Re: comments on QUIC Recovery -24.

Ian Swett <ianswett@google.com> Sat, 16 November 2019 11:05 UTC

Return-Path: <ianswett@google.com>
X-Original-To: quic@ietfa.amsl.com
Delivered-To: quic@ietfa.amsl.com
Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id CD3D91200CD for <quic@ietfa.amsl.com>; Sat, 16 Nov 2019 03:05:46 -0800 (PST)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -17.5
X-Spam-Level:
X-Spam-Status: No, score=-17.5 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIMWL_WL_MED=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, ENV_AND_HDR_SPF_MATCH=-0.5, HTML_MESSAGE=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, USER_IN_DEF_DKIM_WL=-7.5, USER_IN_DEF_SPF_WL=-7.5] autolearn=ham autolearn_force=no
Authentication-Results: ietfa.amsl.com (amavisd-new); dkim=pass (2048-bit key) header.d=google.com
Received: from mail.ietf.org ([4.31.198.44]) by localhost (ietfa.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id PvFFuWgaiBnF for <quic@ietfa.amsl.com>; Sat, 16 Nov 2019 03:05:43 -0800 (PST)
Received: from mail-ua1-x92c.google.com (mail-ua1-x92c.google.com [IPv6:2607:f8b0:4864:20::92c]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id 7129C120019 for <quic@ietf.org>; Sat, 16 Nov 2019 03:05:43 -0800 (PST)
Received: by mail-ua1-x92c.google.com with SMTP id z9so3812795uan.3 for <quic@ietf.org>; Sat, 16 Nov 2019 03:05:43 -0800 (PST)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=06sl8StJQZ7csImMXJqnA61Mxa75nKiqqFX2+8xjOlA=; b=NjdfcdOTTshvDnmJQ9NZmF0Ymis5pWejQwZbeP4StbVfuTvSiDcIbLc/bghIufNQ5b Y7DT3sbxZ7deSmLHISO1fGrvV73SfWYNpg/74gilkT4I7J2swnvXFDOxSC23UungXJoa t2y9SgttxzVLE5WvPVYY3eftCQIR2KQVBMylWak6H1xVF46msvrggUnQtu2/9Slu/030 Yj8nkjwcz5b8l1OB28aQpsG0pLV+bXjlCbZeA6OfAyCdy4kRlWoZoQUEzVXX2tQitz28 JlH977YAko49waB2k2fLtsneAJCUyXdXri4N07y/7/qdwezEEGYNJr1nv2vsed7+/YF+ Ix4g==
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=06sl8StJQZ7csImMXJqnA61Mxa75nKiqqFX2+8xjOlA=; b=P+tfjgHqySNdY11djGxPrWJ6qsBwkHKYxgF4ec20PefsRNw8qffipKcpvlAHRRzKtQ yLoZGXRn/rgpuwtsg3v7LNPFhAN2p6mX7S0Bwa8DUzD2VMZGdYCa92PR/5JFepUEwCA8 xZRY4pmeYcVuxlugJgK+wD8r294UQSf9ByzwsmTWrcuaoO6ZNCc5RLBJSwpW+Qg5vWvN KquzLrjYQsg6K56p1/bB9ksXYlQjfSjKh5B8+u29Pg5QTUZZX6EznKjTF/JCa8ae4jJh w1q6Zd9ExPnE/BKv8aICJlQm2iJgkavj+3PY9tn8cYAkKrk3+9NRKCixSX9/yAY3WYpY g8+w==
X-Gm-Message-State: APjAAAXWDReTkJFiB9mGmpk0KffxJagpI1JzKyE5qlas8EadC3d4qmjr dB4VCtKbGh0tS+cyKHINoniE/rypRI6RfzPB9u4xwocUGXbHgxsX
X-Google-Smtp-Source: APXvYqz2flhVEF5QMGqWQL5x/Bw/u8Vla2rh8NIbjfb4+aHQpXaIOJOeijKgZqQwZQK230HujnJJZ9yGDgAL2h0+CjY=
X-Received: by 2002:a9f:27a4:: with SMTP id b33mr11982678uab.24.1573902341822; Sat, 16 Nov 2019 03:05:41 -0800 (PST)
MIME-Version: 1.0
References: <5DCD3819.5050300@erg.abdn.ac.uk>
In-Reply-To: <5DCD3819.5050300@erg.abdn.ac.uk>
From: Ian Swett <ianswett@google.com>
Date: Sat, 16 Nov 2019 06:05:29 -0500
Message-ID: <CAKcm_gNX8oMa=0q-RmCQXfwESksS+VH7ZEZhFB23uNM4LCHOLw@mail.gmail.com>
Subject: Re: comments on QUIC Recovery -24.
To: G Fairhurst <gorry@erg.abdn.ac.uk>
Cc: "quic@ietf.org" <quic@ietf.org>
Content-Type: multipart/alternative; boundary="000000000000ff5f75059774b1d4"
Archived-At: <https://mailarchive.ietf.org/arch/msg/quic/2TecSR1s4f9Syp11P9SMj_UlHV0>
X-BeenThere: quic@ietf.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Main mailing list of the IETF QUIC working group <quic.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/quic>, <mailto:quic-request@ietf.org?subject=unsubscribe>
List-Archive: <https://mailarchive.ietf.org/arch/browse/quic/>
List-Post: <mailto:quic@ietf.org>
List-Help: <mailto:quic-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/quic>, <mailto:quic-request@ietf.org?subject=subscribe>
X-List-Received-Date: Sat, 16 Nov 2019 11:05:47 -0000

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.