Hi,
after noticing that some TCP ACKs on my home DOCSIS connection were not
making it to their destination, I after some interaction with cable
Internet people, I found this:
http://www.cedmagazine.com/article/2006/12/docsis-sub-throughput-optimization
"TCP ACK Suppression (TAS)"
"TCP ACK Suppression overcomes the TRGC limitation without actually
affecting the DOCSIS specification or involving the CMTS. It improves
downstream TCP transmissions by taking advantage of TRGC and only sending
the last ACK it receives when its data grant becomes active. Thus, the
number of TCP ACKs is fewer, but the number of bytes acknowledged by each
TCP ACK is increased."
So the DOCSIS modem basically looks at all the ACKs in the queue at the
time of transmission (DOCSIS uses a "grant" system to tell a modem when
it's allowed to transmit on the shared medium), and then basically deletes
all the redundant ACKs (the ones who are just increasing linearly without
indicating packet drop) and keeps the highest ACK only.
Now, this kind of mechanism, how should it be treated when it comes to
AQM? This mechanism is basically done at de-queue, when a number of
packets are emptied from the queue at one time, which is then allowed to
fill up again until the next transmit opportunity arises.
Or is this a non-problem because it's likely that any AQM employed here
would use the buffer fill right after a transmit opportunity has finished
(for those that consider buffer fill as a variable), which would mean that
most likely the TCP ACK purging had already occured so this mechanism
doesn't influence the AQM in any significant manner anyway?
Just as a data point from my home connection, I have 250/50 (down/up) and
when downloading at 250 megabit/s, the upstream traffic is reduced by
approximately 20x, so instead of sending 10 megabit/s (or so) of ACKs, I
see approximately 500 kilobits/s of ACKs.
--
Mikael Abrahamsson email: [email protected]
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