Re: Adding ECN to Transport and Recovery

Ian Swett <ianswett@google.com> Fri, 08 June 2018 16:40 UTC

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References: <26584f2a-230b-c55e-db16-d32225c8ee4d@ericsson.com> <5a82e9ef-971f-6510-866c-9886e73796a9@ericsson.com> <20180608150833.GB13418@ubuntu-dmitri>
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From: Ian Swett <ianswett@google.com>
Date: Fri, 8 Jun 2018 12:40:35 -0400
Message-ID: <CAKcm_gM_OHgWcJ+ktAg9BCQx1rtHc9GFg2bZG40-NcO2MoVG9w@mail.gmail.com>
Subject: Re: Adding ECN to Transport and Recovery
To: Magnus Westerlund <magnus.westerlund@ericsson.com>, IETF QUIC WG <quic@ietf.org>
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As Dmitri said, the num blocks is currently a varint, it's not a single
byte like it was in GQUIC, so stealing a bit from it is a bit awkward and
would not change the limit on the number of ack blocks from 256 to 128, and
instead it would decrease it from 2^62 to 2^61.

Can you make the least significant bit of the ACK frame type indicate
whether the ECN section is present, such as by using the 0x0d and 0x0e code
points?  This is basically equivalent to defining two types, admittedly,
but by making the least significant bit the indicator, one could write up
the text as a single frame if people prefer that.

On Fri, Jun 8, 2018 at 11:08 AM Dmitri Tikhonov <dtikhonov@litespeedtech.com>
wrote:

> On Fri, Jun 08, 2018 at 04:51:36PM +0200, Magnus Westerlund wrote:
> > My understanding of the views at the interim was that eliminating the
> > additional ACK_ECN frame was desired.
>
> That is my understanding as well.
>
> > I will next update the pull request to use an updated ACK frame with a
> > signaling bit for the present or absence of the ECN counters. The plan
> > is to steal a bit in the ACK Block count and make that into a fixed
> > 7-bit field rather than a variable length encoding.  Limiting an ACK
> > Frame to 128 ACK blocks.
>
> Even though I don't have strong feelings about this, I wonder if there
> is another way.  The ACK frame (Section 7.15 of
> [draft-ietf-quic-transport-12]) is all varints.  Changing a field to a
> fixed encoding would go against the "flavor of the frame," if one may
> say so.  Perhaps we could keep the varint encoding and interpret the
> decoded value as an ORed block count with the ECN bit (the low bit)?
>
> Introducing an artifial limit to where there was none is a separate
> discussion.  Perhaps 128 blocks is really enough for all practical
> purposes.  And yet: at some point the block limit was removed as we
> went from gQUIC to IETF QUIC...
>
>   - Dmitri.
>
>