2: More complex topologies with seven hops. In these cases, the link information up to
hop h4 is directly conveyed by the overlay network infrastructure, while other link information
is inferred based on theoretical estimates using average or past information (where mij
indicates the estimated or average value for the metric mij, m = {g,e,d}).
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Under the existence of feedback from an overlay network infrastructure, the BER and
queuing delay per link can be disseminated to the remaining network hops at frequent
intervals (via a hop-to-hop feedback mechanism3), or when the incurred change in network
parameters is larger than a preset threshold. Thus, they can be considered to be known
(Figure 5.1). However, in certain cases, feedback from remote hops may arrive with an
intolerable delay, or, alternatively, it can be deemed unreliable due to the rapidly-changing
network conditions. As a result, a certain ???horizon??? of information retrieval can be
envisaged for each hop (Figure 5.2), where network information within the horizon is
deemed reliable and can be received in a timely manner, while information beyond the
horizon can only be theoretically estimated based on average or previous measurements.
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