On Wed, 7 Oct 2015, [email protected] wrote:
Date: Wed, 7 Oct 2015 15:52:18 -0400 (EDT)
From: [email protected]
To: [email protected]
Subject: Re: [aqm] ACK Suppression
I presume this is not a serious focus of the AQM group. So maybe I'm wasting
my time by writing about this.
Nonetheless I am troubled by the very fact of the discussion taking place, for
two reasons:
1) TCP ACKs are TCP's business only. It's not a gateway or router's job to
get involved in or to understand end-to-end protocols, *even if* the router
thinks it knows exactly what the endpoints' goals are. And the router cannot
know that for every protocol, not even the many higher level protocols on top
of TCP, which use TCP quite differently. The idea that routers can be
omniscient, merely by looking at packets and taking the designers' personal
prejudices into account, seems ridiculous. TCP endpoints on both ends of a
connection can reduce the number of ACKs they send if they want. If ACKs are
filling up buffers in intermediate routers, just drop them or mark them to
notify that they are contributing to congestion. The endpoints have to slow
down something, and they can slow down ACKs by mutual agreement.
2) The hypothetical that there will be a sufficiently long sequence of ACKs
for a single end-to-end flow buffered in a single router queue may seem
plausible, *until it becomes clear that in the big picture, having so many
packets in a queue means that the network is extremely congested by that
point*. In other words, in order for this "optimization" to apply, you would
have to operate the network at an unacceptable operating point! It's like
saying that when a highway has slowed to a crawl, we can load all the cars
going to a particular destination onto single "car carrier" to save gas. Far
better to insure that queues are not built up! The purpose of queueing is to
absorb bursts that can't be anticipated, not to build up congestion in order
to have enough data to perform a dubious optimization that can best be done at
the source of traffic in cooperation with the destination.
Look at Wifi, you have close to 1Gb/s data rates, but it's still half-duplex. So
while a station/AP is receiving, there can be a substantial number of ack
packets that queue up while recieving even a 'small' bundle of data packets.
This is normal operation, not 'operate the network at an unacceptable operating
point'
David Lang
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