INDEX
| Raweb 2003 / Project-Team : reso
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In TCP/IP networks, the end to end principle aims at simplifying the network level while pushing all the complexity on the end host level. This principle has been proved to be very valuable in the context of the traditional low capacity Internet. In packet networking, congestion events are the natural counterpart of the flexibility to interconnect mismatched elements and freely multiplex flows. Managing congestion in packet networks is a very complex issue. This is especially true in IP networks where, at best, congestion information is very limited (e.g., ECN) or, at worst, non-existent, forcing the transmitter to infer it instead (e.g., based on losses or delay) in TCP.
The conservative behavior of TCP with respect to congestion in IP networks (RFC 2581) is at the heart of the current performance issues faced by the high-performance networking community. Several theoretical and experimental analyses have shown that the dynamic of the traditional feedback based approach is too low in very high speed networks that may lose packets. Consequently network resource utilization is not optimal and the application performances are poor and disappointing. Proposed enhancements to TCP tackle this problem in different ways, while retaining backwards compatibility. Highspeed TCP [62] and Scalable TCP [69] increase the aggressiveness in high-throughput situations while staying fair to standard TCP flows in legacy contexts. FAST [57] leverages the queueing information provided by round-trip time variations, in order to efficiently control buffering in routers and manage IP congestion optimally. Since two year, these propositions are actively analyzed and experimented by the international community. Several issues have been already enlightened. Considering the traditional feedback loop will not scale with higher rate level under loss or congesting traffic conditions, it seems judicious to start examining alternative radical solutions.
On the other hand, tools for measuring the end-to-end performance of a link between two hosts are very important for transport protocol and distributed application performance optimization. Bandwidth evaluation methods aim to provide a realistic view of the raw capacity but also of the dynamic behavior of the interconnection that may be very useful to evaluate the time for bulk data transfer. Existing methods differ according to the measurements strategies and the evaluated metric. These methods can be active or passive, intrusive or non-intrusive. Non-intrusive active approaches, based on packet train or on packet pair provide available bandwidth measurements and/or the total capacity measurements. None of the proposed tools, based on these methods, enable the evaluation of both metrics, while giving an overview of the link topology and characteristics.
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