Study Material
Unit
:V
Semester : VI
Class
: III BSc (A, B & C)
Staff
: R.Balakrishnan
The Presentation Layer:
Presentation layer is the 6th layer in the OSI reference model. Presentation layer is
concerned with preserving the meaning of the information transported. Each computer
has its own way of representing data internally like ASCII or EBCDIC. It is the job of
presentation layer to encode structured data from the internal format used on the sending
machine to a bit stream suitable for transmission and then to decode it to the required
representation at the destination.
Design Issues of the presentation layer:
The presentation layer has four primary functions.
1. Giving users a way to execute the session service primitives
2. Providing a way to specify complex data structures
3. Managing set of data structures currently required
4. Converting data between internal and external form
Data Representation:
Different computers have different internal representation for data. All IBM
mainframes use EBCDIC as the character code whereas all PC use ASCII. When these
two incompatible system exchanges data then complexity will arise. It is up to the
presentation layer to represent data in structured way such that smooth exchange of
information takes place between incompatible systems
Data Compression:
Data compression refers to the technique of reducing the number of bits required to
represent and transmit the information thru network without changing the original
meaning.
There are different types of compression techniques used. One is encoding a finite set
of equally likely symbols. Another one is replacing frequently occurring information
with equivalent code.
Cryptography:
The art of devising ciphers and breaking them is collectively called as cryptography.
Intruder
Plain
Tex t P Encryption
method
Encryption key k
Decryption
method
Plain Text
Cipher text C=Ek(p) Decryption key
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The message to be encrypted is known as plain text is transformed by a function that is
parameterized by a key. The output of the encryption process known as the cipher text or
cryptogram is then transmitted. Intruder refers to the unauthorized person accessing
information flowing thru the network without the permission. There are two kinds of
intruder. Passive Intruder is an intruder who can read information that is passing thru the
network. Active Intruder is an intruder who can read and can alter the message that is
passing thru the network.
Encryption methods is divided into two categories
1. Substitution ciphers
2. Transposition ciphers
Substitution ciphers:
In substitution cipher each letter or group of letter is replaced by another letter
In Caesar substitution cipher a becomes D, b becomes E, etc. and the key is 3. For
example attack becomes DWWDFN.
The next type of substitution cipher is that each alphabet is mapped into some other
alphabet. This system is called as Mono Alphabetic substitution.
The next method is Digrams in which common and frequently occurring two letter
alphabets like th,in are replaced by another alphabet. Similarly three letter combinations
like the, ing are replaced by another symbols and called as Trigrams.
Codes:
If the units encrypted become longer and longer the cipher begins to resemble a
Code. The main difference between the cipher and code is that the former encrypts a
fixed size unit of plaintext where as code encrypts a single variable-length linguistic unit.
Encoding a coded message is called super encipherment.
Transposition cipher:
Transposition ciphers reorder the letters present in a plain text. The cipher is keyed by
a word or phrase not containing any repeated letters. For example MEGABUCK is the
key. The purpose of the key is to number the columns. The plaintext is written
horizontally in rows. The cipher text is read out by columns starting with the column
whose key is the lowest.
The Data Encryption Standard (DES):
Transposition and substitution ciphers can be implemented with simple circuits.
The P box stands for permutation used to effect a transposition on an 8 bit input. If the 8bit input are designated from top to bottom as 01234567 the output of the P box is
36071245.
The Substitutions are performed by S boxes. The 3-bit input is given and the output
is a 3-bit cipher text. The 3-bit input selects one of the eight lines exiting from the first
stage and sets it to 1 and all other lines are 0. The second stage is a P box. The third stage
encodes the selected input line in binary.
Both P and S box can be combined to have a effective encryption and the whole process
can be called as Product Cipher.
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Product Cipher
Combination of P and S box. P stands for P box and S stands for S box.
P1
S1
S5
S2
P2
S9
S6
P3
S10
S3
S7
S11
S4
S8
S12
P4
A Standard encryption was developed called Data Encryption Standard (DES). Plaintext
is encrypted in blocks of 64 bits yielding 64 bits cipher text. The algorithm, which is
parameterized by a 56-bit key, has 19 distinct stages. The first stage is a key independent
transposition on the 64 bit key plaintext. The last stage is the exact inverse of this
transposition. The stage prior to the last one exchanges the leftmost 32 bits with the
rightmost 32 bits. The remaining 16 stages functionally identical but are parameterized
by different functions of the key. The algorithm will allow decryption
64 bit plain text
Initial transposition
56
bit
key
Iteration 1
Iteration 16
32 bit swap
Inverse transposition
64 bit cipher text
The data encryption standard
Stream Encryption:
Effective encryption can be achieved through stream cipher instead of block cipher.
