Depuis plusieurs ann´ees, on annonce le d´eploiement de solutions multipoints dans l’inter-net. Cependant, `a part quelques exceptions, force est de constater que la transmission multipoint n’est pas accessible au grand public. Le multipoint dans l’internet est bas´e sur IP multipoint qui n’a pas ´et´e con¸cu avec l’id´ee d’ˆetre exploit´e commercialement [21]. De nombreuses fonction-nalit´es manquent `a IP multipoint : la gestion de groupes multipoints, la s´ecurit´e multipoint, l’allocation d’adresses multipoints, la facturation de services multipoints, etc. Bien que d’ac-tives recherches soient men´ees dans tous ces domaines, la communaut´e des r´eseaux se divise entre ceux qui pensent que tˆot ou tard le multipoint sera d´eploy´e dans l’internet et ceux qui pensent que le multipoint n’est plus qu’un sujet acad´emique sans aucun avenir. ´Etant donn´e qu’une grande partie de cette th`ese est sur la transmission multipoint, doit-on consid´erer notre travail comme inutile si la transmission multipoint ne devient pas une fonctionnalit´e offerte au grand public?
D’autre part, pour appliquer le paradigme FS il faut avoir un r´eseau FS, c’est-`a-dire un r´eseau o`u tous les routeurs impl´ementent une politique d’ordonnancement de type FS. Or, il s’agit d’une hypoth`ese forte puisque, actuellement, soit les routeurs ne poss`edent pas un m´e-canisme de type FS, soit les routeurs poss`edent effectivement un m´em´e-canisme de type FS mais qui n’est pas activ´e. Doit-on consid´erer le paradigme FS comme un paradigme irr´ealiste et par cons´equent sans int´erˆet?
Notre r´eponse `a ces deux questions est (( non ! ))Trop souvent, les pressions ´economiques et politiques poussent la communaut´e scientifique `a consid´erer que les recherches qui ne sont pas applicables `a court terme ne sont pas dignes d’int´erˆet. Cette habitude est dangereuse car il est tr`es difficile, en travaillant `a court terme, d’apporter des id´ees originales. Or c’est justement le rˆole des chercheurs d’apporter des id´ees originales qui permettent de trouver de nouvelles orientations pour demain.
La transmission multipoint a pos´e et pose toujours de grands d´efis `a la communaut´e scien-tifique. Cependant, les r´esultats de recherche sur la transmission multipoint ont souvent un champ d’application beaucoup plus large que la transmission multipoint elle mˆeme. L’exemple le plus flagrant est celui des r´eseaux overlay (overlay network). Cette technique permet, entre autres choses, la diffusion de contenus multim´edias sur l’internet. Or si l’on regarde qui sont les pionniers des r´eseaux overlay, on retrouve des sp´ecialistes de la transmission multipoint (par exemple Steven McCanne avec Fast Forward Networks [25] ou J¨org Nonnenmacher avec Castify Networks [10]). La technique des r´eseaux overlay, actuellement appliqu´ee dans l’inter-net, n’aurait sans doute pas ´et´e possible sans la compr´ehension du probl`eme de la diffusion de contenus multim´edias avec la transmission multipoint.
Le paradigme FS, bien que bas´e sur une contrainte r´eseau qui n’est pas r´ealisable pour le
4.2. DISCUSSION SUR LES CONTRIBUTIONS 67 moment, a permis de montrer qu’il ´etait possible de consid´erablement simplifier et am´eliorer la conception des protocoles de contrˆole de congestion. Ce r´esultat fondamental montre qu’il peut ˆetre tr`es profitable de prendre du recul par rapport au paradigme TCP-friendly et va, on l’esp`ere, encourager les recherches dans cette voie.
En conclusion, bien que cette th`ese ne pr´esente pas de solutions que l’on puisse directement appliquer au contexte actuel de l’internet, elle donne un aper¸cu des solutions qui feront, on l’esp`ere, le succ`es des r´eseaux best effort de demain.
Appendix A
Pathological Behaviors for RLM and RLC
Abstract
RLM [55] and RLC [87] are two well known receiver-driven cumulative layered multi-cast congestion control protocols. They both represent an indisputable advance in the area of congestion control for multimedia applications. However, there are very few studies that evaluate these protocols, and most of the time, these studies conclude that RLM and RLC perform reasonably well over a broad range of conditions.
In this paper, we evaluate both RLM and RLC and show that they exhibit fundamental pathological behaviors. We explain in which context these pathological behaviors happen, why they are harmful, and why they are inherent to the protocols themselves and cannot be easily corrected. Our aim is to shed some light on the fundamental problems with these protocols.
Keywords: RLM, RLC, Pathological behaviors, Congestion Control, Multimedia, Multi-cast, Cumulative layers.
A.1 Introduction
Multimedia applications will probably become some of the most popular applications in the Internet. One fundamental problem when introducing a new application in the Internet is to find an efficient way (for both the application and the network) to do congestion control. Cu-mulative layered multicast congestion control protocols are presented as the best solution for the dissemination of multimedia content to a heterogeneous set of receivers (see for instance [55, 87, 85, 50]). Therefore, these applications are the subject of active research.
Steven McCanne et al. introduced the first receiver-driven cumulative layered multicast congestion control protocol called RLM [55]. The behavior of RLM is determined by a state machine where transitions among the states are triggered by the expiration of timers (the join-timer and the detection-join-timer) or the detection of losses. The maintenance of the join-timers and the
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loss estimator are fundamental parts of the RLM protocol. In order to scale with the number of receivers, RLM needs an additional mechanism called shared learning. McCanne evaluated RLM for simple scenarios and only considered inter-RLM interaction. He found that RLM can result in high inter-RLM unfairness. Bajaj et al. [2] explored the relative merits of uniform versus priority dropping for the transmission of layered video. They found that RLM performs reasonably well over a broad range of conditions, but performs poorly in extreme conditions like bursty traffic. Gopalakrishnan et al. [35] studied the behavior of RLM for VBR traffic and show that RLM exhibits high instability for VBR traffic, has very poor fairness properties in most of the cases, and achieves a low link utilization with VBR traffic.
A TCP-friendly version of RLM, called RLC, was introduced by Vicisano et al. [87]. RLC is based on the generation of periodic bursts that are used for bandwidth inference and on synchro-nization points (SP) that indicate when a receiver can join a layer. The TCP-friendly behavior is mainly due to the exponential distribution of the layers that results in an exponential decrease of the bandwidth consumed (like TCP) in case of losses. While the exponential distribution of the layers is not a requirement for the TCP-like behavior if the protocol drops the layers in an exponential way, it considerably simplifies the protocol. We are not aware of any study con-sidering another layer distribution. Vicisano found that RLC can be unfair with TCP for large packet sizes.
According to these previous studies, RLM and RLC seem to perform reasonably well in a broad range of cases. However, in this paper, we evaluate both RLM and RLC with very simple scenarios and show that they exhibit pathological behaviors. We explain in which context these pathological behaviors happen, why they are harmful, and why they are inherent to the protocols themselves and cannot be easily corrected. Our aim is to shed some light on the fundamental problems with RLM or RLC.
The paper is organized as follows. In section A.2 we present the scenarios considered for the simulations. We discuss the results of the simulation for RLM in section A.3, and for RLC in section A.4. We conclude the paper in section A.5.