1. Cs457 Computer Networks
Introduction

2. The Internet A global network of computers who runs it?
who pays for it?
who invented the internet?
How do the computer’s communicate?
There are many levels of abstraction, just like speech
local ISP
UVa
network
regional ISP
router
workstation
server
mobile
Introduction

3. What’s a protocol? Hi Hi 2:00 <file> time TCP connection request
response
Got the
time?
Get
2:00
<file>
time
Introduction

4. What’s a protocol?
protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt
Introduction

5. A closer look at network structure:
network edge: applications and hosts
network core:
routers
network of networks
access networks, physical media: communication links
Introduction

6. Distributed Applications
end systems (hosts):
run application programs
e.g. Web,
at “edge of network”
client/server model
client host requests, receives service from always-on server
e.g. Web browser/server; client/server
peer-peer model:
minimal (or no) use of dedicated servers
e.g. Skype, BitTorrent, KaZaA
Introduction

7. Connection-oriented Vs Connectionless Services
TCP: supports connection oriented services
UDP: supports connectionless services
what's the difference?
Reliability
What’s needed to make this difference?
flow control: slows down when receiver is slow
congestion control: slows down when network is slow
Trade-off?
UDP is faster
Introduction

8. Connnection vs Connectionless services
App’s using TCP:
HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP ( )
App’s using UDP:
streaming media, teleconferencing, DNS, Internet telephony
Introduction

9. The Network Core
mesh of interconnected links
Introduction

10. Switching How to transfer data between these four nodes?
Reserved bandwidth
On-demand bandwidth
Introduction

11. Circuit Switching End-end resources reserved for “call”
dedicated resources: no sharing
circuit-like (guaranteed) performance
call setup required
Introduction

12. Circuit Switching: FDM and TDM
network resources (e.g., bandwidth) divided into “pieces”
4 users
FDM
frequency
time
TDM
frequency
time
Two simple multiple access control techniques.
Each mobile’s share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling.
As we will see, used in AMPS, GSM, IS-54/136
Introduction

13. 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 Mbps
Each link uses TDM with 24 slots/sec
500 msec to establish end-to-end circuit
1,536,000/24 = 64000
640,000/64,000=10 seconds
= 1-.5seconds
Introduction

14. Bandwidth division into “pieces”
Packet Switching
each end-end data stream divided into packets
user A, B packets share network resources
each packet uses full link bandwidth
resources used as needed
disadvantages:
aggregate resource demand can exceed amount available
store and forward delay: packets move one hop at a time
Node receives complete packet before forwarding
Queue delay: must wait for link use
Bandwidth division into “pieces”
Dedicated allocation
Resource reservation
Introduction

15. Packet Switching vs TDM
100 Mb/s
Ethernet C A
statistical multiplexing
1.5 Mb/s B
queue of packets
waiting for output
link D E
Sequence of A & B packets does not have fixed pattern, shared on demand  statistical multiplexing.
TDM: each host gets same slot in revolving TDM frame.
Introduction

16. Propagation delay Example: L = 640,000 bits R = 1.5 Mbps
Do we need to receive entire packet before forwarding?
Takes L/R seconds to transmit (push out) packet of L bits on to link or R bps
Entire packet must arrive at router before it can be transmitted on next link: store and forward
delay = 3L/R (assuming zero propagation delay)
Example:
L = 640,000 bits
R = 1.5 Mbps
delay = 3*640,000/1.5=1.28s
Introduction

17. Propagation delay L R R R
1.28 is much faster than 10.5 seconds for circuit switched routing
What if there were other users??
--revisit this question later
Introduction

18. Packet switching versus circuit switching
Is packet switching a “slam dunk winner?”
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
still an unsolved problem (chapter 7)
Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?
Introduction

19. Chapter 1: roadmap 1.1 What is the Internet?
1.2 Distributed applications
1.3 Routing
1.4 Physical media
1.5 Internet structure
1.6 Delay & loss
1.7 Protocol layers
1.8 History
Introduction

20. Access networks and physical media
Q: How to connect end systems together?
Introduction

21. Home networks Typical home network components: ADSL or cable modem
router/firewall/NAT
Ethernet
wireless access
point
wireless
laptops
to/from
cable
headend
cable
modem
router/
firewall
wireless
access
point
Ethernet
Introduction

22. Physical Media Twisted Pair (TP) two insulated copper wires
Category 3: traditional phone wires, 10 Mbps Ethernet
Category 5: 100Mbps Ethernet
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
Introduction

23. Sharing the Medium Ethernet protocol: Listen If channel is clear, send
If collision, back off
packet 1 2 3 4 5 6 7
cable headend
home
cable distribution
network (simplified)
Introduction

24. Getting more data on the wire
FDM:
Channels V I D E O A T C N R L 1 2 3 4 5 6 7 8 9
cable headend
home
cable distribution
network
Introduction

25. Chapter 1: roadmap 1.1 What is the Internet?
1.2 Distributed Applications
1.3 Routing
1.4 Physical media
1.5 Internet structure
1.6 Delay & loss
1.7 Protocol layers
1.8 History
Introduction

