1. Chapter 1
Introduction
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Introduction 1-1

2. Goals
Examine the Types of Network Models
Understand nature of an Internet Protocol
Introduction 1-2

3. Postal System • Verify postage
• “Route” letter
Take letter to
local post office
• Write address Drop letter
• Put stamp
• Close envelop
Take letter to
destination’s
postal office
Take letter to Check destination
• Check address destination address
3
• Open envelope

4. Other Information Networks
Sending messages by birds Fire signals
4

5. Other Networks
5

6. Taxonomy of Communication Networks
Communication networks can be classified based on
the way in which the nodes exchange information:
Skeleton View
of Internet
6

7. Taxonomy of Communication Networks
Communication networks can be classified based on
the way in which the nodes exchange information:
Communication
Network
Broadcast
Communication
Network
7

8. Broadcast Communication Networks
• Information transmitted by any node is
received by every other node in the
network
– Usually only in LANs (Local Area Networks)
 E.g., WiFi, Ethernet (classical, but not current)
 E.g., lecture!
• What problems does this raise?
• Problem #1: limited range (the killer)
• Problem #2: privacy of communication
• Problem #3: coordinating access to the
shared communication medium (Multiple
Access Problem)
8

9. Taxonomy of Communication Networks
Communication networks can be classified based on
the way in which the nodes exchange information:
Communication
Network
Switched Broadcast
Communication Communication
Network Network
9

10. Switched Communication Networks
Information transmitted along a path of
intermediary nodes
“switches” or “routers”
Each switch acts as a small switchboard
Information comes in on one link
Switch directs it out some other link
Basic issue: how do the switches figure out
the next hop along the path?
10

11. Taxonomy of Communication Networks
Communication networks can be classified based on
the way in which the nodes exchange information:
Communication
Network
Switched Broadcast
Communication Communication
Network Network
Circuit-Switched
Communication
Network
11

12. Circuit Switching
End-end resources
reserved for “call”
 link bandwidth, switch
capacity
 dedicated resources:
no sharing
 circuit-like
(guaranteed)
performance
 call setup required
Introduction 1-12

13. Circuit Switching
network resources  dividing link bandwidth
(e.g., bandwidth) into “pieces”
divided into “pieces”  frequency division
 pieces allocated to calls  time division
 resource pieceidle if
not used by owning call
(no sharing)
Introduction 1-13

14. Circuit Switching (e.g., Phone Network)
• Establish: source creates circuit to destination
– Nodes along the path store connection info
– Nodes generally reserve resources for the
connection
– If circuit not available: “Busy signal”
• Transfer: source sends data over the circuit
– No destination address, since nodes know path
• Teardown: source tears down circuit when done
14

15. Circuit Switching With Human Operator
15

16. Telephone Network
Almon Brown Strowger (1839 - 1902)
1889: Invents the mechanical switching system for
telephone network

17. Timing in Circuit Switching
Host 1 Host 2
Switch 1 Switch 2
time
17

18. Timing in Circuit Switching
Host 1 Host 2
Switch 1 Switch 2
propagation delay
between Host 1
Circuit and Switch1
Establishment
time
18

19. Timing in Circuit Switching
Host 1 Host 2
Switch 1 Switch 2
Transmission delay
propagation delay
between Host 1
Circuit and Switch1
Establishment
time
19

20. Timing in Circuit Switching
Host 1 Host 2
Switch 1 Switch 2
Transmission delay
propagation delay
between Host 1
Circuit and Switch1
Establishment
time
20

21. Timing in Circuit Switching
Host 1 Host 2
Switch 1 Switch 2
Transmission delay
propagation delay
between Host 1
Circuit and Switch1
Establishment propagation delay
between Host 1
and Host 2
time
21

22. Timing in Circuit Switching
Host 1 Host 2
Switch 1 Switch 2
Transmission delay
propagation delay
between Host 1
Circuit and Switch1
Establishment propagation delay
between Host 1
and Host 2
Transfer
Information
time
22

23. Timing in Circuit Switching
Host 1 Host 2
Switch 1 Switch 2
Transmission delay
propagation delay
between Host 1
Circuit and Switch1
Establishment propagation delay
between Host 1
and Host 2
Transfer
Information
time
Circuit
Teardown
23

