Introduction 1-1

Chapter 1Computer Networksand the Internet

Textbook: Computer Networking: A Top Down Approach ,5th edition. Jim Kurose, Keith RossAddison-Wesley, April 2009.

http://www.aw-bc.com/kurose-ross

These slides are adapted from the slides that accompany the textbook

Introduction 1-2

Chapter 1: IntroductionOur goal: get context,

overview, “feel” of networking

more depth, detail later in course

approach: descriptive use Internet as

example

Overview: what’s the Internet what’s a protocol? network edge & access network core performance: loss, delay,

throughput, capacity protocol layers, service

models security

Introduction 1-3

Chapter 1: roadmap

1.1 What is the Internet?1.2 Network edge1.3 Network core1.4 Delay & loss in packet-switched

networks1.5 Protocol layers, service models1.6 Security1.7 History

Introduction 1-4

What’s the Internet: “nuts and bolts” view

millions of connected computing devices: hosts, end-systems PCs workstations, servers PDAs phones, toasters

running network apps communication links

fiber, copper, radio, satellite

transmission rate = bandwidth

routers: forward packets (chunks of data)

local ISP

companynetwork

regional ISP

router workstation

servermobile

Introduction 1-5

What’s the Internet: “nuts and bolts” view protocols control sending,

receiving of msgs e.g., TCP, IP, HTTP, FTP,

PPP

Internet standards RFC: Request for comments IETF: Internet Engineering

Task Force

Internet: “network of networks” loosely hierarchical public Internet versus

private intranet

local ISP

companynetwork

regional ISP

router workstation

servermobile

Introduction 1-6

Internet structure: network of networks

roughly hierarchical at center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T,

Cable and Wireless), national/international coverage treat each other as equals (peering points)

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-1 providers interconnect (peer) privately

Introduction 1-7

Internet structure: network of networks

“Tier-2” ISPs: smaller (often regional) ISPs Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet tier-2 ISP is customer oftier-1 provider

Tier-2 ISPs also peer privately with each other.

Introduction 1-8

Internet structure: network of networks

“Tier-3” ISPs and local ISPs last hop (“access”) network (closest to end systems)

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Local and tier- 3 ISPs are customers ofhigher tier ISPsconnecting them to rest of Internet

Introduction 1-9

Tier-1 ISP: e.g., Sprint

…

to/from customers

peering

to/from backbone

….

………

POP: point-of-presence

POPs reside in buildings like this London IXP

10

Internet Core Routers Look Like These

11

Router on“paper”

Introduction 1-12

A closer look at network structure: network edge:

applications and hosts network core:

routers network of networks

access networks, physical media: communication links

Introduction 1-13

What’s the Internet: a service view communication infrastructure

enables distributed applications: Web, email, games, e-commerce,

database., voting, file (MP3) sharing

communication services provided to apps: connectionless connection-oriented

services provided by protocols running on hosts and routers.

Introduction 1-14

What’s a protocol?human protocols: “what’s the time?” “I have a question” introductions

… specific msgs sent… specific actions

taken when msgs received, or other events

network protocols: machines rather than

humans all communication

activity in Internet governed by protocols

protocols define format, order of msgs sent and

received among network entities, and actions taken on msg transmission, receipt

Introduction 1-15

What’s a protocol?a human protocol and a computer network protocol:

Q: Other human protocols?

Hi

Hi

Got thetime?

2:00

TCP connection req

TCP connectionresponseGet http://www.awl.com/kurose-ross

<file>time

Introduction 1-16

Internet protocol stack application: supporting network

applications FTP, SMTP, HTTP

transport: process-process data transfer TCP, UDP

network: routing of datagrams from source to destination IP, routing protocols

link: data transfer between neighboring network elements PPP, Ethernet

physical: bits “on the wire”

application

transport

network

link

physical

2: Application Layer 17

Why stack or layering?

Dealing with complex systems (e.g., big c/c++ program)

Need structured, modular design for easy maintenance (layers = *.c files)

Layer n uses the services provided by layer n-1 (e.g., error/flow control, connection set up)

Interface between two layers, e.g., Application Programmer’s Interface (API) or sockets

Logical connection between two layer n entities (peers)

layering harmful (can’t do global/customized optimization) ?