26. 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
NAP
Tier-1 providers also interconnect at public network access points (NAPs)
Tier 1 ISP
Tier-1 providers interconnect (peer) privately
Tier 1 ISP
Tier 1 ISP
Introduction

27. Tier-1 ISP: e.g., Sprint Sprint US backbone network … …. DS3 (45 Mbps)
OC3 (155 Mbps)
OC12 (622 Mbps)
OC48 (2.4 Gbps)
Seattle
Atlanta
Chicago
Roachdale
Stockton
San Jose
Anaheim
Fort Worth
Orlando
Kansas City
Cheyenne
New York
Pennsauken
Relay
Wash. DC
Tacoma …
to/from customers
peering
to/from backbone ….
POP: point-of-presence
Introduction

28. 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-2 ISPs also peer privately with each other, interconnect at NAP
Tier-2 ISP
Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet
tier-2 ISP is customer of
tier-1 provider
Tier 1 ISP
NAP
Tier 1 ISP
Tier 1 ISP
Introduction

29. Internet structure: network of networks
“Tier-3” ISPs and local ISPs
last hop (“access”) network (closest to end systems)
local
ISP
Tier 3
Local and tier- 3 ISPs are customers of
higher tier ISPs
connecting them to rest of Internet
Tier-2 ISP
Tier 1 ISP
NAP
Tier 1 ISP
Tier 1 ISP
Introduction

30. Internet structure: network of networks
a packet passes through many networks!
local
ISP
Tier 3
ISP
local
ISP
local
ISP
local
ISP
Tier-2 ISP
Tier 1 ISP
NAP
Tier 1 ISP
Tier 1 ISP
local
ISP
local
ISP
local
ISP
local
ISP
Introduction

31. Review Transport Layer Network Layer Link Layer The Internet
Connection-oriented
Connectionless
Network Layer
Circuit switching
Packet switching
Link Layer
Ethernet
FDM
The Internet
Introduction

32. Transport Layer Review
Connection-oriented (TCP)
Acknowledgements (can have retries)
Flow control
Congestion control
Better for most protocols
Connectionless (UDP)
No acknowledgements
Send as fast as needed
Some packets will get lost
Better for video, telephony, etc
Human speech?
Introduction

33. Network Layer Review Circuit-switching Packet switching
Divide resources into “slots”
Reserve all end-to-end resources (a “circuit”)
Provides known latency
Better for phones networks, etc
Packet switching
Divide resources into “packets”
Send a packet whenever needed
Maximizes throughput
Better for random traffic, eg. internet, etc
Introduction

34. Packet Switching Virtual Circuit Networks Datagram Networks
Initialize: tell each router it is on the circuit
Send each packet with the Virtual Circuit ID
Datagram Networks
No initialization
Send each packet with the destination address
Trade offs?
VC networks minimize routing tables
Datagram networks minimize setup time
Introduction

35. Network Layer Taxonomy
Introduction

36. Link Layer Physical media Sharing techniques Human sharing protocols?
Ethernet
Fiber optics
DSL
Cable
Sharing techniques
FDM
TDM
Ethernet protocol
Etc.
Human sharing protocols?
Introduction

37. Transport Layer Network Layer Link Layer TCP UDP IP Connection-
oriented
Connection-
less
Network Layer IP
Link Layer
Ethernet
Cable
Wireless
DSL
Fiber
FDMA
TDMA
CSMA
CDMA
Introduction

38. 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
Introduction

39. Four sources of packet delay
1. nodal processing:
check bit errors
determine output link
2. queueing
time waiting at output link for transmission
depends on congestion level of router A B
nodal
processing
queueing
Introduction

40. 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
Note: s and R are very different quantities!
transmission A
propagation B
nodal
processing
queueing
Introduction

41. Caravan analogy
toll
booth
toll
booth
100 km
100 km
ten-car
caravan
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
Introduction

42. Caravan analogy (more)
toll
booth
toll
booth
100 km
100 km
ten-car
caravan
1st bit of packet can arrive at 2nd router before packet is fully transmitted at 1st router!
Cars now “propagate” at km/hr
Toll booth now takes 1 min to service a car
Total transmit time is: 10*1+.1 = 10.1 minutes
Which scenario is more like internet routers?
Introduction

43. Nodal delay dproc = processing delay dqueue = queuing 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
Introduction

44. Queueing delay (revisited)
R=link bandwidth (bps)
L=packet length (bits)
a=average packet arrival rate
traffic intensity = La/R
Output Rate
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

45. How does loss occur?
packet arrival rate to link exceeds output link capacity
Queue grows
When no more space in queue, packets are lost
lost packet may be retransmitted by previous node, by source end system, or not retransmitted at all A
free (available) buffers: arriving packets
dropped (loss) if no free buffers
packets queueing (delay) B
Introduction