24. Circuit Switching
• Node (switch) in a circuit switching network
incoming links Node outgoing links
How do the black and orange circuits share the
outgoing link? 24

25. Circuit Switching: Multiplexing a Link
• Time-division • Frequency-division
– Each circuit allocated – Each circuit allocated
certain time slots certain frequencies
frequency
time time
25

26. Time-Division Multiplexing Illustration
Frames
Slots = 0 1 2 3 4 5 0 1 2 3 4 5
• Time divided into frames; frames into slots
• Relative slot position inside a frame determines to which
conversation data belongs
– E.g., slot 0 belongs to orange conversation
• Requires synchronization between sender and receiver—
surprisingly non-trivial!
• In case of non-permanent conversations
– Need to dynamically bind a slot to a conversation
• If a conversation does not use its circuit the capacity is lost!
26

27. Taxonomy of Communication Networks
Communication networks can be classified based on
the way in which the nodes exchange information:
Communication
Network
Switched Broadcast
Communication Communication
Network Network
Circuit-Switched Packet-Switched
Communication Communication
Network Network
27

28. Packet Switching
each end-end data stream resource contention:
divided into packets  aggregate resource
 user A, B packets share demand can exceed
network resources amount available
 each packet uses full link  congestion: packets
bandwidth queue, wait for link use
 resources used as needed  store and forward:
packets move one hop
at a time
Bandwidth division into “pieces”  Node receives complete
Dedicated allocation packet before forwarding
Resource reservation
Introduction 1-28

29. Packet Switching
• Data sent as chunks of formatted bit-sequences
(Packets)
• Packets have following structure:
Header Data Trailer
(sometimes)
 Header and Trailer carry control information
(e.g., destination address, checksum)
• Each packet traverses the network from node to node
along some path (Routing) based on header info.
• Usually, once a node receives the entire packet, it
stores it (hopefully briefly) and then forwards it to the
next node (Store-and-Forward Networks) 29

30. Packet Switching
• Node in a packet switching network
incoming links Node outgoing links
Memory
30

31. Packet-switching: store-and-forward
L
R R R
 takes L/R seconds to Example:
transmit (push out)  L = 7.5 Mbits
packet of L bits on to  R = 1.5 Mbps
link at R bps
 transmission delay = 15
 store and forward:
sec
entire packet must
arrive at router before
it can be transmitted
on next link
 delay = 3L/R (assuming more on delay shortly …
zero propagation delay)
Introduction 1-31

32. Packet Switching: Statistical Multiplexing
100 Mb/s
A Ethernet statistical multiplexing C
1.5 Mb/s
B
queue of packets
waiting for output
link
D E
Sequence of A & B packets does not have fixed pattern,
bandwidth shared on demand ² statistical multiplexing.
TDM: each host gets same slot in revolving TDM frame.
Introduction 1-32

33. Packet Switching: Multiplexing/Demultiplexing
• Data from any conversation can be transmitted at any
given time
– Single conversation can use the entire link capacity if it is alone
• How to tell them apart?
– Use meta-data (header) to describe data
33

34. Taxonomy of Communication Networks
Communication networks can be classified based on
the way in which the nodes exchange information:
Communication
Network
Switched Broadcast
Communication Communication
Network Network
Circuit-Switched Packet-Switched
Communication Communication
Network Network
Datagram
Network
34

35. Datagram Packet Switching
Each packet is independently switched
Each packet header contains full destination address
No resources are pre-allocated (reserved) in
advance
Leverages “statistical multiplexing” (or stat-
muxing)
Gambling that packets from different conversations won’t
all arrive at the same time, so we don’t need enough
capacity for all of them at their peak transmission rate
Assuming independence of traffic sources, can compute
probability that there is enough capacity
Example: IP networks; postal system
35

36. Timing of Datagram Packet Switching
Host 1 Host 2
Node 1 Node 2
propagation
delay between
Host 1 and
Packet 1
Node 1
36

37. Timing of Datagram Packet Switching
Host 1 Host 2
Node 1 Node 2
propagation
delay between
transmission Host 1 and
time of Packet 1 Packet 1
at Host 1
Node 1
37

