Reliable Multicast and active networking

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Multicast is the process of sending every single packet to multiple destinations. Motivations behind multicast facilities are to handle one-to-many communications in a wide-area network with the lowest network and end-system overheads. In contrast to best-effort multicast, that typically tolerates some data losses and is more suited for real-time audio or video for instance, reliable multicast requires that all packets are safely delivered to the destinations. Desirable features of reliable multicast include, in addition to reliability, low end-to-end delays, high throughput and scalability.

In active networking, routers themselves play an active role by executing application-dependent services on incoming packets. Recently, the use of active network concepts where routers themselves could contribute to enhance the network services by customized functionalities have been proposed in the multicast research community . Contributing mainly on feedback implosion problems, retransmission scoping and cache of data, these active reliable multicast protocols open new perspectives for achieving high throughput and low latency on wide-area networks:

DyRAM  "Dynamic Replier Active reliable Multicast"

DyRAM is a reliable multicast protocol with a recovery strategy based on a tree structure constructed on a per-packet basis with the assistance of routers. It is an incremental step from the existing propositions but it has been designed to provide low-overhead active functionalities with a dynamic replier election on a per-packet basis. The protocol uses a NACK-based scheme with receiver-based local recoveries where receivers are responsible for both the loss detection and the retransmission of repair packets. Routers play an active role in DyRAM which consists in the following services: The local recovery in DyRAM is performed by repliers elected amongst a sub-set of receivers. Instead of an approximate solution based on timers as in SRM, or a complex DLR discovery as in PGM, the elected replier in our case is dynamically determined at each lost packet (without much overhead as it will be described later on), and not determined at the beginning of the multicast session, thus justifying the ``dynamic replier'' property of our active reliable multicast protocol (noted DyRAM). Since a specific replier can be chosen for each lost packet, it is therefore possible to have several logical subtrees at the same time for the recovery process. In addition, this very dynamic choice of the replier provide load balance features that decreases the end-to-end latency and reduce the receiver overhead.  As opposed to LMS and PGM, DyRAM uses the concept of active networking with active services within routers for implementing several advanced mechanisms including the replier election.

Motivations behind DyRAM can be summerized in th following design goals :


    Moufida Maimour , PhD student (sep. 2001-nov 2003), now at University of Paris 13.
    Cong-Duc Pham , Assistant Professor.



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