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Networking Primer - The Internetand the Link Layer

ECE 256

Romit Roy ChoudhuryDept. of ECE and CS

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Slides are from ECE 156Designed to help you recall undergrad

material

Please be patient if you remember most of this …

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On the Shoulders of Giants 1961: Leonard Kleinrock published a work on

packet switching

1962: J. Licklider described a worldwide network of computers called Galactic Network

1965: Larry Roberts designed the ARPANET that communicated over long distance links

1971: Ray Tomilson invents email at BBN

1972: Bob Kahn and Vint Cerf invented TCP for reliable packet transport

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On the Shoulders of Giants … 1973: David Clark, Bob Metcalfe

implemented TCP and designed ethernet at Xerox PARC

1975: Paul Mockapetris developed DNS system for host lookup

1980: Radia Perlman invented spanning tree algorithm for bridging separate networks

Things snowballed from there on …

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What we have today is beyond any of the inventors’ imagination …

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And YOU are here

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And by “YOU” I mean …

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“Cool” internet appliances

World’s smallest web serverhttp://www-ccs.cs.umass.edu/~shri/iPic.html

IP picture framehttp://www.ceiva.com/

Web-enabled toaster +weather forecaster

Internet phones

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And Of Course real people …

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InterNetwork Millions of end points (you, me, and

toasters) connected across a mesh of links Many end points can be addressed by numbers Many others lie behind a virtual end point

Many networks form a bigger network

The overall strcture called the Internet With a capital I Defined as a network of networks

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Internet structure: network of networks

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

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

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-1 providers interconnect (peer) privately

NAP

Tier-1 providers also interconnect at public network access points (NAPs)

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Tier-1 ISP: e.g., Sprint

Sprint US backbone network

Seattle

Atlanta

Chicago

Roachdale

Stockton

San Jose

Anaheim

Fort Worth

Orlando

Kansas City

CheyenneNew York

PennsaukenRelayWash. DC

Tacoma

DS3 (45 Mbps)OC3 (155 Mbps)OC12 (622 Mbps)OC48 (2.4 Gbps)

…

to/from customers

peering

to/from backbone

….

………POP: point-of-presence

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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

France telecome, Tiscali, etc. buys from Sprint

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISPTier-2 ISP

Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet

Tier-2 ISPs also peer privately with each other, interconnect at NAP

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Internet structure: network of networks

“Tier-3” ISPs and local ISPs (Time Warner, Earthlink, etc.) last hop (“access”) network (closest to end systems)

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISPTier-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

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Internet structure: network of networks

a packet passes through many networks! Local ISP (taxi) -> T1 (bus) -> T2 (domestic) -> T3 (international)

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISPTier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

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Organizing the giant structureNetworks are

complex! many “pieces”:

hosts routers links of various

media applications protocols hardware,

software

Question: Is there any hope of organizing structure

of network?

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Turn to analogies in 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 routingairplane routing

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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

gate

takeoff/landing

airplane routing

Layering of airline functionality

Layers: each layer implements a service layers communicate with peer layers rely on services provided by layer below

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Why layering? explicit structure allows identification,

relationship of complex system’s pieces

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

transparent to rest of system e.g., change in aircraft runway does not

affect boarding gate

layering considered harmful?

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Protocol “Layers” Service of each layer encapsulated

Universally agreed services called PROTOCOLS

A large part of this course will focus on designing protocols for

networking systems

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Internet protocol stack application: supporting network

applications FTP, SMTP, HTTP

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, WiFi, Bluetooth

physical: bits “on the wire”

application

transport

network

link

physical

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messagesegment

datagramframe

sourceapplicatio

ntransportnetwork

linkphysical

HtHnHl MHtHn MHt M

M

destinationapplicatio

ntransportnetwork

linkphysical

HtHnHl MHtHn MHt M

Mnetwork

linkphysical

linkphysical

HtHnHl MHtHn M

HtHnHl MHtHn M

HtHnHl M HtHnHl M

router

switch

Encapsulation

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PHY and Link Layer

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PHY and Link Layer The Layers that make the connections

Sends signals on physical media Schedules who gets to transmit Detects transmission errors and collisions Etc.

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Physical Link / Media Guided media

Twisted pair

Coaxial cable

Fiber optics

Unguided media terrestrial microwave up to 45 Mbps WiFi LAN 11Mbps, 54 Mbps Cellular Wide-area 3G: hundreds of kbps Satellite Kbps to 45Mbps

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Laying out Access networksQ: How to connect end

systems to edge router?