38. Timing of Datagram Packet Switching
Host 1 Host 2
Node 1 Node 2
propagation
delay between
transmission Host 1 and
time of Packet 1 Packet 1 processing
at Host 1
Node 1
Packet 1 delay of
Packet 1
at Node 2
Packet 1
38

39. Timing of Datagram Packet Switching
Host 1 Host 2
Node 1 Node 2
propagation
delay between
transmission Host 1 and
time of Packet 1 Packet 1 processing
at Host 1
Node 1
Packet 2
Packet 1 delay of
Packet 3 Packet 1
Packet 2 at Node 2
Packet 1
Packet 3
Packet 2
Packet 3
39

40. Datagram Packet Switching
Host C
Host A Host D
Node 1 Node 2
Node 3
Node 5
Host B
Node 7 Host E
Node 6
Node 4
40

41. Datagram Packet Switching
Host C
Host A Host D
Node 1 Node 2
Node 3
Node 5
Host B
Node 7 Host E
Node 6
Node 4
41

42. Datagram Packet Switching
Host C
Host A Host D
Node 1 Node 2
Node 3
Node 5
Host B
Node 7 Host E
Node 6
Node 4
42

43. Taxonomy of Communication Networks
Communication networks can be classified based on
the way in which the nodes exchange information:
Communication
Network
Switched Broadcast
Communication Communication
Network Network
A hybrid of circuits and
Circuit-Switched Packet-Switched packets; headers include a
Communication Communication
“circuit identifier” established
Network Network
during a setup phase
Datagram Virtual Circuit
Network Network
43

44. Advantages of Circuit Switching
• Guaranteed bandwidth
– Predictable communication performance
– Not “best-effort” delivery with no real guarantees
• Simple abstraction
– Reliable communication channel between hosts
– No worries about lost or out-of-order packets
• Simple forwarding
– Forwarding based on time slot or frequency
– No need to inspect a packet header
• Low per-packet overhead
– Forwarding based on time slot or frequency
– No IP (and TCP/UDP) header on each packet
44

45. Disadvantages of Circuit
Switching
• Wasted bandwidth
– Bursty traffic leads to idle connection during silent period
– Unable to achieve gains from “statistical multiplexing”
• Blocked connections
– Connection refused when resources are not sufficient
– Unable to offer “okay” service to everybody
• Connection set-up delay
– No communication until the connection is set up
– Unable to avoid extra latency for small data transfers
• Network state
– Network nodes must store per-connection information
– Unable to avoid per-connection storage and state
– This makes failures more disruptive!
45

46. Packet-Switching vs. Circuit-Switching
• Performance advantage of packet-switching over circuit
switching: Exploitation of statistical multiplexing
• Reliability advantage: since routers don’t know about
individual conversations, when a router or link fails, it’s:
Easy to fail over to a different path
• Deployability advantage: easier for different parties to
link their networks together because they’re not
promising to reserve resources for one another
• Disadvantage: packet-switching must handle congestion
– More complex routers (more buffering, sophisticated dropping)
– Harder to provide good network services (e.g., delay and
bandwidth guarantees)
46

47. Packet switching versus circuit switching
Packet switching allows more users to use network!
 1 Mb/s link
 each user:
 100 kb/s when “active”
 active 10% of time
N users
 circuit-switching: 1 Mbps link
 10 users
 packet switching:
 with 35 users,
probability > 10 active
at same time is less
than .0004 (leave as
Introduction
exercise) 1-47

48. Packet switching versus circuit switching
Is packet switching an outright 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 1-48

49. Combinations
Can a packet switched network run over a circuit-
switched one?
Can a circuit-switched network run over a packet-
switched one?
49

50. What’s a protocol?
human protocols: network protocols:
 “what’s the time?”  machines rather than
 “I have a question” humans
 introductions  all communication
activity in Internet
governed by protocols
… specific msgs sent
… specific actions protocols define format,
taken when msgs order of msgs sent and
received, or other received among network
events entities, and actions
taken on msg
transmission, receipt
Introduction 1-50

