Multi-Mode TCP for Wireless Overlay Networks

1. Multi-Mode TCP for Wireless Overlay Networks • Motivation • TCP has been tuned to traditional networks comprising wired links and stationary hosts • Wireless overlay networks • Heterogeneous overlays • User mobility: vertical handoffs • Sudden, frequent, and significant changes in bandwidth and delay • Study TCP behavior over wireless overlay networks and improve its performance • New Ideas • TCP manages multiple modes • A mode is a subset of entire TCP state • corresponds to an overlay network • dynamic • TCP is notified of the handoff events • TCP switches between modes when the mobile host vertical handoffs between different overlay networks • Different actions for upward and downward handoffs • Key Design/Approach • A mode includes cwnd, ssthresh, rtt, srtt, rttvar, rtxcur, etc. • Generally, save the current mode and load a new mode for a vertical handoff • Schedule a handoff timer for a downward handoff to avoid fast retransmission caused by out-of-order delivery • Reset mode for an upward handoff to an unvisited overlay network • An extension to New-reno TCP • Results/Lesson Learned • Implemented in ns2 • Simulation results in ns2 • Eliminate unnecessary retransmissions caused by timeouts or duplicate acks • TCP goodput improved (> 7%) • Lesson learned: • Implementation of TCP stack • Limitation of the simulation • Need of more realistic experiments Project team: Fengfeng Tu, Hongxia Tian URL: http://www.cs.utexas.edu/users/ftu//project.html