Notes01 Intro

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SEG 3255

Communicationand Networking

Winter 2008 course notes

A. Williams


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What is a Network?

A set of processing nodes connected by communication links. Many topologies possible:

Many types of communication media:

twisted (copper) pair

coaxial (copper) cable

radio infrared

fiber optic cable

satellites

busring mesh star


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Types of Communication Links

Point-to-point

Shared / broadcast


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Circuit Switching

All resources (e.g. communication links) needed by a call arededicated to that call for its duration.

Example: a voice telephone call

Call from A to F blocks calling from B to E.

Resource reservation: resources are always available when needed

by a call, providing a guaranteed quality of service.

F

B C D

EA


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Packet Switching

Data entering network is divided into small chunks calledpackets.

Packets traversing the network share network resourceswith other packets.

Demand for resources may exceed resources available:

Contention: two packets arrive simultaneously at D

destined for E or F Queuing (waiting) for resources.

Statistical sharing of resources.

F

B C D

EA

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Why resource sharing?

To save/make money! Example: 1 Mbit/sec link; each user requires 100 Kbits/sec

when transmitting; each user has data to send only 10% ofthe time.

Circuit switching: give each caller 100 Kbits/sec capacity.10 callers can be supported.

Packet switching: with 35 calls in progress, the probabilitythat 10 or more callers are simultaneously active is less than0.0004. Many more callers can be supported with only a

small probability of contention. If users are bursty, then packet switching is advantageous.


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Elements of a Network

Communication links

Buffers: to hold packets when contention for communications link.

Network: set of nodes (hosts, routers, gateways) within a singleadministrative domain (e.g. university department, company).

A

CBD

internal

view of C

Three networks forming an internetwork


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Elements of a network (2)

Internetwork: a collection of interconnected networks

Active network elements: hardware running protocols:

Host: hardware running applications which use network (e.g. A).

Router: hardware (often without application level functions)routing packets from input line to output line (e.g. C).

Gateway: a router connected directly to two or more networks(e.g. B and D).

A

CBD


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Protocols

Rules by which active network elementscommunicate with each other is a protocol

Protocols define the formats and timing ofmessages exchanged, and actions taken on receiptof messages for peer entities

Protocols in everyday life:

Rules by which two or more people communicateto provide a service, or to get something done

Example: traffic lights guiding traffic flow Example: military precedence for entering a

vehicle


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Layered Architecture

Architecture of a complex system canbe simplified by layering.

Layer N relies on services of layerN 1 to provide a service to layerN + 1

Service required from lower layer isindependent of how that service isimplemented

Interfaces define how services arerequested

Benefits:

Information/complexity hiding

Layer N change doesnt affectother layers

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interface


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Layering and Protocols

The network consists of geographically distributed hardware andsoftware components

A distributed, layered view:

Principal challenge: how to provide services when:

resources and information needed are distributed

communication via unreliable medium

A B C

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Layering and Protocols

Peer entities (processes) in layer N provide service bycommunicating (via messages) with each other using thecommunication service provided by layer N 1

logical versus physical communication:

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A B C

Logical communication:

Layer 4 to Layer 4

Actual

communication

path


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Generic Layer Issues (1)

Error control: making a channel more reliable, and handlinglost or out of sequence messages.

Flow control: avoid flooding a slower peer entity.

Resource allocation: mediating contention for physical (e.g.buffers) or logical (e.g. data structures) resources

Fragmentation: dividing chunks of data into smaller pieces,and subsequent reassembly

Multiplexing: combining several higher layer sessions


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Generic Layer Issues (2)

Connection setup: initiating logical communicationwith peer entity

Addressing / naming: managing identifiers

Compression: reducing data rate

Encryption: provide data security

Timer management: bookkeeping and error

recovery


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OSI Reference Model

OSI (Open Systems Interconnect) referencemodel adopted in 1984

Model consists of a 7 layer stack:

Application

Presentation

Session

TransportNetwork

Data Link

Physical


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Functions of the OSI Layers

Physical The bits that are transmitted over the

communication media.

Deals with network hardware, bit encoding.

Examples: copper, fibre, radio, satellite.

Data Link

Activates, maintains, and deactivates the

physical link between two adjacent nodes. Deals with framing, windowing, flow control,

error detection and recovery.


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Functions of OSI Layers (2)

Network Determines how best to route packets of data

from source to destination via intermediatenetwork nodes.

Deals with addressing, routing, fragmentation,and congestion.

Transport

Ensures that data is transmitted reliablybetween source and destination.

Deals with end to end integrity and quality ofservice.


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Functions of OSI Layers (3)

Session (often omitted) Controls the dialogue between two host applications.

Provides check points and error recovery.

Reports exceptions to upper layers.

Presentation (often done by application) Resolves data representation differences.

Performs data compression and encryption.

Application

Perform functions to implement network applications. Examples: e-mail, teleconferencing.


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Data Transmission in the OSI model

Send side layer N takes a protocol data unit (PDU) from layer N + 1, addsits own fields to form a new PDU, and passes it to layer N 1

Receive side layer N takes the PDU from layer N 1, strips the layer Nfields, and passes it to layer N + 1

T: transport header: e.g. sequence numbers, error correction bits, timestamp info

N: network header: e.g. source and destination addresses

L: link header: e.g. error detection bits, acknowledgment field

A

network

Application

PresentationSession

Transport

Network

Data Link

Physical

AS

AST

ASTN

ASTN L2L1


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The Internet Reference Model

De facto model that was defined after ARPANETwas up and running.