46. “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

47. “Real” Internet delays and routes
traceroute: gaia.cs.umass.edu to
Three delay measurements from
gaia.cs.umass.edu to cs-gw.cs.umass.edu
1 cs-gw ( ) 1 ms 1 ms 2 ms
2 border1-rt-fa5-1-0.gw.umass.edu ( ) 1 ms 1 ms 2 ms
3 cht-vbns.gw.umass.edu ( ) 6 ms 5 ms 5 ms
4 jn1-at wor.vbns.net ( ) 16 ms 11 ms 13 ms
5 jn1-so wae.vbns.net ( ) 21 ms 18 ms 18 ms
6 abilene-vbns.abilene.ucaid.edu ( ) 22 ms 18 ms 22 ms
7 nycm-wash.abilene.ucaid.edu ( ) 22 ms 22 ms 22 ms
( ) 104 ms 109 ms 106 ms
9 de2-1.de1.de.geant.net ( ) 109 ms 102 ms 104 ms
10 de.fr1.fr.geant.net ( ) 113 ms 121 ms 114 ms
11 renater-gw.fr1.fr.geant.net ( ) 112 ms 114 ms 112 ms
12 nio-n2.cssi.renater.fr ( ) 111 ms 114 ms 116 ms
13 nice.cssi.renater.fr ( ) 123 ms 125 ms 124 ms
14 r3t2-nice.cssi.renater.fr ( ) 126 ms 126 ms 124 ms
15 eurecom-valbonne.r3t2.ft.net ( ) 135 ms 128 ms 133 ms
( ) 126 ms 128 ms 126 ms
17 * * *
18 * * *
19 fantasia.eurecom.fr ( ) 132 ms 128 ms 136 ms
trans-oceanic
link
* means no response (probe lost, router not replying)
Introduction

48. 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
Introduction

49. How do they all fit together?
Protocol “Layers”
Networks are complex!
many “pieces”:
hosts
routers
links of various media
applications
protocols
hardware, software
Question:
How do they all fit together?
Introduction

50. Organization of air travel
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
Introduction

51. Layering of airline functionality
ticket (purchase)
baggage (check)
gates (load)
runway (takeoff)
airplane routing
departure
airport
arrival
intermediate air-traffic
control centers
ticket (complain)
baggage (claim
gates (unload)
runway (land)
ticket
baggage
gate
takeoff/landing
Layers: each layer implements a service
Providing an interface to higher layers
And relying on services provided by layer below
Introduction

52. Why layering? Dealing with complex systems:
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?
Introduction

53. 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
Introduction

54. Encapsulation source destination application transport network link
message M
application
transport
network
link
physical
segment Ht M Ht
datagram Ht Hn M Hn
frame Ht Hn Hl M
link
physical
switch
destination
network
link
physical Ht Hn M Ht Hn Hl M M
application
transport
network
link
physical Ht Hn M Ht M Ht Hn M
router Ht Hn Hl M
Introduction

55. 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
Introduction

56. Internet History 1961-1972: Early packet-switching principles
1961: Kleinrock - queueing theory shows effectiveness of packet-switching
1964: Baran - packet-switching in military nets
1967: ARPAnet conceived by Advanced Research Projects Agency
1969: first ARPAnet node operational
1972:
ARPAnet public demonstration
NCP (Network Control Protocol) first host-host protocol
first program
ARPAnet has 15 nodes
Introduction

57. Internet History 1972-1980: Internetworking, new and proprietary nets
1970: ALOHAnet satellite network in Hawaii
1974: Cerf and Kahn - architecture for interconnecting networks
1976: Ethernet at Xerox PARC
ate70’s: proprietary architectures: DECnet, SNA, XNA
late 70’s: switching fixed length packets (ATM precursor)
1979: ARPAnet has 200 nodes
Cerf and Kahn’s internetworking principles:
minimalism, autonomy - no internal changes required to interconnect networks
best effort service model
stateless routers
decentralized control
define today’s Internet architecture
Introduction

58. Internet History 1980-1990: new protocols, a proliferation of networks
1983: deployment of TCP/IP
1982: smtp protocol defined
1983: DNS defined for name-to-IP-address translation
1985: ftp protocol defined
1988: TCP congestion control
new national networks: Csnet, BITnet, NSFnet, Minitel
100,000 hosts connected to confederation of networks
Introduction

59. Internet History 1990, 2000’s: commercialization, the Web, new apps
Early 1990’s: ARPAnet decommissioned
1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995)
early 1990s: Web
hypertext [Bush 1945, Nelson 1960’s]
HTML, HTTP: Berners-Lee
1994: Mosaic, later Netscape
late 1990’s: commercialization of the Web
Late 1990’s – 2000’s:
more killer apps: instant messaging, P2P file sharing
network security to forefront
est. 50 million host, 100 million+ users
backbone links running at Gbps
Introduction

60. Introduction: Summary
Course in a nutshell
Internet overview
what’s a protocol?
network edge, core, access network
packet-switching versus circuit-switching
Internet/ISP structure
performance: loss, delay
layering and service models
history
Rest of course:
Ch 2: applications
Ch 3: transport
Ch 4: network
Ch 5: link
Ch 6: wireless
Ch 8: security
Introduction