51. What’s a protocol?
a human protocol and a computer network protocol:
Hi
TCP connection
request
Hi
TCP connection
Got the response
time? Get http://www.awl.com/kurose-ross
2:00
<file>
time
Q: Other human protocols?
Introduction 1-51

52. What Is A Protocol?
• A protocol is an agreement on how to
communicate
• Includes syntax and semantics
– How a communication is specified & structured
 Format, order messages are sent and received
– What a communication means
 Actions taken when transmitting, receiving, or when a timer
expires
52

53. Examples of Protocols in Human Interactions
• Telephone
1. (Pick up / open up the phone.)
2. Listen for a dial tone / see that you have service.
3. Dial.
4. Should hear ringing …
5. Callee: “Hello?”
– Caller: “Hi, it’s Alice ….”
Or: “Hi, it’s me” (← what’s that about?)
– Caller: “Hey, do you think … blah blah blah …” pause
– Callee: “Yeah, blah blah blah …” pause
53

54. Examples of Protocols in Human Interactions
• Asking a question
1. Raise your hand.
2. Wait to be called on.
– Or: wait for speaker to pause and vocalize
54

55. Example: HyperText Transfer Protocol
GET /courses/archive/spring06/cos461/ HTTP/1.1
Host: www.cs.princeton.edu
User-Agent: Mozilla/4.03 Request
CRLF
HTTP/1.1 200 OK
Date: Mon, 6 Feb 2006 13:09:03 GMT
Server: Netscape-Enterprise/3.5.1
Last-Modified: Mon, 6 Feb 2006 11:12:23 GMT
Response Content-Length: 21
CRLF
Site under construction
55

56. Example: The Internet Protocol (IP)
Problem:
Many different network technologies
e.g., Ethernet, Token Ring, ATM, Frame Relay, etc.
How can you hook them together?
• n x n translations?
IP was invented to glue them together
n translations
Minimal requirements (datagram)
The Internet is founded on IP
“IP over everything”
56

57. What does an internet protocol
do?
What would the protocol need to communicate?
57

58. Example: IP Packet
4-bit
4-bit Header 8-bit Type of
16-bit Total Length (Bytes)
Version Length Service (TOS)
3-bit
16-bit Identification 13-bit Fragment Offset
Flags
8-bit Time to 8-bit Protocol 16-bit Header Checksum 20-byte
Live (TTL) header
32-bit Source IP Address
32-bit Destination IP Address
Options (if any)
Payload
58

59. Example: IP: “Best-Effort” Packet
Delivery Protocol
Datagram packet switching
Send data in packets
Header with source & destination address
Service it provides:
Packets may be lost
Packets may be corrupted
Packets may be delivered out of order
source destination
IP network
59

60. Example: Transmission Control Protocol
• Communication service
– Ordered, reliable byte stream
– Simultaneous transmission in both directions
• Key mechanisms at end hosts
– Retransmit lost and corrupted packets
– Discard duplicate packets and put packets in order
– Flow control to avoid overloading the receiver buffer
– Congestion control to adapt sending rate to network load
TCP connection
source network destination 60

61. Protocol Standardization
• Ensure communicating hosts speak the same protocol
– Standardization to enable multiple implementations
– Or, the same folks have to write all the software
• Standardization: Internet Engineering Task Force
– Based on working groups that focus on specific issues
– Produces “Request For Comments” (RFCs)
 Promoted to standards via rough consensus and running
code
– IETF Web site is http://www.ietf.org
– RFCs archived at http://www.rfc-editor.org
• De facto standards: same folks writing the code
– P2P file sharing, Skype, <your protocol here>…
61

62. Summary
Examined the types of Communication
Networks
Got introduced to the concept of protocol

63. Acknowledgment & Copyright
The instructor duly acknowledges the authors of
the text book “Computer Networking: A Top-
down approach”, James Kurose & Keith Ross
and the instructors of EE122 “Computer
Networks” course at UC Berkeley, Ian Stoica,
Scott Shenker, Jennifer Rexford, Vern Paxson
and other instructors at Princeton University
for the course material.
Please note: certain modifications may have
been done to adapt the slides to the current
audience -Bruhadeshwar @cs335 Spring
2011(C)