1. TCP-LP: A Distributed Algorithm for Low Priority Data Transfer
Aleksandar Kuzmanovic
&
Edward W. Knightly
Rice Networks Group

2. Motivation Traditional view of service differentiation:
High priority: real-time service
Best-effort: everything else
What’s missing?
Low-priority (receiving only excess bandwidth)
Lower than best-effort!
Non-interactive apps, bulk download
Speeds up best-effort service
Inference of available bandwidth for resource selection
Routers could achieve via a low (strict) priority queue
Objective: realize low-priority via end-point control
Premise: routers will not help

3. Applications for Low Priority Service
LP vs. rate-limiting:
P2P file sharing
Often rate limited
Isolation vs. sharing
LP vs. fair-share:
Bulk downloads
Improve my other
applications
Data-base replication
across the Internet

4. Problem Formulation & Design Objectives
Low-priority service objectives
Utilize the “excess/available” capacity
What no other flows are using
TCP-transparency (non-intrusiveness)
Inter-LP flow fairness (fair-share of the available bandwidth)

5. Origins of the Available Bandwidth
Why is excess bandwidth available when TCP is greedy?
TCP is imperfect
Cross-traffic burstiness
Delayed ACKs due to reverse traffic frees up available bandwidth
Short-lived flows
Majority of traffic consists of short-lived flows (web browsing)
Bandwidth gaps between short lived-flows

6. Illustration of TCP Transparency
LP flow utilizes only
excess bandwidth
Does not reduce the throughput of TCP flows

7. How Is This Different from TCP?
In presence of TCP
cross-traffic:
TCP achieves fairness
LP achieves
TCP-transparency

8. Fairness Among LP Flows
Inter-LP-fairness is
essential for
simultaneous
file transfers
estimates of available
bandwidth

9. TCP-LP: A Congestion Control Protocol
Key concepts
Early congestion indication
One-way delay thresholds
Modified congestion avoidance policy
Less aggressive than TCP
Implication: Sender-side modification of TCP
incrementally deployable and easy to implement

10. Early Congestion Indication
For transparency, TCP-LP must know of congestion before TCP
Idealized objective: buffer threshold
indication
Endpoint inference: one-way delay threshold
RFC1323
Source - destination time stamping
Synchronized clocks not needed
Eliminates bias due to reverse traffic

11. TCP-LP Congestion Avoidance
Objectives: LP-flow fairness and TCP transparency
LP-flow fairness
AIMD with early congestion indication
Transparency
Early congestion indication
Inference phase goals:
Infer the cross-traffic
Improve dynamic properties
“MD” not conservative enough

12. TCP-LP Timeline Illustration
- Send 1 pkt/RTT
- Ensure available x bandwidth > 0

13. TCP-LP Timeline Illustration
- AI phase
- CWND/2 upon __early congestion xxindication
- Inference phase

14. TCP-LP Timeline Illustration
2nd CI => CWND=1
- Inference phase

15. TCP-LP Timeline Illustration

16. Low-Aggregation Regime
Hypothesis: TCP cannot attain 1.5 Mb/s throughput due to reverse cross-traffic
How much capacity remains and can TCP-LP utilize it?

17. TCP-LP is invisible to TCP traffic!
TCP-LP in Action
TCP alone Kb/s
TCP vs Kb/s
TCP-LP Kb/s
TCP-LP is invisible to TCP traffic!

18. High-Aggregation Regime with Short-Lived Flows
Bulk FTP flow using TCP-LP vs. TCP
Explore delay improvement to web traffic
Explore throughput penalty to FTP/TCP-LP flow

19. TCP Background Bulk Data Transfer
Web response times
are normalized

20. TCP-LP Background Bulk Data Transfer
Web response times improved
3-5 times
FTP throughput: TCP: 58.2%
TCP-LP: 55.1%

21. Conclusions http://www.ece.rice.edu/networks/TCP-LP
TCP-LP adds a new service to the Internet
General low priority service (compared to “best-effort”)
TCP-LP is easy to deploy and use
Sender side modification of TCP without changes to routers
TCP-LP is attractive for many applications: ftp, web updates, overlay networks, P2P
Significant benefits for best effort traffic, minimal throughput loss for bulk flows