Evaluation and optimisation of Less-than-Best-Effort TCP congestion control mechanisms

Increasing use of online software installation, updates, and backup services, as well as the popularity of user-generated content, has increased the demand for band-width in recent years. Traffic generated by these applications — when receiving a ‘fair-share’ of the available bandwidth — can impact the responsiveness of delay-sensitive applications. Less-than-Best-Effort TCP congestion control mechanisms aim to allow lower-priority applications to utilise excess bandwidth with minimum impact to regular TCP carrying delay-sensitive traffic. However, no previous study has evaluated the performance of a large number of this class of congestion con-trol mechanisms. This thesis quantifies the performance of existing Less-than-Best-Effort TCP congestion control mechanisms, and proposes a new mechanism to im-prove the performance of these mechanisms with high path delay. This study first evaluated the performance of seven Less-than-Best-Effort conges-tion control mechanisms in realistic scenarios under a range of network conditions in a Linux testbed incorporating wired Ethernet and 802.11n wireless links. The seven mechanisms evaluated were: Apple LEDBAT, CAIA Delay-Gradient (CDG), RFC6817 LEDBAT, Low Priority, Nice, Westwood-LP, and Vegas. Of these mecha-nisms, only four had existing implementations for modern operating systems. The remaining three mechanisms — Apple LEDBAT, Nice, and Westwood-LP — were implemented based on published descriptions and available code fragments to fa-cilitate this evaluation. The results of the evaluation suggest that Less-than-Best-Effort congestion control mechanisms can be divided into two categories: regular TCP-like mechanisms, and low-impact mechanisms. Of the low-impact mechanisms, two mechanisms were identified as having desirable performance characteristics: Nice and CDG. Nice pro-vides background throughput comparable to regular TCP while maintaining low queuing delay in low path delay settings. CDG has the least impact on regular TCP traffic, at the expense of reduced throughput. In high path-delay settings, these reductions to throughput experienced by CDG are exacerbated, while Nice has a greater impact on regular TCP traffic. To address the very low throughput of existing Less-than-Best-Effort congestion control mechanisms in high path-delay settings, a new Less-than-Best-Effort TCP congestion control algorithm was developed and implemented: Yield TCP. Yield utilises elements of a Proportional-Integral controller to better interpret and re-spond to changes in queuing delay to achieve this goal while also reducing the impact on regular TCP traffic over TCP-like mechanisms. Source code for the im-plementation of Yield developed for this research has also been made available. The results of evaluating Yield indicate that it successfully addresses the low through-put of low-impact Less-than-Best-Effort mechanisms in high delay settings, while also reducing the impact on foreground traffic compared to regular TCP-like con-gestion control mechanisms. Yield also performs similarly to Nice in low delay settings, while also achieving greater intra-protocol fairness than Nice across all settings. These results indicate that Yield addresses the weaknesses of Nice and CDG, and is a promising alternative to existing Less-than-Best-Effort congestion control algorithms

Control de Congestión TCP y mecanismos AQM

En los últimos años se ha ido poniendo énfasis particularmente en la importancia del retraso sobre la capacidad. Hoy en día, nuestras redes se están volviendo más y más sensibles a la latencia debido a la proliferación de aplicaciones y servicios como el VoIP, la IPTV o el juego online donde un retardo bajo es esencial para un desempeño adecuado y una buena experiencia de usuario. La mayor parte de este retraso innecesario se debe al mal funcionamiento de algunos búferes que pueblan internet. En vez de desempeñar la tarea para la que fueron creados, absorber eventuales ráfagas de paquetes con el fin de prevenir su pérdida, hacen creer al mecanismo de control de congestión que la ruta hacia el destino actual tiene más ancho de banda que el que posee realmente. Cuando la pérdida de paquetes ocurre, si es que lo hace, es demasiado tarde y el daño en el enlace, en forma de tiempo de transmisión adicional, ya se ha producido. En este trabajo de fín de grado intentaremos arrojar luz sobre una solución específica cuyo objetivo es el de reducir el retardo extra producido por esos hinchados búferes, la Gestión Avanzada de Colas o Active Queue Management (AQM). Hemos testeado un grupo de estos algoritmos AQM junto con diferentes modificaciones del control de congestión de TCP con el fín de entender las interacciones generadas entre esos dos mecanismos, realizando simulaciones en varios escenarios caracterísiticos tales como enlaces transoceánicos o enlaces de acceso a red, entre otros.In recent years, the relevance of delay over throughput has been particularly emphasized. Nowadays our networks are getting more and more sensible to latency due to the proliferation of applications and services like VoIP, IPTV or online gaming where a low delay is essential for a proper performance and a good user experience. Most of this unnecessary delay is created by the misbehaviour of many bu ers that populate Internet. Instead of performing the task for what they were created for, absorbing eventual packet bursts to prevent loss, they deceive the sender's congestion control mechanisms into believing that the current path to the destination has more bandwidth than it really has. When the loss event occurs, if it does, it's too late and the damage on the path, in terms of additional transmission time, has been done. On this bachelor thesis we will try to throw light over an speci c solution that aims to reduce the extra delay produced by these bloated bu ers: Active Queue Management. We have tested a bunch of AQM algorithms with di erent TCP modi cations in order to understand the interactions between these two mechanisms. We performed simulations testing various characteristic scenarios like Transoceanic links or Access link scenarios, among other.Ingeniería Telemátic

