Conventional wisdom has been that the performance limitations in the
current Internet lie at the edges of the network -- {\em i.e} last
mile connectivity to users, or access links of stub ASes. As these
links are upgraded, however, it is important to consider where new
bottlenecks and hot-spots are likely to arise.  In this paper, we
address this question through an investigation of {\em non-access}
bottlenecks.  These are links within carrier ISPs or between
neighboring carriers that could {\em potentially} constrain the
bandwidth available to long-lived TCP flows.  Through an extensive
measurement study, we discover, classify, and characterize bottleneck
links (primarily in the U.S.) in terms of their location, latency, and
available capacity.

We find that about 50\% of the Internet paths explored have a
non-access bottleneck with available capacity less than 50 Mbps, many
of which limit the performance of well-connected nodes on the
Internet today.  Surprisingly, the bottlenecks identified are roughly
equally split between intra-ISP links and peering links between ISPs.
Also, we find that low-latency links, both intra-ISP and peering, have
a significant likelihood of constraining available bandwidth.
Finally, we discuss the implications of our findings on related issues
such as choosing an access provider and optimizing routes through the
network.  We believe that these results could be valuable in guiding
the design of future network services, such as overlay routing, in
terms of which links or paths to avoid (and how to avoid them) in
order to improve performance.