Introduction 1-18

sourceapplicatio

ntransportnetwork

linkphysical

HtHn M

segment Ht

datagram

destination

application

transportnetwork

linkphysical

HtHnHl M

HtHn M

Ht M

M

networklink

physical

linkphysical

HtHnHl M

HtHn M

HtHn M

HtHnHl M

router

switch

encapsulation at each layermessage M

Ht M

Hn

frame

2: Application Layer 19

An analogy (Fig.1-10 of Tanenbaum)

The Edge

20

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

Introduction 1-21

Network edge: connection-oriented service

Goal: data transfer between end systems

handshaking: setup (prepare for) data transfer ahead of time Hello, hello back

human protocol set up “state” in two

communicating hosts

TCP - Transmission Control Protocol Internet’s connection-

oriented service

TCP service [RFC 793] reliable, in-order byte-

stream data transfer loss: acknowledgements

and retransmissions

flow control: sender won’t overwhelm

receiver

congestion control: senders “slow down

sending rate” when network congested

Introduction 1-22

Network edge: connectionless service

Goal: data transfer between end systems same as before!

UDP - User Datagram Protocol [RFC 768]: Internet’s connectionless service unreliable data

transfer no flow control no congestion

control

App’s using TCP: HTTP (Web), FTP (file

transfer), Telnet (remote login), SMTP (email)

App’s using UDP: streaming media,

teleconferencing, DNS, Internet telephony

2: Application Layer 23

Access networks and physical media

Q: How to connect end systems to edge router?

residential access nets institutional access

networks (school, company)

mobile access networks

2: Application Layer 24

Wireless access networks wireless LANs:o 802.11b (2.4GHz): 11 Mbps; o 802.11a (5GHz), 802.11g (2.4GHz): 54Mps o 802.11n (2.4 or 5GHz): up to 600Mps

(40Mhz channel and MIMO)

Personal Area Networks (PANs)o Bluetooth: upto 1 Mbps within ~ 10m

access point

router

Wide-area Cellular Accesso 2G (GSM, TDMA, CDMA, iDEN) 10-20kpbs (circuit-switched)o 2.5G system (GPRS, 1xRTT, EDGE ) 30-50kbps ( up to

114kbps)o 3G (UMTS, W-CDMA) 300 – 500kbps, (up to a few Mbps)

o 1xEvolutional Data Only (EV-DO) 2.4M/700K)o 1XEV – DV (Data and Voice) (3.1M/1.5M) and HSPDA

o 4G: LTE (and LTE-Advanced) by 3GPP, TD-LTE in China, and WiMax (by IEEE 802.16): about 100Mbps down and 50Mbps up.

2: Application Layer 25

Home networks

Typical home network components: cable (or DSL) modem router/firewall/NAT Ethernet wireless access point

DD-WRT, Tomato…

wirelessaccess point

wirelesslaptops

router/firewall

cablemodem

to/fromcable

headend

Ethernet(switched)

Introduction 1-26

The Network Core

mesh of interconnected routers (wide area networks or WANs)

the fundamental question: how is data transferred through net? circuit switching (CS)

dedicated circuit per call: e.g. telephone net

packet-switching (PS) data sent thru net in discrete “chunks” called “packets”

Introduction 1-27

Network Core: Circuit Switching

End-end resources reserved for “call”

call setup required link bandwidth, switch

capacity dedicated resources to

each call: no sharing multiple calls can be

multiplexed onto a link guaranteed

performance

Introduction 1-28

Network Core: Circuit Switching

network resources (e.g., bandwidth) divided into “pieces”

pieces allocated to calls resource piece idle if

not used by owning call (dedicated, no sharing)

not bandwidth efficient if data is sporadic (or bursty with a variable bit rate)

dividing link bandwidth into “pieces” (for call grooming, aggregating or multiplexing) frequency division (wavelength division)

• f x C (speed of light) time division code division (e.g.,

wireless/cellular) space division (with a

multi-fiber link)

Introduction 1-29

Circuit Switching: TDMA and TDMA

FDMA

frequency

time

TDMA

frequency

time

4 users

Example:

