Seminar in Computer Networks and Distributed Systems — Hadassah College — Spring 2019 TCP over Wireless Networks Dr. Martin Land 1 TCP Westwood for Wireless תוכן.1 מבוא.2 טכני רקע— ב עומס בקרת- TCP .3 TCP אלחוטי קשר על.4 פרוטוקול עם תפוקה שיפורTCP Westwood .5 סיכום
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Seminar in Computer Networks and Distributed Systems — Hadassah College — Spring 2019 TCP over Wireless Networks Dr. Martin Land 1
TCPWestwood
for Wirelessתוכן
מבוא1.
TCP - בקרת עומס ב—רקע טכני 2.
.3TCPעל קשר אלחוטי
TCP Westwoodשיפור תפוקה עם פרוטוקול 4.
סיכום5.
Seminar in Computer Networks and Distributed Systems — Hadassah College — Spring 2019 TCP over Wireless Networks Dr. Martin Land 2
מבואמתאר בקצרה
מקומו של הנושא בתחום התקשורת
הבעיות שהנושא פותר
המצב שהיה לפני קיומו
כיצד הנושא מקדם את המדע
Seminar in Computer Networks and Distributed Systems — Hadassah College — Spring 2019 TCP over Wireless Networks Dr. Martin Land 3
TCP Over WirelessWireless internet
Widely usedRoutine workflowRequires TCP/IP
ProblemTCP handles packet loss as congestion
Lost packet ⇒ bit error or congestionTCP designed for cable (BER < 10-9)Congestion ⇒ slow packet transmission
Wireless BER ≈ 10-3
Many random errors without congestionNo need (or efficacy) in lower packet rate
Westwood Modified TCP SEQ/ACK mechanismImproved treatment of random errors http://cdn.androidpolice.com/wp-content/uploads/
Seminar in Computer Networks and Distributed Systems — Hadassah College — Spring 2019 TCP over Wireless Networks Dr. Martin Land 18
דיון ממו קדחומר המ קצועי הבסיסי שהוסכם עליו עם המנחה
תיאור של החומר שנקרא ומטרתו
סקירה על תוכן החומר שנלמד
דוגמאות מקוריות להמחשת ה נושא
Seminar in Computer Networks and Distributed Systems — Hadassah College — Spring 2019 TCP over Wireless Networks Dr. Martin Land 19
TCP WestwoodReference
Saverio Mascolo, Claudio Casetti, Mario Gerla, M. Y. Sanadidi and RenWang, TCP Westwood: Bandwidth Estimation for Enhanced Transport over Wireless Links (2001) ACM SIGMOBILE.
Modifies TCP senderNot dependent on negotiation with TCP senderNot dependent on support in router or receiver
Estimates available bandwidthCounts dupACKs as successful traffic
On packet loss set cwnd = available bandwidthImproves on Reno cwnd = cwnd / 2
Seminar in Computer Networks and Distributed Systems — Hadassah College — Spring 2019 TCP over Wireless Networks Dr. Martin Land 20
Average Transmission Rate in RenoSenders must measure bandwidth estimate R
Know cwndMeasure <RTT>
TCP average RTT (simplified)R'= last RTT measurement
SRTT ← (1 - α) * SRTT + α * R'
RTO ← max(1 sec, SRTT)
α = 1/8
= =packets sent cwnd
Rtime to ACK RTT
Seminar in Computer Networks and Distributed Systems — Hadassah College — Spring 2019 TCP over Wireless Networks Dr. Martin Land 21
Noise Filtering in WestwoodModel bandwidth as noisy signal
BWmeas (time) = BW (time) + noise (time)
Filter out noise with low-pass filter
( ) ( )
( ) ( ) ( )
( )
2
2
20
0
1
1
Fourier transform input signal
Filter in frequency domain
Low-pass filter
Output signal "cut off" at frequency
ift
ift
x t X f e df
y t H f X f e df
H fff
f
π
π
∞
−∞
∞
−∞
=
=
=⎛ ⎞
+ ⎜ ⎟⎝ ⎠
∫
∫
Seminar in Computer Networks and Distributed Systems — Hadassah College — Spring 2019 TCP over Wireless Networks Dr. Martin Land 22
Low Pass Filter Example
( ) ( ) ( )
( ) ( ) ( )2 2
1sin 2 sin 20
101 1 1
sin 2 sin 20101 10
1 12 2
x t t t
y t t t
π π
π π
= +
= +⎛ ⎞ ⎛ ⎞+ +⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠
Filter input
Filter output
( )( )
x t
y t
( ) 02
0
12
1
, H f ff
f
= =⎛ ⎞
+ ⎜ ⎟⎝ ⎠
( )X f
f1 10
Seminar in Computer Networks and Distributed Systems — Hadassah College — Spring 2019 TCP over Wireless Networks Dr. Martin Land 23
DSP — Low Pass Filter for Sampled Signals
1
1
, 0,1, 2,...
