On Wed, 7 Oct 2015, Jonathan Morton wrote:
On 7 Oct, 2015, at 23:40, Agarwal, Anil <anil.agar...@viasat.com> wrote:
Since the cable modem link will lead to clumped ACKs the difference between
sending 100 ACKs vs. 1 ACK is probably not that big... (except w.r.t.
reliability).
The difference may not be big in the spacing of new packets that a sender
will send, unless the sender implements some sort of pacing or if the return
link is very thin.
But with ABC, there will be a difference in the amount of cwnd increase at
the sender.
There is also a potential difference for detecting packet loss in the forward
direction. It’s entirely possible that thinning would cause a DupAck
condition to be recognised only after three MAC grants in the reverse
direction have elapsed, rather than one. Receivers are REQUIRED to send an
ack for every received packet under these conditions, but that would be
subverted by the modem. AckCC would not induce this effect, because the
receiver would still produce the extra acks as required.
Packet loss causes head-of-line blocking at the application level, which is
perceived as latency and jerkiness by the end-user, until the lost packet is
retransmitted and actually arrives. Hence the addition of two MAC grant
delays (60ms?) may make the difference between an imperceptible problem and a
noticeable one.
and excessive ack traffic causes congestion and results in packet loss on
real-world hightly asymmetric links.
So things that reduce the flow of acks can result in very real benefits to
users.
David Lang
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