Introduction 1-30

Electromagnetic Spectrum

Introduction 1-31

Network Core: Packet Switching

each end-end data stream divided into packets

user A & B’s packets share network resources

each packet uses full link bandwidth

resources used as needed

each packet has a “header” (containing e.g., destination address) in addition to “payload” (data)

Store and Forward (requires buffer and introduces delay)

Statistical multiplexing (support an aggregate demand that exceeds link bandwidth)

Bandwidth division into “pieces”

Dedicated allocationResource reservation

Introduction 1-32

Packet-switching: store-and-forward

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

must have a large enough buffer

incurs transmission delay per hop

alternative: cut-through switching as in CS

Example: transmit (push out) a

packet of L bits on to link of R bps

L = 7.5 Mbits R = 1.5 Mbps Transmission delay

per hop = L/R = 5 sec Total transmission

delay (3 hops) = 15 sec

R R RL

Introduction 1-33

Four sources of delay (I)

1. Transmission delay: R=link bandwidth

(bps) L=packet length (bits) time to send bits into

link = L/R

2. Propagation delay: d = length of physical

link c = propagation speed in

medium (~2x108 m/sec) propagation delay = d/c

A

B

propagation

transmission

nodalprocessing queueing

Note: c and R are very different quantities!

Introduction 1-34

Four sources of delay (II)

3. nodal processing: check bit errors determine output link

(based on e.g., the destination address)

A

B

propagation

transmission

nodalprocessing queueing

4. queueing time waiting at output

link for transmission (and at input buffer too)

depends on congestion level of router

Introduction 1-35

Nodal delay

dproc = processing delay typically a few microsecs or less

dqueue = queuing delay depends on congestion

dtrans = transmission delay = L/R, significant for low-speed links

dprop = propagation delay a few microsecs to hundreds of msecs

proptransqueueprocnodal ddddd

Introduction 1-36

Queueing delay (revisited)

R=link bandwidth (bps) L=packet length (bits) a=average packet

arrival rate

traffic intensity = La/R

La/R ~ 0: average queueing delay small La/R -> 1: delays become large La/R > 1: more “work” arriving than can

be serviced, average delay infinite!

Introduction 1-37

“Real” Internet delays and routes

What do “real” Internet delay & loss look like? Traceroute program: provides delay

measurement from source to router along end-end Internet path towards destination. For all i: sends three packets that will reach router i on path

towards destination router i will return packets to sender sender times interval between transmission and reply.

3 probes

3 probes

3 probes

Introduction 1-38

“Real” Internet delays and routes

1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms3 cht-vbns.gw.umass.edu (128.119.3.130) 6 ms 5 ms 5 ms4 jn1-at1-0-0-19.wor.vbns.net (204.147.132.129) 16 ms 11 ms 13 ms 5 jn1-so7-0-0-0.wae.vbns.net (204.147.136.136) 21 ms 18 ms 18 ms 6 abilene-vbns.abilene.ucaid.edu (198.32.11.9) 22 ms 18 ms 22 ms7 nycm-wash.abilene.ucaid.edu (198.32.8.46) 22 ms 22 ms 22 ms8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms9 de2-1.de1.de.geant.net (62.40.96.129) 109 ms 102 ms 104 ms10 de.fr1.fr.geant.net (62.40.96.50) 113 ms 121 ms 114 ms11 renater-gw.fr1.fr.geant.net (62.40.103.54) 112 ms 114 ms 112 ms12 nio-n2.cssi.renater.fr (193.51.206.13) 111 ms 114 ms 116 ms13 nice.cssi.renater.fr (195.220.98.102) 123 ms 125 ms 124 ms14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms15 eurecom-valbonne.r3t2.ft.net (193.48.50.54) 135 ms 128 ms 133 ms16 194.214.211.25 (194.214.211.25) 126 ms 128 ms 126 ms17 * * *18 * * *19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms

traceroute: gaia.cs.umass.edu to www.eurecom.frThree delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu

* means no response (probe lost, router not replying)

trans-oceaniclink

Introduction 1-39

Throughput

throughput: rate (bits/time unit) at which bits transferred between sender/receiver instantaneous: rate at given point in time average: rate over longer period of time

server, withfile of F bits

to send to client

link capacity

Rs bits/sec

link capacity

Rc bits/sec pipe that can carry

fluid at rate

Rs bits/sec)

pipe that can carryfluid at rate

Rc bits/sec)

server sends bits

(fluid) into pipe

Introduction 1-40

Throughput (more)

Rs < Rc What is average end-end throughput?

