Dr. Yingwu Zhu

1
Introduction

• Dr. Yingwu Zhu

2
Introduction

• Goal
• get context, overview, feel of networking
• more depth, detail later in course
• approach
• descriptive
• use Internet as example

• Overview
• whats the Internet
• whats a protocol?
• network edge
• network core
• access net, physical media
• performance loss, delay
• protocol layers, service models
• history

3
Whats the Internet nuts and bolts view

• millions of connected computing devices hosts,
  end-systems
• pcs workstations, servers
• PDAs phones, toasters
• running network apps
• communication links
• fiber, copper, radio, satellite
• routers forward packets (chunks) of data thru
  network

4
Whats the Internet nuts and bolts view

• protocols control sending, receiving of msgs
• e.g., TCP, IP, HTTP, FTP, PPP
• Internet network of networks
• loosely hierarchical
• public Internet versus private intranet
• Internet standards
• RFC Request for comments
• IETF Internet Engineering Task Force

router
workstation
server
mobile
local ISP
regional ISP
company network
5
Whats the Internet a service view

• communication infrastructure enables distributed
  applications
• WWW, email, games, e-commerce, database., voting,
  
• more?
• communication services provided
• connectionless
• connection-oriented

6
Perspective (I)

• Network users Does the network support the
  users applications
• Reliability
• Error-free service
• Performance speed of data transfer
• Even more security? Privacy?
• Network designers Cost-efficient network design
• Good utilization of network resources
• Cost of building the network
• Types of services to be supported

7
Perspective (II)

• Network providers Network administration and
  customer service
• Maximize Revenue
• Minimize Operations Expenses
• Survivability and Resiliency

8
A closer look at network structure

• network edge applications and hosts
• network core
• routers
• network of networks
• access networks, physical media communication
  links

9
Connectivity

• We may want a set of hosts (or devices) to be
  directly connected! WHY ?
• We may not always strive for global
  connectivity! WHY?
• Building Blocks to connect at the physical level
• links coax cable, optical fiber...
• nodes general-purpose workstations (though
  sometimes very specialized)

10
Basic Connectivity

• Two types of direct connectivity
• point-to-point
• multiple access

11
Discussions

• If all computers had to be directly connected,
  networks would be either very limited or
  expensive and unmanageable!
• Question If n computers were to be completely
  and directly connected by point-to-point links of
  cost C, what would be the total cost of the net?
• Question Would it be less expensive to use a
  multiple-access network? What are the drawbacks
  and limitations?

12
Building Blocks

• Switches, Routers, Gateways
• Special network components responsible for
  moving packets across the network from source
  to destination.
• Network hosts, workstations, etc.
• they generally represent the source and sink
  (destination) of data traffic (packets)
• We can recursively build large networks by
  interconnecting networks via gateways and
  routers.

13
An Interconnection Network
14
Whats a protocol?

• human protocols
• whats the time?
• I have a question
• introductions
• specific msgs sent
• specific actions taken when msgs received, or
  other events

• network protocols
• machines rather than humans
• all communication activity in Internet governed
  by protocols

protocols define format, order of msgs sent and
received among network entities, and actions
taken on msg transmission receipt
15
Whats a protocol?

• a human protocol and a computer network protocol

Hi
TCP connection req.
Hi
Q Other human protocol?
16
Protocols

• Building blocks of a network architecture
• Each protocol object has two different interfaces
• service interface defines operations on this
  protocol
• peer-to-peer interface defines messages
  exchanged with peer
• Term protocol is overloaded
• specification of peer-to-peer interface
• module that implements this interface

17
The network edge

• end systems (hosts)
• run application programs
• e.g., WWW, email
• at edge of network
• client/server model
• client host requests, receives service from
  server
• e.g., WWW client (browser)/ server email
  client/server
• peer-peer model
• host interaction symmetric
• e.g. teleconferencing, Gnutella, Kazza,
  BitTorrent

18
Network edge connection-oriented service

• Goal data transfer between end sys.
• handshaking setup (prepare for) data transfer
  ahead of time
• Hello, hello back human protocol
• set up state in two communicating hosts
• TCP - Transmission Control Protocol
• Internets connection-oriented service

• TCP service RFC 793
• reliable, in-order byte-stream data transfer
• loss acknowledgements and retransmissions
• flow control
• sender wont overwhelm receiver
• congestion control
• senders slow down sending rate when network
  congested

19
Network edge connectionless service

• Goal data transfer between end systems
• same as before!
• UDP - User Datagram Protocol RFC 768
  Internets connectionless service
• unreliable data transfer
• no flow control
• no congestion control