On The Existence Of Optimal LEDBAT Parameters

The Low Extra Delay Background Transport (LEDBAT) protocol is a recently standardized protocol that aims to offer a scavenger service (i.e. the goal is to exploit the remaining and unused capacity of a link). LEDBAT is a delay-based protocol mainly defined by two parameters: a target queuing delay and a gain. The RFC 6817 provides guidelines to configure both parameters that strongly impact on the LEDBAT behavior in terms of fairness with other protocols. However, these guidelines are questioned by several studies as they might lead to the generation of a non-LBE (Less-than-Best-Effort) traffic. This paper explores the set of optimal parameters allowing LEDBAT protocol to effectively perform as an LBE traffic. We conclude that the optimal couple of target and decrease gain is (5ms; 10). However, we observe that the aggregated use of optimized LEDBAT sources still disturb the overall traffic performance and that the exponential backoff is not an answer to this issue. As a result, we believe that additional strategies to limit the number of LEDBAT flows are required for integrating this protocol at a large scale

Transport Architectures for an Evolving Internet

In the Internet architecture, transport protocols are the glue between an application’s needs and the network’s abilities. But as the Internet has evolved over the last 30 years, the implicit assumptions of these protocols have held less and less well. This can cause poor performance on newer networks—cellular networks, datacenters—and makes it challenging to roll out networking technologies that break markedly with the past. Working with collaborators at MIT, I have built two systems that explore an objective-driven, computer-generated approach to protocol design. My thesis is that making protocols a function of stated assumptions and objectives can improve application performance and free network technologies to evolve. Sprout, a transport protocol designed for videoconferencing over cellular networks, uses probabilistic inference to forecast network congestion in advance. On commercial cellular networks, Sprout gives 2-to-4 times the throughput and 7-to-9 times less delay than Skype, Apple Facetime, and Google Hangouts. This work led to Remy, a tool that programmatically generates protocols for an uncertain multi-agent network. Remy’s computer-generated algorithms can achieve higher performance and greater fairness than some sophisticated human-designed schemes, including ones that put intelligence inside the network. The Remy tool can then be used to probe the difficulty of the congestion control problem itself—how easy is it to “learn” a network protocol to achieve desired goals, given a necessarily imperfect model of the networks where it ultimately will be deployed? We found weak evidence of a tradeoff between the breadth of the operating range of a computer-generated protocol and its performance, but also that a single computer-generated protocol was able to outperform existing schemes over a thousand-fold range of link rates

Stochastic Forecasts Achieve High Throughput and Low Delay over Cellular Networks

Sprout is an end-to-end transport protocol for interactive applications that desire high throughput and low delay. Sprout works well over cellular wireless networks, where link speeds change dramatically with time, and current protocols build up multi-second queues in network gateways. Sprout does not use TCP-style reactive congestion control; instead the receiver observes the packet arrival times to infer the uncertain dynamics of the network path. This inference is used to forecast how many bytes may be sent by the sender, while bounding the risk that packets will be delayed inside the network for too long. In evaluations on traces from four commercial LTE and 3G networks, Sprout, compared with Skype, reduced self-inflicted end-to-end delay by a factor of 7.9 and achieved 2.2 the transmitted bit rate on average. Compared with Google’s Hangout, Sprout reduced delay by a factor of 7.2 while achieving 4.4 the bit rate, and compared with Apple’s Facetime, Sprout reduced delay by a factor of 8.7 with 1.9 the bit rate. Although it is end-to-end, Sprout matched or outperformed TCP Cubic running over the CoDel active queue management algorithm, which requires changes to cellular carrier equipment to deploy. We also tested Sprout as a tunnel to carry competing interactive and bulk traffic (Skype and TCP Cubic), and found that Sprout was able to isolate client application flows from one another.National Science Foundation (U.S.) (NSF Grant 1040072

FLOWER, an Innovative Fuzzy Lower-than-Best-Effort Transport Protocol

We present a new delay-based transport protocol named FLOWER, that aims at providing a Lower-than-Best-Effort (LBE) service. The objective is to propose an alternative to the Low Extra Delay Background Transport (LEDBAT) widely deployed within the official BitTorrent client. Indeed, besides its intra-fairness problem, known as latecomer unfairness, LEDBAT can be too aggressive against TCP, making it ill suited for providing LBE services over certain networks such as constrained wireless networks. By using a fuzzy controller to modulate the sending rate, FLOWER aims to solve LEDBAT issues while fulfilling the role of a LBE protocol. FLOWER operates to a modification of the standard LEDBAT protocol implementation by replacing its proportional controller by a fuzzy controller. Thanks to this modification, our simulation results show that FLOWER can carry LBE traffic in network scenarios where LEDBAT cannot while solving the latecomer unfairness problem. The presented algorithm is simple to implement and does not require complex computation that would prevent its deployment. Finally, we show that FLOWER remains compliant when used over an AQM-based network and remains LBE while not increasing the bufferbloat