How Fast Is the Internet Or How Long Will It Take To Download That File

1
How Fast Is the Internet-- Or How Long Will
It Take To Download That File?

• John DeDourek
• July 7-8, 2003

2
Day 1 Schedule

• How does the Internet work?
• Introductions
• Presentation
• Discussion
• Tour
• Network tools

3
Day 2 Schedule

• How fast is the Internet?
• Presentation
• More tools
• Network measurements

4
A Network
Medium
5
Two Types Of Network
Point-to-point (full duplex)
6
Frames
Network data is packaged in frames
Data
Header
Trailer
(e.g. 1500 bytes for Ethernet)
7
A more complex Ethernet
Store and Forward Switch
Store and Forward Switch
8
Definition
Network an interconnection of hosts such that
any host can send a frame directly to any
other host
9
Limitation

• For various technical reasons, a network is
  restricted in various ways
• Maximum number of hosts
• Maximum length of wire
• Maximum number of switches (etc.) between two
  hosts

10
The Internet Idea

• Interconnect individual networks
• Any host can send data to any other host, whether
  on the same network or another network, in the
  same way

11
Internet
Network
Network
Network
12
IP Packet
data
header
host
packet
net
frame
router
packet
Etc.
13
Queues
out
in
Packets or frames
14
- Shared 15
UNBs Campus Network

• Mainly Ethernet at 100 Mbs over copper and 1,000
  Mbs (1 Gbs) over fiber
• Switches (various) and switches combined with
  routers (Nortel Passport 8600)

16
UNBs External Connections

• NB/PEI Educational Research Network (other
  institutions of higher learning in provinces)
• Alliant (about 45 Mbs over ATM over fiber)
• CANet4 (about 2.5 Gbs over WDM over fiber)

17
UNBs big routers

• For connections to outside networks
• Juniper M20 routers

18
Tour

• Steven Birch

19
Tools

• Linux
• Command window
• netstat
• netstat i
• netstat rn
• ping
• traceroute

20
Miscellaneous unix tools

• Shell and pipes ()
• awk
• grep
• head
• tail
• grep
• Etc.

21
SoHow Fast Is the Internet-- Or How Long
Will It Take To Download That File?

• Day 2

22
What is the Speed of the Internet?

• Internet data flows across
• Copper wire
• Fiber
• Wireless (radio)
• Physics tells us that these all represent
  electromagnetic propagation

23
Einstein Says

• The speed of light, hence the speed of
  electromagnetic propagation is a constant
• C 300 x 106 m/s
• Hence the speed of the Internet is
• 300 x 106 m/s

24
Well Not Exactly!

• C is the speed of propagation in free space
• From Peterson
• Copper 2.3 x 108 m/s
• Fiber 2.0 x 108 m/s
• Radio 3.0 x 108 m/s
• Speed of the Internet is between 2.0 x 108 and
  3.0 x 108 m/s

25
A File to Download
62,728 bytes from Stanford University pipe
26
Another File to Download
14,554 bytes from Petosa, Inc., Seattle ???
accordion
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How Long Will It Take To Download?

• One bit!
• Distance (driving distance mapquest)
• Stanford 5288 km
• Seattle 5342 km
• Speed 2 x 108 m/s
• D / S 26.4 x 10-3 s pipe organ
• D / S 26.7 x 10-3 s accordion

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How Long Will It Take To Download n Bits?

• Size 62,728 bytes (pipe organ jpeg)
• Time for n bits
• 62,728 x 8 x 26.4 x 10-3
• 13,248 s
• 3.7 hours!
• Is this correct?

29
One bit at a time
30
One bit at a time
31
One bit at a time
32
Keeping the Pipe Full
33
Width of a bit

• Depends on communications technology
• On 100 Mbps Ethernet 2.3 m
• Time to send one bit (length of a bit)
• 2.3 m / ( 2.3 x 108 m/s ) 1 x 10-8 s
• Bits per second
• 1 bit / (1 x 10-8 s) 100 x 106 b/s

34
Propagation Delay and Transmission Delay
To get all the bits on the wire Number of
bits / Transmission rate
Transmission delay To get the last bit to the
other end Distance / speed
Propagation delay 62,728 x 8 / (100 x 106)
5,288 x 103 / (2.3 x 108) 5.0 x
10-3 23 x 10-3 28 x 10-3 s
35
Error Detection and Packets

• Data divided into packets each packet has a
  header with error detection information,etc.
• Packet overhead (typical)
• 1460 bytes of data
• 66 bytes of header
• Adds 66 / 1460 4.5 overhead
• Pipe organ jpeg
• 62,728 4.5 x 62,728 65,551 bytes

36
Full Mesh Connections

• My machine has a wire to every other machine on
  the Internet!!!
• Consider diameter of conduit to run this many
  wires into my office!!!

