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 droponinput 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
proofofconcept 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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 Fall '10
 Eugene Ng
 Scheduling algorithm, Roundrobin scheduling, Scheduling algorithms, Ow, Fair queuing, ows

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