Revisiting the IETF multipath extensions on transport layer

Load sharing on the transport layer of the OSI reference model is an important topic in the IETF standardization. This approach is also supported by the industry to optimize the use of the resources in a network like the Internet. After many trials, two basic sets of mechanisms and functionalities on the transport layer have been proposed by the IETF to achieve load sharing. These basic sets extend the protocol mechanisms that were originally designed for the use in singlepath dominated networks and represent only a first step to introduce a real end-to-end multipath transfer on the Internet. These first basic sets must be investigated and improved for the next steps. The Transmission Control Protocol (TCP) and the Stream Control Transmission Protocol (SCTP) provide the basis for the two IETF end-to-end multipath extensions. Both singlepath transport protocols have a different historical background but similar goals. These can be characterized by a reliable, connection-oriented and ordered data transport. However, initial experiments with the IETF multipath extensions in real networks show unexpected and in some cases clearly inadequate results. It is becoming rather apparent that the singlepath transport protocol specifications with their singlepath goals have a significant impact on the effectiveness of the load sharing mechanism and, furthermore, that the severity of the influence depends on the topology. The new mechanisms for multipath transfer include, in particular, an extended “path management” and “scheduling” task. The mechanisms addressing the path management organize the new, alternative paths and the scheduling mechanisms sup- port their effective use. For both protocol extensions of TCP and SCTP, an interaction can be identified between the new load sharing mechanisms and the existing specifications for singlepath transfer. This thesis systematically identifies the impact factors of the singlepath specifications on the new load sharing mechanisms and demonstrates their effects. In addition to the focus on the optimal use, the fair distribution of resources across all connections must be taken into account in the IETF standardization process. This so-called “fairness” discus- sion is mandatory for a transport protocol in the IETF context and has a direct impact on the overall system performance. Furthermore, this thesis discusses the currently implemented load sharing extensions and analyzes their weaknesses. Moreover, in this work new design approaches are developed to decrease the impact

On the Fairness of Transport Protocols in a Multi-Path Environment

Abstract—Today, a steadily growing number of devices contains multiple network interfaces. For example, nearly all smartphones are equipped with at least W-LAN as well as 3G/4G interfaces. In consequence, there is a rising demand for so-called multi-path transfer, which utilizes all of these interfaces simultaneously in order to maximize the payload throughput of applications. Currently, this so-called multi-path transfer is very actively discussed by the IETF, in form of the Multi-Path TCP (MPTCP) extension for TCP as well as the Concurrent Multi-path Transfer extension for SCTP (CMT-SCTP). Their larger-scale deployment in the Internet is expected for the near future. A key issue that prevents the standardization of these approaches is the fairness to concurrent TCP flows. A multipath transfer should behave “TCP-friendly”, i.e. cause no harm to the performance of the very widely deployed TCP-based applications. In this paper, we first extend the notion of “fairness” from single-path transport to multi-path transport. Furthermore, we introduce the relevant congestion control approaches in the IETF context for single-path as well as multi-path transfer. We simulatively analyze these approaches in a couple of interesting network configuration scenarios, in order to show their behavior with special regard to the fairness definitions. Particularly, we also point out items of further discussion which are the result of the current approaches. 12