ( 31^77) mod 119 = 5

So you must have noticed that , RSA is secure in the sense, there are numerous number of prime numbers are there for computation. Hence it will be a very brainstorming task for an Intruder to break the code and get the plain text.

Transport Layer 4

Introduction to Transport Layer

Now after covering almost every aspect of the Application Layer and the Application Layer protocols, we have now here to start our discussion on the 2nd layer of the Internet stack , The Transport Layer.

Before starting with our discussion on Transport Layer, I want to give you a live scenario, that will make you clear with the working of Transport Layer and the other layers of the Internet stack.

Let us suppose that there are two bungalows. One is in India and Other is in America. In the bungalow in India, lives a person James along with his 5 children. And in the bungalow in America, lives a person Steve along with his 4 children. Now all 5 children of James write a letter to every children of Steve on every Sunday. Therefore total number of letters will be 20. Thus, all the children writes the letter , put them in envelopes and hand over it to James. Then James write source house address and the destination house address on the envelope and give it to the postal service of India. Now the postal service of India puts some other addresses corresponding to the country and delivers it to the America postal Service. The American Postal sees the destination address on the envelopes and will deliver those 20 letters to the Steve House. Steve collects the letter from the postman and after considering the name of his respective children on the envelopes, he gives the letter to each of them.

In this example we have processes and the layers. Let me explain.

Processes = children

Application Layer messages = envelopes

Hosts = The two Bungalows

Transport Layer Protocol = James and Steve

Network Layer protocol = Postal Service

• Here you can see, that according to children, James and Steve are the mail services, but in real they are just a part of the delivery process.

Suppose James and Steve went on a holiday for 10 days. Then as their susbstitute, Angelina comes at place of James and Ashley comes at place of Steve. But now for the two bungalows, unfortunately Angelina and Ashley are not able of providing services as James and Steve. Angelina and Ashley often drops letters, lose them, which are often eaten by the dogs. Thus in the same way , services provided by the Transport Layer depends on type of protocol you use in your network.

• In this scenario, for example  if the postal service cannot guarantee the maximum time taken in the delivery of the envelopes, thus there is no way that James and Steve can guarantee the maximum delays in the delivery of mails between the children. In the same way, the services provided by the Transport Layer are often subjected to the services provide by the underlying network layer. If the network layer cannot guarantee the delay in delivery of messages, then surely transport layer can never.

But still, a Transport Layer protocol guarantees certain services that are not guaranteed by the underlying Network Layer. 

Thus, in our coming posts on transport layer and the other layers, you must always remember this example, making it easier for you to understand the layers concept.

Overview of Transport Layer Protocol:

Transport Layer acts as a medium between the Application Layer and the the Network Layer. Transport Layer provides a logical communication medium between the processes running on different end systems. What does logical communication means?

Logical communication means that, from an Application perspective, it is like the hosts are directly connected to each other, but in reality , the hosts may be on two opposite sides of the earth, connected by routers and links. As the physical infrastructure of the two hosts and the intermediate can be different, so for this reason , the Application processes use the logical communication of the Transport Layer to send message from one host to other and be free from the worry of the physical infrastructure used.

Different Protocols of Transport Layer:

There are basically two types of protocols that Transport provides to give services to Application Layer. These are:

1. Transmission Control Protocol (TCP)

2. User Datagram Protocol (UDP)

TCP provides a reliable and connection-oriented service to the Application. On the other hand, UDP provides a unreliable and connection-less service to the Application. Thus, it is upto the Application Developer, which transport Layer protocol has to be used. For example: In case of HTTP, it is built over TCP.

 Transmission Control Protocol (TCP):

1. TCP provides a reliable delivery of data. Irrespective of the fact, Network layer doesn't provides reliable data transfer, TCP guarantees that. But TCP doesn't guarantee anything about the time taken to deliver the packet.

2. TCP delivers the packet in their respective order. This means, from the source, in the order in which you will send the packet, they will reach in the same order at the destination.

