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A Transport Layer Approach for Achieving Aggregate Bandwidths on Multi-homed Mobile Hosts

This research paper discusses a transport layer approach to achieve aggregate bandwidths on multi-homed mobile hosts. It explores the design and protocol of a transport layer protocol that allows for simultaneous use of multiple wireless access technologies and provides reliable and sequenced delivery.

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A Transport Layer Approach for Achieving Aggregate Bandwidths on Multi-homed Mobile Hosts

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  1. A Transport Layer Approach for Achieving Aggregate Bandwidths on Multi-homed Mobile Hosts Hung-Yun Hsieh and Raghupathy Sivakumar GNAN Research Group School of Electrical and Computer Engineering Georgia Institute of Technology September 26, 2002

  2. Outline • Introduction • Motivation • Design • Protocol • Evaluation • Conclusions

  3. Introduction • A multitude of wireless access technologies • How can these technologies co-exist in providing the best service to a multi-homed mobile user? • Use only the interface with the highest data rate at any given time • Objective • Allow simultaneous use of multiple wireless access technologies (striping) • Design a transport layer protocol that provides reliable and sequenced delivery like in TCP, and achieves effective bandwidth aggregation

  4. Why Not Lower Layers? • Link layer striping [Adiseshu ’96, Snoeren ’99] • Stripe according to estimates of link bandwidths • Optimal link layer striping  optimal bandwidth aggregation across multi-hop paths • Not applicable to multi-homed mobile hosts • Network layer striping [Phatak ’02] • IP performs tunneling and striping • TCP’s RTT estimates lose meaning • TCP adversely reacts to packet reordering • Lower layer approaches fail due to the multi-hop nature of paths and TCP’s reliance on FIFO delivery!

  5. pipe 1 pipe 2 Why Not Multiple Sockets? • Application layer striping • Open multiple TCP sockets • Each socket is in charge of only one path (pipe) • Perform bandwidth estimation at the sending end and packet re-sequencing at the receiving end • Optimal aggregate bandwidth = sum of maximum achievable bandwidths on the individual pipes • Problems • Data rate differential • Data rate fluctuations • Blackouts

  6. Sequence Number (Unaware Application) 6 Throughput vs. Bandwidth Ratio Pipe 1 (2500kbps) Pipe 2 (500kbps) 6 5 Ideal Smart Application 5 Unaware Application 4 Sequence Number (x1000) 4 3 Throughput (Mbps) 3 2 2 1 1 35 36 37 38 39 40 Time (sec) 0 1 2 3 4 5 6 7 8 9 10 Bandwidth Ratio (to 500kbps) Data Rate Differential • Head of line blocking will result in the slower pipes stalling the faster pipes • Example • Two pipes with mismatched bandwidths • Unaware application • Write until blocked • Smart application • Coarse-grained bandwidth estimation • The aggregate bandwidth is limited by the slower pipe in the unaware application

  7. Data Rate Fluctuations 3.5 3.3 Bandwidth Fluctuations (5Mbps Pipe) 3.1 6 2.9 2.7 5 Throughput (Mbps) 2.5 4 2.3 Ideal 2.1 Bandwidth (Mbps) Smart Application 3 Unaware Application 1.9 1.7 2 1.5 10 9 8 7 6 5 4 3 2 1 1 Fluctuation Period (sec) 0 0 20 40 60 80 100 Time (sec) Data Rate Fluctuations • Data rate fluctuations cause disproportional striping • Example • Twopipes with random data rate fluctuations every t seconds • Smart application • Performs bandwidth estimation every T seconds • Smart application’s performance is limited by disproportional striping

  8. Smart Application during Blackouts 25 Pipe 1 (does not experience blackout) Pipe 2 (experiences blackout) 20 15 Sequence Number (x1000) stalled due to pipe 1 blackout 10 5 blackout 0 0 10 20 30 40 50 60 70 80 90 100 Time (sec) Blackouts • Undelivered packet in the blackout pipe will cause head of line blocking • Example • One of the two pipes experiences blackout • The other pipe does not experience any blackout • Blackouts on one pipe will stall the entire aggregate connection

