M. Y. Sanadidi

TCP westwood: Bandwidth estimation for enhanced transport over wireless links

TCP Westwood (TCPW) is a sender-side modification of the TCP congestion window algorithm that improves upon the performance of TCP Reno in wired as well as wireless networks. The improvement is most significant in wireless networks with lossy links, since TCP Westwood relies on end-to-end bandwidth estimation to discriminate the cause of packet loss (congestion or wireless channel effect) which is a major problem in TCP Reno. An important distinguishing feature of TCP Westwood with respect to previous wireless TCP “extensions” is that it does not require inspection and/or interception of TCP packets at intermediate (proxy) nodes. Rather, it fully complies with the end-to-end TCP design principle. The key innovative idea is to continuously measure at the TCP source the rate of the connection by monitoring the rate of returning ACKs. The estimate is then used to compute congestion window and slow start threshold after a congestion episode, that is, after three duplicate acknowledgments or after a timeout. The rationale of this strategy is simple: in contrast with TCP Reno, which “blindly” halves the congestion window after three duplicate ACKs, TCP Westwood attempts to select a slow start threshold and a congestion window which are consistent with the effective bandwidth used at the time congestion is experienced. We call this mechanism faster recovery. The proposed mechanism is particularly effective over wireless links where sporadic losses due to radio channel problems are often misinterpreted as a symptom of congestion by current TCP schemes and thus lead to an unnecessary window reduction. Experimental studies reveal improvements in throughput performance, as well as in fairness. In addition, friendliness with TCP Reno was observed in a set of experiments showing that TCP Reno connections are not starved by TCPW connections. Most importantly, TCPW is extremely effective in mixed wired and wireless networks where throughput improvements of up to 550% are observed. Finally, TCPW performs almost as well as localized link layer approaches such as the popular Snoop scheme, without incurring the O/H of a specialized link layer protocol.

TCP westwood: end-to-end congestion control for wired/wireless networks

TCP Westwood (TCPW) is a sender-side modification of the TCP congestion window algorithm that improves upon the performance of TCP Reno in wired as well as wireless networks. The improvement is most significant in wireless networks with lossy links. In fact, TCPW performance is not very sensitive to random errors, while TCP Reno is equally sensitive to random loss and congestion loss and cannot discriminate between them. Hence, the tendency of TCP Reno to overreact to errors. An important distinguishing feature of TCP Westwood with respect to previous wireless TCP “extensions” is that it does not require inspection and/or interception of TCP packets at intermediate (proxy) nodes. Rather, TCPW fully complies with the end-to-end TCP design principle. The key innovative idea is to continuously measure at the TCP sender side the bandwidth used by the connection via monitoring the rate of returning ACKs. The estimate is then used to compute congestion window and slow start threshold after a congestion episode, that is, after three duplicate acknowledgments or after a timeout. The rationale of this strategy is simple: in contrast with TCP Reno which “blindly” halves the congestion window after three duplicate ACKs, TCP Westwood attempts to select a slow start threshold and a congestion window which are consistent with the effective bandwidth used at the time congestion is experienced. We call this mechanism faster recovery. The proposed mechanism is particularly effective over wireless links where sporadic losses due to radio channel problems are often misinterpreted as a symptom of congestion by current TCP schemes and thus lead to an unnecessary window reduction. Experimental studies reveal improvements in throughput performance, as well as in fairness. In addition, friendliness with TCP Reno was observed in a set of experiments showing that TCP Reno connections are not starved by TCPW connections. Most importantly, TCPW is extremely effective in mixed wired and wireless networks where throughput improvements of up to 550% are observed. Finally, TCPW performs almost as well as localized link layer approaches such as the popular Snoop scheme, without incurring the overhead of a specialized link layer protocol.

CapProbe: a simple and accurate capacity estimation technique

We present a new capacity estimation technique, called CapProbe. CapProbe combines delay as well as dispersion measurements of packet pairs to filter out samples distorted by cross-traffic. CapProbe algorithms include convergence tests and convergence speed-up techniques by varying probing parameters. Our study of CapProbe includes a probability analysis to determine the time it takes CapProbe to converge on the average. Through simulations and measurements, we found CapProbe to be quick and accurate across a wide range of traffic scenarios. We also compared CapProbe with two previous well-known techniques, pathchar and pathrate. We found CapProbe to be much more accurate than pathchar and similar in accuracy to pathrate, while providing faster estimation than both. Another advantage of CapProbe is its lower computation cost, since no statistical post processing of probing data is required.

