Introduction to Networking 14-740: Fundamentals of Computer - - PowerPoint PPT Presentation

Material from Computer Networking: A Top Down Approach, 7th edition. J.F. Kurose and K.W. Ross

Introduction to Networking

14-740: Fundamentals of Computer Networks Bill Nace

Introduction to Networking

• Our goal:
• Get a “feel” for networks and the

terminology

• A quick skim through the course --

more detail and depth later

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Roadmap

• What is the Internet?
• Network Edge
• Network Core

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A Series of Tubes?

• According to Senator Ted Stevens

(R-Alaska) in a speech 28 Jun 06

• “the Internet is not something

that you just dump something

• n. It's not a big truck. It's a

series of tubes.”

• Widely ridiculed as overly

simplistic and incorrect

• Stevens lost 2008 election

• 12.5 billion connected computing devices (as of 2010

according to Cisco)...

• “hosts” = end systems
• ... running network applications
• ... connected with communication links
• fiber, copper, radio, satellite
• transmission rate = bandwidth
• ... sending packets (chunks of data)
• ... through routers (which forward them to the correct

destination)

The Internet: Nuts and Bolts

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Local ISP Regional ISP Company Network

“Nuts & bolts” view

Internet consists of:

• Protocols: control sending and

receiving of messages

• eg: TCP

, IP , HTTP , FTP , PPP

• Network of networks
• loosely hierarchical
• public Internet vs. private intranet
• Standards
• RFC: Request for Comments
• IETF: Internet Engineering Task

Force

Local ISP Regional ISP Company Network

A service view

Internet consists of:

• Communication infrastructure
• enables distributed applications
• web, games, email, file sharing
• Communication services
• provided to applications
• connectionless (unreliable)
• connection-oriented (reliable)

What’s a Protocol?

• Human protocols
• “what’s the time?”
• “I have a question”
• introductions
• A defined sequence of actions and

utterances

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Network Protocols

• Define the format, order of messages

sent and received among network entities and the actions taken on message transmission or receipt

• All communication activity on the Internet

is governed by protocols

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Protocols: Examples

Hi Hello Got the time? 2:00

TCP Connection Request TCP Connection Response GET http://www.cmu.edu/~wnace <file>

A Human Example A Network Example

Time

Q: Other human protocols?

Roadmap

• What is the Internet?
• Network Edge
• Network Core

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Local ISP Regional ISP Company Network

Network Structure: A Closer Look

• Network Edge
• Hosts running applications
• Clients and servers
• Network Core
• Routers
• Interconnected Networks
• “Network of networks”
• Communication Links
• Physical Media

Local ISP Regional ISP Company Network

At the Edge

• End Systems (hosts)
• Run application programs
• Web, email
• Source or destination for data
• Client / Server model
• client asks for and gets service ...
• ... from an “always-on” server
• ex: web browser / server
• Peer to peer model
• minimal use of dedicated servers

Connection-oriented Service

• Goal: data transfer between end systems
• Requires handshaking
• setting up data transfer ahead of time
• “Hi” “Hello” of human protocol
• Prepare “state” in two hosts
• Protocol: TCP
• Transmission Control Protocol

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Connectionless Service

• Goal: data transfer between end systems
• same as connection-oriented
• Works by sending data
• No handshaking
• Protocol: UDP
• User Datagram Protocol

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Different strokes

• Some apps use TCP
• HTTP (web)
• FTP (file transfer)
• Telnet (remote login)
• SMTP (email)
• Some apps use UDP
• DNS (Naming)
• Skype (VOIP)
• streaming media
• teleconferencing

Roadmap

• What is the Internet?
• Network Edge
• Network Core

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Local ISP Regional ISP Company Network

Network Core

• Mesh of interconnected routers
• “How is data transferred through

the net?”

