Chapter 1- Introduction to Computer Networks_Second PPT

8/13/2019 Chapter 1- Introduction to Computer Networks_Second PPT

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Protocol Hierarchies

To reduce the complexity, most of the networks areorganized as a series of layers or levels, each one built upon

the one below it.

The number of layers, the name of layer, the contents of

layer and function of layer differ from network to network.

In general the purpose of each layer is to offer services to

higher layers.

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Layer non one machine carries on a conversation with layern on another machine.

The rules and convention used in this conversation are

correctively known as layer n protocol.

Following figure illustrates example of five layer protocol:

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Host 1Layer 5

Layer 4

Layer 3

Layer 2

Layer 1

Host 2Layer 5

Layer 4

Layer 3

Layer 2

Layer 1

Layer 4 / 5 interface




Layer 5 Protocol

Layer 4 Protocol

Layer 3 Protocol

Layer 2 Protocol

Layer 1 Protocol

Physical Medium


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The entities comprising the corresponding layers on differentmachines are called peers.

In reality no data are directly transferred from layer n on one

machine to layer n on another machine.

Each layer passes data and control information to the layer

immediately below it, until the lowest layer is reached.

Below layer 1 is the physical mediumthrough which actual

communication occurs.

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These layers are also capable of doing virtual communicationshown by dotted lines.

The solid line presents physical communication.

Between each pair of adjacent layer there is an interface.

The interface defines which primitive operation and

services the lower layer offers to the upper one and viseversa.

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Goal and Use of Network Architecture

1. To provide assistance and guidance to implement highquality network.

2. Not only helps to deploy the network but also assists in

troubleshoot and maintaining architecture.

3. Helps for security management and disaster recovery.

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Design Issues For the Layers

Every layer needs a mechanism for identifying senders andreceivers.

Since a network normally has many computers, some of

which have multiple processes, a means is needed for a

process on one machine to specify with whom it wants to

talk.

As a consequences of having multiple destinations, some

form of addressing is needed in order to specify a specific

destination.

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Another set of design decisions concerns the rules for datatransfer.

In some systems, data travel in one direction (simplex

communication).

In others, they can travel in either direction, but not

simultaneously (half duplex communication).

In others, they travel in both directions at once (full duplex

communication).

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Error control is an important issue because physicalcommunication circuits are not perfect.

Many error detecting and error correcting codes are known,

but both ends of connection must agree on which one is

being used.

In addition, the receiver must have some way of telling the

sender which messages have been correctly received and

which have not.

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Not all communication channels are preserve the order of

messages sent on them.

To deal with a possible loss of sequencing, the protocol must

make explicit provision for the receiver to allow the pieces tobe put back together properly.

An obvious solution is to number the pieces, but this solution

still leaves open the question of what should be done with

pieces that arrive out of order.

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An issue that occurs at every level is how to synchronize thesenders and receivers clock.

To handle this protocol must have facility of feedback from

the receiver to sender, either directly or indirectly.

Protocol must give the facility of disassembling, transmitting

and then reassembling messages.

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Message passing in n layer model

How to provide communication to the top layer of five-layer model?

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Layers

M

H4

5

4

Layer 5 Protocol

Layer 4 Protocol

Layer 3 Protocol

Layer 2

Protoco

l

M

H3 M1H4 H3 M2

3

H2 M1H4H3 T2 H2 M2H3 T2 H2 M1H4H3 T2 H2 M2H3 T2

M

H4 M

H3 M1H4 H3 M2

2

Source

Machine

Destination

Machine


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A message M is provided by an application of layer 5 andgiven to layer 4 for transmission.

Layer 4 puts a headerin front of the message to identify the

message and passes the result to layer 3.

The header contains the control information, such as

sequence numbers to allow layer 4 on the destination

machine to deliver messages in the right order.

Sometimes it has sizes, times, and other fields.

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In many networks there is no limit to the size of messagestransmitted in the layer 4, but there is limit imposed by layer

3 protocol.

So layer 3 must break the incoming messages into smaller

units, packets pretending layer 3 header to each packet. So

M is break as M1 and M2.

Layer 3 decides which of outgoing lines to use and passes the

packet to layer 2.

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Layer 2 adds not only header to each piece, but also trailerand gives the resulting unit to layer 1 for physical

transmission.

At receiving machine the message moves upward from layer

to layer, with headers being stripped off as it progress.

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Connection-oriented and Connectionless service

The function of layers is to offer services to the layers aboveit.

This can be done in two modes:-

1. Connection-oriented service

2. Connectionless service

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Connection-oriented service

Services are modeled after telephone system.

Consider the case when you make a telephone call.

Here are the steps you would take in order to connect to the

person at the other end.

1. Pick up your telephone

2. Dial the destination number which is unique

3. Say what you want to convey

4. Hang up your phone

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A similar mode of operation is followed in the case ofconnection oriented services.