TCP Westwood: congestion window control using bandwidth estimation

We study the performance of TCP Westwood (TCPW), a new TCP protocol with a sender-side modification of the window congestion control scheme. TCP Westwood controls the window using end-to-end rate estimation in a way that is totally transparent to routers and to the destination. Thus, it is compatible with any network and TCP implementation. The key innovative idea is to continuously estimate, at the TCP sender, the packet rate of the connection by monitoring the ACK reception rate. The estimated connection rate is then used to compute congestion window and slow start threshold settings after a congestion episode. Resetting the window to match available bandwidth makes TCPW more robust to sporadic losses due to wireless channel problems. These often cause conventional TCP to overreact, leading to unnecessary window reduction. Experimental studies of TCPW show significant improvements in throughput performance over Reno and SACK, particularly in mixed wired/wireless networks over high-speed links. The contributions of this paper include a model for fair and friendly sharing of the bottleneck link and a Markov Chain performance model in presence of link errors/loss. TCPW performance is compared to that of TCP Reno, and analytic results are validated against simulation results. Internet and laboratory measurements using a Linux TCPW implementation are also reported, providing further evidence of the gains achievable via TCPW.

TCP with sender-side intelligence to handle dynamic, large, leaky pipes

Transmission control protocol Westwood (TCPW) has been shown to provide significant performance improvement over high-speed heterogeneous networks. The key idea of TCPW is to use eligible rate estimation (ERE) methods to intelligently set the congestion window (cwnd) and slow-start threshold (ssthresh) after a packet loss. ERE is defined as the efficient transmission rate eligible for a sender to achieve high utilization and be friendly to other TCP variants. This work presents TCP Westwood with agile probing (TCPW-A), a sender-side only enhancement of TCPW, that deals well with highly dynamic bandwidth, large propagation time/bandwidth, and random loss in the current and future heterogeneous Internet. TCPW-A achieves this goal by adding the following two mechanisms to TCPW. 1) When a connection initially begins or restarts after a timeout, instead of exponentially expanding cwnd to an arbitrary preset sthresh and then going into linear increase, TCPW-A uses agile probing, a mechanism that repeatedly resets ssthresh based on ERE and forces cwnd into an exponential climb each time. The result is fast convergence to a more appropriate ssthresh value. 2) In congestion avoidance, TCPW-A invokes agile probing upon detection of persistent extra bandwidth via a scheme we call persistent noncongestion detection (PNCD). While in congestion avoidance, agile probing is actually invoked under the following conditions: a) a large amount of bandwidth that suddenly becomes available due to change in network conditions; b) random loss during slow-start that causes the connection to prematurely exit the slow-start phase. Experimental results, both in ns-2 simulation and lab measurements using actual protocols implementation, show that TCPW-A can significantly improve link utilization over a wide range of bandwidth, propagation delay, and dynamic network loading.

The probe gap model can underestimate the available bandwidth of multihop paths

The Probe Gap Model (PGM) was proposed as a lightweight and fast available bandwidth estimation method. Measurement tools such as Delphi and Spruce are based on PGM. Compared to estimation methods that require multiple iterations with different probing rates, PGM uses a single probing rate and it infers the available bandwidth from a direct relation between the input and output rates of measurement packet pairs. An important assumption behind the PGM model is that the measured path has a single bottleneck link that determines the available bandwidth of the end-to-end path. In this letter, we show that, even though PGM is accurate in the case of a single queue, it cannot estimate the available bandwidth of multi-hop paths, even if there is a single bottleneck in the path. Whether PGM is accurate or not depends on the routing of cross traffic relative to the measurement traffic. PGM is accurate when the cross traffic follows the same path with the measurement traffic. In the general case, however, PGM can significantly underestimate the available bandwidth of an end-to-end path.

