The Internet consists of many administrative domains, or
\emph{Autonomous Systems} (ASes), each owned by an economic entity
(Microsoft, AT\&T, The Hebrew University, etc.). The task of
ensuring interconnectivity between ASes, known as \emph{interdomain
routing}, is currently handled by the \emph{Border Gateway Protocol}
(BGP).
ASes are self-interested and might be willing to manipulate BGP for
their benefit. In this paper we present the strategic justification
for using BGP for interdomain routing in today's Internet: We show
that, in the realistic Gao-Rexford setting, BGP is immune to almost
all forms of rational manipulation by ASes, and can easily be made
immune to all such manipulations. The Gao-Rexford setting is said to
accurately depict the current commercial relations between ASes in
the Internet. Formally, we prove that a slight modification of BGP
is incentive-compatible in \emph{ex-post Nash equilibrium}.
Moreover, we show that, if a certain reasonable condition holds,
then this slightly modified BGP is also \emph{collusion-proof} in
ex-post Nash -- i.e., immune to rational manipulations even by
\emph{coalitions} of \emph{any} size.
Unlike previous works on achieving incentive-compatibility in
interdomain routing, our results \emph{do not require any monetary
transfer between ASes} (as is the case in practice). We also
strengthen the Gao-Rexford constraints by proving that one of the
three constraints can actually be enforced by the rationality of
ASes if the two other constraints hold