  Peer-to-peer (P2P) locality has recently raised a lot of interest in the
community. Indeed, whereas P2P content distribution enables financial savings
for the content providers, it dramatically increases the traffic on inter-ISP
links. To solve this issue, the idea to keep a fraction of the P2P traffic
local to each ISP was introduced a few years ago. Since then, P2P solutions
exploiting locality have been introduced. However, several fundamental issues
on locality still need to be explored. In particular, how far can we push
locality, and what is, at the scale of the Internet, the reduction of traffic
that can be achieved with locality? In this paper, we perform extensive
experiments on a controlled environment with up to 10,000 BitTorrent clients to
evaluate the impact of high locality on inter-ISP links traffic and peers
download completion time. We introduce two simple mechanisms that make high
locality possible in challenging scenarios and we show that we save up to
several orders of magnitude inter-ISP traffic compared to traditional locality
without adversely impacting peers download completion time. In addition, we
crawled 214,443 torrents representing 6,113,224 unique peers spread among 9,605
ASes. We show that whereas the torrents we crawled generated 11.6 petabytes of
inter-ISP traffic, our locality policy implemented for all torrents could have
reduced the global inter-ISP traffic by up to 40%.