  9. Agenda • Introduction • Motivation • pTCP Design • pTCP Protocol • Evaluation • Conclusions

  10. Design Elements • Decoupling of functionalities • Per-pipe (TCP-v) behavior vs. aggregate connection (pTCP) behavior • TCP-v: virtual packets; pTCP: data packets • TCP-v: congestion control; pTCP: reliability • TCP-v: how much to send; pTCP: what to send • Allow plug-in of different congestion control schemes for different pipes

  11. Design Elements • Congestion window based striping • A packet is given to a TCP-v only when its congestion window has space • Reuse TCP’s congestion control for bandwidth estimation • Dynamic reassignment • Reassign packets that fall out of TCP-v’s window • Avoid packets being held up by one pipe • Redundant striping • Redundantly stripe the first packet after timeouts • Allow the concerned pipe to probe while not potentially stalling the rest

  12. data packet virtual packet Application control write read pTCP open/close TCP-v established/closed receive send bindings active virtual send buffer send buffer resume pipes virtual recv buffer recv buffer shrunk ip-output ptcp-recv IP pTCP Architecture

  13. data packet virtual packet Application control write read pTCP open/close TCP-v established/closed receive send bindings active virtual send buffer send buffer resume pipes virtual recv buffer recv buffer shrunk ip-output ptcp-recv IP pTCP Architecture

  14. pTCP Protocol • Congestion control • Performed by each TCP-v on a per-pipe basis • Any congestion control mechanism can be used • Flow control • Managed by pTCP through window advertisement • No flow control by TCP-v • Reliability • pTCP delegates reliability to each TCP-v unless the packet has to be reassigned • Connection management • pTCP manages the aggregation connection while TCP-v manages the individual pipes

  15. client server open socket nIF interfaces S Y N , n T x = n I F ( T C P - v ) 1 ) v - P C T ( F 1 I n = x R n , K C A + N Y S send data A C K ( T C P - v ) first pipe 1 S Y N ( T C P - v ) established 2 S Y N (connection ( T C P - v ) established) I n F ) v - P C T ( K 2 C A + N Y S ) v - P C T ( F I K n C A + N Y S A C K ( T C P - v ) 2 A C K ( T C P e - v ) all pipes m I n F i t established pTCP Connection Handshake

  16. Agenda • Introduction • Motivation • Design • Protocol • Evaluation • Conclusions

  17. Simulation Environment • Topology • Pipe 1: BW = 500kbps ~ 5Mbps, RTT = 100ms • Pipe 2: BW = 2Mbps, RTT = 400ms • Pipe 3: BW = 500kbps, RTT = 400ms • Approaches • Ideal • pTCP • Smart application • Unaware application

  18. Throughput vs. Bandwidth Ratio 6 Ideal pTCP 5 Smart Application Unaware Application 4 Throughput (Mbps) 3 2 1 0 1 2 3 4 5 6 7 8 9 10 Bandwidth Ratio (to 500kbps) Data Rate Differential

  19. Throughput vs. Number of Pipes 30 Ideal pTCP 25 Smart Application Unaware Application 20 Throughput (Mbps) 15 10 5 0 2 3 4 5 6 7 8 9 10 Number of Pipes Number of Pipes

  20. Throughput vs. Number of Pipes (with Fluctuations) 16 Ideal 14 pTCP Smart Application 12 Unaware Application 10 Throughput (Mbps) 8 6 4 2 0 2 3 4 5 6 7 8 9 10 Number of Pipes Data Rate Fluctuations

  21. pTCP during Blackouts 60 Pipe 1 (does not experience blackout) Pipe 2 (experiences blackout) 50 40 Sequence Number (x1000) continues progression 30 20 blackout 10 0 0 10 20 30 40 50 60 70 80 90 100 Time (sec) Blackouts (pTCP)

