Transmission Control Protocol (TCP) is considered one of the most important protocols in the Internet. An important mechanism in TCP is the congestion control mechanism which controls TCP sending rate and makes TCP react to congestion signals. Nowadays in heterogeneous networks, TCP may work in networks with some links that have lossy nature (wireless networks for example). TCP treats all packet loss as if they were due to congestion. Consequently, when used in networks that have lossy links, TCP reduces sending rate aggressively when there are transmission (non-congestion) errors in an uncongested network. One solution to the problem is to discriminate between errors; to deal with congestion errors by reducing TCP sending rate and use other actions for transmission errors. In this work we investigate the problem and propose a solution using an end-to-end error discriminator. The error discriminator will improve the current congestion window mechanism in TCP and decide when to cut and how much to cut the congestion window. We have identified three areas where TCP interacts with drops: congestion window update mechanism, retransmission mechanism and timeout mechanism. All of these mechanisms are part of the TCP congestion control mechanism. We propose changes to each of these mechanisms in order to allow TCP to cope with transmission errors. We propose a new TCP congestion window action (CWA) for transmission errors by delaying the window cut decision until TCP receives all duplicate acknowledgments for a given window of data (packets in flight). This will give TCP a clear image about the number of drops from this window. The congestion window size is then reduced only by number of dropped packets. Also, we propose a safety mechanism to prevent this algorithm from causing congestion to the network by using an extra congestion window threshold (tthresh) in order to save the safe area where there are no drops of any kind. The second algorithm is a new retransmission action to deal with multiple drops from the same window. This multiple drops action (MDA) will prevent TCP from falling into consecutive timeout events by resending all dropped packets from the same window. A third algorithm is used to calculate a new back-off policy for TCP retransmission timeout based on the network's available bandwidth. This new retransmission timeout action (RTA) helps relating the length of the timeout event with current network conditions, especially with heavy transmission error rates. The three algorithms have been combined and incorporated into a delay based error discriminator. The improvement of the new algorithm is measured along with the impact on the network in terms of congestion drop rate, end-to-end delay, average queue size and fairness of sharing the bottleneck bandwidth. The results show that the proposed error discriminator along with the new actions toward transmission errors has increased the performance of TCP. At the same time it has reduced the load on the network compared to existing error discriminators. Also, the proposed error discriminator has managed to deliver excellent fairness values for sharing the bottleneck bandwidth. Finally improvements to the basic error discriminator have been proposed by using the multiple drops action (MDA) for both transmission and congestion errors. The results showed improvements in the performance as well as decreases in the congestion loss rates when compared to a similar error discriminator.

Para beneficiarse de una reducción en la latencia así como disminuir tanto el consumo como el coste, el número óptimo de puertos de un conmutador ha ido aumentando a lo largo del tiempo. Sin embargo, las arquitecturas tradicionales se han quedado atrás bien por bajo rendimiento o bien por problemas de escalabilidad con el número de puertos. En esta Tesis se propone una nueva arquitectura de conmutador válida para conmutadores de elevado grado llamada Partitioned Crossbar Input Queued (PCIQ). Esta arquitectura resuelve el problema de los excesivos requerimientos de memoria en el diseño de arquitecturas de elevado grado. Además PCIQ define una nueva familia de arquitecturas de conmutador. PCIQ se basa en un particionado inteligente del crossbar, dividiéndolo en sub-crossbars, requiriendo menos recursos de memoria que las otras propuestas para conmutadores de elevado grado y que consigue una mayor eficiencia debido en parte a un incremento en la eficiencia de los árbitros empleados en el diseño. En este sentido, PCIQ emplea dos árbitros con prioridad rotativa (uno para cada sub-crossbar) que presentan un coste lineal y una respuesta en el tiempo logarítmica conforme aumenta el número de puertos del conmutador. Además PCIQ tiene un coste (medido en términos de requerimientos de memoria, complejidad del crossbar y complejidad en el arbitraje) similar o incluso menor que organizaciones básicas como CIOQ. No obstante PCIQ es capaz de conseguir máxima eficiencia para distribuciones de tráfico uniforme. El bloqueo por paquete al principio de cola (o HOL en inglés) reduce dramáticamente el rendimiento del conmutador. Las soluciones tradicionales para eliminar el bloqueo por HOL no son escalables con el número de puertos o requieren arquitecturas complejas. En esta Tesis se propone una técnica de control de la congestión que elimina el bloqueo por HOL llamada RECN-IQ. RECN-IQ está diseñada para conmutadores con memorias sólo a la entrada y es una técnica altamente eficiente / Mora Porta, G. (2009). High-performance architectures for high-radix switches [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/4335 / Palancia