[LOOPS] Transport recovery and interaction with, link, and LOOPS loss recovery
Gorry Fairhurst <gorry@erg.abdn.ac.uk> Mon, 27 July 2020 15:22 UTC
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From: Gorry Fairhurst <gorry@erg.abdn.ac.uk>
To: loops <loops@ietf.org>
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Date: Mon, 27 Jul 2020 16:21:37 +0100
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Subject: [LOOPS] Transport recovery and interaction with, link, and LOOPS loss recovery
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Thank you for taking the trouble to update draft-li-tsvwg-loops-problem-opportunities, the new text does help in many places, and I'm sorry not to have responded earlier to this new text. I am still not clear yet what is antcipated from the transport congestion-control response. Please see comments below: (1) Moving figure 1 seems helpful to introduce the context. Thanlks. (2) In Section 6: I read section 6 as describing two recovery cases: One is about DupACK-based recovery. The other says it is about RACK recovery, I wonder if this is mainly the case where the dupACK, where I think this is about TLP (which uses RACKL) to short-cut the RTO by trying to recover after ~2RTTs? (3) I think the text suggests these might have different implications on LOOPS, which seems reasonable, although I have questions: “If LOOPS network does not buffer the out-of-order packets caused by packet loss, TCP sender which uses a time based loss detection like RACK [I-D.ietf-tcpm-rack] will perform well here. It uses the notion of time to replace the conventional DUPACK threshold approach to detect losses. Hence it prevents the TCP sender from invoking fast retransmit too early. “ (4) In Section 4.1: “When a sender does not receive an ACK for a given segment within a certain amount of time called retransmission timeout (RTO), it re- sends the segment [RFC6298]. RTO can be as long as several seconds.” … “Even when the lost packet is not an exact tail, it can possibly add another RTT because there may not be enough packets in flight to trigger the fast retransmit).” - I think this text argues simply for TLP? - How would the method imporve performance, if TLP were to be widely deployed, since it's been promoted for TCP and specified for QUIC? (5) Also, maybe I misunderstand, can’t RACK trigger retransmission earlier than timeout for a TLP - which the start of the ID said was an important use-case? “Local retransmission will not interfere the sender's retransmission generally in this case. If time based loss detection is not supported at the sender, end to end retransmission may be invoked as usual. It consumes extra bandwidth Because the lost packets (i.e. recovered packet) is normally a very small percentage of the total packets. Then extra bandwidth cost is not significant.” - I don’t really understand this argument. Is this saying there will be few losses, and hence the additional capacity used by retransmission is small. If that was my correct interpretation, how does LOOPS know there are few losses? Does it measure this and rate-control it’s retransmissions to ensure this, or maybe some other method is used to avoid there being significant load from two levels of retransmission? (6) Is some sort of check needed anyway perhaps? Because LOOPS could potentially work over a path with L2 retransmissions (e.g. WiFi, and any loops retransmissions multiply the number of L2 retransmissions? - Can LOOPS be tunnelled over LOOPS in any cases? (7) I now think I might understand the scope of the proposed work is constrained only to cases where ECN has been deployed and is enabled end-to-end. In Section 5.2: “LOOPS can CE(Congestion Experienced) marks its recovered packets as the loss signal to end-to-end. Converting a packet loss signal to CE marking signal brings the benefits of reducing Head-of-Line blocking and probability of RTO expiry [RFC8087] without affecting TCP sender's loss based congestion control behaviour while enjoying the faster local recovery. ECN based indication is equivalent to a loss event at the TCP sender [RFC3168].” … “ In this way, a requirement is set for applying LOOPS.” I still think this would benefit from being clearer at the start of the document, because only ECT (ECN-Capable Transport) flows should be directed to an LOOPS enabled path segment, as I think it says? Best wishes, Gorry
- [LOOPS] Transport recovery and interaction with, … Gorry Fairhurst
- Re: [LOOPS] Transport recovery and interaction wi… Liyizhou
- Re: [LOOPS] Transport recovery and interaction wi… Gorry Fairhurst