1. TCP Throughput Modeling

2. Modeling TCP Throughput
Hard to predict TCP throughput due to its congestion/flow control.
Congestion control: adjusting its sending rate responding to loss event.
Flow control: preventing overflow of receiver’s buffer.
To define a fair share throughput of a non TCP flow.
Developing a analytical model of TCP throughput as function of loss rate and RTT.

3. What Impacts on TCP Throughput
Sender/Receiver’s buffer size
Loss rate and distribution
Round-trip time
Amount of data to be transferred

4. General TCP Model cwnd Time Ai-1 Ai Ai+1 Ai+2 TTO Ni-1 Ni Ni+1 Ni+2
Wi-1 Wi
Wi+1
Wi+2

5. Assumption No ACK loss Independent packet loss Steady state
No delayed ACK

6. Modeling Fast Retransmission
RTT*C/2
time
cwnd C
C/2
Drop rate: p
No. of pkts sent between two consecutive loss = 1/p
1/p = 3/8 * C2
C = sqrt(8/3p)

7. An Example: Timeout
From [FF96].

8. Modeling Timeout Probability
Fact: Timeout occurs when packets are lost, and less than three duplicated ACKs are received.
Assumption: Timeout occurs when more then three packets are lost in a congestion window. [Fal96]

9. Modeling Timeout Period
TCP Implementation: RTO = A + 4D
Where A is the estimated RTT, and D is the deviation of RTT.
Model: RTO = 3 (or 4) RTT

10. Model Validation

11. Detecting “Bad” Flows The “TCP-friendly” test: The “responsive” test:
does the flow bandwidth exceed the rate of an aggressive TCP in comparable circumstances?
The “responsive” test:
does the flow reduce its arrival rate in response to an increase in the packet drop rate?

12. The “TCP-Friendly” Test
A flow is not “TCP-friendly” if its rate exceeds a multiple of:

13. The “Unresponsive” Test
TCP throughput equation suggests a relationship between packet drop rate and flow arrival rate: