The dynamics of peer participation, or churn, are an inher-
ent property of Peer-to-Peer (P2P) systems and critical for
design and evaluation. Accurately characterizing churn re-
quires precise and unbiased information about the arrival
and departure of peers, which is challenging to acquire.
Prior studies show that peer participation is highly dynamic
but with conflicting characteristics. Therefore, churn re-
mains poorly understood, despite its significance.
In this paper, we identify several common pitfalls that
lead to measurement error. We carefully address these dif-
ficulties and present a detailed study using three widely-
deployed P2P systems: an unstructured file-sharing system
(Gnutella), a content-distribution system (BitTorrent), and
a Distributed Hash Table (Kad). Our analysis reveals sev-
eral properties of churn: (i) overall dynamics are surprisingly
similar across different systems, (ii) session lengths are not
exponential, (iii) a large portion of active peers are highly
stable while the remaining peers turn over quickly, and (iv)
peer session lengths across consecutive appearances are cor-
related. In summary, this paper advances our understanding
of churn by improving accuracy, comparing different P2P file
sharing/distribution systems, and exploring new aspects of
churn.