1. Establish a connection between the sender and the

receiver.

2. Make use of the connection by sending and receiving

information.

3. When done, terminate the existing connection.

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Connectionless service

Services are modeled after postal services.

A connectionless services is just a service with no pre-

established connection.

It is more like a postal system wherein you send and receive

mails with the help of destination address and several

offices that route your mails to the correct destination.

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The packets send need not arrive in the same order in whichthey are sent.

This is basically because all packets might not take the same

route since there is no preset connection in such services.

The routers route the packets according to their built in

algorithms and the receiver might need to reorder the

packet.

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Reliable and Unreliable Service

Both connection oriented and connectionless serviceshavereliable as well as unreliable services.

Reliable services in the sense that we are assured of the

receiver receiving the data and in unreliable the receiver

might not actually get the data.

This is monitored by usually setting a feedback required or

not mechanism.

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In cases like FTP, we might require that the senderacknowledge the receipt.

For this we use reliable connection oriented services.

The acknowledgement process introduces overhead and

delays, which are often worth it but are sometimes

undesirable.

In other cases such as database queries, it is not really

required that the receiver sends an acknowledgement.

In these cases, we use unreliable connectionless services.

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Unreliable (meaning not acknowledged) connectionlessservice is often called datagram service, in analogy with

telegram service, which also does not return an

acknowledgement to the sender.

TCPis one protocol that uses connection oriented service

concept and UDPis another protocol that uses

connectionless service concept.

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Six different types of services

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Reliable Message Stream:-The message boundaries are preserved.

When two 1-KB messages are sent, they arrive as two

distinct 1-KB messages, not as 2 KB messages.

Reliable Byte Stream:-

Message boundaries are not preserved.

When 2KB message arrive at receiver, there is no way to tell

if they were sent as one 2KB message or two 1KB messages.

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Unreliable connection:-In some application the delays introduced by

acknowledgements are unacceptable.

Unreliable datagram:-

Unreliable connectionless service is often called as datagram

service,which does not provide an acknowledgement back

to the sender.

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Acknowledged datagram:-

In some applications, without establishing connection

reliability is important.

The acknowledged datagram service can be provided for

these applications.

It is like sending register letter and requesting a return

receipt.

When receipt comes back, the sender is absolutely sure that

letter was delivered to intended party.

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Request-reply:-The sender transmits a single datagram containing a request;

the reply contains the answer.

Request-reply is commonly used to implement

communication in the client server model: the client issues a

request and the server responds to it.

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Service Primitives

A service is formally specified by a set of primitives(operations) available to a user process to access the service.

These primitives tell the service to perform some action or

report on an action taken by a peer entity.

If the protocol stack is located in the operating system, as it

often is, the primitives are normally system calls.

These calls cause a trap to kernel mode, which then turns

control of the machine over to the operating system to send

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The set of primitives available depends on the nature of theservice being provided.

The primitives for connection-oriented service are different

from those of connectionless service.

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Five service primitives for Conn-Oriented


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First ,the server executes the LISTEN to indicate that it isprepared to accept incoming connections.

A common way to implement LISTEN is to make it a blocking

system call.

After executing the primitive, the server process is blocked

until a request for connection appears.

Next the client process executes CONNECT to establish a

connection with the server.

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The CONNECT call needs to specify who to connect to so itmight have a parameter giving the servers address.

The operating system then typically sends a packet to the

peer asking it to connect.

The client process is suspended until there is a response.

When the packet arrives at the server, it is processed by theoperating system there.

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When the system sees that the packet is requesting a

connection, it checks to see if there is a listener.

If so, it does two things :-

1. Unblocks the listener and2. Sends back an acknowledgement

The arrival of this acknowledgement then releases the client.

At this point the client and server are both running and they

have a connection established.

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Note that the acknowledgement is generated by the protocol

code itself.

If a connection request arrives and there is no listener the

result is undefined.

In some systems the packet may be queued for a short time

in anticipation of a LISTEN.

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The Relationship between Services and Protocols

Def:-

A service is a set of primitives (operations) that a layer

provides to the layer above it.

It defines what operations the layer is prepared to performon behalf of its users.

Doesnt specify the implementation details of these

operations.

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It relates an interface between two layers, with the lower layer being the

service provider and upper layer being the service user.

Definition of a protocol:-

A protocol in contrast , is a set of rules governing the format and meaning

of packets or messages that are exchanged by the peer entities within alayer.

Entities use protocols to implement their service definitions.

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A service is like an abstract data type or an object in an

object-oriented language.

It defines operations that can be performed on an object but

does not specify how these operations are implemented.

A protocol relates to the implementation of the service and

as such is not visible to the user of the service.

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In short,

Services relate to the interfaces between layers.

Protocols relate to the packets sent between peer entities on

different machines.

Fig. The relationship betweena serviceand a protocol.

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Chapter 1- Introduction to Computer Networks_Second PPT