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Improving TCP Performance over Mobile Networks

Improving TCP Performance over Mobile Networks. Zahra Imanimehr Rahele Salari. Outline. Problems with TCP Class of solutions Review some of the proposals TCP Reno 3-dupacks (3DA) Freeze TCP ATCP References. FH. BS1. BS2. MH. MH. Mobile Networks Topology. FH – Fixed Host

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Improving TCP Performance over Mobile Networks

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  1. Improving TCP Performance over Mobile Networks Zahra Imanimehr Rahele Salari

  2. Outline • Problems with TCP • Class of solutions • Review some of the proposals • TCP Reno • 3-dupacks (3DA) • Freeze TCP • ATCP • References

  3. FH BS1 BS2 MH MH Mobile Networks Topology FH – Fixed Host BS – Base Station MH – Mobile Host

  4. Wireless Networks • Communication characterized by • sporadic high bit-error rates (10-4 to 10-6) • disconnections • intermittent connectivity due to handoffs • Limited and variable bandwidth

  5. TCP Performance with BER

  6. The congestion control in regular TCP • Assume congestion to be the primary cause for packet losses and unusual delays • Invoke congestion control and avoidance algorithms, resulting in significant degraded end-to-end performance and very high interactive delays.

  7. The congestion control Algorithm • Slow start • Congestion avoidance • Fast Retransmit

  8. Classification of Schemes • End-to-End protocols • loss recovery handled by sender • Link-layer protocols • Split-connection protocols

  9. Link-Layer Protocols • Hides the characteristics of the wireless link from the transport layer and tries to solve the problem at the link layer • Uses technique like forward error correction (FEC) • Snoop, AIRMAIL(Asymmetric Reliable Mobile Access In Link-layer)

  10. Link-layer Protocols (cont.) • Advantages: • The wireless link is made more reliable • Doesn’t change the semantics of TCP • Fits naturally into the layered structure of network protocols • Disadvantage: • If the wireless link is very lossy, sender times-out waiting for ACK, and congestion control algorithm starts

  11. Split Connection Protocols • Split the TCP connection into two separate connections. • 1st connection: a wired connection between fixed host and base station • 2nd connection: a wireless connection between base station and mobile host

  12. Split Connection • Advantages: • Isolate mobility and wireless related problems from the existing network protocols • Better throughput can be achieved by fine tuning the wireless protocol link. • Disadvantages: • Violates the semantics of TCP. • Extra copying at the Base station.

  13. Proposed Protocols Link Layer End to End Split Connection AIRMAIL Snoop RLP MTC I-TCP M-TCP WAP EBSN FR New Reno SACK Reno Freeze 3-dupacks ATCP

  14. TCP Reno • TCP Reno is like regular TCP except it includes fast recovery • Fast recovery: • Reduces the value of the congestion window (cwnd) by half • Increments cwnd by one for each duplicate acknowledgement received • When a “new” ACK is received, the sender exits fast recovery, sets cwnd to ssthresh and enters the congestion avoidance phase

  15. 3-dupacks • After disconnection and upon subsequent reconnection, the MH sends three duplicate acknowledgements to the fixed host • These dupacks cause the TCP sender at the FH immediately enter the fast recovery phase

  16. Freeze TCP • Upon receipt of an indication of impending disconnection, Freeze TCP at the MH sends a zero window advertisement to the FH • Upon reconnection, it uses 3DA to restart transmission

  17. ATCP • Unlike earlier work, ATCP improve the performance when the TCP sender is FH or MH • ATCP assumes that the network layer sends connection event signal and disconnection event signal to TCP

  18. ATCP (cont.) • MH to FH data transfer: • Upon disconnection event: • If sending window is open, ATCP does not wait for packet sent before disconnection and cancel the retransmission timer (RTX). • If sending window is closed and it was waiting for ACKs, ATCP does not cancel RTX but waits for the occurrence of an RTO event.

  19. ATCP (cont.) • Upon connection event • If the sending window is open, ATCP sends data and sets a new RTX • If the sending window is closed and RTO has occurred, ATCP retransmits • If RTO has not occurred, ATCP waits for an RTO • Upon RTO event • If a disconnection has occurred, ATCP sets ssthresh to the value of cwnd at the time of disconnection and sets cwnd to one • If MH is connected, ATCP retransmits lost packet

  20. ATCP (cont.) • FH to MH data transfer: ATCP delays the ACK for the last two bytes by d milliseconds. • Upon disconnection event: • the network connectivity status is updated • Upon disconnection event: • ATCP ACKs the first of these bytes with zero window advertisement and ACKs the second byte with the full window advertisement

  21. Comparison • MH to FH transfer: • A percentage improvement of up to 40% is observed for short RTT connections over TCP Reno • An improvement of up to 150% is observed for long RTT connections over TCP Reno

  22. Comparison (cont.) • FH to MH transfer: • ATCP shows uniform improvement in throughput over TCP Reno and 3DA • In WLAN environment, the performance of ATCP is very close to that of Freeze TCP • In WWAN environment, the performance of ATCP is very close to Freeze TCP for small disconnection intervals but for longer disconnection intervals Freeze TCP work better than ATCP

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