3.1. From peers to ISPs
According to the definition of request rate, Θi
is the minimum value of the aggregate rate that partition Mi must receive
from the outside such that all the peers of Mi receive a streaming rate supporting a continuous video playout. On the other
hand, if the rate at which data are actually received, i.e., Γi
is smaller than Θi
, there are peers of the ISP that cannot correctly
play the stream. The widespread diffusion of P2P streaming applications yields significant implications for ISPs: on the one
hand the increasing amount of P2P traffic results in additional costs for the ISP, that is requested to provide a satisfactory level
of service to its subscribers (not only the P2P streaming users). On the other hand, these applications represent an important
driving force for the adoption of broadband access, which is a significant source of revenue for the ISPs. A successful strategy
to manage the traffic generated by P2P streaming applications must address both the ISP perspective of costs and the
subscriber perspective of quality of service. From the ISP side the rate at which data are received, i.e., Γi
, represents a gage
of the offered quality of service. That is, if Γi ≥ Θi
, the P2P ISP subscribers may correctly receive the streaming. Obviously
when Γi < Θi there are peers belonging to the ISP that cannot correctly play the stream. Hence the ISP aiming at fulfilling its
requests tries to get an appropriate Γi
. Note that the reduction of the ISP requests, i.e., Θi can be pursued by using techniques
enforcing locality. We define locality awareness as the peer ability to exploit local resources (e.g., chunks) over remote ones
whenever possible (see for instance [9] or [3]). Hence we have that an ISP defines its policies for selecting peer neighborhood
and this has an impact on the ISP requests, i.e., on Θi
.
On the other hand the costs due to content uploads can be drastically reduced by setting Φi = 0. This can be obtained
by dropping all the communications from peers that belong to the ISP towards peers that are outside (e.g., bandwidth
throttling). It is clear that this extreme ISP free-riding behavior cannot be socially acceptable from the point of view of the
P2P application.
If we assume that the resources offered by an ISP (and by the streaming server) can be used by any other ISP we have
that a universal streaming1
service is possible iff