Flow control

Yes, TCP flow control is a big, big problem for high-speed links.  For a (commercial but)
quantitative analysis, see http://asperasoft.com/technology/problem/.

It turns out packet loss rate is a poor way to measure congestion on wide-area links, where
there are many causes of packet loss other than congestion.  Fortunately, better measures are
known now.  In places where there's money to spend (particularly e.g. movie & music studios,
lately, but increasingly in military and oil exploration) most data goes via non-TCP
protocols.  For example, all the studios copy music files to iTunes download servers via the
FASP flow control method mentioned in the link above, implemented at user-level on top of UDP,
using ordinary PCs running Linux, Windows, or Macosix.