  In this paper, we aim to model the formation of data dissemination in online
social networks (OSNs), and measure the transport difficulty of generated data
traffic. We focus on a usual type of interest-driven social sessions in OSNs,
called \emph{Social-InterestCast}, under which a user will autonomously
determine whether to view the content from his followees depending on his
interest. It is challenging to figure out the formation mechanism of such a
Social-InterestCast, since it involves multiple interrelated factors such as
users' social relationships, users' interests, and content semantics. We
propose a four-layered system model, consisting of physical layer, social
layer, content layer, and session layer. By this model we successfully obtain
the geographical distribution of Social-InterestCast sessions, serving as the
precondition for quantifying data transport difficulty. We define the
fundamental limit of \emph{transport load} as a new metric, called
\emph{transport complexity}, i.e., the \emph{minimum required} transport load
for an OSN over a given carrier network. Specifically, we derive the transport
complexity for Social-InterestCast sessions in a large-scale OSN over the
carrier network with optimal communication architecture. The results can act as
the common lower bounds on transport load for Social-InterestCast over any
carrier networks. To the best of our knowledge, this is the first work to
measure the transport difficulty for data dissemination in OSNs by modeling
session patterns with the interest-driven characteristics.