3. As TCP provides reliable data transfer and connection oriented services, it is a bit heavy and complex protocol.

4. TCP provides congestion control or the flow control within a network.


User Datagram Protocol (UDP):

1. UDP provides non-reliable data delivery. It is a connection-less service provider.

2. UDP doesn't guarantee the delivery of packets in the order they are sent.

3. Its a light protocol. 

Relationship Between Network and Transport layer:

On one hand , the Transport Layer provides communication between processes, whereas on the other hand, the Network Layer provides communication between hosts. Recall the above example of two bungalows and compare it with that. I am sure, you will understand it

TCP

Transmission Control Protocol (TCP)

Coming to the 2nd Transport Layer Protocol i.e. Transmission Control Protocol (TCP). The Transmission Control Protocol is such an important protocol in the Internet protocol model, that the whole Internet stack is called as the TCP/IP stack.  TCP was 1st introduced in 1974.

A number of applications use TCP for their execution such as HTTP, FTP, SMTP. Web Browser uses TCP to connect to the World Wide Web (www). TCP is used to deliver mails and transfer files from one host to another.
TCP is called as Connection-Oriented Protocol. Because in TCP, there is a 3-way handshake between the communicating hosts before exchanging the data or the message. There are lots of services provided by TCP that were not available by UDP. TCP provides Reliable Delivery of data, checks for error in the data and tries for its correction, delivers the packet in their respective order as they were sent by the sender. TCP also provides congestion control and flow control.

TCP provides a full-duplex mode of transmission i.e. if there is a connection between a Process Y on one host and a Process Z on another host. Then the Application message can be sent from Process Y to Process Z and from Process Z to Process Y within the same TCP connection.

A TCP connection is between a single sender and a single receiver. That means, TCP is a point-to-point connection control protocol. Multi-casting is not possible with TCP. Multi-catsing is a procedure of sending of data from a single sender to different receiver in a single send operation. 

Problems with TCP:

Due to large traffic and congestion in the network, packets might get lost or they can get corrupt. TCP being the reliable data delivery protocol, detects all these problems, try to resolve them, asks for the re-transmission of the lost or corrupt packets, arranges the packet in order and send them to the receiver. The TCP receiver arranges all these packets in order and send them to the Application.

Thus in all this, re-transmissions , in order packet delivery, sometimes TCP results in long delays.


How TCP provides Reliable Delivery of Data:

TCP does it by a process of positive Acknowledgement. The receiver has to acknowledge the sender by sending a message to the sender, if it has receive the data correctly. The sender keeps the record of every packet it sends and maintains a clock with every packet. If the acknowledgment doesn't comes before the clock expires, the sender will re-transmit that packet, assuming that the packet is either lost or corrupted.


TCP Packet Structure:

1. Source Port Number: It is used to identify the sending port number.

2. Destination Port Number: It is used to identify the receiving port Number.

3. Sequence Number and Acknowledgement Number : These are used for the reliable Delivery of data. Each Sequence and Acknowledgement Number is of 32 bits.

4. Header Length : This field is of 4 bytes. It tells the length of the TCP Header. The TCP header can be of variable lengths due to the presence of Option Fields. (In most cases, option field is empty, then the TCP header length is 20 bytes).

5. Unused or Reserved : This is reserved for future use. It is of 3 bit.

6. Pointers: 

• ACK: This bit is used to indicate that whether the value in Acknowledgement field is valid or not. That means, whether it contains an acknowledgment number for a packet that has been successfully received.

• RST, SYN and FIN : These 3 bits are used for establishing a connection between and to stop a connection.

• PSH : This bit indicates whether the data should be pass to the upper layer or not. If  PSH=1, it indicates the receiver should pass the data immediately to the upper layer.

• URG: This bit is used to indicate , if there is any data , that the host has marked as urgent. If URG=1, then it shows that some data has been marked as urgent. To know the address of that Urgent data, the 16-bit Urgent Data Pointer field is used which gives that information.