TCP with delayed ack for wireless networks

This paper studies the TCP performance with delayed ack in wireless networks (including ad hoc and WLANs) which use IEEE 802.11 MAC protocol as the underlying medium access control. Our analysis and simulations show that TCP throughput does not always benefit from an unrestricted delay policy. In fact, for a given topology and flow pattern, there exists an optimal delay window size at the receiver that produces best TCP throughput. If the window is set too small, the receiver generates too many acks and causes channel contention; on the other hand, if set the window too high, the bursty transmission at the sender triggered by large cumulative acks will induce interference and packet losses, thus degrading the throughout. In wireless networks, packet losses are also related to the length of TCP path; when traveling through a longer path, a packet is more likely to suffer interference. Therefore, path length is an important factor to consider when choosing appropriate delay window sizes. In this paper, we first propose an adaptive delayed ack mechanism which is suitable for ad hoc networks, then we propose a more general adaptive delayed ack scheme for ad hoc and hybrid networks. The simulated results show that our schemes can effectively improve TCP throughput by up to 30% in static networks, and provide more significant gain in mobile networks. The proposed schemes are simple and easy to deploy.

Defense against low-rate TCP-targeted denial-of-service attacks

Low-rate TCP-targeted denial-of-service (DoS) attacks aim at the fact that most operating systems in use today have a common base TCP retransmission timeout (RTO) of 1 sec. An attacker injects periodic bursts of packets to fill the bottleneck queue and forces TCP connections to timeout with near-zero throughput. This work proposes randomization on TCP RTO as defense against such attacks. With RTO randomization, an attacker cannot predict the next TCP timeout and consequently cannot inject the burst at the exact instant. An analytic performance model on the throughput of randomized TCP is developed and validated. Simulation results show that randomization can effectively mitigate the impact of such DoS attacks while maintaining fairness and friendliness to other connections.

Streaming Media Congestion Control Using Bandwidth Estimation

The fundamental challenge in streaming media over the Internet is to transfer the highest possible quality, adhere to the media play out time constraint, and efficiently and fairly share the available bandwidth with TCP, UDP, and other traffic types. This work introduces the Streaming Media Congestion Control protocol (SMCC), a new adaptive media streaming congestion management protocol in which the connections packet transmission rate is adjusted according to the dynamic bandwidth share of the connection. In SMCC, the bandwidth share of a connection is estimated using algorithms similar to those introduced in TCP Westwood. SMCC avoids the Slow Start phase in TCP. As a result, SMCC does not exhibit the pronounced rate oscillations characteristic of traditional TCP, thereby providing congestion control that is more suitable for streaming media applications. Furthermore, SMCC is fair, sharing the bandwidth equitably among a set of SMCC connections. An important advantage is robustness when packet losses are due to random errors, which is typical of wireless links and is becoming an increasing concern due to the emergence of wireless Internet access. In the presence of random errors, SMCC is also friendly to TCP New Reno. We provide simulation results using the ns2 simulator for our protocol running together with TCP New Reno.

Enhancing Bluetooth TCP throughput via link layer packet adaptation

TCP throughput limitations over wireless links have received considerable attention in the last few years. One of the problems is that TCP congestion control interprets packet losses as an indication of congestion, whereas in wireless links, losses could be due to transient link quality degradations. In this paper, we propose and study a link layer solution and evaluate its effects on TCP in the context of Bluetooth. We enhance the Bluetooth link layer to make use of channel state information and accordingly adapt the Bluetooth packet type to enhance TCP throughput We propose a simple analytical method to determine the optimal packet type for a given channel state by adding FEC support or changing packet size. Since wireless interfaces, such as 802.11 or Bluetooth, can provide information regarding the channel state using relevant APIs, this simple enhancement can easily be added to the link layer. We implemented this functionality in the Bluetooth link layer. Our simulation experiments show that the proposed adaptive packet type solution significantly improves TCP throughput. The throughput enhancement increases with the error rate. For high error rates close to 0.1%, the link layer enhanced with the adaptive scheme is able to maintain good TCP throughput, whereas throughput is almost zero when the adaptive scheme is not used.