When stream cipher is used both sender and receiver operate their the DES chips in
encryption mode. Each DES chip has 64 bit input register, which operates as a shift
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register and a 64 bit output register. When a plain text arrives it is EX – ORed with 8 bits
of output register O1.The character thus created is both transmitted to the receiver and
shifted into the input register pushing I8 off the end. The chip is activated and the output
is computed for the new input.
At the receiving end the incoming character is first EX –ORed with O1 and then
shifted into I1.Since the incoming plain text character at the transmitter is being EX-Ored
with the same character as the incoming cipher text character at the receiver, the output at
the receiver is the plaintext.
Public key cryptography:
Key distribution is one important problem to be solved. Both the receiver and the
transmitter should know the key to decrypt the message. Identical key pair is generated
and should be distributed to both transmitter and receiver but distribution of key thru
network or thru personal courier is a vulnerable one.
Public key cryptography solves this key distribution problem. The encryption
algorithm in which the key is made public is called Public key Cryptography.
There are three requirements
1. D(E(P))=P
2. It is difficult to deduce D from E
3. E cannot be broken by a chosen plain text attack.
Digital signatures with public key cryptography:
In this A and B can have its own private key and still can use public key cryptography.
A can send a plaintext message with its own secret decryption key Da as well as the
public key B. When B receives the message it transforms it using its private key and then
decrypts it using Ea i.e. the public key to get the original plain text.
The MIT algorithm:
Due to the advantages of public key cryptography many algorithms are developed
based on it. One method is MIT algorithm. It is based on some principles from number
theory.
Algorithm:
1. Choose two large primes, p and q each greater than 10 power 100.
2. Compute n=p*q and z=(p-1)*(q-1)
3. Choose a number relatively prime to z and call it d
4. Find e such that e*d=1 mod z
Divide the plain text into blocks, so that plaintext message p falls in the interval 0<=P<n.
This can be done by grouping the plaintext into blocks of k bits where k is the largest
integer for which 2 k<n is true.
To encrypt a message P, compute C=Pe(mod n). To decrypt a message compute
P=cd(mod n).
Application Layer
It is the top most layer in the OSI reference model. It holds the user programs that do the
actual work. These programs make use of services offered by the presentation layer.
Design Issues :
The important design issues are
1. File Transfer, access and Management
2. E Mail
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3. Virtual terminals
File Transfer, Access and Management:
File handling is the principal services in any network and important design issue of
application layer.
File Server:
A file server can be characterized by three properties
1.File Structure
2.File attributes and
3.File Operations
File Structure:
Every file server has differrent file models. Three models are widely used.
a) Unstructured: In this model file is an unstructured lump of data without any
substructure known to the file server. File server cannot perform multiple
operations on this kind of file. Only read and write operations can be performed.
b) Flat File: It consists of ordered sequence of records and the records need not all be
of the same size.
c) Hierarchical file: Takes the shape of a tree. Each node of the tree may have label,
data or both.
File Attributes:
All files have attributes that describe them. File Name, Date and time of creation
last modified owner, file availability are some of the file attributes.
File Operations:
Various operations can be performed on a file. Creation deletion, selection, open
close, read, insert, replace, extend are the different types of operations that can be
performed on a file.
Concurrency Control:
Network file servers have multiple clients. If more than one client access a same
file then problems will occur. One widely implemented solution to this problem is to
permit clients to lock files before using the files. Two types of locks are there.
1. Shared Lock:
Shared locks are typically used for reading. When a client just wants to
read a file-shared lock can be applied which will not create too much
problem.
2. Exclusive Lock:
If exclusive lock is used no other client can access the file till it is
unlocked.
Some file servers support a concept called transactions. Transaction is
nothing but an atomic actions in the context of file servers. Client can initiate a
transaction followed by any number of open and file operations and finished by a
command to end the transaction. It is upto the server to carry out all the requests in an
atomic way without the interference from other client requests. At any time transaction
can be aborted and file can be restored to its original state.
