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

Delayed congestion response protocols

Due to the character of the original source materials and the nature of batch digitization, quality control issues may be present in this document. Please report any quality issues you encounter to [email protected], referencing the URI of the item.Includes bibliographical references (leaves 61-63).Issued also on microfiche from Lange Micrographics.There has been a surge of new real-time multimedia applications over the IP network which cannot use TCP because the congestion control algorithm of TCP results in drastic variations in the sending rate, which in turn affect the user perceived quality. As a result several of these applications use UDP. The fact that UDP does not respond to congestion in any way has raised several questions about possible congestion collapse of the Internet and fairness to existing TCP-based applications. Several congestion control algorithms suited for such applications have been proposed which claim bandwidth in a "TCP-friendly" manner. However all of them reduce their sending rate once and as soon as allowed by the protocol design, when a packet drop occurs. In this thesis we have proposed a novel scheme by which response to congestion is deliberately delayed by [] RTTs. We have provided the framework for this class of protocols in general and examined three cases in particular. For these three cases we have developed analytical models and derived the conditions under which they are TCP-friendly. With these conditions we have run simulations on the ns-2 platform to show that they are indeed TCP-friendly. By showing that delayed congestion response is possible, we have laid the ground work for the development of a whole new class of protocols which are not just TCP-friendly and capable of providing early warning to the application regarding an impending reduction in the sending rate but also can be designed to have a smooth congestion response

TCP-DCR: A

novel protocol for tolerating wireless channel error

Robustness to Packet Reordering in High-speed Networks

In this paper we investigate the impact of packet reordering on the performance of high-speed protocols. Our results show that even small fraction of packet reordering can severely impair the performance of these protocols. We then investigate the benefits of using delayed congestion response (TCP-DCR) with one of the high-speed protocols (LTCP). Our results indicate that the benefits in terms of avoiding performance degradation is significant, even at very high levels of packet reordering. In the absence of any packet reordering, the protocol behavior in terms of fairness among competing flows or impact on bottleneck link drop rates is remains unmodified

Improving TCP performance in high bandwidth high RTT links using layered congestion control

In this paper, we propose Layered TCP (LTCP for short), a simple layering technique for the congestion window response of TCP to make it more scalable in highspeed, high RTT networks. LTCP is a two dimensional congestion control framework- the macroscopic control uses the concept of layering to quickly and efficiently make use of the available bandwidth whereas microscopic control extends the existing AIMD algorithms of TCP to determine the per-ack behavior. We provide the general intuition and framework for the LTCP protocol modifications in this paper. Then, using a simple design, we illustrate the effectiveness of using layering for improving the efficiency, without sacrificing the convergence properties of TCP. We assess the RTT unfairness with the chosen design and show that by using a simple RTT compensation factor, the RTT unfairness can be assured to be no worse than that of unmodified TCP. Evaluation of the specified design is based on analyses and ns-2 based simulations. We show that LTCP has promising convergence properties, is about an order of magnitude faster than TCP in utilizing high bandwidth links, employs few parameters and is easy to understand. The choice of parameters can be influenced to reduce the RTT unfairness, compared to TCP or other highspeed solutions. The flexible framework opens a whole class of design options for improving the performance of TCP in highspeed networks

TCP-DCR: A novel protocol for tolerating wireless channel errors

This paper presents a new TCP protocol, TCP-DCR, designed to tolerate channel errors in wireless networks. TCP-DCR employs the simple solution of allowing a link level retransmission scheme to recover the packets lost due to channel errors thereby limiting the response of the transport protocol to mostly congestion losses. TCP-DCR delays responding to a packet loss indication for a small period of time (one RTT) to allow the channel errors to be recovered by link level retransmission. We analyze TCP-DCR to show that the congestion response delay does not impact its performance and its fairness. We evaluate TCP-DCR through simulations and compare its performance with TCP-Reno, TCP-SACK and TCP-Westwood under different network conditions. Our results show that TCP-DCR offers significantly better performance when channel errors make a large contribution to packet losses in the network and when the round trip delays are large. We also present an analysis to show that protocol evaluation in the wireless networks is significantly impacted by the number of flows in the network

LTCP: A Layering Technique for Improving the Performance of TCP . . .

In this paper, we propose Layered TCP (LTCP for short), a simple layering technique for the congestion window response of TCP to make it more scalable in highspeed networks. LTCP uses two dimensional congestion control: at the macroscopic level, the layers are added/dropped based on dynamic network conditions and at the microscopic level, the congestion window behavior is defined for operating at any given layer. We provide the design, analysis and preliminary results based on ns-2 simulations. Our experiments show that LTCP has promising convergence properties, is about an order of magnitude faster than TCP in utilizing high bandwidth links and can be made to operate with smaller window fluctuations than normal TCP. LTCP employs few parameters and is easy to understand