Gh

1. Basic
Concepts
(part
2):
packet
delays
and
layering
CS168,
Fall
2014
Sylvia
Ratnasamy
hCp://inst.eecs.berkeley.edu/~cs168/fa14/

2. Administrivia
‣ Discussion
secLons
start
Monday,
Sep
15
‣ Homework#1
out
tonight
-­‐ announce
homework
logis1cs
2

3. Plan
of
aCack
‣ What
is
a
network
made
of?
‣ How
is
it
shared?
‣ How
do
we
evaluate
a
network?
‣ How
is
communicaLon
organized?
3

4. 4
end-­‐system
switch
link
Internet
Service
Provider

5. 5
phone
company

6. 6
home
PC
switch
DSL
modem
DSLAM
central
office
phone
line
telephone
telephone
network
...

7. Two
approaches
to
sharing
‣ ReservaLons
à
circuit
switching
‣ On
demand
à
packet
switching
7

8. Circuit
vs.
Packets
‣ Circuits
-­‐ predictable
performance
-­‐ inefficient
use
of
network
resources
-­‐ complex
(state
in
the
network)
‣ Packets
-­‐ unpredictable
performance
-­‐ efficient
use
of
network
resources
-­‐ simple
(no
state
in
the
network)

9. ‣ What
physical
infrastructure
is
already
available?
‣
Reserve
or
on-­‐demand?
9

10. Today
‣ What
is
a
network
made
of?
‣ How
is
it
shared?
‣ How
do
we
evaluate
a
network?
‣ How
is
communicaLon
organized?
10

11. Performance
Metrics
‣ Delay
‣ Loss
‣ Throughput

12. Delay
‣ How
long
does
it
take
to
send
a
packet
from
its
source
to
des1na1on?

13. Delay
‣ Consists
of
four
components
-­‐ transmission
delay
-­‐ propaga1on
delay
-­‐ queuing
delay
-­‐ processing
delay
due
to
link
proper1es
due
to
traffic
mix
and
switch
internals

14. A
network
link
l Link
bandwidth
l number
of
bits
sent/received
per
unit
Lme
(bits/sec
or
bps)
l PropagaLon
delay
l Lme
for
one
bit
to
move
through
the
link
(seconds)
l Bandwidth-­‐Delay
Product
(BDP)
l number
of
bits
“in
flight”
at
any
Lme
l BDP
=
bandwidth
×
propagaLon
delay
bandwidth
PropagaLon
delay
delay
x
bandwidth

15. Examples
l Same
city
over
a
slow
link:
l bandwidth:
~100Mbps
l propagaLon
delay:
~0.1msec
l BDP:
10,000bits
(1.25KBytes)
l Cross-­‐country
over
fast
link:
l bandwidth:
~10Gbps
l propagaLon
delay:
~10msec
l BDP:
108bits
(12.5MBytes)

16. time=0
A B
100Byte packet
Time
1Mbps, 1ms
Time to transmit
one bit = 1/106s
Time to transmit
800 bits=800x1/106s
Time when that
bit reaches B
= 1/106+1/103s
The last bit
reaches B at
(800x1/106)+1/103s
= 1.8ms
Packet
Delay
Sending
100B
packets
from
A
to
B?

17. Packet
Delay
Sending
100B
packets
from
A
to
B?
A B
100Byte packet
Time
1Mbps, 1ms
1Gbps, 1ms?
The last bit
reaches B at
(800x1/106)+1/103s
= 1.8ms
1GB file in 100B packets
The last bit
reaches B at
(800x1/109)+1/103s
= 1.0008ms
The last bit in the file
reaches B at
(107x800x1/109)+1/103s
= 8001ms
107 x 100B packets

18. A B
100Byte packet
Time
1Mbps, 10ms
100Byte packet
100Byte packet
time à
BW
à
pkt tx
time
Packet
Delay:
The
“pipe”
view
Sending
100B
packets
from
A
to
B?

19. Packet
Delay:
The
“pipe”
view
Sending
100B
packets
from
A
to
B?
1Mbps, 10ms (BDP=10,000)
time à
BW
à
10Mbps, 1ms (BDP=10,000)
time à
BW
à
1Mbps, 5ms (BDP=5,000)
time à
BW
à

20. Packet
Delay:
The
“pipe”
view
Sending
100B
packets
from
A
to
B?
1Mbps, 10ms (BDP=10,000)
time à
BW
à
200B?
1Mbps, 10ms (BDP=10,000)
time à
BW
à

21. Computer
Networks,
Fall
2013
1.
Transmission
delay
‣ How
long
does
it
take
to
push
all
the
bits
of
a
packet
into
a
link?
‣ Packet
size
/
Link
bandwidth
-­‐ e.g.
1000
bits
/
100
Mbits
per
sec
=
10
-­‐5
sec
21

22. Computer
Networks,
Fall
2013
2.
PropagaLon
delay
‣ How
long
does
it
take
to
move
one
bit
from
one
end
of
a
link
to
the
other?
‣ Link
length
/
Link
propagaLon
delay
-­‐ E.g.
30
kilometers
/
3
108
meters
per
sec
=
10-­‐4
sec
22