Rs bits/sec Rc bits/sec

Rs > Rc What is average end-end throughput?

Rs bits/sec Rc bits/sec

link on end-end path that constrains end-end throughput

bottleneck link

Introduction 1-41

Throughput: Internet scenario

10 connections (fairly) share backbone bottleneck link R

bits/sec

Rs

Rs

Rs

Rc

Rc

Rc

R

per-connection end-end throughput: min(Rc,Rs,R/10)

in practice: Rc or Rs is often bottleneck

Introduction 1-42

Packet Switching: Statistical Multiplexing

Sequence of A & B packets does not have fixed pattern statistical (time division) multiplexing.

Unlike in TDM, where each host gets same slot in revolving TDM frame.

A

B

C10 MbsEthernet

1.5 Mbs

D E

statistical multiplexing

queue of packetswaiting for output

link

Introduction 1-43

Packet switching versus circuit switching

1 Mbit link each user:

100 kbps when “active”

active 10% of time

circuit-switching: 10 users

packet switching: with 35 users,

probability > 10 active less than .0004

Packet switching allows more users to use network!

N users

1 Mbps link

Introduction 1-44

Packet switching versus circuit switching

Great for bursty data resource sharing (statistical multiplexing) simpler, no call setup

Excessive congestion: 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 (chapter 6)

Is packet switching a “slam dunk winner?”

Introduction 1-45

Message Switching (A mesg = 7.5Mbits) at 1.5Mbps transmission rate

Introduction 1-46

Packet Switching: Message Segmenting

Now break up the message into 5000 packets

Each packet 1,500 bits

1 msec to transmit packet on one link

pipelining: each link works in parallel

Delay reduced from 15 sec to 5.002 sec

Q: the smaller the packets, the better?

Introduction 1-47

Packet-switched networks: forwarding Goal: move packets through routers from source to

destination we’ll study several path selection (i.e. routing) algorithms (ch. 4)

datagram network: destination address in packet determines next hop (according

to the network’s routing algorithm) routes may change during session (out of order delivery) due to

congested/failed links/routers analogy: driving, asking directions at each/every intersection

virtual circuit network: each packet carries tag (virtual circuit ID), tag determines next

hop fixed path determined at call setup time, remains fixed thru call

(can be manually chosen for traffic engineering purposes) routers maintain per-call state

Introduction 1-48

VC vs Datagram

With VC, two packets with the same destination can be assigned two different VC# and forced to take different paths.

Introduction 1-49

Network Taxonomy

Telecommunicationnetworks

Circuit-switchednetworks

FDM TDM

Packet-switchednetworks

Networkswith VCs

DatagramNetworks

• One may have a circuit-switched network (e.g. optical WDM) below, over which a packet-switched network runs • Internet provides both connection-oriented (TCP) and connectionless services (UDP) to apps.

Introduction 1-50

Network Security The field of network security is about:

how bad guys can attack computer networks how we can defend networks against attacks how to design architectures that are immune

to attacks Internet not originally designed with

(much) security in mind original vision: “a group of mutually trusting

users attached to a transparent network” Internet protocol designers playing “catch-up

” Security considerations in all layers!

Introduction 1-51

The bad guys can sniff packetsPacket sniffing:

broadcast media (shared Ethernet, wireless) promiscuous network interface reads/records all packets

(e.g., including passwords!) passing by

A

B

C

src:B dest:A payload

Wireshark software used for end-of-chapter labs is a (free) packet-sniffer

Introduction 1-52

The bad guys can use false source addresses IP spoofing: send packet with false source

addressA

B

C

src:B dest:A payload

Introduction 1-53

The bad guys can record and playback

record-and-playback: sniff sensitive info (e.g., password), and use later password holder is that user from system point of

view

A

B

C

src:B dest:A user: B; password: foo

Introduction 1-54

Network Security more throughout this course A chapter focuses on security crypographic techniques: obvious uses

and not so obvious uses