Download How Fast Is the Internet - Faculty of Computer Science

How Fast Is the Internet
-- Or –
How Long Will It Take To
Download That File?
John DeDourek
July 7-8, 2003
Day 1 Schedule
How does the Internet work?
Introductions
Presentation
Discussion
Tour
Network tools
Day 2 Schedule
How fast is the Internet?
Presentation
More tools
Network measurements
A Network
Medium
Host
Host
Host
Two Types Of Network
Host
Host
Point-to-point (full duplex)
Multiple Access Network
Host
Host
Host
Frames
Network data is “packaged” in frames
Trailer
Data
(e.g. 1500 bytes
for Ethernet)
Header
A more complex Ethernet
Store and Forward
Switch
Host
Store and Forward
Switch
Host
Definition
Network: an interconnection
of hosts such that any
host can send a frame
“directly” to any other
host
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
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
Internet
Host
Host
Network
Network
Router
Host
Host
Network
Router
IP Packet
data
header
host
packet
net
frame
router
packet
Etc.
Queues
out
in
Packets or frames
-> Shared <- Internet
H
S
R
R
S
H
H
H
S
R
H
H
UNB’s 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)
UNB’s 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)
UNB’s “big” routers
For connections to outside networks
Juniper M20 routers
Tour
Steven Birch
Tools
Linux
Command window
netstat
netstat –i
netstat –rn
ping
traceroute
Miscellaneous “unix” tools
Shell and pipes (“|”)
awk
grep
head
tail
grep
Etc.
So
How Fast Is the Internet
-- Or –
How Long Will It Take To
Download That File?
Day 2
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
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
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
A File to Download
62,728 bytes from Stanford
University [pipe]
Another File to Download
14,554 bytes from Petosa, Inc.,
Seattle ??? [accordion]
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
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?
One bit at a time
One bit at a time
One bit at a time
Keeping the Pipe Full
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
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
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
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!!!
Packet Switches
Switches (“layer 2”) and routers (“layer
3”)
Store and forward packets
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
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
Sharing Cost and Resources
I don’t 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
Flow Of Data
Screen
Disk
Server
Application
Internet
Send
Buffers
Client
Application
Receive
Buffers
advertised window
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
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
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
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”
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
Pipe Organ
Accordion
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
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
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-csfaculty.stanford.edu/~knuth/organ/jpeg Accessed, Nov. 22,
2002.
[accordion] Petosa Model AM-1100 “Marocco Jazz” Accordion.
http://www.petosa.com/artistmodels/images/frankmarocco2002
_med.jpg Accessed Nov 22, 2002.
[mapquest] Mapquest Driving Directions.
http://www.mapquest.com/directions/ Accessed Nov 22, 2002.