Re: [aqm] ACK Suppression

[Jonathan Morton <[e-mail]>]

> On 7 Oct, 2015, at 23:40, Agarwal, Anil <[e-mail]> 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.

 - Jonathan Morton