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CSC 401Data and Computer Communications Networks

Computer Networks and The Internet

Prof. Lina Battestilli

Fall 2017

Sec 1.3

Outline

Computer Networks and the Internet (Ch 1)1.1 What is the Internet?1.2 network edge

end systems, access networks, links

1.3 network core circuit switching,

packet switching,

network structure

1.4 delay, loss, throughput in networks1.5 protocol layers, service models1.6 networks under attack: security1.7 history

NCSU CSC401 © Lina Battestilli 2

Previous Lecture

A closer look at network structure:


mobile network

National or

global ISP

Local or

regional ISP

home network

institutionalnetwork

• network edge:applications and hostsservers in data centers

• access networks, physical media: wired, wireless communication links

• network core: interconnected routersnetwork of networks

Outline




packet switching,

network structure



The network core


• mesh of interconnected routers

• Two possibilities

– Circuit switched

– Packet switched

Circuit Switching: Telephone Network


DedicatedWire

DedicatedWire

• telephones are connected by a dedicated wire to a local exchange

• Early days, switchboard operators used a big patch-panel to manually connect

• the wire is dedicated to the phone conversation from the start to the end of the phone call.



DedicatedWire

DedicatedWire

Circuit Switch

1. Dial a number, which creates a dedicated circuit between the two phones. Each switch

maintains state to map the incoming circuit to the correct outgoing circuit.

2. Talk: digital phone system, our voice is sampled and digitized, and sent over the dedicated

circuit, typically 64kb/s for voice. Our phone conversation has a dedicated circuit

3. Hang up: the circuit is removed, and any state is removed at the switches along the path.

10


Source“Caller”

Central Office(C.O.)

Destination“Callee”

Central Office(C.O.)

TrunkExchange

Each phone call allocated 64kb/s. A 10Gb/s trunk line can carry over

150,000 calls.

“big fat pipes”2.4Gb/s-100Gb/s

Circuit switching for the Internet’s core?

• dedicated resources: no sharing

– circuit-like (guaranteed) performance

• resource piece idle if not used by owning call (no sharing)

• dividing link bandwidth into “pieces”

– frequency division

– time division

end-end resources reserved for, “call” between source, destination:

Restaurant Analogy

Circuit switching: FDM versus TDM

FDM

frequency

timeTDM

frequency

time

4 users

Example:

frame

e.g 8000 frames/sec, each slot 8bits -> 64Kbps circuit

FM radio stations use FDM

time is divided into frames of certain duration

Circuit Switching Example


• Host A to Host B• Each link is 1.536Mbps and uses TDM, 24 slots• End-to-end circuit establishment 500msec• How long will it take to transmit a 640 Kbits file?

A

B

dedicated resources: no sharing, circuit-like (guaranteed) performance, idle if not used

Circuit Switching Summary

• Each call has its own private, guaranteed,

isolated data rate from end-to-end.

• A call has three phases:1. Establish circuit from end-to-end (“dialing”)2. Communicate3. Close circuit (“tear down”)

• Originally, a circuit was an end-to-end physical

wire.

• Nowadays, a circuit is like a virtual private wire.

15

Circuit Switching Issues for the Internet

1. Inefficient. Computer communication tends to be very bursty. e.g. typing over an ssh connection, or viewing a sequence of web pages. If each communication has a dedicated circuit, it will be used very inefficiently.

2. Diverse Rates. Computers communicate at many different rates. e.g. a web server streaming video at 6Mb/s, or typing at 1 character per second over ssh. A fixed rate circuit will not be much use!

3. State Management. Circuit switches maintain per-communication state, which must be managed.

16

Outline




packet switching,

network structure



Packet Switching

19

Address Next-hop

B S2

C S3

D S3

AB

Packet Switch

S1

S2

S4

S3

C D

Data HeaderData B

• hosts break application-layer messages into packets

• mesh of interconnected routers• forward packets from one

router to the next, across links on path from source to destination

Packet Switching

20

Packet Switch

21

Packet Switching Advantages

A

R1

R2

R4

R3

B

Source Destination

- Packets are routed individually, by looking up

address in router’s local table.

- All packets share the full capacity of a link.

- The routers maintain no per-communication state.

Internet uses Packet Switching

22

• Efficient use of expensive links– Links were assumed to be expensive and scarce.

– Packet switching allows many, bursty flows to share the same link efficiently.