37
Packet Switches

• Switches (layer 2) and routers (layer 3)
• Store and forward packets

38
Delay Through Store and Forward Networks

• Delay through each link
• Transmission delay Propagation delay
• Assume
• All links have the same transmission rate
• All links have the same propagation speed
• Delay n x (PackSize / TRate) TotDist / Speed
• Number of links n

39
Calculation

• traceroute shows number of routers
• For pipe organ jpeg 15 routers
• No way to know number of switches
• Guess 14
• Number of links 30
• Calculation
• 30 x (65,551 x 8 / 2.5 Gbs) 5,288 km / (2 x 108
  m) 6.32 x 10-3 26.4 x 10-3 s 32.7 x 10-3
  s

40
Sharing Cost and Resources

• I dont want to pay for the whole Internet myself
• The others who share the cost insist on using
  it to transmit their data too
• Contention at switches and routers when packets
  arrive destined for the same outgoing link
• Queuing delay introduced
• TotalDelay TransDelay PropDelay QueueDelay
• Unable to predict queuing delay
• Pipe organ typical measured RTT (Round TripTime)
  90 ms
• TransDelay 6.32 ms PropDelay 26.4 ms

41
Flow Of Data
Internet
Screen
Disk
Server Application
Client Application
Send Buffers
Receive Buffers
advertised window
42
Flow Control

• Receive buffer size
• In operating system
• Value may be set by application
• Default determined by operating system
• Advertised window
• In every acknowledgement sent from receiver to
  sender
• Amount of buffer free
• BufferSize AmountStillFull
• Application may not have read all data yet

43
Effect of Window On Transmission Time

• Assume long path i.e. large delay across
  network (trans prop queue)
• Sender transmits one window full of packets,
  then waits
• Receiver receives a window full and
  acknowledges receipt and advertises full window
• I.e., application is infinitely fast
• Sender receives acknowledgements and sends
  another full window of data
• Effect one window per round trip time

44
Calculation

• Typical maximum advertised window (my Windows
  machine) 8760 bytes
• Number of window fulls for pipe organ
• 62,728 bytes / 8760 bytes 7
• Round trip time is 90 ms
• Total download time is 7 x 90 360 ms
• Effective transmission time
• 62,728 bytes / 360 ms 174 KBs 1.4 Mbs

45
Congestion and Packet Loss

• To much contention for a switch or router output
  port exhausts buffer queue
• Called congestion
• Switch or router discards packets
• TCP attempts to avoid waste involved in
  transmitting packets which will be discarded
• Each TCP connection reduces effective
  transmission rate until no congestion occurs
  (congestion avoidance)
• Each connection gets a fair share

46
TCP Congestion Avoidance

• Sender maintains a congestion window
• Actual window used for transmission is minimum of
  window advertised by the receiver and the
  congestion window
• Congestion window starts at one packet
• When an acknowledgement is received, congestion
  window is increased
• When a packet is lost (and retransmitted)
  congestion window is reduced
• Net transmission rate is WinSize / RTT

47
Pipe Organ
48
Accordion
49
Performance Suggestions

• All links in a path must be considered
• Provisioning by your ISP, and their ISPs, etc.
  are important
• All system components must be considered
• Disk, processor, display, etc.
• The advertised window (under control of the
  receiver) is important

50
Research Areas

• Performance of various TCP algorithms under
  actual Internet conditions
• Congestion avoidance, selective ack, explicit
  congestion notification
• Incorporation of wireless links at the network
  edge
• Interaction of TCP traffic with UDP traffic
• RTP for real time multimedia flow

51
References

• Peterson Peterson, Larry L. and Davie, Bruce S.
  Computer Networks A Systems Approach. Morgan
  Kaufmann, San Francisco, 2000.
• pipe The Organ of Don and Jill Knuth.
  http//www-cs-faculty.stanford.edu/knuth/organ/jp
  eg Accessed, Nov. 22, 2002.
• accordion Petosa Model AM-1100 Marocco Jazz
  Accordion. http//www.petosa.com/artistmodels/ima
  ges/frankmarocco2002_med.jpg Accessed Nov 22,
  2002.
• mapquest Mapquest Driving Directions.
  http//www.mapquest.com/directions/ Accessed Nov
  22, 2002.