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Analytic Modeling of Load Balancing
SCTP
Zhengliang Yi
Tarek Saadawi, Myung Lee
CUNY Graduate Center
Overview
1.
SCTP, Stream Control Transmission Protocol
(RFC2960).
• Multiple interfaces (possible multiple distinct
multiple paths)
• Current specification: one primary path, others
secondary paths to improve reachibility and
reliability.
• May be used as multiple path load balancing to
dramatically improve throughput as well (current
work).
Analytic Model of Standard SCTPassumptions
• Assumptions about the endpoints
– Assuming sender is using SCTP congestion control mechanism
defined by RFC2960.
– Senders have unlimited amount of data to send so that all the flows
can reach steady states.
– Sender sends full-sized (fixed-sized) segments as fast as its
congestion window allows.
– Data are sent out via primary path and secondary paths only used
for retransmission purpose.
Analytic Model of Standard SCTPassumptions
• Assumptions about network
– Modeling SCTP behavior in terms of “rounds”, starts when the
sender begins the transmission of a window of packets and ends
when the sender receives first ACK for the packets in that window.
“Round”=RTT
– Assuming the time to send all packets in a window is smaller than
the RTT.
– Duration of a round (RTT) is independent of the window size.
– Losses of one round are independent of the losses of any other
rounds. Losses in one round is dependent: all packets in same round
following a loss will be lost. Ideal of drop-tail queue, widely used on
Internet
SCTP throughput model -Integrated Period
W
Wi2
Wi-1
Wi3
Wi1
QDP1
ZiTO
ZiSS
QDP2
QDP3
ZiCA
T
Si
Figure An integrated period.
Method: Bandwidth 
Ave No of pkts transmitted in integrated period
Ave No of rounds  RTT
Total number of packets transmitted Yi = YiTO+ YiSS + YiCA
Length of integrated period i
Zi = ZiTO+ ZiSS + ZiCA
Throughput can be expressed as
B = E[Y] / E[S]
Find average Y and Z for each period
Time-Out period ZTO
•
•
–
Current size and position at cwnd
A1
TO
B2 cwnd=6
TW
Zi
TO
TR
B2 cwnd=7
E[ZTO]= E[To] + E[TR]
– E[To] is first time-out value
– E[TR] is the average time for
retransmitting all lost packets


 E [ RP ](  1)

E[TR ]  log r 
 1  E[ RTT r ]


W1


– W1 = 1 and γ = 1+1/b
A2
B1 cwnd=4
Retransmit lost packets via
alternative path from slow start
E[YiTO] = E[RP]
– E[RP] is the average number of
lost packets when time-out
happens.
– Important in modeling SCTP
because of its unique
retransmission
•
B1
B1 cwnd=1
B2 cwnd=7
Slow start period
ZSS
B1
•
•
E[ Z
SS
 E[W ]
]  log 
 2W 1
E[Y
SS

 1  E[ RRT ]

b 1
]
E[W ]  bW 1
2
E[W], average congestion
window size in CA period.
A2
B1 cwnd=4
TO
Starts with one MSS
Increases cwnd by one
MSS each ack.
• Ends till encountering
packet loss or ssthresh.
A1
B2 cwnd=6
TW
Zi
TO
TR
B2 cwnd=7
B1 cwnd=1
B2 cwnd=7
Congestion avoidance period
W
Win
Wi1
Wi2
QDP1i
QDP2i
ZiCA
QDPni
T
• E[ZCA] = E[n]E[ZQDP]
• E[YCA] = E[n]E[YQDP]
– E[n]: average number of QDPs in each CA
– 1/ E[n]= Q: Given pkt loss(es), the probability that
the packet loss(es) result in TO, not QDP (fast rtx
process is triggered or not) .
Probability packet loss(es) result in TO, not QDP
• Time out can only happen when packets get lost but fast
retransmission can not be triggered, two situation:
– Sender successfully sends out less than 4 packets in last round and
following packets all lost
– Sender successfully sends out 4 packets or more in last round but can
only successfully sends out less than 4 packets in the round after.
• Considering these two situations, we can find that:
 (1  (1  p) 4 )(1  (1  p) 4 (1  (1  p) w3 ) 

Q( w)  min1,
w
1  (1  p)


E[RP]: Ave No. of packets lost when time out happens.
(3  w) p 3  (4w  11) p 2  (14  6w) p  (4w  6)
RP ( w) 
 p3  4 p 2  6 p  4
Average congestion window size
E[W]
W
E[W]/2
E[W]
E[W]/2
b*RTT
T
b*RTT*E[W]/2
QDP
QDP
4
E
[
W
]


Use above graph, we can easily find:
3b
 
8
4

3bp
3b
2
SCTP throughput model
-throughput expression
• Now we have everything we need
Qj  (1.5W max  4)  W max 
B j ( p) 
1 p
p



E[ RP j ] 
W max
b
1 
Qj  To  E[ RTT jr ] 0.5 

E
[
RTT
j ] Qj  log 1.5(

1
)

W
max 



r 

4
E
[
W
]
8
pW
max 

j




Thag g 

N
j 1
Bj(pj)
Simulation Environment
• Using the NS2 network simulator
• Drop-tail as the queuing scheme
• Primary path for data, secondary path for
retransmission
SCTP multihomed node
SCTP multihomed node
RTT of path1 to 20 ms, RTT of path2 to 50 ms and path 3 to 35ms.
After 10 seconds, path3 is intentionally shut down
Our SCTP model compare with NS2
simulation
Two lines are very close which indicates that our model can accurately
predict the throughput of SCTP for a wide range of PER and RTT
DelAck with bytes oriented vs. acks oriented
• Bytes oriented show significant throughput gain
• Throughput gain decreases as PER increase. Reason: PER increase,
CA decrease and TO increase. Two cwnd updating mechanisms act the
same way during slow start, different during congestion avoidance.
(RFC2960: max cwnd increase = 1 MSS)
Conclusions
• Our analytic model precisely predicts steady
state throughput of SCTP.
• SCTP demonstrates better throughput
compared with TCP, especially when packet
error rate is high.
• SCTP may be a better choice transport layer
protocol for error prone networks such as
wireless networks.
Thank you!