Lecture-3 1

Administrative Things Textbook in the bookstore ? Lecture notes on the web ?? Any questions and suggestions so

far ??? …

Lecture-3 2

Chapter 1: roadmap1.1 What 1.1 What isis the Internet? the Internet?1.2 Network edge1.3 Network core1.4 Network access and physical media1.5 Internet structure and ISPs1.5 Internet structure and ISPs1.6 Delay & loss in packet-switched

networks1.7 Protocol layers, service models1.8 History1.8 History

Lecture-3 3

A Review of Last Lecture network edge:

applications and hosts connection-oriented and connectionless services Internet protocols: ??? and ???

network core: routers, network of networks data switching: circuit switching and packet

switching comparison of the two switching approaches virtual circuit networks and datagram networks

access networks, physical media Dail-up, DSL, Cable modem, LAN, wireless LAN, 3G, Satellite Twisted pair copper, Coaxial cable, fiber optics, ratio

Lecture-3 4

Internet structure: network of networks

roughly hierarchical at center: “tier-1” ISPs

e.g., UUNet, BBN/Genuity, Sprint, AT&T national/international coverage

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)

Lecture-3 5

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

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 ISP is customer oftier-1 provider

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

Lecture-3 6

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

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

Lecture-3 7

A packet passes through many networks

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

Lecture-3 8

Chapter 1: roadmap1.1 What 1.1 What isis the Internet? the Internet?1.2 Network edge1.2 Network edge1.3 Network core1.3 Network core1.4 Network access and physical media1.4 Network access and physical media1.5 Internet structure and ISPs1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched

networks1.7 Protocol layers, service models1.8 History1.8 History

Lecture-3 9

How do loss and delay occur?packets queue in router buffers, wait for turn

A

B

packet being transmitted (delay)

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

Lecture-3 10

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

Lecture-3 11

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!

Lecture-3 12

Caravan analogy

Cars “propagate” at 100 km/hr

Toll booth takes 12 sec to service a car (transmission time)

car~bit; caravan ~ packet Q: How long until caravan

is lined up before 2nd toll booth?

Time to “push” entire caravan through toll booth onto highway = 12*10 = 120 sec

Time for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr)= 1 hr

A: 62 minutes

toll booth

toll booth

ten-car caravan

100 km

100 km

Lecture-3 13

Caravan analogy (more)

Cars now “propagate” at 1000 km/hr

Toll booth now takes 1 min to service a car

Q: Will cars arrive to 2nd booth before all cars serviced at 1st booth?

Yes! After 7 min, 1st car at 2nd booth and 3 cars still at 1st booth.

1st bit of packet can arrive at 2nd router before packet is fully transmitted at 1st router!

toll booth

toll booth

ten-car caravan

100 km

100 km

Lecture-3 14

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

Lecture-3 15

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!

Lecture-3 16

“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

Lecture-3 17

“Real” Internet delays and routes

traceroute to www.sun.com (64.124.140.199), 30 hops max, 40 byte packets 1 137.99.15.129 (137.99.15.129) 5.902 ms 1.946 ms 4.148 ms 2 10.12.250.1 (10.12.250.1) 0.682 ms 0.618 ms 0.578 ms 3 137.99.24.193 (137.99.24.193) 0.891 ms 0.871 ms 0.852 ms 4 uconn-edge.net.uconn.edu (137.99.22.41) 1.478 ms 1.866 ms 2.736 ms 5 159.247.232.129 (159.247.232.129) 2.162 ms 1.983 ms 2.068 ms 6 65.115.97.193 (65.115.97.193) 6.237 ms 6.785 ms 6.199 ms 7 205.171.28.29 (205.171.28.29) 6.399 ms 8.517 ms 6.036 ms 8 205.171.8.19 (205.171.8.19) 11.142 ms 10.784 ms 11.431 ms 9 205.171.30.14 (205.171.30.14) 11.137 ms 11.752 ms 10.963 ms10 pos4-1.pr1.lga1.us.mfnx.net (208.185.156.25) 13.200 ms 11.095 ms 11.963 ms11 so-3-0-0.cr1.lga1.us.mfnx.net (208.185.0.237) 11.542 ms 12.008 ms 12.055 ms12 so-1-0-0.cr1.iad1.us.mfnx.net (208.184.233.61) 15.587 ms 16.120 ms 16.459 ms13 so-1-0-0.cr1.dca2.us.mfnx.net (208.184.233.125) 17.613 ms 17.086 ms 16.744 ms14 so-3-0-0.mpr3.sjc2.us.mfnx.net (208.184.233.133) 80.789 ms 82.137 ms 82.055 ms15 so-0-0-0.cr1.sjc3.us.mfnx.net (208.185.175.153) 82.030 ms 82.679 ms 82.160 ms16 pos0-0.er2a.sjc3.us.mfnx.net (208.185.175.190) 81.658 ms 81.977 ms 80.668 ms17 alt1-1.java.sun.com (64.124.128.212) 83.412 ms 82.077 ms 87.823 ms

traceroute: fester.engr.uconn.edu to www.sun.comThree delay measurements from Fester.engr.uconn.edu to www.sun.com

Lecture-3 18

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

Lecture-3 19

Chapter 1: roadmap1.1 What 1.1 What isis the Internet? the Internet?1.2 Network edge1.2 Network edge1.3 Network core1.3 Network core1.4 Network access and physical media1.4 Network access and physical media1.5 Internet structure and ISPs1.5 Internet structure and ISPs1.6 Delay & loss in packet-switched

networks1.7 Protocol layers, service models1.8 History1.8 History

Lecture-3 20

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?

Lecture-3 21

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 routingairplane routing

Lecture-3 22

Organization of air travel: a different view

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

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routingairplane routing

Lecture-3 23

Layered air travel: servicesCounter-to-counter delivery of person+bags

baggage-claim-to-baggage-claim delivery

people transfer: loading gate to arrival gate

runway-to-runway delivery of plane

airplane routing from source to destination

Lecture-3 24

Distributed implementation of layer functionality

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing

Depa

rting

ai

rpor

t

arriv

ing

airp

ort

intermediate air traffic sitesairplane routing airplane routing

Lecture-3 25

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?

Lecture-3 26

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

link

physical

Lecture-3 27

Layering: logical communication applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical application

transportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

Each layer: distributed “entities”

implement layer functions at each node

entities perform actions, exchange messages with peers

Lecture-3 28

Layering: logical communication applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical application

transportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

data

dataE.g.: transport take data from

app add addressing,

reliability check info to form “datagram”

send datagram to peer

wait for peer to ack receipt

analogy: post office

data

transport

transport

ack

Lecture-3 29

Layering: physical communication applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical application

transportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

data

data

Lecture-3 30

Protocol layering and dataEach layer takes data from above adds header information to create new data unit passes new data unit to layer below

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

source destinationMMMM

HtHtHnHtHnHl

MMMM

HtHtHnHtHnHl

messagesegmentdatagramframe

Lecture-3 31

Chapter 1: roadmap1.1 What 1.1 What isis the Internet? the Internet?1.2 Network edge1.2 Network edge1.3 Network core1.3 Network core1.4 Network access and physical media1.4 Network access and physical media1.5 ISPs and Internet backbones1.5 ISPs and Internet backbones1.6 Delay & loss in packet-switched

networks1.7 Protocol layers, service models1.8 History1.8 History

Lecture-3 32

Homeworks Reading assignments

Chapter 1: a good overview of computer networks

Written assignments Part of 1st written assignment Chapter 1: Problem #5, 10, 16 (a, b, c) More (& due date) will be posted on the web

Try traceroute program again and do some delay analysis choose whatever destination you like