23. Computer
Networks,
Fall
2013
3.
Queuing
delay
‣ How
long
does
a
packet
have
to
sit
in
a
buffer
before
it
is
processed?
23

24. Queuing
delay:
“pipe”
view

25. No
queuing
delay!
Queuing
delay:
“pipe”
view

26. Transient
Overload
Not
a
rare
event!
Queue
Queuing
delay:
“pipe”
view

27. Queue
Queuing
delay:
“pipe”
view

28. Queue
Queuing
delay:
“pipe”
view

29. Queue
Queuing
delay:
“pipe”
view

30. Queue
Queuing
delay:
“pipe”
view

31. Queue
Queuing
delay:
“pipe”
view
Queues
absorb
transient
bursts
but
introduce
queuing
delay

32. What
about
persistent
overload?
Will
eventually
drop
packets
(“loss”)
Queuing
delay:
“pipe”
view

33. Computer
Networks,
Fall
2013
Queuing
delay
‣ If
arrival
rate
>
departure
rate
-­‐ approaches
infinity
(assuming
an
infinite
buffer)
33

34. Computer
Networks,
Fall
2013
34
arrival
rate
/departure
rate
1
Average
q
ueuing
d
elay

35. Computer
Networks,
Fall
2013
Queuing
delay
‣ If
arrival
rate
>
departure
rate
-­‐ approaches
infinity
(assuming
an
infinite
buffer)
-­‐ in
prac1ce,
finite
buffer
à
loss
‣ If
arrival
rate
<
departure
rate
35

36. e.g.,
arrival
rate
<
departure
rate
Queue

37. Computer
Networks,
Fall
2013
Queuing
delay
‣ If
arrival
rate
>
departure
rate
-­‐ approaches
infinity
(assuming
an
infinite
buffer)
-­‐ in
prac1ce,
finite
buffer
à
loss
‣ If
arrival
rate
<
departure
rate
-­‐ depends
on
burst
size
37

38. Queuing
Delay
l How
long
does
a
packet
have
to
sit
in
a
buffer
before
it
is
processed?
l Depends
on
traffic
paCern

39. Queuing
Delay
l How
long
does
a
packet
have
to
sit
in
a
buffer
before
it
is
processed?
l Depends
on
traffic
paCern
l Characterized
with
staLsLcal
measures
l average
queuing
delay
l average
arrival
rate
l average
departure
rate

40. Basic
Queuing
Theory
Terminology
l Arrival
process:
how
packets
arrive
l Average
rate
A
l W:
average
Lme
packets
wait
in
the
queue
l W
for
“waiLng
Lme”
l L:
average
number
of
packets
waiLng
in
the
queue
l L
for
“length
of
queue”

41. LiCle’s
Law
(1961)
L
=
A
x
W
l Compute
L:
count
packets
in
queue
every
second
l How
ooen
does
a
single
packet
get
counted?
W
Lmes
l Why
do
you
care?
l Easy
to
compute
L,
harder
to
compute
W

42. Computer
Networks,
Fall
2013
4.
Processing
Delay
‣ How
long
does
the
switch
take
to
process
a
packet?
42
• typically
assume
this
is
negligible

43. Delay
‣ Consists
of
four
components
-­‐ transmission
delay
-­‐ propaga1on
delay
-­‐ queuing
delay
-­‐ processing
delay
due
to
link
proper1es
due
to
traffic
mix
and
switch
internals

44. transmission
44
End-­‐to-­‐end
delay
propagaLon
queuing
processing
transmission
propagaLon
queuing
processing
transmission
propagaLon

45. Loss
‣ What
frac1on
of
the
packets
sent
to
a
des1na1on
are
dropped?

46. Throughput
‣ At
what
rate
is
the
des1na1on
receiving
data
from
the
source
-­‐ Data
size
/
transfer
Lme

47. F/R
+
propagaLon
delay
47
transmission
rate
R
bits/sec
Average
throughput
=
Transfer
Lme
=
file
of
size
F
bits
R
packets
of
size
L
bits

48. 48
transmission
rate
R’
>
R
transmission
rate
R
file
of
size
F
bits
packets
of
size
L
bits

49. F/R
+
propagaLon
delay
+
L/R’
49
bo@leneck
link
min
{
R,
R’
}
=
R
transmission
rate
R’
>
R
transmission
rate
R
file
of
size
F
bits
Average
throughput
=
Transfer
Lme
=
packets
of
size
L
bits

50. 50
bo@leneck
link
min
{
R,
R’
}
=
R
transmission
rate
R’
>
R
transmission
rate
R
file
of
size
F
bits
Average
throughput
=
packets
of
size
L
bits

51. 51
bo@leneck
link
transmission
rate
R1
transmission
rate
R2

52. 52
transmission
rate
R1
transmission
rate
R2
bo@leneck
link

53. Throughput
‣ At
what
rate
is
the
desLnaLon
receiving
data
from
the
source?
‣ Later
in
the
semester
-­‐ TCP
throughput,
applica1on-­‐level
throughput,
etc.
-­‐ throughput
vs.
“goodput”
53

54. ‣ What
physical
infrastructure
is
already
available?
‣
Reserve
or
on-­‐demand?
‣ Where’s
my
delay
coming
from?
54