Material from Computer Networking: A Top Down Approach, 6th edition. J.F. Kurose and K.W. Ross

Introduction to Networking

14-740: Fundamentals of Computer NetworksBill Nace

14-740: Fall 2017

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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14-740: Fall 2017

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 on. It's not a big truck. It's a series of tubes.”

• Widely ridiculed as overly simplistic and incorrect

• Stevens lost 2008 election

14-740: Fall 2017

• 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

CompanyNetwork

“Nuts & bolts” viewInternet 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

CompanyNetwork

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)

14-740: Fall 2017

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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14-740: Fall 2017

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: ExamplesHi

Hello

Got the time?

2:00

TCP ConnectionRequest

TCP ConnectionResponse

GET http://www.cmu.edu/~wnace

<file>

A Human Example A Network Example

Tim

e

Q: Other human protocols?

14-740: Fall 2017

Roadmap

• What is the Internet?

• Network Edge

• Network Core

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Local ISP Regional ISP

CompanyNetwork

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

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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

14-740: Fall 2017

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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14-740: Fall 2017

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

14-740: Fall 2017

Roadmap

• What is the Internet?

• Network Edge

• Network Core

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Local ISP Regional ISP

CompanyNetwork

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

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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

14-740: Fall 2017

• 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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14-740: Fall 2017

FDM and TDM ExampleFDM

frequency

timeTDM

frequency

time

4 users

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14-740: Fall 2017

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 linksFor communication links, K / M / G are

powers of 10 (M means 1,000,000)

14-740: Fall 2017

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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14-740: Fall 2017

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 allocationResource reservation

14-740: Fall 2017

The Ugly

• Resource Contention exists• Aggregate demand can exceed

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

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14-740: Fall 2017

• 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 Ethernet1.5 Mbps T1 line

queue of packets waiting for output link

statistical multiplexing

14-740: Fall 2017

• 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

14-740: Fall 2017

• 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

14-740: Fall 2017

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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14-740: Fall 2017

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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14-740: Fall 2017

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?

14-740: Fall 2017

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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14-740: Fall 2017

Lesson Objectives• By the end of this lesson, the student 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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14-740: Fall 2017

By the end of this lesson, the student will 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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14-740: Fall 2017

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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