  22. Multiple Congestion Control Schemes 7 6 5 4 Throughput (Mbps) 3 Ideal: TCP (Pipe 1) + ELN (Pipe 2) 2 pTCP: TCP (Pipe 1) + ELN (Pipe 2) Smart Application: TCP (Pipe 1) + ELN (Pipe 2) 1 Smart Application: TCP (Pipe 1) + TCP (Pipe 2) 0 1.00E-05 1.00E-04 1.00E-03 1.00E-02 Packet Drop Probability (Pipe 2) Multiple Congestion Control Schemes • Use TCP-ELN for lossy link (Pipe 2)

  23. Related Work • Link layer • Scalable striping [Adiseshu ’96] • Adaptive inverse multiplexing [Snoeren ’99] • Network layer • Multi-IP links streaming [Phatak ’02] • Transport layer • SCTP [Stewart ’00] • RMTP [Magalhaes ’01] • Application layer • PSockets [Sivakumar ’00]

  24. Conclusions • Bandwidth aggregation can be used to provide better service to multi-homed mobile hosts • A transport layer approach is more effective than other layers in the target environment • pTCP achieves effective bandwidth aggregation despite the data rate mismatch, fluctuations, and blackouts of different pipes • For more information: http://www.ece.gatech.edu/research/GNAN

  25. Application Complexity • Sophisticated implementation • Need to re-implement most TCP functionalities • Very complicated bandwidth estimation scheme at least the time-granularity of TCP • It is not desirable to overload application • Not application specific functionalities • Data rate differential can be solved using a larger re-sequencing buffer • Need larger buffer than pTCP uses (transport layer buffering) • Buffer over-provisioning

  26. Throughput vs. Socket Buffer Ratio (5Mbps vs. 500kbps) 6 5 4 Throughput (Mbps) 3 2 1 Ideal Unaware Application 0 1 2 3 4 5 6 7 8 9 10 Socket Buffer Ratio (5Mbps to 500kbps) Buffer Over-provisioning

  27. CLOSED OPEN [ TCP-v open( ), p = nIF ] 1 CLOSE ESTABLISH WAIT [ TCP-v close( ) ] 1 TCP-v established( ) 1 [ TCP-v open( ), n = 1 ] 2...nIF TCP-v established( ) ESTABLISHED ( n ) n +1 [ n = n + 1 ] CLOSE [ TCP-v close( ), m = n ] 1... n TCP-v closed( ) CLOSE WAIT ( m ) m [ m = m - 1 ] m == 0 pTCP State Diagram

  28. Multi-homed Mobile Hosts

  29. Throughput Aggregation vs. Bandwidth Ratio (Two Links) Bandwidth Aggregation vs. Number of Links (Fluctuations) 5500 11 Ideal Ideal pTCP 5000 10 pTCP Multiple Sockets Multiple Sockets 4500 9 4000 8 3500 7 Application Throughput (kbps) Application Throughput (Mbps) 3000 6 2500 5 2000 4 1500 3 1000 2 500 1 1 2 3 4 5 6 7 8 9 10 2 (5M+2M) 3 (+500k) 4 (+1M) 5 (+10M) Bandwidth Ratio (to 500kbps) Number of Links Multi-homed Mobile Hosts

  30. RMTP • Coupling of congestion control and reliability • Does not have fine-grained handling of fluctuations & blackout • Does not have delayed binding (fine-grained packet assignment) • Does not have dynamic reassignment • Does not have redundant stripingA

  31. Smart Application during Blackouts 25 Pipe 1 (does not experience blackout) Pipe 2 (experiences blackout) 20 15 stalled due to pipe 1 blackout Sequence Number (x1000) 10 5 blackout 0 0 10 20 30 40 50 60 70 80 90 100 Time (sec) Blackouts (Smart Application)

  32. Issues • Bandwidth-delay product • TCP friendliness • Backward compatibility • Complexity • Handoffs

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