7. Checksum: It is of 16 bits. It is used to check that whether the data has arrived correctly or not. It is also used to detect any error in the data.


8. Application Message: It is of 32 bits. It contains the actual Application that the user wants to transmit.

9. Options : This field is of 32 bits. If the user need to send a data that is larger that the Application Message field size, then option field is used by the TCP header.

• It is recommended that you must send small packets, because if the packet size is larger, than the packet fragmentation will take place at the network layer, resulting in more number of lost packets.

TCP Implementation:

TCP is implemented in different ways. These are:

i) TCP TAHOE

ii) TCP RENO

iii) TCP VEGAS

iv) TCP SACK

We will discuss all these in details in our coming posts. 

Concept of Sequence Number and Acknowledgement Number

These two numbers are of great importance in TCP. As they enable TCP for reliable Data Delivery and in order Delivery of Packets. Lets take a look and try to understand, what are these two fields .

First of all, I would like to tell you that, TCP Sequence and Acknowledgement Number doesn't hold the size or number of transmitted packet , instead they contain the byte stream of those packets. That means , Sequence and Acknowledgement number contains the 1st byte of the transmitted packet.

The sender and the receiver decides on , what should be the starting sequence number of the 1st packet.

Sequence Numbers:

For Example : If you have a 50,000 bytes of data to send and the Maximum Segment Size (MSS) is 1000 bytes, you have to send 50 packets. Suppose you and the receiver decide the starting sequence number of 1st packet as 0. The second packet will have sequence number as 1000, 3rd will have 2000 and so on till 50000.

• MSS refers to the maximum size of a TCP packet that can be sent.

Acknowledgement Number:

As we have already discussed that TCP is full Duplex, so at the same time both sender and receiver can send packets to each other simultaneously. Let us Suppose that James is the sender and Steve is the receiver.

All the packets arriving from James, has a sequence number for the data flowing from James to Steve.  Now the Acknowledgment  number that Steve puts in its packet is the sequence number of the next packet, that Steve is expecting from James.

For Example: If Steve has received all bytes numbered from 0 to 499 from James. Then Steve will put the acknowledgement number as 500 in the packet , it will send to James. 

Cumulative Acknowledgement:

For Example: If Steve has received bytes numbered from 0 to 499 and bytes from 1000 to 1099. Due to some failure or packet loss, Steve has not received packet from 500 to 999 and Still he is waiting for 500 segment, to build the whole message from James in a proper order. Thus Steve's next packet will contain acknowledgment number as 500. Since TCP only acknowledges the 1st byte missing, this is known as Cumulative Acknowledgement.

This also brings one more concern, that when Steve receives packet from 0 to 499 and then from 1000 to 1099, but packet from 500 to 999 is still missing. Thus the 3rd packet arrives out of order. Now a question arises: What should the receiver do when it receives a packet out of order?

Thus , I would like to bring the fact to you that, Computer networking doesn't impose any restriction on this. It totally depends on the people programming the TCP implementation. They can choose either of these:

1. The receiver can discard the out-of-order packet as soon as he receives it.

or

2. The receiver can save the out of order packet in its buffer and waits for the missing packet to arrive.

• Hopefully maximum people will choose the 2nd option , in order to use the network bandwidth to the maximum, and to shorten the delay process.   In Internet, the 2nd choice is used .

*****There are various interesting cases, where you sometimes have to re-transmit and sometimes you need not re-transmit a packet due to acknowledgment. Lets have a look at these scenarios.

Scene 1: Re-transmission due to lost Acknowledgment :

Scene 2: Cumulative Acknowledgment avoids re-transmission of the 1st packet.

 Suppose, sender sends a packet of 15 bytes and a packet of 20 bytes back to back and they are received at the receivers side and the receiver sends an acknowledgement for the same. But the acknowledgment for the 1st packet is lost in the network. But before the timeout occurs, the sender receives an acknowledgment for the 2nd packet. Thus, the sender will understand, that the sender has received all the packets till Sequence number 54. Hence it will not resend the 1st packet.