Quoting r. David S. Miller [off-list ref]:

The path an SKB can take is opaque and unknown until the very last
moment it is actually given to the device transmit function.

Why, I was proposing looking at dst cache. If that's NULL, well,
we won't stretch ACKs. Worst case we apply the wrong optimization.
Right?

People need to get the "special case this topology" ideas out of their
heads. :-)

Okay, I get that.

What I'd like to clarify, however: rfc2581 explicitly states that in some cases
it might be OK to generate ACKs less frequently than every second full-sized
segment. Given Matt's measurements, TCP on top of IP over InfiniBand on Linux
seems to hit one of these cases.  Do you agree to that?

-- 
Michael S. Tsirkin
Staff Engineer, Mellanox Technologies