Layered Architectures 26-11-10

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Layered Architecture

Protocol Hierarchies

To reduce their design complexity, most

networks are organized as a stack of layers orlevels, each one built upon the one below it.

each layer is a kind ofvirtual machine, offering

certain services to the layer above it.

The above layer does not know how the

services are actually implemented


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Layer n on one machine carries on a

conversation with layer n on another machine.

The rules and conventions used in thisconversation are collectively known as the

layer n protocol.

a protocol is an agreement between the

communicating parties on how

communication i s to proceed.


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The entities comprising the corresponding

layers on different machines are called peers.


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In reality, no data are directly transferred fromlayer n on one machine to layer n on anothermachine.

Instead, each layer passes data and controlinformation to the layer immediately below it,until the lowest layer is reached.

Between each pair of adjacent layers is aninterface.

The interface defines which primitive operationsand services the lower layer makes available tothe upper one.


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A message, M, is produced by an application processrunning in layer 5 and given to layer 4 for transmission.Layer 4 puts a header in front of the message toidentify the message and passes the result to layer 3.

In many networks, there is no limit to the size ofmessages transmitted in the layer 4 protocol, but thereis nearly always a limit imposed by the layer 3 protocol.

Consequently, layer 3 must break up the incomingmessages into smaller units, packets,prepending alayer 3 header to each packet. In this example, M issplit into two parts, M1 and M2.


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Design Issues for the Layers

Every layer needs a mechanism for identifying sendersand receivers. some form of addressing is needed inorder to specify a specific destination.

Error control is an important issue because physical

communication circuits are not perfect. An issue that occurs at every level is how to keep a fast

sender from swamping a slow receiver with data. Flowcontrol should b e employed.

Another problem that must be solved at several levelsis the inability ofall processes to accept arbitrarily longmessages. Mechanisms for dissembling , reassemblingetc., must be used


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Design Issues for the Layers

It is inconvenient or expensive to set up a

separate connection for each pair of

communicating processes, multiplexing can be

used for multiple conversations over single

connection

When there are multiple paths between

source and destination, a route must be

chosen. This is known as routing.


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Connection-Oriented and

Connectionless Services

Layers can offer two different types of service tothe layers above them: connection-oriented andconnectionless.

Connection-oriented service is modeled after thetelephone system. To talk to someone, you pickup the phone, dial the number, talk, and thenhang up.

Similarly, to use a connection-oriented networkservice, the service user first establishes aconnection, uses the connection, and thenreleases the connection.


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In connection oriented service, generally, the

order ofthe data is preserved. Eg; telephone

call

In contrast, connectionless service is modeled

after the postal system. Each message (letter)

carries the full destination address, and each

one is routed through the system independent

ofall the others.

Data may not arrive in order.


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Example ofreliable connection oriented service is file transfer.

byte stream(remote login), message stream(sequence of

pages)

Example of non-reliable connection oriented service isdigitized voice

Unreliable (meaning not acknowledged) connectionless

service is often called datagram service, in analogy with

telegram service, which also does not return an

acknowledgement to the sender. Eg;, is electronic junk mail.

The acknowledged datagram service can be provided for

applications where connection is not required for a small

message , but reliability is required. It is like sending a

registered letter and requesting a return receipt.


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The ISO OSI model


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Principles ofOSI

Each layer should perform a well-defined function.

The function of each layer should be chosen with an

eye toward defining internationally standardized

protocols.

The layer boundaries should be chosen to minimize

the information flow across the interfaces. The number of layers must be chosen large enough to

prevent too much functionality being performed by

one layer.


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The Physical Layer

The physicallayer is concerned with

transmitting raw bits over a communicationchannel. The design issues have to do with

making sure that when one side sends a 1 bit,

it is received by the other side as a 1 bit, not

as a 0 bit.


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The Data Link Layer

The main task ofthe data link layer is to transform a raw

transmission facility into a line that appears free of

undetected transmission errors to the network layer. Sender breaks the data into frames and transmits them

sequentially.

If frame is correctly received, receiver acknowledges the

receipt ofthe frames.

Another issue is how to prevent fast sender from

drowning a slow receiver


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The Network Layer

The network layer controls the operation of

the subnet. A key design issue is determininghow packets are routed from source to

destination.

Routes can be based on static tables that are

''wired into'' the network and rarely changed.

They can be highly dynamic, being determined

anew for each packet, to reflect the current

network load.


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The Transport Layer

The basic function ofthe transport layer is to

accept data from above, split it up into smaller

units ifneed be, pass these to the network

layer, and ensure that the pieces all arrive

correctly at the other end.

The transport layer also determines what type

ofservice toprovide to the session layer, and,

ultimately, to the users ofthe network.


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The Session Layer

The session layer allows users on different

machines to establish sessions between them.

