Dr. L. Christofi 1

Local & Metropolitan Area Networks

ACOE322

Lecture 7Upper OSI layers

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Upper OSI layers

7 Application

6 Presentation

5 Session

4 Transport

3 Network

2 Data Link

1 Physical

Upper OSI (Host layers): Upper OSI (Host layers):

Provide accurate data delivery between computers

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Transport Layer• It is responsible for source-to-destination

(end-to-end) delivery of the entire message. —Whereas the network layer oversees end-to-end

delivery of individual packets, it does not recognize any relationship between those packets.

• Ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-to-destination level.

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Transport Layer (Cont.)

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The responsibilities of Transport Layer• Service-point addressing

—Source-to-destination delivery means delivery not only from one computer to the next but also from a specific process(running program) on one computer to a specific process(running program) on the other.

—The transport layer header must include a type of address called a service-point address (or port address).

—The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer.

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The responsibilities of Transport Layer (cont.)

• Segmentation and reassembly—A message is divided into transmittable segments,

each segment containing a sequence number.—These numbers enable the transport layer to

reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in the transmission.

• Flow control—The transport layer is responsible for flow control.— It is performed end-to-end rather than across a

single link.

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The responsibilities of Transport Layer (cont.)

• Connection control—Can be either connectionless or connection-

oriented.—A connectionless transport layer treats each

segment as an independent packet and delivers it to the transport layer at the destination machine.

—A connection-oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets.

—After all the data is transferred, the connection is terminated.

• Error control— It is performed end-to-end rather than across a

single link.

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User Layers• The session, presentation and application

layers are known as user layers

• Mainly implemented by software

• In most protocols like TCP/IP and Novell, these layers are implemented by a single layer called application layer

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Session Layer• It is responsible for establishing, maintaining,

and synchronising dialogs between communication upper layers

• Also helps to handle upper level problems such as inadequate disk space or out of paper for the printer

• Although it is considered as user layer, but it is often implemented within the operating system

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Session layer

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Services of the Session Layer• Coordinate connection and disconnection of

dialogs between application

• Provide synchronisation points for data exchange

• Coordinate who sends first and when

• Ensure that the data exchange is complete before the session closes

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Session and Transport Interaction

• The transport layer can make an abrupt disconnection while session layer has an obligation to the user and cannot disconnect until the session can be brought to a conclusion

• The communication with the transport layer can be of 3 types:—One-to-one: there is one session layer

connection for each transport layer connection—Many-to-one: multiple session layer

connections share the services of one transport layer connection

—One-to-many: one session layer connection needs many transport layer connections to handle the task

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Synchronization points• Mechanism for recovering data that have

been delivered but mishandled

• Reference points are introduced into the data to control flow of information and allow recovery from software or operator errors

• These reference points may call for user acknowledgment or just may provide a go-back facility for data recovery

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Two types of synchronization points• Major synchronization points:

—Divide an exchange into a series of dialogs—Each point must be acknowledged before the

session can continue—If an error occurs, data can be recovered only up

to the last major point—A session layer activity can be a single dialog or

several dialogs separated by major synchronization points

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Two types of synchronization points• Minor synchronization points:

—Are inserted into the middle of dialogs and may or may not require confirmation

—If an error occurs, the control can go back one or more minor synchronization points within a dialog to recover the data

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Presentation Layer• Functions performed includes:

—Translation

—Encryption/decryption

—Authentication

—Compression

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Presentation Layer

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Translation• The internal representation of a piece of

information might vary enormously from one machine to the other (e.g. one may be using ASCII and the other using EBCDIC)

• Therefore translation is require so that the two machines can communicate

• Two methods: direct or indirect

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Direct and Indirect Translation

• Direct translation performs the translation at the receiver

• Indirect translation performs the translation at the sender and at the receiver

• The direct method is not acceptable in most cases, because if a computer is communicating with several other computers, it may need several conversion tables

• The indirect is recommended by OSI and the recommended model is called abstract syntax notation 1 (ASN.1)

• ASN.1 not only takes care of translation but also handles other formatting problems such as the diverse nature of data (text, program) and the diversity in data storage (store data in different format)

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Encryption/Decryption

• To assure privacy for transmitting sensitive information

• Microwave, satellite and other wireless media cannot be protected from unauthorised reception of the transmission, cable transmission cannot totally avoid that either.