• The fundamental question
• Circuit switching
• dedicated circuit per call
• Packet switching
• discrete chunks of data over

many paths

Local ISP Regional ISP Company Network

Circuit Switching

• End to end resources reserved for

“the call”

• link bandwidth
• switch capacity
• Dedicated resources: no sharing
• results in waste when not being

used

• Guaranteed performance
• Call setup required

• Network bandwidth divided into “pieces”
• pieces allocated to calls
• piece is idle if not used by owning call
• How to divide?
• FDM: Frequency Division Multiplexing
• TDM: Time Division Multiplexing
• Others: Code division, etc

Circuit switching

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FDM and TDM Example

FDM frequency time TDM frequency time 4 users

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Numerical Example

How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network?

• All links are 1.536 Mbps (e.g. T1 line)
• Each link uses TDM with 24 slots
• 500 msec to establish end-to-end circuit

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Transmission rates are bits per second for communication links For communication links, K / M / G are powers of 10 (M means 1,000,000)

Answer

• TDM splits transmission rate among all slots
• Single circuit transmission rate is
• File is 640,000 bits
• Time to transmit the file is
• Add set-up time

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Packet Switching

• Each end-to-end data stream gets divided into

chunks or “packets”

• each packet uses full bandwidth of link
• all users share network resources
• resources used as needed
• Store and Forward
• Packets move hop-at-a-time
• Each node gets complete packet before

forwarding

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Bandwidth division into “pieces” Dedicated allocation Resource reservation

The Ugly

• Resource Contention exists
• Aggregate demand can exceed

available resources

• Congestion happens
• packets queue, waiting to use a link

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• Sequence of packets from different

sources don’t have a fixed pattern

• Note: in TDM, each host would get the

same slots in the revolving TDM frame

• Pattern would be fixed

Statistical Multiplexing

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10 Mbps Ethernet 1.5 Mbps T1 line queue of packets waiting for output link statistical multiplexing

• For a packet of L bits
• ... on a link of R bps throughput
• Takes L / R seconds to transmit the

packet

• Entire packet must arrive at the router

before it can be transmitted on the next link

Store-and-forward

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L bit packet R bps R bps R bps

• What is transmission time for a 7.5 Mb

file from end-to-end on this network?

• All links are 1.5 Mbps
• Negligible processing, queueing and

switching times

• No connection establishment
• File sent as a single packet

Example

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L bit packet R bps R bps R bps

Answer

• Time for host to forward the file
• There are 3 hops in the network
• Total time to move the file end-to-end

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Message Segmentation

• Now, break the file into 5000 packets of

1500 bits each

• Time to forward a packet
• Time at which first packet reaches end
• Time at which second packet reaches end
• Time at which the whole file is transferred

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Packet vs circuit switching

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n users

1 Mbps link

• Packet switching allows for more users
• Imagine a 1Mbps link
• with n users
• active 10% of the time
• using 100Kbps when active
• Max n for circuit switching?
• Max n for packet switching?

Packet vs circuit switching

• Is packet switching a “slam dunk” winner?
• Great for bursty data
• resources can be shared
• simpler, no circuit set-up / reservations
• But, when congestion hits?
• packet delay and loss
• must add protection for reliable

transmission and congestion control

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Lesson Objectives

• By the end of this lesson, you will be able to:
• describe the components of the internet

according to the “nuts and bolts” model, including the role of each component

• describe the role of the following

components in the internet: protocol, network, hierarchy, standards, packet, router, communication link, application, connection-oriented service, connectionless service

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By the end of this lesson, you should be able to:

• identify the network components that belong to

the core and those that belong to the edge

• differentiate between circuit-switched and

packet-switched networks, including the pros and cons of each, performance differences of each, and the mechanisms each use to share the network bandwidth

• calculate end-to-end transmission time for data

sent on a store-and-forward network with no delays

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Next Lecture

• Physical Media
• Delay and loss in packet-switched networks
• Architecture of the Internet
• Reading
• The Design Philosophy of the DARPA Internet

Protocols by David Clark

• Published in 1988, but a retrospective paper on

protocol suite designed 15 years prior

• Influenced design of ISO OSI model
• Website ➙ Readings

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