1. Networks & Protocols EE 122: Intro to Communication Networks
Fall 2010 (MW 4-5:30 in 101 Barker)
Scott Shenker
TAs: Sameer Agarwal, Sara Alspaugh, Igor Ganichev, Prayag Narula
Materials with thanks to Jennifer Rexford, Ion Stoica, Vern Paxson and other colleagues at Princeton and UC Berkeley

2. TA: Sara Alspaugh Taking over Igor’s section
Will post office hours, etc.

3. Goals for Today’s Class
Type of Networks
And the key concept of multiplexing
What’s a Protocol?

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

5. Other Information Networks
Source routing
Broadcast
Sending messages by birds
Fire signals

6. Other Networks
Circuit switched.

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

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

9. 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)
Do Multiaccess problem for: smoke signals, radio, ethernet
When can I tell there is a collision?
Does everyone know there is a collision? [if not, I might get screwed]

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

11. 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?

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

13. 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. Circuit Switching Node (switch) in a circuit switching network
incoming links
Node
outgoing links
How does the node connect the incoming link to the outgoing?

15. Telephone Network Alexander Graham Bell
1876: Demonstrates the telephone at US Centenary Exhibition in Philadelphia

16. Circuit Switching With Human Operator

17. Telephone Network Almon Brown Strowger (1839 - 1902)
1889: Invents the “girl-less, cuss-less” telephone system -- the mechanical switching system

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

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

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

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

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

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

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

25. 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?

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

27. Time-Division Multiplexing/Demultiplexing
Frames
Slots = 1 2 3 4 5 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!

28. Questions Before We Proceed?
5 Minute Break
Questions Before We Proceed?

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

30. Packet Switching
Data sent as chunks of formatted bit-sequences (Packets)
Packets have following structure:
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)
Header
Data
Trailer (sometimes)

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

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

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

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

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

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

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

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

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

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

43. 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
No IP (and TCP/UDP) header on each packet

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

47. What kinds of networks are these?

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

49. Other Information Networks
Source routing
Broadcast
Sending messages by birds
Fire signals

50. Other Networks
Circuit switched.

51. Questions?

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

53. Examples of Protocols in Human Interactions
Telephone
(Pick up / open up the phone.)
Listen for a dial tone / see that you have service.
Dial.
Should hear ringing …
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

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

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

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”

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

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

59. Which fields are critical?
IPv6 kept:
Addresses, payload length, hop limit, traffic class
IPv6 dropped:
Header length, total length, identification, flags
Fragmentation, protocol, header checksum
IPv6 added:
Next header
Flow label

60. IP: “Best-Effort” Packet Delivery
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

61. 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

62. 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
RFCs archived at
De facto standards: same folks writing the code
P2P file sharing, Skype, <your protocol here>…

63. Summary Types of communication networks
Broadcast, circuit-switched, packet-switched
Advantages & disadvantages of each
Protocols: agreement on how to communicate
For networks, often requires formal standardization

64. Next Lecture(s) Holiday on Monday Sockets on Wednesday
HW #1 assigned on Wednesday
If you haven’t already:
Pick up class computer account forms
Take the survey
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Office hours: I will hang around after class and then will move back to Soda when no one else is waiting (or they kick me out)