Gettys: Whose house is of glasse, must not throw stones at another
Gettys: Whose house is of glasse, must not throw stones at another
Jim Gettys has been on the path of a number of network pathologies for some
time; he has now summarized
his findings. The problem: too much buffering in Internet routers.
"The buffers are confusing TCP's RTT estimator; the delay caused by
the buffers is many times the actual RTT on the path. Remember, TCP is a
servo system, which is constantly trying to "fill" the pipe. So by not
signalling congestion in a timely fashion, there is *no possible way* that
TCP's algorithms can possibly determine the correct bandwidth it can send
data at (it needs to compute the delay/bandwidth product, and the delay
becomes hideously large). TCP increasingly sends data a bit faster (the
usual slow start rules apply), reestimates the RTT from that, and sends
data faster. Of course, this means that even in slow start, TCP ends up
trying to run too fast. Therefore the buffers fill (and the latency
rises). Note the actual RTT on the path of this trace is 10 milliseconds;
TCP's RTT estimator is mislead by more than a factor of 100. It takes 10-20
seconds for TCP to get completely confused by the buffering in my modem;
but there is no way back.
"