ppt

canoeornithologistΔίκτυα και Επικοινωνίες

26 Οκτ 2013 (πριν από 3 χρόνια και 9 μήνες)

86 εμφανίσεις

Reduced TCP Window Size

for VoIP in Legacy LAN Environments

Nikolaus F
ä
rber,

Bernd Girod, Balaji Prabhakar


Nikolaus F
ärber

Legacy LAN VoIP

Scenario


SOHO with < 30 nodes


Switched, full duplex LAN architecture


10/100 BASE
-
T, single switch

WAN

Legacy LAN, IP best
-
effort

T1/DSL/Cable

“Last Mile”

R

S

10 BASE
-
T

Host

IP Phone

T2


Problems:


Data (TCP) interferes

with voice (UDP)


Queuing delay


Loss

Nikolaus F
ärber

Legacy LAN VoIP

Goal

Control TCP traffic from network edge (T2)

such that voice delay is reduced

Nikolaus F
ärber

Legacy LAN VoIP

Overview


TCP flow control basics


Window based flow control


Bandwidth
-
delay product


TCP’s congestion avoidance


Bandwidth
-
delay product


LFNs


LANs


Rule of thumb for setting advertised window size


Results: voice delay and data throughput for


File transfer


LAN at different loads


TCP window control by T2


Limitations and work around


Nikolaus F
ärber

Legacy LAN VoIP

TCP Flow Control Basics


TCP flow control based on window size
W

(number of packets
source is allowed to send without ACK)


Receiver signals advertised window size
W
max

in ACK


Steady state:
W

N

=
B

D


TCP does not know
B
,
D
,
N
!


Use loss as implicit sign for congestion:

B

= 40 packet/sec

D

= 0.1 sec

N

= 2 connections

Increase until loss

Back off

W
= 2

(incremental increase)

(multiplicative decrease)

tx

rx

Nikolaus F
ärber

Legacy LAN VoIP

B

D

for LFNs


“Long Fat Networks” (LFNs) have big
B

D

requiring big windows


Example: cross
-
country ATM



B

= 155 Mbps


D

= 70 ms



Original TCP only supports 64 Kbyte (16 bit filed in header)


New “window scaling” option allows up to 1 Gbyte


Common values still 32
-
64 KByte

B

D
= 1.3 MByte

Nikolaus F
ärber

Legacy LAN VoIP

B

D

for LANs


W
max

for LFNs way too big for LANs!


Example, single Ethernet link:


B

D
<! 512 bit


Main delay on switched LAN:


Packet transmission
d
data

= P/B


Store
-
and
-
forward operation


Queuing delay
d
Q


Estimate for low loads:


D

< 2

H

P
/B


N
= 1


Rule of thumb based on
W

N

=
B

D


W
max

= 2

H

P



Typical settings:
W
max

=

4
-
16 KByte

d
data

= P/B

d
data

= P/B

rx

sw

tx

d
Q,fwd

d
Q,bwd

d
ack

= A/B

d
ack

= A/B

P

= 1500 Byte

A

= 60 Byte

B

= 10 Mbps

Nikolaus F
ärber

Legacy LAN VoIP

File Transfer: Scenario


Voice traffic: UDP, 30 ms, 240 Byte, 10 s


Data traffic: TCP, 8 MB file, 1500 Byte packets, start at 3.5 s


Links: 10/100 Mbps, full duplex, 0.1 ms delay


Switch: 30 KByte buffer, Drop
-
Tail

R

S

File Server

100BASE
-
T

10BASE
-
T

Host

UDP voice

TCP data

bottle neck

Nikolaus F
ärber

Legacy LAN VoIP

File Transfer: Simulation Results

2

3

4

5

6

7

8

9

10

0

5

10

15

20

25

30

2

3

4

5

6

7

8

9

10

0

2

4

6

8

10

12

voice delay

[ms]

data throughput

[Mbps]

time [s]

ftp

start

W
max

= 8 KB

W
max

= 32 KB

Nikolaus F
ärber

Legacy LAN VoIP

File Transfer: Measurements

2

3

4

5

6

7

8

9

10

0

10

20

30

40

50

60

70

2

3

4

5

6

7

8

9

10

0

2

4

6

8

10

voice delay

[ms]

data throughput

[Mbps]

time [s]

ftp

start

W
max

= 32 KB

W
max

= 8 KB

Nikolaus F
ärber

Legacy LAN VoIP

LAN: Simulation Scenario


Balanced N
-
N communication


Traffic model:


File size PDF
f
F
(F):

Log
-
Normal
[Arlit 99, Douceur 99]


Idle time T ~
F

/



Evaluation:


Voice QoS: 95 percentile of voice delay (
d
95
)


Data QoS: goodput
G

=
S
F
i
/
S
T
i


R

S

1

2

3

L

load [0,1]


10BASE
-
T only


Average data
rate on each link
in each direction
is ~


10 Mbps

UDP voice

TCP data

Nikolaus F
ärber

Legacy LAN VoIP

LAN: Simulation Results


L
=4,
B
sw

= 200 KByte

1

2

3

4

5

6

7

8

0

10

20

30

40

50

60

70

80

90

95 percentile of voice delay,
d
95

[ms]

data goodput,
G

[Mbps]



= 0.5

0.4

0.3

0.1

0.2

W
max

= 32

16

8

4

2

1


Low load


Voice uncritical


Data critical


High load


Voice critical


Data uncritical



W=4 is good choice
for all loads!

Nikolaus F
ärber

Legacy LAN VoIP

LAN: Simulation Result (Cont.)


L
=16,
B
sw

= 100 KByte

1

2

3

4

5

6

7

8

0

10

20

30

40

50

60

70

80

90

95 percentile of voice delay,
d
95

[ms]

data goodput,
G

[Mbps]



= 0.5

W
max
= 1

2

4

8

16

32

0.4

0.3

0.1

0.2


General behavior

also applies for


L

= {4,8,16}


B
sw

= {100,200,


300}


H

= {2,4}


For
H
=4 (each host
has T2) optimal
window size is

W
max

= 8

Nikolaus F
ärber

Legacy LAN VoIP

TCP Window Control by T2


Advertised window size
W
max

is signaled in ACK


T2 can intercept all TCP ACKs and reduce
W
max
before butting it
back onto LAN


TCP flows of WAN traffic is not changed


This “packet spoofing” technique is also used by
Packeteer Inc.
for TCP rate control


No need to modify server/client software


Particular simple for single switch LAN


Even for single T2, the connected host can take full advantage of
the technique


Allows gradual deployment without need to use T2 for all hosts


Nikolaus F
ärber

Legacy LAN VoIP

Limitations


Control from network edge is limited for general LAN topology


T2 cannot control traffic through “remote” switches


S2

S1

S3

R

UDP voice

TCP data

Nikolaus F
ärber

Legacy LAN VoIP

Work Around


Avoid queuing delays in between switches


Keep inter
-
switch traffic low


Use faster links for inter
-
switch connections



generate virtual big switch


Allow communication amoung T2s

S2

S1

S3

R

UDP voice

TCP data

Nikolaus F
ärber

Legacy LAN VoIP

Conclusions


Reduced TCP window size is advantageous for local data traffic


Reduced voice delay and jitter


Improves data goodput


Rule of thumb for switched LAN:
W
max

= 2

H

P


W=4 [packets] is good choice for single switch LAN


Window size can be spoofed by T2 without awareness of client
or server


Control from network edge has inherent limitations


For medium sized LANs, optimization of topology may help