– “Circuit switching is rarely used for data networks, ... because of very inefficient use of the links”, Bertsekas/Gallager

• Resilience to failure of links & routers– “For high reliability, ... [the Internet] was to be a datagram

subnet, so if some lines and [routers] were destroyed, messages could be ... rerouted”, Tanenbaum

Host: sends packets of data

1-23

host sending function:

• takes application message

• breaks into smaller chunks, known as packets, of length Lbits

• transmits packet into access network at transmission rate R– link transmission rate, aka link

capacity, aka link bandwidth

R: link transmission ratehost

12

two packets,

L bits each

packettransmission

delay

time needed totransmit L-bit

packet into link

L (bits)

R (bits/sec)= =

Packet-switching: store-and-forward


• takes L/R seconds to transmit (push out) L-bit packet into link at R bps

• store and forward: entire packet must arrive at router before it can be transmitted on next link

sourceR bps

destination123

L bitsper packet

R bps

(assuming zero propagation delay)

𝑑𝑒𝑙𝑎𝑦 =2𝐿𝑅

Packet-switching: store-and-forward


sourceR bps

destination123

L bitsper packet

R bps

Q: Time between when the source begins to send the first packet until the destination has received all three packets?

(assuming zeropropagation delay)

Packet Switching: queueing delay, loss


A

B

CR = 100 Mb/s

R = 1.5 Mb/sD

Equeue of packetswaiting for output link, output buffer

resource contention: • aggregate resource demand (use of transmission link) can exceed amount

available

• congestion:

packets will queue, wait for link use

packets can be dropped (lost) if no memory to store them

Forwarding Tables and Routing Protocols

forwarding: move packets from router’s input to appropriate router output

routing: determines source-destination route taken by packets

routing algorithms

routing algorithm

local forwarding table

header value output link

0100

0101

0111

1001

3

2

2

1

1

23

dest address in arriving

packet’s header

Analogues to a driver that wants to ask for directions instead of using a map.

Packet Switching versus Circuit Switching

example:

1 Mb/s link

each user:

• 100 kb/s when “active”

• active 10% of time

packet switching allows more users to use network!

circuit-switching:10 users

packet switching:with 35 users, probability > 10 active

at same time is less than .0004

Nusers

1 Mbps link

Packet switching versus circuit switching

• great for bursty data– resource sharing

– simpler, no call setup

• excessive congestion possible: packet delay and loss– protocols needed for reliable data transfer, congestion control

Q: How to provide circuit-like behavior?– bandwidth guarantees needed for audio/video apps

– still an unsolved problem (ch 7, Multimedia Networking)

is packet switching a “slam dunk winner?”

Q: human analogies of reserved resources

(circuit switching) versus on-demand allocation

(packet-switching)?

Outline




packet switching,

network structure



Internet structure: network of networks

End systems connect to Internet via access ISPs (Internet Service Providers) Residential, company and university ISPs

Access ISPs in turn must be interconnected. So that any two hosts can send packets to each other

Resulting network of networks is very complex Evolution was driven by economics and national policies

Let’s take a stepwise approach to describe current Internet structure


Question: given millions of access ISPs, how to connect them together?

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnetaccess

net

accessnet


Option: connect each access ISP to every other access ISP?

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnetaccess

net

accessnet

connecting each access ISP

to each other directly doesn’t

scale: O(N2) connections.


accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnetaccess

net

accessnet

Option: connect each access ISP to a global transit ISP? Customerand provider ISPs have economic agreement.

global

ISP


accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnetaccess

net

accessnet

But if one global ISP is viable business, there will be competitors ….

ISP B

ISP A

ISP C


accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnetaccess

net

accessnet

But if one global ISP is viable business, there will be competitors …. which must be interconnected

ISP B

ISP A

ISP C

IXP

IXP

peering link

Internet exchange point


accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnetaccess

net

accessnet

… and regional networks may arise to connect access nets to ISPS

ISP B

ISP A

ISP C

IXP

IXP

regional net


accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnetaccess

net

accessnet

… and content provider networks (e.g., Google, Microsoft, Akamai ) may run their own network, to bring services, content close to end users

ISP B

ISP A

ISP B

IXP

IXP

regional net

Content provider network


at center: small # of well-connected large networks

– “tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national & international coverage

– content provider network (e.g, Google): private network that connects it data centers to Internet, often bypassing tier-1, regional ISPs

access

ISP

access

ISP

access

ISP

access

ISP

access

ISP

access

ISP

access

ISP

access

ISP

Regional ISP Regional ISP

IXP IXP

Tier 1 ISP Tier 1 ISP Google

IXP

Tier-1 ISP: e.g., Sprint

…

to/from customers

peering

to/from backbone

…

………

POP: point-of-presence

References

Some of the slides are identical or derived from

1. Slides for the 7th edition of the book Kurose & Ross, “Computer Networking: A Top-Down Approach,”

2. Slides by Jim Kurose for his CSC453 course at Umass

3. Slides from Nick McKeown, CS144 at Stanford University

NCSU CSC401 © Lina Battestilli

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