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Improving TCP Performance over Mobile Networks

Improving TCP Performance over Mobile Networks. ACM Computing Surveys 2002. HALA ELAARAG Stetson University Speaker : Aron. Overview. Regular TCP The Problems ? Why regular TCP is not suitable? The solutions to improve the performance Link layer End to end Split connection

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Improving TCP Performance over Mobile Networks

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  1. Improving TCP Performance over Mobile Networks ACM Computing Surveys 2002 HALA ELAARAG Stetson University Speaker : Aron

  2. Overview • Regular TCP • The Problems ? • Why regular TCP is not suitable? • The solutions to improve the performance • Link layer • End to end • Split connection • Conclusions

  3. Introduction • Mobile users would like to use the same applications over the wireless link and with the same quality of service (QoS) they are getting over a wired link. • Objective: to improve the performance of TCP over mobile wireless networks.

  4. The behavior of regular TCP • Congestion control • Slow-start • Congestion avoidance • Fast Retransmit

  5. Problems with wireless and mobile networks • High bit error rates • Disconnections • Limited and variable bandwidth • Cell size • Power scarcity • Dynamic network topology

  6. Why regular TCP is not suitable? • TCP’s main problem is the delay caused by packet losses due to congestion. • Wired links have low bit error rates (BER), as opposed to wireless links that suffer from high bit error rates. • If regular TCP is used on a mobile network, it can severely degrade performance.

  7. Problems of mobile TCP implement • Non-congestion delay • Serial timeouts • Packet size variation

  8. The solutions to improve the performance • Link layer protocols • RLP • AIRMAIL • Snoop • End-to-end protocols • Reno • New-Reno • SACK • FR • EBSN • Split-connection • MTCP • I-TCP • M-TCP • WAP

  9. Link layer protocols • Objective – • Increase the quality of the lossy wireless link • Solve the problem at the link layer • Transport layer protocol is too slow to recover from losses • Congestion control mechanisms of transport layer are unnecessarily triggered, throughput is decreased

  10. Link layer protocols – RLP • Radio Link Protocol (RLP) • Automatic Repeat reQuest (ARQ) for radio channels. • Retransmit a packet when transmitter make sure it was not received. • May Solve High bit error rates at link layer

  11. Link layer protocols –AIRMAIL • Approaches to improve link layer protocol performance • Automatic Repeat reQuest (ARQ) • Forward Error Correction (FEC) • Mobility and handoff processing by window management and state transfer

  12. Link layer protocols – Snoop • Snoop protocol (TCP-aware link-layer schemes) • Introduce a module--snoop agent at the base station • monitors every packet that passes through the TCP connection in both directions • Maintains a cache of TCP packets sent from the sender that haven’t yet been acknowledged by the receiver. • If detect packet loss (use duplicate ACKs or local timeout), retransmit the packet if it is in the cache and suppress the duplicate ACKs

  13. End-to-end protocols –Reno、 New-Reno、SACK TCP • Change Fast Retransmit to include Fast Recovery • New-Reno、SACK TCP improve the performance when multiple packets lost in the same window • Improvement of regular TCP in wireless is expected to be limited

  14. End-to-end protocols – FR • Fast retransmission scheme • Providing smooth hand-offs on networks that lose packet during handoff. • When mobile IP software signal hand-off complete, mobile host signals fixed host to invoke retransmission scheme. • Focus on hand-off

  15. EBSN End-to-end protocols – EBSN • Explicit Bad State Notification • Base Station sends EBSN message to sender if packets cannot be transmitted successfully • Sender changes Timeout based on current RTT • Timeout is reset to original on receipt of new ack. • Eliminates unnecessary timeouts

  16. Split Connection Schemes • Divide TCP connection into 2 connections • Isolate wired network from wireless network TCP II TCP I Wired Link Wireless Link

  17. Split-connection – MTCP • Protect the wired connection from the impact of the erratic behavior of wireless connection • Use session layer protocol at BS and MH • Selective Repeat Protocol (SRP) – recover quickly packet loss TCP II TCP I Wired Link Wireless Link

  18. Split-connection – I-TCP • I-TCP (Indirect TCP) • The idea is the same with MTCP

  19. Split-connection – M-TCP • Three-level hierarchy architecture High-Speed Network Supervisor Host SH SH Cell Mobile Support Station(MSS) Mobile Host(MH)

  20. M-TCP TCP SH Fixed Host (Sender) Mobile Host (Receiver) SH-TCP M-TCP M-TCPcont. • End-to-end TCP connection • TCP connection is split at the SH • The SH does not send an ack to FH unless SH has received an ack from MH • Maintains end-to-end semantics

  21. M-TCPcont. • TCP Persist Mode • When a new ack is received with receiver’s advertised window = 0, the sender enters persist mode • Sender does not send any data in persist mode • When a positive window advertisement is received, sender exits persist mode • On exiting persist mode, RTO and congestion window are same as before the persist mode

  22. M-TCPcont. • Advantages • Maintains the TCP end-to-end semantics • In case disconnection, avoids useless retransmission and slow start • Need not buffer at SH • Efficient handoff • Adapt to dynamically changing bandwidth over starved link • Disadvantages • SH does not act as proxy • Packet loss on wireless link is propagated to the sender • Requires modifications to MH protocol software and new network elements like the bandwidth management module

  23. Split-connection – WAP

  24. Split-connection – WAP cont.

  25. RLP Airmail Snoop FR EBSN MTCP I-TCP M-TCP High BER √ √ √ √ √ √ Bursty error √ √ √ √ Handoff √ √ √ √ √ Long Disconnections √ √ Frequent Disconnections √ √ Bandwidth √ √ √ Cell size √ Power scarcity √ √ √ Serial timeouts √ √ √ √ Packet size variation √ √ End-to-end TCP semantics √ √ √ √ √ √ Compatability √ √ √ √ √ √ Comparison of categories

  26. Conclusion • Avoid erroneously triggering congestion control mechanisms on the fixed host. • Avoid the serial timeout problem on the fixed host. • Be reliable, by solving the problems arising from the lossy wireless links and their bursty high BER.

  27. Conclusions cont. • Can efficiently deal with handoff. • Can handle frequent and long disconnections of the mobile host. • Take into consideration the limited bandwidth and power scarcity of mobile hosts.

  28. Conclusions cont. • Use a dynamic packet size depending on the dynamic bandwidth available for mobile hosts. • Preferably provide compatibility; that is, do not require any software on the fixed hosts.

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