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6.033 Computer System Engineering
Spring 2009
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L9: Intro Network Systems
Dina Katabi
Some slides are from lectures by
Nick Mckeown, Ion Stoica, Frans
Kaashoek, Hari Balakrishnan, Sam
Madden, and Robert Morris
What have you seen so far?
Systems
Naming systems
Complexity
Modularity
Dtechnology/dt
Gluing systems
Client/service
design
Operating
systems
Enforced
modularity
Client/service
with in a
computer
Performance
Coping with
bottlenecks
Hierarchy
Therac-25
File system
name space
X windows
Eraser and Unix
MapReduce
Client/service using network
client
OS
request
network
server
OS
• Sharing irrespective of geography
• Strong modularity through geographic separation
Network is a system too!
domain-1
domain-3
domain-2
• Network consists of many networks, many links,
many switches
• Internet is a case study of successful network
system
Today’s topic: challenges
• Economical:
• Universality
• Topology, Sharing, Utilization
• Organizational
• Routing, Addressing, Packets, Delay
• Best-effort contract
• Physical
• Errors, speed of light, wide-range of parameters
Circuit Switching
• It’s the method
used by the
telephone network
• A call has three
phases:
Establish circuit from
end-to-end
(“dialing”),
Communicate,
Close circuit (“tear
down”).
•
If circuit not
available: “busy
signal”
Boston
Switch
Caller
LA
Switch
Callee
processing delay at switch
propagation
delay
between
caller and
and Boston
switch
(1)
(2)
DATA
(3)
Isochronous Multiplexing/Demultiplexing
Switch
interval
frames = 0 1 2 3 4 5
0 1 2 3 4 5
One way for sharing a link is TDM:
• A time interval is divided into n frames
• Each frame carries the data of a particular conversation
• E.g., frame 0 belongs to the red conversation
Circuit Switching
•
•
•
•
•
•
Assume link capacity is C bits/sec
Each communication requires R bits/sec
#frames = C/R
Maximum number of concurrent communications is C/R
What happens if we have more than C/R communications?
What happens if the a communication sends less/more than
R bits/sec?
Æ Design is unsuitable for bursty communications
Packet Switching
• Used in the Internet
• Data is sent in Packets
(header contains control
info, e.g., source and
destination addresses)
Header
Host 1
Host 2
switch 1
Data
• Per-packet routing
• At each node the entire
packet is received,
buffered, and then
forwarded)
• No capacity is allocated
transmission
time of
Packet 1
at Host 1
Packet 1
Packet 2
Packet 3
switch 2
propagation
delay
between
Host 1 &
Node 2
Packet 1
processing
delay of
Packet 1
at Node 2
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3
Asynchronous
Multiplexing/Demultiplexing
Switch
Queue
• Multiplex using a queue
• Switch need memory/buffer
• Demultiplex using information in packet header
• Header has destination
• Switch has a forwarding table that contains
information about which link to use to reach a
destination
Aggregate Internet Traffic Smooths
5-min average traffic rate at an MIT-CSAIL router
Max In:12.2Mb/s
Avg. In: 2.5Mb/s
Max Out: 12.8Mb/s
Avg. Out: 3.4 Mb/s
Statistical
multiplexing
Best Effort
No Guarantees:
• Variable Delay (jitter)
• Variable rate
• Packet loss
• Duplicates
• Reordering
Bits/s
T
W el m
ire
le ode
ss
m
C
Te DP
lm D
Te od
em
l.
2. mo
Bl 5G de
u
m
C etoo wire
ab
le th w less
m
od irele
em ss
/D
SL
80
2.
11 T1
ba
E t se
he
rn
80
2. In et
11 fra
r
b
w ed
ire
le
80
ss
2.
11
a
T
Fa wire 3
st
le
Et ss
G he
ig
r
Et net
h
O
C ern
et
O 48
C
o
19 pt
2c ica
10 op l
G
ti
Et cal
he
rn
O et
C
76
8
Networks are heterogeneous
1E+11
1E+10
1E+09
1E+08
1E+07
1E+06
100000
10000
1000
Link technology
Normalized Growth since 1980
d(technology)/dt for networks
1,000,000
Highest Link Capacity
2x / 7 months
100,000
# Internet hosts
2x / 13.3 months
10,000
Aggregate Internet Traffic
2x / 12 months
1,000
100
10
Moore’s Law
2x / 18 months
Bits/s per dollar
2x / 79 months
(crude estimate)
Speed of light
0x / 18 months!
1
1980
1983
1986
1989
1992
1995
1998
Thanks to Nick Mckeown @ Stanford for some of these data points
2001
Plan for studying network systems
Sharing and
challenges
Layering
7.A
Ethernet
7.B+C
End-to-end
Routing
7.D
End-to-end
reliability
Congestion
control
7.E
Internet
routing
Network file
system
NATs
7.F