Re: [tcpPrague] [tcpm] Fwd: I-D Action: draft-bagnulo-tcpm-tcp-low-rtt-00.txt

[Yuchung Cheng <[e-mail]>]

On Fri, Aug 12, 2016 at 4:30 AM, Karen Elisabeth Egede Nielsen
<[e-mail]> wrote:
>
> Hi Marcello,
>
> I very much appreciate your draft.
>
> Even more so as it addresses one of the things I find to be a bit
> "disturbing"  in the l4s drafts - Namely the claim that the finer saw
> tooth functionality of l4s/DCTCP alone solves the latency problems (while
> DCTCP indeed also is tuning the RTO timeout aggressively).
> E.g., the formulation in  section 1.1 of
> https://datatracker.ietf.org/doc/draft-briscoe-tsvwg-ecn-l4s-id/?include_t
> ext=1
>
> It turns out that a TCP algorithm like DCTCP that solves TCP's
>    scalability problem also solves the latency problem, because the
>    finer sawteeth cause very little queuing delay.  A supporting paper
>    [DCttH15] gives the full explanation of why the design solves both
>    the latency and the scaling problems, both in plain English and in
>    more precise mathematical form.  The explanation is summarised
>    without the maths in [I-D.briscoe-aqm-dualq-coupled].
>
> could benefit from a reference to your draft or the issue that you
> addresses here (at least for comprehensiveness).
>
> An additional comment is  that new approaches to retransmissions - like
> TCP TLP and TCP RACK (also SCTP TLR which however is not progressing at
> the moment) might fundamentally alter the picture. I.e., if
> retransmissions are sent pro-actively in tail loss situations then more
> conservative RTOs may be kept for situations where it is prudent to wait
> longer. Don't know if TCP TLP is so widely deployed that it is something
> that you should relate to even if it may be superseded by RACK. Just a
> thought.
TLP and RACK help reduce timeout cases in DC environment for sure. But
still it can not completely avoid timeouts. So I agree we should keep
timeouts conservative, but the current RFCs are way too conservative:
min RTO of 1 second is 4-5 orders of magnitude of DC RTTs.

The issue is, the "right" min-RTO depends on the actual DC and stacks:
they all have different RTTs and timer granularity. The draft cites
Glenn's paper, but Morgan Stanley's DC could be different than others.


Other comments on the draft:

1. min ssthresh or cwnd: with very large incast ((tens of) thousands
of senders into one receiver), cwnd of 0.1 pkt can still be too big. the
only way is to pace the packets over N*RTT intervals.


2. delayed acks:
as mentioned the actual time depends a lot on the DC implementation,
which is really "vender/owner"-specific. instead of perhaps an option
during setup to inform the sender about the max delay in the ack works
better.



>
> BR, Karen
>
> > -----Original Message-----
> > From: tcpm [mailto:[e-mail]] On Behalf Of marcelo bagnulo
> > braun
> > Sent: 8. juli 2016 23:19
> > To: tcpm IETF list <[e-mail]>
> > Subject: [tcpm] Fwd: I-D Action: draft-bagnulo-tcpm-tcp-low-rtt-00.txt
> >
> > Hi,
> >
> > We just submitted this draft for consideration of the WG. Comments are
> > appreciated.
> >
> > Regards, marcelo
> >
> >
> >
> >
> > -------- Mensaje reenviado --------
> > Asunto:       I-D Action: draft-bagnulo-tcpm-tcp-low-rtt-00.txt
> > Fecha:        Fri, 08 Jul 2016 14:16:35 -0700
> > De:   [e-mail]
> > Responder a:  [e-mail]
> > Para:         [e-mail]
> >
> >
> >
> > A New Internet-Draft is available from the on-line Internet-Drafts
> directories.
> >
> >
> >          Title           : Recommendations for increasing TCP
> performance in low RTT
> > networks.
> >          Authors         : Marcelo Bagnulo
> >                            Koen De Schepper
> >                            Glenn Judd
> >       Filename        : draft-bagnulo-tcpm-tcp-low-rtt-00.txt
> >       Pages           : 7
> >       Date            : 2016-07-08
> >
> > Abstract:
> >     This documents compiles a set of issues that negatively affect TCP
> >     performance in low RTT networks as well as the recommendations to
> >     overcome them.
> >
> >
> > The IETF datatracker status page for this draft is:
> > https://datatracker.ietf.org/doc/draft-bagnulo-tcpm-tcp-low-rtt/
> >
> > There's also a htmlized version available at:
> > https://tools.ietf.org/html/draft-bagnulo-tcpm-tcp-low-rtt-00
> >
> >
> > Please note that it may take a couple of minutes from the time of
> submission
> > until the htmlized version and diff are available at tools.ietf.org.
> >
> > Internet-Drafts are also available by anonymous FTP at:
> > ftp://ftp.ietf.org/internet-drafts/
> >
> > _______________________________________________
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> > Internet-Draft directories: http://www.ietf.org/shadow.html or
> > ftp://ftp.ietf.org/ietf/1shadow-sites.txt
> >
> >
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