Replicated Files:
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Network can also have multiple file servers 1. To split the workload over multiple
servers 2.To allow access to occur even if one file servers is down and 3. To increase
reliability by having independent backups of each file.
Problem:
If the multiple copies of file in different file servers are not modified no problem will
arise. Lot of complexity will arise if one copy of file is updated by the client in one file
server such that any other client accessing the same copy of the file will get old copy and
not the updated one.
Solutions:
Primary Copy Replication:
The simplest solution is primary copy replication. One copy is considered as a
master file and all others as slaves. If a client modifies a file updates are made to the
master and propagates to all the slaves. So client irrespective of different file server will
get the updated copy
Voting:
Another solution is voting. To read a file client must obtain a read quorum i.e. it has
to get permission from all other file servers. To modify a file client has to get a write
quorum. Since all file servers know about the operation and its status since it was the one
which is giving the permission updates can be made easily. If any file server is down then
a dummy file server is created whose main task is to give permission to clients and it is
called voting with ghosts.
Implementation Issues:
1. Simple File Server:
The file server handles one request at a time. While it is busy with the current
request all new requests are simply queued until the server completes the current job.
So a file server can the viewed as simple loop
Begin
GetMessage(buffer);
DoWork(buffer,result);
SendReply(result);
End
The main problem is because of the clients invalid request server can be forced to enter
an indefinite loop.
2. Multithreaded Server:
In this type of server file server is split into several tasks that share a common
address space. Each task is a separate thread of control with its own program counter and
stack. When a request message form client comes the kernel accepts it and passes to the
dispatcher task. The dispatcher task then inspects it and hands it off to an idle worker
task. Each task handles a single request to completion. If a task blocks or goes into
indefinite loop till other task can work well since each having its own stack and program
counter.
Various methods can be followed to improve the performance of the file server. One is
to have the server maintain a buffer cache in memory. The buffer contains most recently
and frequently accessed files. When client request a specific file it can access it from
buffer if it was there instead of doing lengthy disk access. Another method is that client
system to have the buffer but frequent updation has to be done to maintain integrity.
E-Mail:
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Architecture and services of MOTIS and X.400 E Mail systems:
There are 6 aspects of email
1. Composition: Refers to the process of creating messages and answers. Any text
editor can be used.
2. Transfer: Refers to the process of moving messages from the originator to the
recipient
3. Reporting: Refers to the process of report on the successful delivery of the email
message
4. Conversion: Refers to the process of converting the message suitable for display on
the recipient’s terminal.
5. Formatting: Refers to the process of formatting the data that is displayed on the
recipient’s terminal
6. Disposition: Concerned with what the recipient does with the message after
receiving it
Many other mail systems allow different features like mailboxes probes etc.
General model of E-mail system:
The user agent is a program that provides the interface to the mail system. It
allows user to compose, send and receive and manipulate messages.
The message transfer agent accepts mail from user agents and sees to it that the
mail is transferred thru the network. Some computers may have limited storage
capabilities and the user may not be logged in when message arrives. To sort out this
type of the problem MTA contains a message store, which is used to store the mail
messages temporarily.
THE USER AGENT:
The UA has three different kinds of interactions with the user. They are
1. It manages the dialog with the user at the terminal.
2. It talks to the MTA about accepting and delivering messages
3. It deals with the message server
The UA is typically invoked by calling a program that accepts variety of commands that
relate to composing, sending and receiving mails. For example the command s-> Stands
for sending a message, f-> For forwarding a message and d-> Stands for deleting a
message.
The UA’s use a common protocol while exchanging mail. The protocol defines some
standard headers, which each UA must adhere. Subject is one of the headers the UA will
have. Similarly some common headers are used by UA, which ultimately helps UA to
transfer mail in a effective way.
THE MESSAGE TRANSFER AGENT:
The MTA is concerned with relaying the message from the originator to the
recipient. If the message is form UA the MTA will check the syntax for its validity. The
next step is to see if the recipients UA is local. If so message is delivered. If not message
is forwarded to another neighboring MTA and so on till it reaches the recipient UA.
The important aspect of the MTA is the envelope which contains come important
information like originator address , recipient address, priority etc.
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