• Apps using TCP
• HTTP (WWW), FTP (file transfer), Telnet (remote
  login), SMTP (email)
• Apps using UDP
• streaming media, teleconferencing, Internet
  telephony,
• Skype

20
The Network Core

• mesh of interconnected routers
• the fundamental question how is data transferred
  through net?
• circuit switching dedicated circuit per call
  telephone net
• packet-switching data sent thru net in discrete
  chunks

21
Different Types of Switching

• Different Types of Switching
• Circuit Switching (telephone network)
• dedicated circuit, sending and receiving bit
  streams
• Packet Switching
• store and forward, sending and receiving packets
• Message Switching
• Virtual Circuit Switching
• Cell Switching (ATM)
• What are Packets?
• Data to be transmitted is divided into discrete
  blocks

22
Network Core Circuit Switching

• End-end resources reserved for call
• link bandwidth, switch capacity
• dedicated resources no sharing
• circuit-like (guaranteed) performance
• call setup required

23
Cost-Effective Resource Sharing

• Must share (multiplex) network resources among
  multiple users.
• Common Multiplexing Strategies
• Time-Division Multiplexing (TDM)
• Frequency-Division Multiplexing (FDM) Frequency
  band ? bandwidth
• Multiplexing multiple logical flows over a single
  physical link.

24
Network Core Circuit Switching

• network resources (e.g., bandwidth) divided into
  pieces
• pieces allocated to calls
• resource piece idle if not used by owning call
  (no sharing)
• dividing link bandwidth into pieces
• frequency division
• time division

25
Network Core 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,

• resource contention
• aggregate resource demand can exceed amount
  available
• congestion packets queue, wait for link use
• store and forward packets move one hop at a time
• transmit over link
• wait turn at next link

26
Network Core Packet Switching
On-demand sharing
10 Mbs Ethernet
C
A
statistical multiplexing
1.5 Mbs
B
queue of packets waiting for output link
45 Mbs
27
Network Core Packet Switching

• Packet-switching
• store and forward behavior

28
Packet switching versus circuit switching

• Packet switching allows more users to use network!

• 1 Mbit link
• each user
• 100Kbps when active
• active 10 of time
• circuit-switching
• 10 users
• packet switching
• with 35 users, probability gt 10 active less than
  .004

N users
1 Mbps link
29
Packet switching versus circuit switching

• Is packet switching a slam dunk winner?

• Great for bursty data
• resource sharing
• 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!

30
Packet-switched networks routing

• Goal move packets among routers from source to
  destination
• well study several path selection algorithms
• datagram network
• destination address determines next hop
• routes may change during session
• analogy driving, asking directions
• virtual circuit network
• each packet carries tag (virtual circuit ID),
  tag determines next hop
• fixed path determined at call setup time, remains
  fixed thru call
• routers maintain per-call state
• ATM

31
Access networks and physical media

• Q How to connect end systems to edge router?
• residential access nets
• institutional access networks (school, company)
• mobile access networks
• Keep in mind
• bandwidth (bits per second) of access network?
• shared or dedicated?

32
Residential access point to point access

• Dialup via modem
• up to 56Kbps direct access to router
  (conceptually)
• ISDN intergrated services digital network
  128Kbps all-digital connect to router
• ADSL asymmetric digital subscriber line
• up to 1 Mbps home-to-router
• up to 8 Mbps router-to-home

33
Residential access cable modems

• HFC hybrid fiber coax
• asymmetric up to 10Mbps upstream, 1 Mbps
  downstream
• network of cable and fiber attaches homes to ISP
  router
• shared access to router among home
• issues congestion, dimensioning
• deployment available via cable companies, e.g.,
  MediaOne, Comcast

34
Institutional access local area networks

• company/univ local area network (LAN) connects
  end system to edge router
• Ethernet
• shared or dedicated cable connects end system and
  router
• 10 Mbs, 100Mbps, Gigabit Ethernet
• deployment institutions, home LANs soon

35
Wireless access networks

• shared wireless access network connects end
  system to router
• wireless LANs
• radio spectrum replaces wire
• e.g., Lucent Wavelan 10 Mbps
• wider-area wireless access
• CDPD wireless access to ISP router via cellular
  network (base stations)

36
Physical Media

• Twisted Pair (TP)
• two insulated copper wires
• Category 3 traditional phone wires, 10 Mbps
  ethernet
• Category 5 TP 100Mbps ethernet

• physical link transmitted data bit propagates
  across link
• guided media
• signals propagate in solid media copper, fiber
• unguided media
• signals propagate freely, e.g., radio

