Lecture 1Internet

CPE 401 / 601

Computer Network Systems

slides are modified from Dave Hollinger and Daniel Zappala

Lecture 1

Introduction

Network

“ ... communication system for connecting end-systems”

End-systems a.k.a. “hosts”• PCs, workstations• dedicated computers• network components

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Multiaccess vs. Point-to-point• Multi-access means shared medium– many end-systems share the same physical

communication resources (wire, frequency, ...)– There must be some arbitration mechanism.

• Point-to-point– only 2 systems involved– no doubt about where data came from !

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Internetwork• Connection of 2 or more distinct (possibly dissimilar)

networks.• Requires some kind of network device to facilitate the

connection.

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Net A Net B

The Internet

• millions of connected computing devices: hosts = end systems – running network apps

• communication links– fiber, copper, radio,

satellite

• routers: – forward packets

(chunks of data)

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Home network

Institutional network

Mobile network

Global ISP

Regional ISP

router

PC

server

wirelesslaptop

cellular handheld

wiredlinks

access points

The network edge:

• end systems (hosts):– run application programs– e.g. Web, email– at “edge of network”

client/server

peer-peer

client/server model client host requests, receives

service from always-on server e.g. Web browser/server;

email client/server peer-peer model:

minimal (or no) use of dedicated servers

e.g. Skype, BitTorrent

The Internet

7Internet Mapping Project, Bill Cheswick

A Network of Networks• roughly hierarchical

– Tier-1 ISPs provide national, international coverage

– Tier-2 ISPs provide regional coverage

– Tier-3 and lower levels provide local coverage

• any tier may sell to business and residential customers

• any ISP may have a link to any other ISP– not strictly hierarchical

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Inernet2 Backbone

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Level3 Backbone

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Many Different Internet Service Providers

• Each network is independent

• Interoperability requires using Internet standards: IP, TCP– the Internet is global and must

run these standards– your private intranet can do

whatever you want it to do

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Internet Design Goals• primary goal: interoperability among existing networks

– a network of networks– obey administrative boundaries

• secondary goals– fault tolerance– multiple transport protocols– support a variety of networks– distributed management– cost effective, low effort for host attachment, accountability

• first three were more important, so remaining four did not receive as much attention

• no mention of security

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Internet Design Principles

• minimal assumptions about services network should support– ability to send packets– no reliability or security

• end-to-end principle– keep the core of the network as simple as possible – put complex functionality at the edges– exception: significant performance improvement

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The Internet Hourglass

Internet 14

Layering• Divide a task into pieces and then solve each

piece independently (or nearly so)

• Establishing a well defined interface between layers makes porting easier

• Major Advantages: Code Reuse Extensibility

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The Internet at each Hop

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Interface and Peer-to-peer Protocols

• Interface protocols describe communication between layers on the same endpoint.

• Peer-to-peer protocols describe communication between peers at the same layer.

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Process

Transport

Network

Data Link

Process

Transport

Network

Data Link

Interface Protocols

Peer-to-peer Protocols

Protocol• An agreed upon convention for communication.

– both endpoints need to understand the protocol.

• Protocols must be formally defined and unambiguous!

• Protocols define – format, – order of msgs sent and received among network entities, – actions taken on msg transmission, receipt

• We will study lots of existing protocols and perhaps develop a few of our own.

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The Physical Layer

• Responsibility:– transmission of raw bits over a communication

channel

• Issues:– mechanical and electrical interfaces– time per bit– distances

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The Data Link Layer • Responsibility:– provide an error-free communication link– Data Link Control sublayer– Medium Access Control sublayer

• Issues:– framing (dividing data into chunks)• header & trailer bits

– addressing

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10110110101 01100010011 10110000001

The Network Layer

• Responsibilities:– path selection between end-systems (routing).– flow control.– fragmentation & reassembly– translation between different network types.

• Issues:– packet headers– virtual circuits

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The Transport Layer

• Responsibilities:– provides virtual end-to-end links between peer

processes.– end-to-end flow control

• Issues:– headers– error detection – reliable communication

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The Application Layer

• Responsibilities:– anything not provided by any of the other layers– TCP/IP model• Session and Presentation Layer functions

• Issues:– application level protocols– appropriate selection of “type of service”

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Layering & Headers • Each layer needs to add some control information to

the data in order to do it’s job.

• This information is typically prepended to the data before being given to the lower layer.

• Once the lower layers deliver the data and control information - the peer layer uses the control information.

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