Bandwidth for High Speed Networks

22 packet algorithm 2 by island we mean a contiguous

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Unformatted text preview: ate of exactly t. This suggests a very simple probabilistic forwarding algorithm that achieves fair allocation of bandwidth: each incoming bit of ow i is dropped with the probability t max 0; 1 , r t 2 i When these dropping probabilities are used, the arrival rate of ow i at the next hop is given by min ri t; t . 2.2 Packet Algorithm The above algorithm is de ned for a bu erless uid system in which the arrival rates are known exactly. Our task now is to extend this approach to the situation in real routers where transmission is packetized, there is substantial bu ering, and the arrival rates are not known. Edge Router Flow 1 Core Router Rate Estimator + Packet Labeling . . . Flow n buffer occupancy Packet Dropping Rate Estimator + Packet Labeling Estimator Figure 1: The architecture of the output port of an edge router, and a core router, respectively. We still employ a drop-on-input scheme, except that now we drop packets rather than bits. Because the rate estimation described below incorporates the packet size, the dropping probability is independent of the packet size and depends only, as above, on the rate ri t and fair share rate t. We are left with two remaining challenges: estimating the rates ri t and the fair share t. We address these two issues in turn in the next two subsections, and then discuss the rewriting of the labels. Pseudocode re ecting this algorithm is described in Figure 2. We should note, however, that the main point of our paper is the overall architecture and that the detailed algorithm presented below represents only an initial prototype. While it serves adequately as a proof-of-concept of our architecture, we fully expect that the details of this design will continue to evolve. 2.2.1 Computation of Flow Arrival Rate Recall that in our architecture, the rates ri t are estimated at the edge routers and then these rates are inserted into the packet labels. At each edge router, we use k exponential averaging to estimate the rate of a th ow. Let ti and lik be the arrival...
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