[tcpm] Some comments on draft-li-tcpm-advancing-ack-for-wireless-00
Gorry Fairhurst <gorry@erg.abdn.ac.uk> Wed, 29 April 2020 13:30 UTC
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From: Gorry Fairhurst <gorry@erg.abdn.ac.uk>
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Date: Wed, 29 Apr 2020 14:30:26 +0100
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Subject: [tcpm] Some comments on draft-li-tcpm-advancing-ack-for-wireless-00
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This email contains comments on comments on draft-li-tcpm-advancing-ack-for-wireless-00 I think this draft touches on a specific topic, but I am unsure of the maturity of the current proposal with respect to the TCPM working group. I suggest you refer to RFC3449, it still contains a list of different in-network modifications to ACKs, which I think remain widely deployed in this type of network. The draft says: “An upper bound of L can also be derived according to the loss rate on the data path (plr_data) and the ack path (plr_ack), i.e., L <= feedback_info/(plr_data*plr_ack), where feedback_info denotes the amount of information carried by an ACK.” - which reads a little like the mechanisms in TCP NCR/ACK-CC/RFC4341. If L is computed from feedback loss information, it can not be set to appropriately compensate for radio resource cost - the ACK rate can grow to fill the available return bottleneck capacity. - In contrast, an L that is based on target asymmetry, e.g. L=10 does not have this drawback, although in our work we argue that this is also important for a large BDP. I agree the value beta = 4 is probably a suitable general update interval, but expect that my thoughts on this are based on sender pacing of the transmission. Have you thought about the pacing requirements of your method? It seems that mitigating bursts is going to be an important design for a stretch-ack proposal, but there are downsides to excessive pacing likely in a wireless case. Have you considering the implications of pacing? Once the cwnd, rwnd >> flow’s required BDP, the effect of L on the growth of cwnd becomes rather small. Of course there can be considerations also in the case of loss for thin flows, where the time to detect a tail loss would be increased by rtt/beta. What do you mean by: “Latency-sensitive transport usually has a small BDP, in the case of application limitation, the latency of thin flows might be enlarged due to a large L.”? When I read this, I did not understand how the two endpoints agreed on the input values that would be needed to perform the mitigation to ACK rate. Changing the ACK frequency dynamically implies the need for signalling, and flows that change between application-limited and cwnd-limited would then need to be considered. Does anything need to be done to avoid oscillation in the ACK policy? One last thing I am curious about is how your proposal will interact with methods such as ACK Thinning at the network layer, I’d hope there are only positive benefits when both schemes are used. Do you have experience of this? Best wishes, Gorry Fairhurst
- [tcpm] Some comments on draft-li-tcpm-advancing-a… Gorry Fairhurst
- Re: [tcpm] Some comments on draft-li-tcpm-advanci… Rahul Jadhav