37
Physical Media coax, fiber

• Coaxial cable
• wire (signal carrier) within a wire (shield)
• baseband single channel on cable
• broadband multiple channel on cable
• bidirectional
• common use in 10Mbs Ethernet

• Fiber optic cable
• glass fiber carrying light pulses
• high-speed operation
• 100Mbps Ethernet
• high-speed point-to-point transmission (e.g., 5
  Gps)
• low error rate

38
Physical media radio

• Radio link types
• microwave
• e.g. up to 45 Mbps channels
• LAN (e.g., waveLAN)
• 2Mbps, 11Mbps
• wide-area (e.g., cellular)
• e.g. CDPD, 10s Kbps
• satellite
• up to 50Mbps channel (or multiple smaller
  channels)
• 270 Msec end-end delay
• geosynchronous versus LEOS

• signal carried in electromagnetic spectrum
• no physical wire
• bidirectional
• propagation environment effects
• reflection
• obstruction by objects
• interference

39
Different Types of Links

• Sometimes you install your own!

40
Bigger Pipes!

• Sometimes leased from the phone company
• STS Synchronous Transport Signal

41
Delay in packet-switched networks

• nodal processing
• check bit errors
• determine output link
• queueing
• time waiting at output link for transmission
• depends on congestion level of router

• packets experience delay on end-to-end path
• four sources of delay at each hop

42
Delay in packet-switched networks

• Propagation delay
• d length of physical link
• s propagation speed in medium (2x108 m/sec)
• propagation delay d/s

• Transmission delay
• Rlink bandwidth (bps)
• Lpacket length (bits)
• time to send bits into link L/R

Note s and R are very different quantitites!
43
Performance

• Bandwidth (throughput)
• Amount of data that can be transmitted per time
  unit
• Example 10Mbps
• link versus end-to-end
• Notation
• KB 210 bytes
• Mbps 106 bits per second

44
Performance

• Bandwidth related to bit width

a)
1 second
b)
1 second
45

• Latency (delay)
• Time it takes to send message from point A to
  point B
• Example 24 milliseconds (ms)
• Sometimes interested in in round-trip time (RTT)
• Components of latency
• Latency Propagation Transmit Queue Proc.
• Propagation Distance / SpeedOfLight
• Transmit Size / Bandwidth

46
Transmission and Propagation Delays

• Propagation delay
• The propagation delay over a link is the time it
  takes a bit to travel from on end of the link to
  the other
• d/s
• Transmission delay
• It is the amount of time it takes to push the
  packet onto the link
• L/B
• Total latency over the link
• transmission delay propagation delay

47
Bandwidth v.s. Latency

• Consider a standard 6250 bpi magnetic tape that
  holds 180 Megabytes. A station wagon can easily
  transport 200 tapes. Suppose the source and
  destination are an hours drive apart.
• Calculate the effective throughput
• 288000 megabits in 3600 seconds
• or 80 Mbps
• What is the moral of this story ?

48
Bandwidth v.s. Latency

• Never underestimate the bandwidth of a station
  wagon full of tapes pacing down the highway.
• But What happens to the latency?

49
The hard limit

• Speed of light
• 3.0 x 108 meters/second in a vacuum
• 2.3 x 108 meters/second in a cable
• 2.0 x 108 meters/second in a fiber
• Notes
• no queuing delays in direct link
• bandwidth not relevant if Size 1 bit
• process-to-process latency includes software
  overhead
• software overhead can dominate when Distance is
  small

50

• Relative importance of bandwidth and latency
• small message (e.g., 1 byte) 1ms vs. 100ms
  dominates 1Mbps vs. 100Mbps
• large message (e.g., 25 MB) 1Mbps vs. 100Mbps
  dominates 1ms vs. 100ms
• Consider two channels of 1Mbps and 100 Mbps
  respectively. For a 1 byte message, the available
  bandwidth is relatively insignificant given a RTT
  of 1 ms. The transmit delay for each channel is 8
  ?s and 0.08 ?s, respectively.

51

• Delay x Bandwidth Product
• e.g., 100ms RTT and 45Mbps Bandwidth 560KB of
  data
• We have to view the network as a buffer. This
  may have interesting consequences
• How much data did the sender transmit before a
  response can be received?

