Re: [tcpm] I-D Action: draft-ietf-tcpm-dctcp-09.txt

Neal Cardwell <ncardwell@google.com> Thu, 20 July 2017 15:37 UTC

MIME-Version: 1.0
In-Reply-To: <150026900560.32439.2656659494607932107@ietfa.amsl.com>
References: <150026900560.32439.2656659494607932107@ietfa.amsl.com>
From: Neal Cardwell <ncardwell@google.com>
Date: Thu, 20 Jul 2017 11:36:54 -0400
Message-ID: <CADVnQyk5Dhi9cfmdwmabTNZdRds_ozmj2ca2SEogoVPimVEhMw@mail.gmail.com>
To: Stephen Bensley <sbens@microsoft.com>, "Eggert, Lars" <lars@netapp.com>
Cc: "tcpm@ietf.org" <tcpm@ietf.org>
Content-Type: text/plain; charset="UTF-8"
Archived-At: <https://mailarchive.ietf.org/arch/msg/tcpm/NGFA-JBOoAFO_egjxbDmhv46fy4>
Subject: Re: [tcpm] I-D Action: draft-ietf-tcpm-dctcp-09.txt
Precedence: list

On Mon, Jul 17, 2017 at 1:23 AM,  <internet-drafts@ietf.org> wrote:
>
> A New Internet-Draft is available from the on-line Internet-Drafts directories.
> This draft is a work item of the TCP Maintenance and Minor Extensions of the IETF.
>
>         Title           : Datacenter TCP (DCTCP): TCP Congestion Control for Datacenters
>         Authors         : Stephen Bensley
>                           Dave Thaler
>                           Praveen Balasubramanian
>                           Lars Eggert
>                           Glenn Judd
>         Filename        : draft-ietf-tcpm-dctcp-09.txt
>         Pages           : 16
>         Date            : 2017-07-16
...
> There are also htmlized versions available at:
> https://tools.ietf.org/html/draft-ietf-tcpm-dctcp-09

I would like to re-raise an issue I raised in Nov 2016:
 https://www.ietf.org/mail-archive/web/tcpm/current/msg10632.html
There I did not get a response, but I think it's an issue worth
at least a sentence in the draft...

This is a nice draft. One quick thought on a minor point:
I couldn't find a passage in the draft about how cwnd is increased.
I didn't see a mention of the word "increase",
or slow start, or congestion avoidance, or a reference to DCTCP
inheriting RFC5681 behavior for cwnd increase.

The SIGCOMM 2010 paper is clear on this point: "The only difference
between a DCTCP sender and a TCP sender is in how each reacts to
receiving an ACK with the ECN-Echo flag set. Other features of TCP
such as slow start, additive increase in congestion avoidance, or
recovery from packet lost are left unchanged."

And the Linux DCTCP code is clear as well, since it uses
tcp_reno_cong_avoid(), meaning it inherits the standard Reno behavior
of slow start and congestion avoidance.

So I presume the intent of the IETF DCTCP would be the same
as the SIGCOMM 2010 DCTCP paper and Linux DCTCP code,
which is to say cwnd increase would be like Reno/RFC5681.

But IMHO it would be useful for the draft to have explicit language to
this effect as well. Particularly since now there are IETF documents
for both Reno and CUBIC, and the details of the DCTCP cwnd
decrease algorithm may or may not depend on the
Reno-style increase.

Apologies if I just missed this discussion.

cheers,
neal

[tcpm] I-D Action: draft-ietf-tcpm-dctcp-09.txt internet-drafts
Re: [tcpm] I-D Action: draft-ietf-tcpm-dctcp-09.t… Neal Cardwell
Re: [tcpm] I-D Action: draft-ietf-tcpm-dctcp-09.t… Eggert, Lars