We call end-hosts behind Network Address Translator (NAT) gateways or firewalls guarded hosts, and otherwise open hosts. Due to mounting security threats and growing population of high-speed Internet users equipped with connection sharing, guarded hosts become increasingly common in today's Internet. However, the presence of guarded hosts has a detrimental effect on the performance of peer-to-peer file sharing systems as they may disallow incoming connections initiated by remote peers to download files or to build an overlay. In this paper, we empirically measure the prevalence of guarded hosts in two popular peer-to-peer file sharing systems, eDonkey and Gnutella, and study the characteristics of their shared files. By performing passive and active probes, we found that about 25-36% of eDonkey and Gnutella users reside on guarded hosts and that the ratio of files shared by guarded hosts is also non-trivial. When discounting guarded hosts, we found that a popular file's availability, i.e., the number of copies available for download, decreases by 25-30%. Our measurement study testifies to the significant impact guarded hosts may have on the performance of current peer-to-peer file sharing systems, and points to a need to consider their presence when designing next generation peer-to-peer systems.
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