A Progressively Reliable Transport Protocol for Interactive Wireless Multimedia

We propose a progressively reliable transport protocol for delivery of delay-sensitive multimedia over Internet connections with wireless access links. The protocol, termed "Leaky" ARQ, initially permits corrupt packets to be leaked to the receiving application and then uses retransmissions to progressively refine the quality of subsequent packet versions. A Web server would employ Leaky ARQ to quickly deliver a possibly corrupt first version of an image over a noisy bandlimited wireless link for immediate display by a Web browser. Later, Leaky ARQ's retransmissions would enable the browser to eventually display a cleaner image. Forwarding and displaying corrupt error-tolerant image data: (1) lowers the perceptual delay compared to fully reliable packet delivery, and (2) can be shown to produce images with lower distortion than aggressively compressed images when the delay budget only permits weak forward error correction. Leaky ARQ supports delaying of re-transmissions so that initial packet transmissions can be expedited, and cancelling of retransmissions associated with "out-of-date" data. Leaky ARQ can be parameterized to partially retransmit audio and video. We propose to implement Leaky ARQ by modifying Type-II Hybrid/"code combining" ARQ

Congestion control algorithms of TCP in emerging networks

In this dissertation we examine some of the challenges faced by the congestion control algorithms of TCP in emerging networks. We focus on three main issues. First, we propose TCP with delayed congestion response (TCP-DCR), for improving performance in the presence of non-congestion events. TCP-DCR delays the conges- tion response for a short interval of time, allowing local recovery mechanisms to handle the event, if possible. If at the end of the delay, the event persists, it is treated as congestion loss. We evaluate TCP-DCR through analysis and simulations. Results show significant performance improvements in the presence of non-congestion events with marginal impact in their absence. TCP-DCR maintains fairness with standard TCP variants that respond immediately. Second, we propose Layered TCP (LTCP), which modifies a TCP flow to behave as a collection of virtual flows (or layers), to improve eficiency in high-speed networks. The number of layers is determined by dynamic network conditions. Convergence properties and RTT-unfairness are maintained similar to that of TCP. We provide the intuition and the design for the LTCP protocol and evaluation results based on both simulations and Linux implementation. Results show that LTCP is about an order of magnitude faster than TCP in utilizing high bandwidth links while maintaining promising convergence properties. Third, we study the feasibility of employing congestion avoidance algorithms in TCP. We show that end-host based congestion prediction is more accurate than previously characterized. However, uncertainties in congestion prediction may be un- avoidable. To address these uncertainties, we propose an end-host based mechanism called Probabilistic Early Response TCP (PERT). PERT emulates the probabilistic response function of the router-based scheme RED/ECN in the congestion response function of the end-host. We show through extensive simulations that, similar to router-based RED/ECN, PERT provides fair bandwidth sharing with low queuing delays and negligible packet losses, without requiring the router support. It exhibits better characteristics than TCP-Vegas, the illustrative end-host scheme. PERT can also be used for emulating other router schemes. We illustrate this through prelim- inary results for emulating the router-based mechanism REM/ECN. Finally, we show the interactions and benefits of combining the different proposed mechanisms