Re: comments on QUIC Recovery -24.
Ian Swett <ianswett@google.com> Sat, 16 November 2019 11:05 UTC
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From: Ian Swett <ianswett@google.com>
Date: Sat, 16 Nov 2019 06:05:29 -0500
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Subject: Re: comments on QUIC Recovery -24.
To: G Fairhurst <gorry@erg.abdn.ac.uk>
Cc: "quic@ietf.org" <quic@ietf.org>
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Thanks for the review, comments below and fixes in PR #3237 <https://github.com/quicwg/base-drafts/pull/3237> On Thu, Nov 14, 2019 at 6:19 AM G Fairhurst <gorry@erg.abdn.ac.uk> wrote: > QUIC Recovery > > I have comments on QUIC Recovery -24. These sets of come join two parts, > this is the first. I hope most of the comments below are easy to resolve: > > Section 3, Para 1 last sentence. This sentence is true from the sender > perspective, but the monotonic increase is not necessarily true at the > receiver. > Fixed. > > Section 3.1.2, Para 1 > /is declared is acknowledged/ > - maybe would be easier to read as /is declared as acknowledged/. > > These were 3.1.3, not 3.1.3 FYI. This has already been improved to "QUIC starts a loss epoch when a packet is lost and ends one when any packet sent after the epoch starts is acknowledged." in the editor's copy. > Section 3.1.2, Para 1 > /QUIC will do it correctly once/ > - There is an inference that TCP does not, and I would argue with that, > because TCP measures the TCP loss recovery, that is often a larger > interval than it could have been had TCP been designed differently, but > this document is not a thesis on TCP. I would like to suggest / QUIC > will appropriately reduce once/ as a more neutral text. > > Removed "correctly" Section 3.1.6, Para 1 > /allowing a peer to maintain a more accurate/ is I think an unnecessary > judgment on TCP again, it would be fairer to say/allowing a peer to > maintain an accurate/ > I disagree. This is a new mechanism(explicit ack delay in the ACK frame) to improve the accuracy of RTT which TCP does not have. If it's not more accurate, QUIC is wasting bytes sending it. > The term /host delay/ appears several times, but there is no discussion > of hosts, only endpoints. The term host delay seems odd, can we call it > endpoint delay or perhaps even “ACK delay” which I think is closer to > the field used for this later in the spec? > Good point. It seems like ACK delay is a reasonable substitute, but you can verify in my PR. > > Section 4.1, final sentence > /Ensuring that RTT estimates retain sufficient history is an open > research question/ > - This is fine for an IRTF experiment, but really rests very > uncomfortably in a standards track specification. I suggest this alone > is not sufficient. An alternate could be to set some bound here that at > least mitigates this - which could otherwise result in odd pathologies. > I'm not sure how to address this, but I also don't have a problem with the existing text. Do you have a specific suggestion? > > Somewhere there is this (it's in several places): > /smoothed RTT = latest RTT/ > - This needs to cite an RFC. > RFC6298 is cited twice in section 4 "Estimating the Round-Trip Time", particularly in regards to smoothed RTT. Is there another RFC you have in mind? - I'm not a huge fan of just assigning a RTT (without taking account of > the variance) as a starting timeout. We've in the past been caught so > many times with things being slightly or vastly different when the next > flow starts and simply assigning the value seems like it could be the > wrong thing to do. > The timeout is twice the initial RTT if there are no RTT samples or the PTO(SRTT+4*RTTVAR) if there is an RTT sample. Which of those cases concerns you? I believe Issue #2789 <https://github.com/quicwg/base-drafts/issues/2789> is open for this? > Section 4 pseudo code uses both Ack_Delay and ack_delay, I don’t > understand the difference. > I can't find a spot in the psuedocode where Ack_Delay is used, but I do see the use of "Ack Delay" in some text and ack_delay in others. Are you suggesting changes there? > > Section 5 > This uses /ack/ and /ACK/ > - is there a difference? > ACK frame is used in some cases and acknowledgement should be used in others. I tried to fix the issues I found in the PR. > > Section 5.1. > /was sent long enough in the past/ > - This isn’t clear, please improve the text here to explain what is > intended. > > Section 5.1.2 is referenced in this section and explains this in detail. Do you have a suggested reword for this sentence? > /sent later was delivered, while …. / > - Text should be improved. I suggest /while/ is reworked to avoid the > inference these occur in parallel. > 'while' changed to 'and'. > > Section 5.1.1 > There is no minimum dupack threshold specified. Why not? I could be > persuaded that the recommendation was for 3, and that more is possible, > but when would less than 3 fit with current other recommendations in the > RFC series ? > - also mentioned in B.1 > Is there an RFC I can cite that specifies a minimum? 5681 specifies 3, but I can't see any discussion of minimum. > > Also 5.4 final sentence > - section 6 (last para -1) reads like this is intended as one “probe” > packet, XXX what does RACK do in this cased with respect to CC? > Maybe you can quote the sentence and clarify your concerns? I'm not sure of the question here. > > Section 5.3, final sentence: > /may never be/ > - seems like it would avoid all possible ambiguity in whether this is > allowed by using /could/. > It's certainly allowed. Are you suggesting 'could never be'? > > /Sending two packets on a PTO expiration increases resilience…/ > - and arguably doubles the non-congestion-controlled load. > Section 6.1 > /endpoints are permitted to experiment/ > - This is much more than intent of RFC8311. The intent was that the IETF > can publish experimental specs for other behaviours. That’s not the > same, and I think the wording needs to be modified a little to reflect > the actual consensus of the RFC. > Do you have suggested text? > > Section 6.2 > /anytime/any time/ > > Good call. > Section 6.9 > - the new text on pacing seems to be heading in the correct direction, > but I don’t really understand how the congestion window will be managed > if pacing is used and the connection becomes application-limited. The > basic TCP spec would respond aggressively reducing the window; cwv would > loosen this; what does QUIC say when pacing *is* used. > I believe this is the relevant text: A sender MAY implement alternative mechanisms to update its congestion window after periods of under-utilization, such as those proposed for TCP in [RFC7661]. ie: You can reduce it, but you don't have to. > Section 7.3 > /as having being marked/ should I think be /as having being CE-marked or > lost/ > - add loss. > > I believe "when the packet is acknowledged" takes care of this case, since if a packet is acknowledged it is not lost. > section A.1 para 2. > - I suggest text is added that mentions the actual reordering could be > larger and how QUIC handles this. > Larger than what? 1RTT? > Section A4 - could the initialisation be clearer? - e.g. showing initial > values for RTT etc. > The initial RTT never changes during a connection, since it's only used prior to measuring the RTT, so the pseudocode uses kIntialRtt and smoothed_rtt is initialized to 0 to indicate its unknown.
- comments on QUIC Recovery -24. G Fairhurst
- Re: comments on QUIC Recovery -24. Ian Swett
- Re: comments on QUIC Recovery -24. Ian Swett
- Re: comments on QUIC Recovery -24. G Fairhurst