tbyh

1. Simulation based
analysis of FAST TCP
using OMNET++
Umair ul Hassan

2. Outline
 TCP Congestion Control
 Variations
 FAST TCP
 OMNET++

3. TCP Congestion Control
 Retransmission Timer Management
 Jacobson’s Algorithm
 Exponential RTO Backoff
 Karn’s Algorthim
 Window Management

4. Window Management
 Slow Start
 Increase cwnd by 1 for each ACK
 Exponential Increase
 Dynamic Window Sizing on Congestion
 Reset cwnd on packet loss and restart slow start
 Linear increase after threshold

5. Window Management
 Fast Retransmit
 On receiving out-of-order segment send an ACK
for last in-order segment.
 Sender should wait atleast 3 duplicate ACKs
before retransmission
 FAST Recovery
 Set cwnd = ssthreshold + 3
 Increase by 1 on each duplicate ACK
 Set cwnd = ssthreshold on ACK

6. TCP Tahoe and Reno
Measure Tahoe Reno
Slow Start
 
Dynamic Window Sizing on
Congestion
 
Fast Retransmit
 
Fast Recovery


7. Problems with TCP Reno
 Packet Level
 Slow additive increase and Fast Multiplicative
decrease
 Oscillations due to binary congestion singal
 Flow Level
 Very small loss probability required
 Instability because of oscillations

8. TCP Vegas
 Delay Based Congestion Control
 New Retransmission Mechanism
 On duplicate ACK retransmit if RTT > RTO, don’t
wait till 3 DACKs
 Congestion Aviodance
 Diff = Expected – Actual
 Modified Slow Start
 Alow exponential growth every other RTT

9. Congestion Window Update
 TCP Vegas
 cwnd + 1 if Diff<α
 Cwnd = cwnd – 1 if Diff>β
 cwnd otherwise
 TCP FAST
 cwnd + γ(α – Diff) if Diff<α
 Cwnd = cwnd – γ(Diff - α) if Diff>α
 cwnd otherwise

10. Implementation
___________________________________________
For earh source i :
 Per ACK 1. Initialization : count i = wi and fastOni = 1
2. On arrival of each ACK of packet l
(a) RTTi (l ) = CurrentTime − TimeStampi (l )
 3 1
(b) ρ = min  , 
 w 8
 i 
(c) ARTTi (l ) = (1 − ρ ) ARTTi (l ) + ( ρ ) RTTi (l )
(d) If fastOni = 1 :
 wi (l ) 
i. Calculate ∆wi = γ  d i + α i − wi 
 ARTTi (l ) 
ii. If ∆wi > β : wi = wi + 1
iii. If ∆wi < − β : wi = wi − 1
(e) count i = counti − 1
(f) If counti ≤ 0 : (One RTT is finished)
i. fastOni = 1 − fastOni
ii. count i = wi
iii. Call Fine Tuning Algorithm
___________________________________________

11. Implementation
 Per RTT
___________________________________________
For each source i, on completetion of each RTT
 wi (l ) 
1. Calculate ∆wi = γ  d i + α i − wi 
 ARTTi (l ) 
2. If ∆wi ≥ 1 : wi = wi + 1
3. If ∆wi ≤ −1 : wi = wi − 1
___________________________________________

12. Problems
 Fairness when used with other variations
 Priority Queues

13. OMNET++
 DiscreteEvent Simulation System
 NED language
 Atomic Modules
 Compound Modules
 Message based communication

14. INET Framework
 Internet
Protocols (IP, TCP, UDP, PPP, ARP)
 TCPBaseAlg
 TCPReno and TCPTahoe
 TCPFast

15. Q&A