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Administrative Things. Textbook in the bookstore ? Lecture notes on the web ?? Any questions and suggestions so far ??? … . Chapter 1: roadmap. 1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 Internet structure and ISPs

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Administrative things

Administrative Things

  • Textbook in the bookstore ?

  • Lecture notes on the web ??

  • Any questions and suggestions so far ???

Lecture-3


Chapter 1 roadmap

Chapter 1: roadmap

1.1 What is the Internet?

1.2 Network edge

1.3 Network core

1.4 Network access and physical media

1.5 Internet structure and ISPs

1.6 Delay & loss in packet-switched networks

1.7 Protocol layers, service models

1.8 History

Lecture-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

A Review of Last Lecture

Lecture-3


Internet structure network of networks

roughly hierarchical

at center: “tier-1” ISPs

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

national/international coverage

NAP

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

Tier-1 providers interconnect (peer) privately

Internet structure: network of networks

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Lecture-3


Internet structure network of networks1

“Tier-2” ISPs: smaller (often regional) ISPs

Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs

NAP

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

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

  • tier-2 ISP is customer of

    tier-1 provider

Tier-2 ISP

Tier-2 ISP

Tier-2 ISP

Tier-2 ISP

Tier-2 ISP

Internet structure: network of networks

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Lecture-3


Internet structure network of networks2

“Tier-3” ISPs and local ISPs

last hop (“access”) network (closest to end systems)

Tier 3

ISP

local

ISP

local

ISP

local

ISP

local

ISP

local

ISP

local

ISP

local

ISP

local

ISP

NAP

Local and tier- 3 ISPs are customers of

higher tier ISPs

connecting them to rest of Internet

Tier-2 ISP

Tier-2 ISP

Tier-2 ISP

Tier-2 ISP

Tier-2 ISP

Internet structure: network of networks

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Lecture-3


A packet passes through many networks

Tier 3

ISP

local

ISP

local

ISP

local

ISP

local

ISP

local

ISP

local

ISP

local

ISP

local

ISP

NAP

Tier-2 ISP

Tier-2 ISP

Tier-2 ISP

Tier-2 ISP

Tier-2 ISP

A packet passes through many networks

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Lecture-3


Chapter 1 roadmap1

Chapter 1: roadmap

1.1 What is the Internet?

1.2 Network edge

1.3 Network core

1.4 Network access and physical media

1.5 Internet structure and ISPs

1.6 Delay & loss in packet-switched networks

1.7 Protocol layers, service models

1.8 History

Lecture-3


How do loss and delay occur

packets queue in router buffers, wait for turn

packet being transmitted (delay)

packets queueing (delay)

free (available) buffers: arriving packets

dropped (loss) if no free buffers

How do loss and delay occur?

A

B

Lecture-3


Four sources of packet delay

1. nodal processing:

check bit errors

determine output link

transmission

A

propagation

B

nodal

processing

queueing

Four sources of packet delay

  • 2. queueing

    • time waiting at output link for transmission

    • depends on congestion level of router

Lecture-3


Delay in packet switched networks

3. 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

transmission

A

propagation

B

nodal

processing

queueing

Delay in packet-switched networks

Note: s and R are very different quantities!

Lecture-3


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

Caravan analogy

100 km

100 km

ten-car

caravan

Lecture-3


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

Caravan analogy (more)

100 km

100 km

ten-car

caravan

Lecture-3


Nodal delay

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

Lecture-3


Queueing delay revisited

R=link bandwidth (bps)

L=packet length (bits)

a=average packet arrival rate

Queueing delay (revisited)

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


Real internet delays and routes

“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


Real internet delays and routes1

“Real” Internet delays and routes

traceroute: fester.engr.uconn.edu to www.sun.com

Three delay measurements from

Fester.engr.uconn.edu to www.sun.com

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 ms

10 pos4-1.pr1.lga1.us.mfnx.net (208.185.156.25) 13.200 ms 11.095 ms 11.963 ms

11 so-3-0-0.cr1.lga1.us.mfnx.net (208.185.0.237) 11.542 ms 12.008 ms 12.055 ms

12 so-1-0-0.cr1.iad1.us.mfnx.net (208.184.233.61) 15.587 ms 16.120 ms 16.459 ms

13 so-1-0-0.cr1.dca2.us.mfnx.net (208.184.233.125) 17.613 ms 17.086 ms 16.744 ms

14 so-3-0-0.mpr3.sjc2.us.mfnx.net (208.184.233.133) 80.789 ms 82.137 ms 82.055 ms

15 so-0-0-0.cr1.sjc3.us.mfnx.net (208.185.175.153) 82.030 ms 82.679 ms 82.160 ms

16 pos0-0.er2a.sjc3.us.mfnx.net (208.185.175.190) 81.658 ms 81.977 ms 80.668 ms

17 alt1-1.java.sun.com (64.124.128.212) 83.412 ms 82.077 ms 87.823 ms

Lecture-3


Packet loss

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


Chapter 1 roadmap2

Chapter 1: roadmap

1.1 What is the Internet?

1.2 Network edge

1.3 Network core

1.4 Network access and physical media

1.5 Internet structure and ISPs

1.6 Delay & loss in packet-switched networks

1.7 Protocol layers, service models

1.8 History

Lecture-3


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?

Protocol “Layers”

Lecture-3


Organization of air travel

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

airplane routing

Organization of air travel

  • a series of steps

Lecture-3


Organization of air travel a different view

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

airplane routing

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

Lecture-3


Layered air travel services

Layered air travel: services

Counter-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


Distributed implementation of layer functionality

airplane routing

airplane routing

airplane routing

Distributed implementation of layer functionality

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

arriving airport

Departing airport

intermediate air traffic sites

Lecture-3


Why layering

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


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

Internet protocol stack

Lecture-3


Layering logical communication

Each layer:

distributed

“entities” implement layer functions at each node

entities perform actions, exchange messages with peers

network

link

physical

application

transport

network

link

physical

application

transport

network

link

physical

application

transport

network

link

physical

application

transport

network

link

physical

Layering: logical communication

Lecture-3


Layering logical communication1

E.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

network

link

physical

application

transport

network

link

physical

application

transport

network

link

physical

application

transport

network

link

physical

application

transport

network

link

physical

data

data

data

ack

Layering: logical communication

transport

transport

Lecture-3


Layering physical communication

network

link

physical

application

transport

network

link

physical

application

transport

network

link

physical

application

transport

network

link

physical

application

transport

network

link

physical

data

data

Layering: physical communication

Lecture-3


Protocol layering and data

M

M

H

H

H

H

H

H

H

H

H

H

H

H

t

t

t

n

l

n

l

t

n

t

n

t

M

M

M

M

application

transport

network

link

physical

application

transport

network

link

physical

M

M

Protocol layering and data

Each layer takes data from above

  • adds header information to create new data unit

  • passes new data unit to layer below

source

destination

message

segment

datagram

frame

Lecture-3


Chapter 1 roadmap3

Chapter 1: roadmap

1.1 What is the Internet?

1.2 Network edge

1.3 Network core

1.4 Network access and physical media

1.5 ISPs and Internet backbones

1.6 Delay & loss in packet-switched networks

1.7 Protocol layers, service models

1.8 History

Lecture-3


Homeworks

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

Lecture-3


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