1. TCP-LP Distributed Algorithm for Low-Priority Data Transfer
Group 5
ECE 4605
Neha Jain
Shashwat Yadav

2. Motivation Service Prioritization Utilize excess network bandwidth
Best Effort Service
All of Internet
Low Priority
Bulk transfer (FTP downloads, P2P)
Utilize excess network bandwidth
End point protocol
Absence of network support
Sender side modification

3. Objectives Utilize unused bandwidth TCP transparency
WHY ?
Caused by TCP congestion control
Cross traffic
ACK delays due to reverse traffic
TCP transparency
Fairness amongst simultaneous
TCP-LP flows

4. TCP-LP at Work

5. Implementation
Low priority service transparent (running simultaneously) to TCP
Two class hierarchical scheduling model
TCP packets have strict priority over TCP-LP packets.
Fairness amongst microflows within each class

6. Congestion Notification
Early Congestion Indication (ECI)
Congestion indication prior to TCP
One way packet delays
Smoothed one way delay
Early congestion indication condition
Where is delay smoothing parameter
is delay threshold
Advantages
Sender receiver clocks need not be synchronized.
Unperturbed by reverse traffic.

7. Congestion Avoidance Linear Increase with TCP
Initial Early Congestion Indicator
W = W/2
Inference State No
Another
ECI ?
Inference Time
Out ?
Yes No
Yes
W = 1

8. Mechanisms Congestion avoidance
Modifies LIMD by addition of an inference
phase
Observes network responses

9. Parameter Settings Delay smoothing parameter( )
Small values degrade ability to detect
congestion
Higher values may cause false ECI’s
balances the two requirements
Inference Time Out Timer (itt)
Smaller values correspond to throughput
aggressive nature
Higher values increase congestion
responsiveness
strikes the balance

10. Delay Threshold Smaller values increase TCP-LP transparency
Tradeoff between TCP
transparency and throughput
Higher values increase the throughput

11. Bulk Data Transfer Simultaneous FTP downloads No reverse traffic
No excess capacity available
TCP-LP slightly perturbs TCP flow
>>WHY?

12. Reverse Background Traffic
10 FTP/TCP flows in reverse direction
Forward direction ACK delays and losses decrease TCP throughput.
Excess capacity utilized by TCP-LP flows.
Nearly perfect TCP transparency.

13. Experimental Results
TCP-LP has additive increase when TCP congestion window collapses
TCP-LP is already in the inference phase when TCP reaches its maximum capacity
TCP-LP encounters ECI. Backs off
>> WHY do TCP-LP peaks occur before TCP?

14. HTTP Background Traffic
Impact on HTTP Response Times
Case I
TCP used for HTTP Vs
TCP used for HTTP; simultaneous bulk transfer using TCP-LP
Slight Increase in mean retrieval time.
Case II
TCP used for HTTP and bulk transfer Vs
TCP used for HTTP; simultaneous bulk transfer using TCP-LP
Web retrieval times decreased by 80%.
Bulk transfer increased by less than 10%
Results
Results

15. RTT Heterogeneity WHY? Case 1: Single bottleneck HTTP uses TCP
When bulk transfer uses TCP-LP the web retrieval rate increases by 80%
WHY?
Case 2:
Multiple bottlenecks
HTTP uses TCP
When bulk transfer uses TCP-LP the web retrieval rate increases by 40%

16. Critique AIMD unsuitable for high speed connections
High speed connections result in
Large window sizes
Faster ACK’s so lesser span to transmit TCP-LP
TCP-LP needs slow start
Quickly make use of the excess bandwidth.

17. Conclusion An end to end protocol – easy to deploy.
Sender side modification of TCP.
Doesn’t require any functionality from network routers, nor any other protocol changes.
Suitable for applications such as ftp, p2p
Utilizing excess network bandwidth without significantly perturbing TCP.