ApplicationPresentation

Session

Transport

Network

Data Link

Physical

Application

Transport

Internet

OSI TCP/IP

Data Link

Physical


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Disadvantages of layering

Layering has many conceptual advantages, but fanaticaladherence to layering is problematic.

Layer N may duplicate lower layer functionality:

Example: error recovery on both a hop by hop and end toend basis.

Different layers may need the same information.

Example: time stamps

Layer N may need to know non-adjacent layer information.

Example: choosing packets to drop if congested

OSI session layer has not proven to be particularly useful.


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Standards Bodies

Formal accredited standards bodies produce national andinternational standards.

National standards bodies

Industry Canada

National Research Council (Canada) (NRC-CNRC)

Canadian Standards Association (CSA)

American National Standards Institute (ANSI)

US National Institute of Standards and Technology(NIST)

International standards bodies International Organization for Standardization (ISO)

International Telecommunications Union (ITU)

Non-governmental organizations


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Standards Bodies

ISO (www.iso.ch)

Non-treaty agency of the United Nations.

Collaborates standards development for informationtechnology.

ITU (www.itu.int) ITU-T: telecom sector of ITU

UN treaty agency that sets telecommunications standards.

ANSI (www.ansi.org)

The US national standards body.

Coordinates and accredits standards development across theUS.

IEEE (www.ieee.org)

US based international professional organization. Develops standards and submits to ANSI for approval.


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Standards Bodies

Telcordia (www.telcordia.com)

Coordinates and develops standards for US telephone service

ETSI (www.etsi.org)

European Telecomunication Standards Institute

Similar to Telcordia, but for Europe

IAB / IETF / IRTF Internet Architecture Board (www.iab.org)

Internet Engineering Task Force (www.ietf.org)

Internet Research Task Force (www.irtf.org)

Object Management Group (OMG) (www.omg.org)

Consists of many companies Develops/co-ordinates CORBA/IDL, UML standards

WWW consortium (www.w3.org)

Develops/co-ordinates standards such as HTTP, HTML, XML,


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Protocol Description Techniques In order to ensure that protocol implementations are

interoperable, there should be an unambiguous definition fora protocol.

The protocol operation is typically described using one ormore of the following techniques:

State diagram State transition table

Standardized communications formal descriptiontechnique (FDT)

Specification and Description Language (SDL)

Message sequence charts (MSCs) Unified Modelling Language (UML)

OSI Service primitives


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Message Sequence Charts (MSCs) An exchange of messages over time is illustrated in order to

describe an aspect of the protocol operation (a scenario).

Since there are typically an infinite set of allowable messagesequences, a set of MSCs cannot completely describe aprotocol.

MSC format has been standardized by ITU-T (standardZ.120, revised 2000)

Phone Switch

lift receiver

dial tone

hang up

dial tone off

time


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OSI Service Primitives

Four classes defined in the OSI model:

Request: An entity wants the service to do some work

Indication: An entity is informed about an event

Response: An entity wants to respond to an event

Confirm: An entity receives confirmation of a previousrequest

Layer

N + 1

Layer

N

Layer

N

Layer

N + 1

1 request

peer protocol

3 response2 indication4 confirm


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OSI Service Primitives (2)

A set of service primitives that could describe a simple connection-based

protocol:

CONNECT.request Request a connection to be established

CONNECT.indication Signal the called party of an incoming request

CONNECT.response Used by the called party to accept or reject a call

CONNECT.confirm Tells the caller whether the call was accepted.

DATA.request Request that data be sent

DATA.indication Signal the arrival of data

DISCONNECT.request Request that the connection be released

DISCONNECT.indication Signal that the connection has been released


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The Internet

Grew out of the US defense department AdvancedProjects Research Agency network (ARPANET)

As other networks were connected to theARPANET, notably the National Science

Foundation network (NSFNET), the resultinginternetwork has become known as the Internet

Foundation is the TCP/IP (1983) protocols forcommunication


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The Internet

What does it mean to be on the Internet? machine runs TCP/IP protocol stack

machine has IP address

machine can send IP packets to other internet

hosts (connected to IP router) Four classic (1980s) Internet applications:

electronic mail

Usenet news

remote login

file transfer


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TCP/IP layers (2)

Internet Layer: Official packet format and protocol: Internet

Protocol (IP).

Layer function is to deliver IP packets to their

destination.

IP Version 4 in use, version 6 is being phased in(slowly)


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TCP/IP layers (2)

T

ransport Layer: Designed to allow source to destination

conversation.

Transmission Control Protocol (TCP) provides a

reliable, connection-oriented service User Datagram Protocol (UDP) is an unreliable,

connectionless protocol, used where promptdelivery is the chief concern


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TCP/IP layers (3)

Application Layer: file transfer (FTP)

e-mail (SMTP)

virtual terminal (T

ELNET

) news (NNTP)

World Wide Web (HTTP)

Documents

Notes01 Intro