• To alter the information before transmitting so that only an authorised receiver can understand it

• Encryption means that the sender transforms the original information to another form and sends the resulting message out over the network

• Decryption reverses the encryption process in order to transform the message back to its original form

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Encryption/Decryption Methods• Conventional Methods: the encryption

key (Ke) and the decryption key (Kd) are the same and secret

• Public Key Methods: every user has the same encryption algorithm and key, however, the decryption algorithm and key are kept secret

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Conventional Method• Can be divided into two categories:

—Character-level encryption—Bit-level encryption

• There are two methods in character-level encryption:—Substitutional—Transpositional

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Substitutional Character-level Encryption• The simplest form of ciphering• In mono-alphabetic substitution, also known

as Caesar Cipher, each character is replaced by another character in the set

• The mono-alphabetic encryption algorithm simply adds a number to the ASCII code of the character; and the decryption algorithm simply subtracts the same number

• Mono-alphabetic substitution is very simple and can be broken easily

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Substitutional Character-level Encryption• In poly-alphabetic substitution, each occurrence

of a character can have different substitute

• One poly-alphabetic encryption technique is to find the position of the character in the text and use that value as the key

• It is not very secure as well, even though the words replaced by different characters but their position is still the same; the code can easily be broken by someone with more experience

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Transpositional Character-level Encryption• Transpositional encryption is a more secure

method in which the characters retain their plaintext form but change their positions to create the ciphertext

• The text is organised into a 2-dimensional table and the columns are interchanged according to a key

• The key defines which columns should be swapped

• Again this encryption is not very secure either, one can still break it through trial and error

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Bit-level Encryption• Data as text, graphics, audio, or video are

first divided into blocks of bits, then altered by —encoding/decoding—permutation—exclusive OR—rotation—others

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Public Key Encryption• In conventional method, the decryption algorithm is

always the inverse of the encryption algorithm and uses the same key

• Anyone who knows the encryption algorithm and key can deduce the decryption algorithm

• Security can only be assured only if the entire process is kept secret

• In public key, anyone can encrypt information but only an authorised receiver can decrypt it

• The decryption algorithm is designed in such a way that it is not the inverse of the encryption algorithm

• Figure 23.21 shows the idea; every customer can use them but the decryption algorithm and key are kept secret and used only by the bank

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Authentication

• Means verifying the identity of a sender• There are many methods, only digital signature

will be discuss here• Digital signature is based on public key

encryption/decryption• See Figure 23.25 to get an idea of how digital

signature works.• If the customer claims never have made such a

transaction, the bank can take C1 out of its file and apply Kp-2 (public key) to it to show that it creates P. This decryption is not possible unless the customer had originally applied Ks-1 (secret key) to P to create C1

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Compression• Reduces the number of bits sent• Becomes important when data that are not

pure text such as audio and video are send• Can be divided into two broad categories:

—lossless—lossy

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Lossless Compression• The compressing and decompressing

algorithms are usually the inverse of each other

• After decompressing, we will get the exact data as they were before compressing

• Some of the techniques used in lossless compression:—Run-length encoding: when data contain strings

of repeated symbols, the strings can be replaced by a special marker, followed by the repeated symbol, followed by the number of occurrences (see Figure 23.27)

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Lossless Compression—Statistical compression: uses short codes for

frequent symbols and long codes for infrequent symbols

—Relative compression: when there is little difference between consecutive frames, only the difference between the consecutive frames are send instead of the entire frame

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Lossy Compression• The decompressed information does not produce

exactly the same information as the original but something very close

• In video transmission, if an image does not have sharp discontinuities, after transformation to a mathematical expression, most of the information is contained in the first few terms

• Using the terms sent, it is possible to reproduce the original frame with enough accuracy

• Some methods are:— Joint photographic experts group (JPEG): for

pictures and graphics— Motion picture experts group (MPEG): for video

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Application Layer• Contains whatever functions are required

by the user• No standardisation in general is possible• However, ITU-T has recognised some

common applications for which standardisation is possible. Two of them are:—Message Handling System (MHS)—File Transfer, Access, and Management

(FTAM)

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Application Layer

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Message Handling System (MHS)• Is the OSI protocol that underlies electronic

mail and store-and-forward handling

• Is the system used to send any message that can be delivered in a store-and-forward manner

• Store-and-forward: instead of opening an active channel between the sender and receiver, the protocol provides a delivery service that forwards the message when a link becomes available

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MHS structure• Each user communicate with a program or

process called a user agent (UA) which is unique to each user

• Each user has message storage (MS), which consists of disk space in a mail storage system and is usually referred to as a mailbox

• The message communicates with a series of processes called message transfer agents (MTAs)

• The combined MTAs make up the message transfer system (MTS)

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File Transfer, Access, and Management (FTAM)• It is used to

—transfer (copy) —access (read, write, or modify) —manage (control) files

• To allow the interaction of different file systems, it uses the concept of virtual files and virtual filestores

• A virtual filestore is a non implementation-specific model for files and databases that can be used as an intermediary for file transfer, access, and management

• Each transaction requires an initiator and responder

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References

• W. Stalling, Local and Metropolitan Area Networks, 6th edition, Prentice Hall, 2000

• F. Halsall, Data Communications, Computer Networks and Open Systems, 4th edition, Addison Wesley, 1995

• B.A. Forouzan, Data Communications and Networking, 3rd edition, McGraw-Hill, 2004

• W. Stallings, Data and Computer Communications, 7th edition, Prentice Hall, 2004