Mobile users

Residential access nets

Institutions, schools

Backbones

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Residential access: point to point access

Dialup via modem up to 56Kbps direct access

to router (often less) Can’t surf and phone at

same time: can’t be “always on”

ADSL: asymmetric digital subscriber line up to 1 Mbps upstream (today typically < 256

kbps) up to 8 Mbps downstream (today typically < 1

Mbps) FDM: 50 kHz - 1 MHz for downstream 4 kHz - 50 kHz for upstream 0 kHz - 4 kHz for ordinary telephone

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Residential access: Networked

Cable modems HFC: hybrid fiber coax asymmetric: up to 30Mbps downstream, 2

Mbps upstream

Network of cable/fiber attach homes to ISP router•Homes share access to router

Deployment: available via cable TV companies

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Residential access: cable modems

Diagram: http://www.cabledatacomnews.com/cmic/diagram.html

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Cable Network Architecture: Overview

home

cable headend

cable distributionnetwork (simplified)

Typically 500 to 5,000 homes

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Cable Network Architecture: Overview

home

cable headend

cable distributionnetwork

server(s)

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Cable Network Architecture: Overview

home

cable headend

cable distributionnetwork (simplified)

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Cable Network Architecture: Overview

home

cable headend

cable distributionnetwork

Channels

VIDEO

VIDEO

VIDEO

VIDEO

VIDEO

VIDEO

DATA

DATA

CONTROL

1 2 3 4 5 6 7 8 9

FDM:

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ADSL Vs Cable DSL is point to point

Thus data rate does not reduce when neighbor uses DSL

But, DSL uses twisted pair Transmission

technology cannot support more than ~10Mbps

Cable modems share pipe to the cable headend Data rate reduces when

neighbor surfing

However, fiber optic lines offer significantly higher data rate (fat pipe) Even with neighbors,

your data rate can be higher

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Wireless Access Networks

shared wireless access network connects end system to router via base station aka “access

point”

wireless LANs: 802.11b/g (WiFi): 11 or 54 Mbps

wider-area wireless access provided by telco operator 4G, WiMax, LTE

•Will it happen??

basestation

mobilehosts

router

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Communication on Links(Delay, Queuing, and Packet Loss)

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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

A

B

packet being transmitted (delay)

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

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Four sources of packet delay

1. nodal processing: check bit errors determine output link

A

B

propagationtransmission

nodalprocessing queueing

2. queueing time waiting at output

link for transmission depends on

congestion level of router

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

A

B

propagationtransmission

nodalprocessing queueing

Note: s and R are very different quantities!

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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 +++=

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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!

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“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

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“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

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Medium Access ControlIn

Computer Networks

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Random Access Protocols Trivial Solution: When node has packet to send

transmit at full channel data rate R. no a priori coordination

Two or more transmitting nodes ➜ “collision” Collision detected by comparing signal with channel content

Random access MAC protocol specifies: how to schedule communications how to recover from collisions

Examples of random access MAC protocols: slotted ALOHA ALOHA CSMA, CSMA/CD, CSMA/CA

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Pure (unslotted) ALOHA unslotted Aloha: simple, no synchronization when frame first arrives

transmit immediately collision probability increases:

frame sent at t0 collides with other frames sent in [t0-1,t0+1]

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Pure Aloha efficiency

P(success by given node) = P(node transmits) . P(no other node transmits in [p0-1,p0] . P(no other node transmits in [p0-1,p0] = p . (1-p)N-1 . (1-p)N-1

= p . (1-p)2(N-1)

… choosing optimum p and then letting n -> infty ...

= 1/(2e) = .18

Very Poor !

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Slotted ALOHAAssumptions all frames same size

time is divided into equal size slots

nodes start to transmit frames only at beginning of slots

nodes are synchronized

Operation when node obtains fresh

frame, it transmits in next slot

no collision, node can send new frame in next slot

if collision, node retransmits frame in each subsequent slot with prob. p until success

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Slotted ALOHA

Pros single active node can

continuously transmit at full rate of channel

highly decentralized: only slots in nodes need to be in sync

simple

Cons collisions, wasting

slots idle slots nodes must be able to

detect collision in less than time to transmit packet

clock synchronizationWhy?