, 0,1, 2,...
ˆ ˆ
number of bytes ACKedtime of ACK time of previous ACK
k
k k k
k kk
k k k
k k k
t k
t t t
d db k
t t t
b b
−
−
=Δ = −
= = = =Δ − −
= α
Sample input at times
Sample stream from input
Output stream (Tustin approximation)
( ) 11
0
12
21 , 2
2
1
parameter related to Nyquist sampling theorem
k kk
kk k
k
b b
tt
t
f
−−
++ − α
τ − Δα = < τ > Δ
τ + Δ
τ =
Cutoff frequency
Seminar in Computer Networks and Distributed Systems — Hadassah College — Spring 2019 TCP over Wireless Networks Dr. Martin Land 24
Bandwidth Sampling in Westwind Bandwidth sample
1
1
2
0 2
number of bytes ACKedtime of ACK time of previous ACK
No ACK for time > insert "virtual ACK"
Insert at time
k kk
k k k
k k k
d db
t t t
d b t τ
−
−
= = =Δ − −
τ ⇒= ⇒ = +
Seminar in Computer Networks and Distributed Systems — Hadassah College — Spring 2019 TCP over Wireless Networks Dr. Martin Land 25
Westwood Packet Counting ExamplePacket transmission times
Time ACKArrived at Receiver
Packets for BWE
Counted
dupACKBWE
t1 1 1 1 0 1 / (t1 – t0)
t2 3 2 + 3 2 0 2 / (t2 – t1)
t3 4 4 1 0 1 / (t3 – t2)
t4 4 6 1 1 1 / (t4 – t3)
t5 4 7 1 2 1 / (t5 – t4)
t6 4 8 1 3 1 / (t6 – t5)
t7 9 5 + 9 2 0 2 / (t7 – t6)
1 2 3 4 5 6 7 8 5 9
t0 t1 t2 t3 t6
ACK jumps to 9 from 4 = 5 but 3 ACKs (6, 7, 8) already counted as dupACKs
Seminar in Computer Networks and Distributed Systems — Hadassah College — Spring 2019 TCP over Wireless Networks Dr. Martin Land 26
Westwood ACK Counter (for BWE)
newACK = ACK – prevACK ; Packets ACKed by new ACK
// if (newACK = 1) do nothing No error condition — report 1 ACK
if (newACK = 0)
count++ ;
newACK = 1 ;
ACK "stuck" on old value (dupACK)
Increment dupACK counter
Count 1 dupACK
if (newACK > 1) ACK advances
if (count >= newACK)
count = count – newACK + 1 ;
newACK = 1 ;
Not all arrived packets ACKed
Remove ACKed from dupACK count
Count as additional dupACK
else if (count < newACK)
newACK = newACK - count ;
count = 0;
All packets ACKed in order
Report ACKed – counted (new ACKs)
Zero dupACK counter
prevACK = ACK ;
return(newACK);
Seminar in Computer Networks and Distributed Systems — Hadassah College — Spring 2019 TCP over Wireless Networks Dr. Martin Land 27