1. Rice Networks Group http://www.ece.rice.edu/networks Aleksandar Kuzmanovic & Edward W. Knightly TCP-LP: A Distributed Algorithm for Low Priority Data Transfer

2. A. Kuzmanovic and E. W. Knightly Motivation l Traditional view of service differentiation: –High priority: real-time service –Best-effort: everything else l 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 l Routers could achieve via a low (strict) priority queue l Objective: realize low-priority via end-point control –Premise: routers will not help

3. A. Kuzmanovic and E. W. Knightly Applications for Low Priority Service l LP vs. rate-limiting: –P2P file sharing  Often rate limited  Isolation vs. sharing l LP vs. fair-share: –Bulk downloads  Improve my other applications  Data-base replication across the Internet

4. A. Kuzmanovic and E. W. Knightly Problem Formulation & Design Objectives l 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. A. Kuzmanovic and E. W. Knightly Origins of the Available Bandwidth l Why is excess bandwidth available when TCP is greedy? 1.TCP is imperfect  Cross-traffic burstiness  Delayed ACKs due to reverse traffic frees up available bandwidth 2.Short-lived flows  Majority of traffic consists of short-lived flows (web browsing)  Bandwidth gaps between short lived-flows

6. A. Kuzmanovic and E. W. Knightly Illustration of TCP Transparency l LP flow utilizes only excess bandwidth –Does not reduce the throughput of TCP flows

7. A. Kuzmanovic and E. W. Knightly How Is This Different from TCP? l In presence of TCP cross-traffic: –TCP achieves fairness –LP achieves TCP-transparency

8. A. Kuzmanovic and E. W. Knightly Fairness Among LP Flows l Inter-LP-fairness is essential for simultaneous –file transfers –estimates of available bandwidth

9. A. Kuzmanovic and E. W. Knightly TCP-LP: A Congestion Control Protocol l 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. A. Kuzmanovic and E. W. Knightly Early Congestion Indication l For transparency, TCP-LP must know of congestion before TCP l Idealized objective: buffer threshold indication l Endpoint inference: one-way delay threshold –RFC1323RFC1323  Source - destination time stamping  Synchronized clocks not needed –Eliminates bias due to reverse traffic

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

12. A. Kuzmanovic and E. W. Knightly TCP-LP Timeline Illustration - Send 1 pkt/RTT - Ensure available x bandwidth > 0

13. A. Kuzmanovic and E. W. Knightly TCP-LP Timeline Illustration - AI phase - CWND/2 upon __early congestion xxindication - Inference phase

14. A. Kuzmanovic and E. W. Knightly TCP-LP Timeline Illustration -2 nd CI => CWND=1 - Inference phase

15. A. Kuzmanovic and E. W. Knightly TCP-LP Timeline Illustration

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

17. A. Kuzmanovic and E. W. Knightly TCP-LP in Action l TCP alone 745.5 Kb/s l TCP vs. 739.5 Kb/s TCP-LP 109.5 Kb/s TCP-LP is invisible to TCP traffic!

18. A. Kuzmanovic and E. W. Knightly High-Aggregation Regime with Short-Lived Flows l Bulk FTP flow using TCP-LP vs. TCP l Explore delay improvement to web traffic l Explore throughput penalty to FTP/TCP-LP flow

19. A. Kuzmanovic and E. W. Knightly TCP Background Bulk Data Transfer l Web response times are normalized

20. A. Kuzmanovic and E. W. Knightly TCP-LP Background Bulk Data Transfer l Web response times improved 3-5 times l FTP throughput: TCP: 58.2% TCP-LP: 55.1%

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