Tom Samplonius wrote:
FreeBSD does not send PAUSE frames, but most of the NICs out there will
process received PAUSE frames.
However ethernet flow control is mainly useful on the switches anyways.
Switches these day (*) have small buffers (sometimes just a shared 3MB buffer
for 24 ports). If several ports send too much to a single port at the same
time, the switch could run out of buffer space in milliseconds and start
dropping frames, so the switch will use flow control to get the transmitting
ports to slow down, and let the buffer drain. Slowing down the senders with
ethernet flow control results in better performance than letting some frames
drop, and force a timeout and retransmit. Of course, deeper buffers would be
even better.
There are switch ASICs out there which implement upstream bandwidth
limiting on ports by sending the PAUSE frame.
I believe thompsa@ recently committed a fix to if_bridge to allow it to
ignore PAUSE frames for the purposes of forwarding (they should never be
forwarded, they are link scope only).
It would be interesting to see a patch which implemented rate limiting
in the same way as these ASICs do, but in if_bridge.
cheers
BMS
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