Sessions offer various services, includingdialog control (keeping track of whose turn it

is to transmit), token management

(preventing two parties from attempting the

same critical operation at the same time), and

synchronization


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The Presentation Layer

Concerned with the syntax and semantics of

the information transmitted.

Provides abstract representation and encodingofdifferent representation ofdata on various

computers.


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The Application layer

The Application Layer

The application layer contains a variety of

protocols that are commonly needed by users.One widely-used application protocol is HTTP

(HyperText Transfer Protocol), which is the

basis for the World Wide Web.


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The TCP/IPmodel

The reference model used in the grandparent ofall

wide area computer networks, the ARPANET, and its

successor, the worldwide Internet.

The Internet Layer

- This layer, called the internet layer, is the linchpin

that holds the whole architecture together.

- Its job is to permit hosts to inject packets into anynetwork and have them travel independently to the

destination (potentially on a d ifferent network).


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The TCP/IPmodel

The internet layer defines an officialpacket

format and protocol called IP (Internet Protocol).

The job ofthe internet layer is to deliver IP

packets where they are supposed to go. Packet

routing is clearly the major issue here, as is

avoiding congestion.

For these reasons, it is reasonable to say that theTCP/IP internet layer is similar in functionality to

the OSI network layer.


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The TCP/IPmodel


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The Transport Layer

- It is designed to allow peer entities on the source

and destination hosts to carry on a conversation, just

as in the OSI transport layer

- Two end-to-end transport protocols have been

defined here.

- The first one, TCP (Transmission ControlProtocol), isa reliable connection-oriented protocol that allows a

byte streamoriginating on one machine to be

delivered without error on any other machine in the

internet.


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The second protocol in this layer, UDP (User

Datagram Protocol), is an unreliable,

connectionless protocol for applications that

do not want TCP's sequencing or flow control

and wish to provide their own.

Used for one-shot messages where reliability

is not important


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The Application Layer

- On topofthe transport layer is the application layer. It

contains all the higher-levelprotocols. The early ones

included virtual terminal (TELNET), file transfer (FTP),and electronic mail (SMTP), as shown in Fig.


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The Host-to-Network Layer

Below the internet layer is a great void. The

TCP/IP reference model does not really saymuch about what happens here, except to

point out that the host has to connect to the

network using some protocol so it can send IP

packets to it.


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Comparison between OSI ISO &

TCP/

IP

Layers The OSI reference model was devised before the

corresponding protocols were invented. This

ordering means that the model was not biased

toward one particular set of protocols,. The downside of this ordering is that the designers

did not have much experience with the subject and

did not have a good idea of which functionality to

put in which layer.

Example datalink layer initially considered only

point to point networks but not broadcast


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With TCP/IP the reverse was true: the

protocols came first, and the model was really

just a description ofthe existing protocols.

There was noproblem with the protocols

fitting the model. They fit perfectly.

The only trouble was that the model did not

fit any other protocol stacks.

Consequently, it was not especially usefulfor

describing other, non-TCP/IP networks.


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Another difference is in the area of connectionless

versus connection-oriented communication. The OSI

model supports both connectionless and connection-

oriented communication in the network layer, butonly connection-oriented communication in the

transport layer, where it counts (because the

transport service is visible to the users).

The TCP/IP model has only one mode in the network

layer (connectionless) but supports both modes in

the transport layer, giving the users a choice. This

choice is especially important for simple request-

response protocols.


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Drawbacks ofOSI model

Bad Timing

- First let us look at reason one: bad timing. The

time at which a standard is established isabsolutely critical to its success.

- By the time OSI came in, TCP/IP already had

widespread use.


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Bad Technology

- The second reason that OSI never caught on is

that both the model and the protocols areflawed.

- The choice ofseven layers was more political

than technical, and twoofthe layers (session

and presentation) are nearly empty, whereas

twoother ones (data link and network) are

overfull.


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Bad Implementations

- Given the enormous complexity ofthe model

and the protocols the initial implementationswere huge, unwieldy, and slow.

- Bad Politics


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Drawbacks of the TCP/IP

Reference Model

The TCP/IP model and protocols have their

problems too. First, the model does not

clearly distinguish the concepts of service,

interface, and protocol.

Second, the TCP/IP model is not at all general

and is poorly suited to describing any protocol

stack other than TCP/IP. Trying to use theTCP/IP model to describe Bluetooth, for

example, is completely impossible.


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Third, the host-to-network layer is not really a layer

at all in the normal sense ofthe term as used in the

context oflayered protocols. It is an interface

(between the network and data link layers). Thedistinction between an interface and a layer is

crucial, and one should not be sloppy about it.

Fourth, the TCP/IPmodel does not distinguish (or

even mention) the physical and data link layers.These are completely different.


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The hybrid reference model to be

used in the book.

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Layered Architectures 26-11-10