odd tcp ack compression when using a lan

[Adon <ahwang@fas.harvard.edu>]

running the test script test-suite-lan.tcl under ns-2.1b5, after
increasing the tcp advertised window to something large (say 1500), the
ack's are strangely compressed when returning to the sender.

for example, if you view the nam animation of this script's output, at
around 0.18 seconds into the playback, i see ack's coming back to the
router feeding the sender, but the ack's are held up and then compressed
into one gigantic feed.

this happens at many other times.

this problem disappears when using normal point-to-point links instead of
a 802.3 lan.

the node at the lan border seems to hold on to the ack packets received
from the lan till the node is finished sending packets into the lan.


any suggestions or pointers would be welcome.

thanks,
adon