On Switchover Performance in Multihomed SCTP

Abstract The emergence of real-time applications, like Voice over IP and video conferencing, in IP networks implies a challenge to the underlying infrastructure. Several real-time applications have requirements on timeliness as well as on reliability and are accompanied by signaling applications to set up, tear down and control the media sessions. Since neither of the traditional transport protocols responsible for end-to-end transfer of messages was found suitable for signaling traffic, the Stream Control Transmission Protocol (SCTP) was standardized. The focus for the protocol was initially on telephony signaling applications, but it was later widened to serve as a general purpose transport protocol. One major new feature to enhance robustness in SCTP is multihoming, which enables for more than one path within the same association. In this thesis we evaluate some of the mechanisms affecting transmission performance in case of a switchover between paths in a multihomed SCTP session. The major part of the evaluation concerns a failure situation, where the current path is broken. In case of failure, the endpoint does not get an explicit notification, but has to react upon missing acknowledgements. The challenge is to distinguish path failure from temporary congestion to decide when to switch to an alternate path. A too fast switchover may be spurious, which could reduce transmission performance, while a too late switchover also results in reduced transmission performance. This implies a tradeoff which involves several protocol as well as network parameters and we elaborate among these to give a coherent view of the parameters and their interaction. Further, we present a recommendation on how to tune the parameters to meet telephony signaling requirements, still without violating fairness to other traffic. We also consider another angle of switchover performance, the startup on the alternate path. Since the available capacity is usually unknown to the sender, the transmission on a new path is started at a low rate and then increased as acknowledgements of successful transmissions return. In case of switchover in the middle of a media session the startup phase after a switchover could cause problems to the application. In multihomed SCTP the availability of the alternate path makes it feasible for the end-host to estimate the available capacity on the alternate path prior to the switchover. Thus, it would be possible to implement a more efficient startup scheme. In this thesis we combine different switchover scenarios with relevant traffic. For these combinations, we analytically evaluate and quantify the potential performance gain from utilizing an ideal startup mechanism as compared to the traditional startup procedure

Desempenho de SCTP em terminais multi-abrigados para transporte de tráfego VoIP em cenários com perdas

Resumo: A multiplicidade de meios de acesso e sua popularização estimula a exploração de sistemas multi-abrigados, que são sistemas com mais de uma porta disponível para acessar a internet. Esta característica pode trazer ganhos à resiliência da conexão e à qualidade de serviços. Serviços multimídia podem ser beneficiados pelo uso mais adequado das rotas disponíveis em sistemas multi-abrigados. O SCTP (Stream Control Transmission Protocol) é um protocolo de transporte que suporta conexões multi-abrigadas, e por esse motivo vem sendo alvo de estudos envolvendo otimização do uso de caminhos para elhoria de qualidade de serviços multimídia. Este trabalho estuda o desempenho do SCTP juntamente com um mecanismo de troca automática de rotas baseado em atraso (delay-centric) para o transporte de tráfego VoIP (Voice over IP) em terminais multi-abrigados. Os cenários simulados envolvem atrasos e perdas variáveis. Dois parâmetros que apresentam forte influência sobre o comportamento do SCTP, o PMR (Path.Max.Retrans) e o RTOmax (Valor máximo do Retransmission TimeOut), têm seus valores variados com o objetivo de investigar sua influência na qualidade de chamadas VoIP. A métrica de desempenho utilizada é o MOS (Mean Opinion Score) estimado a partir do E-model. Os resultados indicam que abordagens mais agressivas, representadas por menores valores para os parâmetros estudados, proporcionam uma melhor qualidade nas chamadas VoIP

SCTP Switchover Performance Issues in WLAN Environments

Abstract- The increased number and diversity of underlying networks have made transparent network migration a necessity. Through its support for multi-homing the Stream Control Transmission Protocol (SCTP) enables seamless network mobility by abstracting multiple underlying physical paths into a single end-to-end association. One of these paths is selected as the primary. When a number of retransmission failures occur on the primary path, switchover is initiated to a secondary path. The number of retransmission attempts before switchover is initiated can be configured; however, the delay between each retransmission is managed internally in SCTP using a Retransmission TimeOut (RTO) value. This paper shows that the current SCTP mechanism for calculating RTO values is inappropriate in WLAN environments, since increased Round Trip Times (RTT) significantly distort RTO calculations. Experimental and simulated results indicate that SCTP behaves in a counterintuitive manner which allows more time for switchover as network conditions degrade: delays of up to 187 seconds can be experienced before switchover occurs. We show that additional SCTP parameters need to be carefully configured in order to reduce this switchover delay to a more acceptable level