Unix Servers Used in This Class Two Unix servers set up in CS department will be used for some...

Unix Servers Used in This Class Two Unix servers set up in CS

department will be used for some programming projects Machine name:

• eustis.eecs.ucf.edu• eustis2.eecs.ucf.edu

You can connect to them only in campus network

• If outside, set up VPN to campus network first• Read the following webpage to learn how to setup

VPN• http://www.cst.ucf.edu/about/telecommunications/

network-services/vpn/ Introduction 1-1

Need to use SSH to remote login SSH free software (many many others):

• Command shell client: PuTTY http://www.putty.org/• File transfer: WinSCP (for windows)

http://winscp.net/eng/index.php

Student can log in using default password Pyymmdd (birth year, month and day). For any login problems, please email

help@eecs.ucf.edu

Introduction 1-2

Basic Usage of Unix

You only need to remember a few basic commands for using the Eustis machine for this class Editor: pico (or edit on your computer and upload

the code to eustis) Command:

• cd, mkdir, rmdir, chdir, • cp, rm, ls, man,

There are many tutorials online http://www.ee.surrey.ac.uk/Teaching/Unix/ http://freeengineer.org/learnUNIXin10minutes.html Command line reference:

http://www.pixelbeat.org/cmdline.html Introduction 1-3

Basic Usage of Linux

You can use PuTTY to open many independent shells

For the program project 1, you can run client code on one eustis machine, and the server code in another eustis machine.

Introduction 1-4

Introduction 1-5

Chapter 1: roadmap

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 and throughput in packet-switched networks

1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History

Introduction 1-6

The Network Core

mesh of interconnected routers

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

dedicated circuit per call: telephone net

packet-switching: data sent thru net in discrete “chunks”

Introduction

Alternative core: circuit switchingend-end resources allocated

to, reserved for “call” between source & dest:

In diagram, each link has four circuits. call gets 2nd circuit in top link

and 1st circuit in right link. dedicated resources: no sharing

circuit-like (guaranteed) performance

circuit segment idle if not used by call (no sharing)

Commonly used in traditional telephone networks

Introduction 1-8

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 (no sharing)

dividing link bandwidth into “pieces” frequency divisiontime division

Introduction 1-9

Circuit Switching: FDM and TDM

frequency

timeTDM (GSM uses this)

frequency

4 users

Example:

TDMA: Time Division Multiplexing Access

Introduction 1-10

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 Each link uses TDM with 24 slots/sec 500 msec to establish end-to-end circuit

Let’s work it out!

Introduction 1-11

Network Core: Packet Switchingeach end-end data stream

divided into packets user A, B packets share

network resources

each packet uses full link bandwidth

resources used as needed

Bandwidth division into “pieces”Dedicated allocationResource reservation

Introduction 1-12

resource contention: aggregate resource demand can exceed amount

available congestion: packets queue, wait for link use store and forward: packets move one hop at a time

Node receives complete packet before forwarding

Network Core: Packet Switching

Introduction

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

one-hop numerical example:

L = 7.5 Mbits R = 1.5 Mbps one-hop transmission

delay = 5 sec

more on delay shortly …1-13

sourceR bps

destination123

L bitsper packet

end-end delay = 2L/R (assuming zero propagation delay)

Introduction 1-14

Packet Switching: Statistical Multiplexing

Sequence of A & B packets does not have fixed pattern, shared on demand statistical multiplexing.

C10 Mb/sEthernet

1.5 Mb/s

statistical multiplexing

queue of packetswaiting for output

Network Layer 4-15

Two key network-core functions

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

0100010101111001

dest address in arrivingpacket’s header

Introduction

Packet switching versus circuit switching

example: 1 Mb/s link each user:

• 100 kb/s when “active”• active 10% of time

circuit-switching: 10 users

packet switching: with 35 users, probability

> 10 active at same time is less than .0004 *

packet switching allows more users to use network!

N users

1 Mbps link

Q: how did we get value 0.0004?

Q: what happens if > 35 users ?

1-16* Check out the online interactive exercises for more examples

Introduction 1-17

Packet switching versus circuit switching

Great for bursty data resource sharing 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

QoS – Quality of Service still an unsolved problem (chapter 7)

Is packet switching a “slam dunk winner?”

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

accessnetaccess

accessnet

………

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

accessnet

accessnetaccess

accessnet

………

connecting each access ISP to each other directly doesn’t scale:

O(N2) connections.

accessnet

accessnetaccess

accessnet

………

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

globalISP

accessnet

accessnetaccess

accessnet

………

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

accessnet

accessnetaccess

accessnet

………

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

peering link

Internet exchange point

accessnet

accessnetaccess

accessnet

………

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

regional net

accessnet

accessnetaccess

accessnet

………

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

regional net

Content provider network

Introduction

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 its data centers to Internet, often bypassing tier-1, regional ISPs

accessISP

Regional ISP Regional ISP

IXP IXP

Tier 1 ISP Tier 1 ISP Google

Introduction

Tier-1 ISP: e.g., Sprint

to/from customers

peering

to/from backbone

………

POP: point-of-presence

Introduction 1-28

Chapter 1: roadmap

Introduction 1-29

How do loss and delay occur?packets queue in router buffers packet arrival rate to link exceeds output link

capacity packets queue, wait for turn

packet being transmitted (delay)

packets queueing (delay)

free (available) buffers: arriving packets dropped (loss) if no free buffers

Introduction 1-30

Four sources of packet delay

1. nodal processing: check bit errors determine output link

propagation

transmission

nodalprocessing queueing

2. queueing time waiting at output

link for transmission depends on congestion

level of router

Introduction 1-31

Delay in packet-switched networks3. Transmission delay: R=link bandwidth

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

link = L/R

4. Propagation delay: d = length of physical

link s = propagation speed

in medium (~2-3x108 m/sec)

propagation delay = d/s

propagation

transmission

nodalprocessing queueing

Note: s and R are very different quantities!

