Well, there is a (real-world) case where your TCP proxy doesn't want
to look at the data and you can't use IP forwarding. If you have TCP
connections between networks that have very different MTU's, using IP
forwarding will result in tiny packets on the large MTU networks.
So who cares? Some machines, notably Crays and NEC's, have a severely
rate-limited network stack and can only transmit up to about 3500
packets per second. That's 40 Mbps on a 1500 byte MTU network, but
greater than line speed on HIPPI (65280 MTU, 800 Mbps).
So, for a rate-limited network stack on a HIPPI network, the best way
to talk to a machine on a gigabit ethernet network is through a TCP
proxy which just doesn't care about the data going through it. Hence
my interest in socket->socket sendfile().
I'll admit this is an odd corner case which isn't important enough to
justify socket->socket sendfile() on its own. But this odd corner
case did make enough money to pay my salary for years to come. :)
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