Delay
Bandwidth
52

• Application Needs
• bandwidth requirements burst versus peak rate
• jitter variance in latency (inter-packet gap)
• Average Bandwidth Requirement is Not enough
• consider a source with an avg. BW-requirement of
  2Mbps. If the application generates 1 Mbit in one
  second interval and 3Mbit in a second, a channel
  that can support 2 Mbps max. will have a tough
  time.
• Other Quality of Service (QOS) Parameters
• max. and min. delay
• max. and min. bandwidth demand
• rates for dynamic increase of demands
• Cell-Loss Rate

53
Queueing delay (revisited)

• Rlink bandwidth (bps)
• Lpacket length (bits)
• aaverage packet arrival rate

traffic intensity La/R
La/R 0 average queueing delay small La/R -gt 1
delays become large La/R gt 1 more work
arriving than can be serviced, average delay
infinite!
54
What Goes Wrong in the Network?

• Different types of Error
• Bit-level errors (electrical interference)
• 1 in 106-107 in copper - 1 in 1012 - 1014 in
  fiber
• Packet-level errors (congestion)
• Link and node failures
• How should we deal with these types of error?
• What are the consequences of errors?

55
Other types of problems in the Network?

• Messages are delayed
• Messages are deliver out-of-order
• Third parties eavesdrop
• The key problem is to fill in the gap between
  what applications expect and what the underlying
  technology provides.

56
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?

57
Organization of air travel

• a series of steps

58
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

59
Layered air travel services
Counter-to-counter delivery of personbags baggag
e-claim-to-baggage-claim delivery people
transfer loading gate to arrival
gate runway-to-runway delivery of plane
airplane routing from source to destination
60
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
arriving airport
Departing airport
intermediate air traffic sites
61
Why layering?

• Dealing with complex systems
• explicit structure allows identification,
  relationship of complex systems pieces
• layered reference model for discussion
• modularization eases maintenance, updating of
  system
• change of implementation of layers service
  transparent to rest of system
• e.g., change in gate procedure doesnt affect
  rest of system
• layering considered harmful?

62
Internet protocol stack

• application supporting network applications
• ftp, smtp, http
• 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

63
Layering logical communication

• Each layer
• distributed
• entities implement layer functions at each node
• entities perform actions, exchange messages with
  peers

64
Layering logical communication

• 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

transport
transport
65
Layering physical communication
66
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
67
Protocol Data Units

• The combination of data from the next higher
  layer and control information is referred to as
  PDU.
• Control Information in the Transport Layer may
  include
• Destination Service Access Point (DSAP)
• Sequence number
• Error-detection code

68
Internet structure network of networks

• roughly hierarchical
• national/international backbone providers (NBPs)
• e.g. BBN/GTE, Sprint, ATT, IBM, UUNet
• interconnect (peer) with each other privately, or
  at public Network Access Point (NAPs) (or
  switching centers)
• regional ISPs
• connect into NBPs
• local ISP, company
• connect into regional ISPs

regional ISP
NBP B
NBP A
regional ISP
69
National Backbone Provider
e.g. BBN/GTE US backbone network
70
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 Reearch
  Projects Agency
• 1969 first ARPAnet node operational

• 1972
• ARPAnet demonstrated publicly
• NCP (Network Control Protocol) first host-host
  protocol
• first e-mail program
• ARPAnet has 15 nodes

71
Internet History
1972-1980 Internetworking, new and proprietary
nets

• 1970 ALOHAnet satellite network in Hawaii
• 1973 Metcalfes PhD thesis proposes Ethernet
• 1974 Cerf and Kahn - architecture for
  interconnecting networks
• late70s proprietary architectures DECnet, SNA,
  XNA
• late 70s switching fixed length packets (ATM
  precursor)
• 1979 ARPAnet has 200 nodes

• Cerf and Kahns internetworking principles
• minimalism, autonomy - no internal changes
  required to interconnect networks
• best effort service model
• stateless routers
• decentralized control
• define todays Internet architecture

72
Internet History
1980-1990 new protocols, a proliferation of
networks

• 1983 deployment of TCP/IP
• 1982 smtp e-mail 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

73
Internet History
1990s commercialization, the WWW

• Early 1990s ARPAnet decomissioned
• 1991 NSF lifts restrictions on commercial use of
  NSFnet (decommissioned, 1995)
• early 1990s WWW
• hypertext Bush 1945, Nelson 1960s
• HTML, http Berners-Lee
• 1994 Mosaic, later Netscape
• late 1990s commercialization of the WWW

• Late 1990s
• est. 50 million computers on Internet
• est. 100 million users
• backbone links runnning at 1 Gbps

74
Summary

• Covered a ton of material!
• Internet overview
• whats a protocol?
• network edge, core, access network
• performance loss, delay
• layering and service models
• backbones, NAPs, ISPs
• history

• You now hopefully have
• context, overview, feel of networking
• more depth, detail later in course