Introduction 1-32

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

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

traffic intensity = La/R

La/R ~ 0: avg. queueing delay small La/R 1: avg. queueing delay large La/R > 1: more “work” arriving than can be serviced, average delay infinite!

La/R ~ 0

Queueing delay (revisited)

La/R 11-33

* Check out the Java applet for an interactive animation on queuing and loss

Introduction 1-34

“ 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

Introduction 1-35

“ 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

Under Windows is “tracert”

Introduction 1-36

Traceroute from My Home Computer (last year)

Traceroute from My Home Computer (another time)

Introduction 1-37

Introduction 1-38

Online Traceroute Tools

Because UCF campus network blocks all ICMP packets, you need an outside machine to try it. Try on http://tools.pingdom.com/ping/ Try from different countries from

www.traceroute.org Check traceroute virtual path at:

• http://traceroute.monitis.com/

and• http://www.yougetsignal.com/tools/visual-tracert/

Introduction 1-39

Introduction 1-40

Packet loss

queue (aka buffer) preceding link in buffer has finite capacity

when packet arrives to full queue, packet is dropped (aka lost)

lost packet may be retransmitted by previous node, by source end system, or not retransmitted at all (UDP)

packet being transmitted

packet arriving tofull buffer is lost

buffer (waiting area)

Introduction 1-41

Throughput

throughput: rate (bits/time unit) at which bits transferred between sender/receiver instantaneous: rate at given point in time average: rate over long(er) 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 1-42

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 1-43

Throughput: Internet scenario

10 connections (fairly) share backbone bottleneck link R

bits/sec

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

in practice: Rc or Rs is often bottleneck

Introduction 1-44

Chapter 1: roadmap

Introduction 1-45

Protocol “Layers”Networks are

complex! many “pieces”:

hosts routers links of various

media applications protocols hardware,

software

Question: Is there any hope of organizing structure of

network?

Or at least our discussion of networks?

Introduction 1-46

Organization of air travel

a series of steps

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

Introduction 1-47

ticket (purchase)

baggage (check)

gates (load)

runway (takeoff)

airplane routing

departureairport

arrivalairport

intermediate air-trafficcontrol centers

airplane routing airplane routing

ticket (complain)

baggage (claim

gates (unload)

runway (land)

airplane routing

ticket

baggage

takeoff/landing

airplane routing

Layering of airline functionality

Layers: each layer implements a service via its own internal-layer actions relying on services provided by layer below

Introduction 1-48

Why layering?Dealing with complex systems: explicit structure allows identification,

relationship of complex system’s pieces layered reference model for discussion

modularization eases maintenance, updating of system change of implementation of layer’s service

transparent to rest of system e.g., change in gate procedure doesn’t

affect rest of system layering considered harmful?

Duplicate functions

Introduction 1-49

Internet protocol stack application: supporting network

applications FTP, SMTP, STTP

transport: host-host 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

physical

Introduction 1-50

ISO/OSI reference model presentation: allow applications

to interpret meaning of data, e.g., encryption, compression, machine-specific conventions

session: synchronization, checkpointing, recovery of data exchange

Internet stack “missing” these layers! these services, if needed, must

be implemented in application needed?

application

presentation

session

transport

network

physical

Introduction 1-51

messagesegment

datagram

sourceapplicatio

ntransportnetwork

linkphysical

HtHnHl M

HtHn M

destination

application

transportnetwork

linkphysical

HtHnHl M

HtHn M

networklink

physical

linkphysical

HtHnHl M

HtHn M

HtHnHl M

HtHn M

HtHnHl M HtHnHl M

router

switch

Encapsulation

Introduction 1-52

Chapter 1: roadmap

Introduction 1-53

Network Security attacks on Internet infrastructure:

infecting/attacking hosts: malware, spyware, worms, unauthorized access (data stealing, user accounts)

denial of service: deny access to resources (servers, link bandwidth)

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 1-54

What can bad guys do: malware? Spyware:

infection by downloading web page with spyware

records keystrokes, web sites visited, upload info to collection site

Virus infection by receiving

object (e.g., e-mail attachment), actively executing

self-replicating: propagate itself to other hosts, users

Worm: infection by passively

receiving object that gets itself executed

self- replicating: propagates to other hosts, usersSapphire Worm: aggregate scans/sec

in first 5 minutes of outbreak (CAIDA, UWisc data)

Introduction 1-55

Denial of service attacks attackers make resources (server, bandwidth)

unavailable to legitimate traffic by overwhelming resource with bogus traffic

1. select target

2. break into hosts around the network (see malware)

3. send packets toward target from compromised hosts

target

Introduction 1-56

Sniff, modify, delete your packetsPacket sniffing:

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

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

src:B dest:A payload

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

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