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Slotted Aloha efficiency

Suppose N nodes with many frames to send, each transmits in slot with probability p

prob that node 1 has success in a slot = p(1-p)N-1

prob that any node has a success = Np(1-p)N-1

For max efficiency with N nodes, find p* that maximizes Np(1-p)N-1

For many nodes, take limit of Np*(1-p*)N-1

as N goes to infinity, gives 1/e = .37

At best: channelused for useful transmissions 37%of time!

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CSMA (Carrier Sense Multiple Access)

CSMA: listen before transmit:If channel sensed idle: transmit entire frame

If channel sensed busy, defer transmission

Human analogy: don’t interrupt others!

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Carrier Sensing

Listen before you talk Carrier sense multiple access (CSMA) Defer transmission when signal on channel

A CB

Don’ttransmit

Can collisions still occur?

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Deterministic MAC protocolsPolling: master node

“invites” slave nodes to transmit in turn

concerns: polling overhead latency single point of

failure (master)

Token passing: control token passed

from one node to next sequentially.

token message concerns:

token overhead latency single point of failure

(token)

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Summary of MAC protocols What do you do with a shared media?

Channel Partitioning, by time, frequency or code•Time Division, Frequency Division

Random partitioning (dynamic), •ALOHA, S-ALOHA, CSMA, CSMA/CD•carrier sensing: easy in some technologies (wire),

hard in others (wireless)•CSMA/CD used in Ethernet•CSMA/CA used in 802.11

Taking Turns•polling from a central site, token passing

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Questions ?

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Backup Slides

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Wired Vs Wireless Media AccessBoth are on shared media.Then, what’s really the problem ?

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Wired

Collision Detection Tx can transmit and listen at the same time

• If (Transmitted_Signal != Sensed_Signal) Collision

Channel Condition ~ identical at Tx and Rx

CBA

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Wireless

Collision Avoidance H/W can either transmit or receive While transmitting, cannot detect a collision Detection is based on SINR

•Thus must take educated decision when to transmit

Channel Condition Tx unaware of signal quality at receiver Channel dispersion large – high uncertainty

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Thoughts Please attend seminars

Equivalent to reading 3 papers in 1 hour Priceless Check out the Comp. Eng Seminar series

•Lot of great speakers are scheduled over the semester

People have begun selecting slots Please start looking

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CSMA collisions

collisions can still occur:propagation delay means two nodes may not heareach other’s transmissioncollision:entire packet transmission time wasted

spatial layout of nodes

note:role of distance & propagation delay in determining collision probability

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CSMA/CD (Collision Detection)

CSMA/CD: carrier sensing, deferral as in CSMA collisions detected within short time colliding transmissions aborted, reducing channel

wastage collision detection:

easy in wired LANs: measure signal strengths, compare transmitted, received signals

difficult in wireless LANs: receiver shut off while transmitting

human analogy: the polite conversationalist

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CSMA/CD collision detection

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Physical Media Bit: propagates between

transmitter/rcvr pairs physical link: what lies

between transmitter & receiver

guided media: signals propagate in solid

media: copper, fiber, coax unguided media:

signals propagate freely, e.g., radio

Twisted Pair (TP) two insulated copper

wires Category 3: traditional

phone wires, 10 Mbps Ethernet

Category 5: 100Mbps Ethernet

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Physical Media: coax, fiber

Coaxial cable: two concentric copper

conductors bidirectional baseband:

single channel on cable legacy Ethernet

broadband: multiple channels on

cable HFC

Fiber optic cable: glass fiber carrying

light pulses, each pulse a bit

high-speed operation: high-speed point-to-point

transmission (e.g., 10’s-100’s Gps)

low error rate: repeaters spaced far apart ; immune to electromagnetic noise

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Physical media: radio

signal carried in electromagnetic spectrum

no physical “wire” bidirectional propagation

environment effects: reflection obstruction by objects interference

Radio link types: terrestrial microwave

e.g. up to 45 Mbps channels

LAN (e.g., Wifi) 11Mbps, 54 Mbps

wide-area (e.g., cellular) e.g. 3G: hundreds of kbps

satellite Kbps to 45Mbps channel

(or multiple smaller channels)

270 msec end-end delay geosynchronous versus low

altitude

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Applying the concepts later Several protocol designs will require solid

understanding of delay Bandwidth estimation TCP congestion control TCP flow control TCP loss discrimination MAC protocols for wireless networks

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Link Layer 5.1 Introduction and

services 5.2 Error detection

and correction 5.3Multiple access

protocols 5.4 Link-Layer

Addressing 5.5 Ethernet

5.6 Hubs and switches 5.7 PPP 5.8 Link Virtualization:

ATM and MPLS