Re: [tcpm] Last Call: <draft-ietf-tcpm-initcwnd-06.txt> (Increasing TCP's Initial Window) to Experimental RFC

[Yoshifumi Nishida <[e-mail]>]

Hi Magnus,

Thanks for the comments.
I think your points are valid. But, it's not very clear to me what you
would like to suggest authors.
IW10 might have an impact on real-time traffic. But, difficult point is we
are not very sure how much it is over the whole Internet. Do we ask authors
to do experiments to address the concerns you pointed out?
There will be situations where IW10 has negative impacts as well as
positive impacts.
But, I'm feeling it might be difficult to discuss this point without large
scale experiments.

Thanks,
--
Yoshifumi

On Thu, Dec 6, 2012 at 2:01 AM, Magnus Westerlund <
[e-mail]> wrote:

> Hi,
>
> I have reviewed draft-ietf-tcpm-initcwnd-06 and have some questions and
> comments.
>
> 1) First of all the experiments done appear to cover the impact on other
> applications, like measuring the RTT variations when using IW10 compared
> to IW3. If one is interested in the impact this proposal have on
> real-time traffic flows over a shared bottle-neck the variations in
> queue time at the bottleneck combined with that flows seen loss rates
> are the most important factors. As a sudden delay spike of sufficient
> magnitude will most likely result in a real-time media packet being
> late, i.e. late loss rather than being lost in the network there might
> be significant impact on such traffic from these IW10 packet bursts.
> Have I missed any material discussing this aspect?
>
> All the latency figures in the parts I was looking at was discussing
> cases when the object transfer takes longer time. But it appear obvious
> that even with a 100 ms increased one-way transfer time for a packet is
> the end of the initial window the total transfer goes faster compared to
> the delay caused by growing the window.
>
> 2) If I assume that most users are behind buffer-bloated links the
> introduction of IW10 on wide scale will additionally disrupt interactive
> applications and cause increased delay and possible late loss depending
> on application. Especially in combination with domain sharding. For
> example a Swedish newspaper's website front page results in that more
> than 40 TCP connections are opened in a current browser. If these all
> was using IW10, the amount of packets being sent initially will grow
> roughly from 40*3 = 120 to 40*10 = 400. There are some distribution over
> time but still this likely results in a larger delay spike.
>
> I don't quite know how I should consider this case. One stand point is
> that the interactive application is anyway buggered and IW10 effects are
> not making it significantly worse. The only remedy is queue separation
> so that what ever happens with the web-downloads doesn't affect the
> interactive traffic. Another is that it will make the existing situation
> worse and we should try to avoid making it worse.
>
> I think I am more in the first camp, but still the second one is
> worrying to me. But I dare to guess that the performance gains might be
> worth the issues, but without real progress on mitigation of the buffer
> bloat issues for interactive real-time traffic this will add
> significantly to the issues real-time has to face.
>
>
> 3) The documents talks in quite loose terms about what can be done to
> avoid to continue cause issues for destinations which has issues with
> IW10. However I think it is a bit unspecific here. I can see that a
> content deliverer can have lists for destination address blocks that
> they see issues with which configures the sending server side with a
> lower IW value. It also talks about the clients can configure to keep
> the window low initially to prevent an IW10 sender to clobber ones link
> if that is known. My question here is if the recommendation for auto
> detection and configuration can be made more explicit. Fortunately the
> issues with misconfiguration in this cases is not that serious. But
> otherwise there is commonly need to be quite careful with such auto
> configs that affects the behavior.
>
> 4) I am also worried that the document is a bit to positive in regards
> to that IW10 will reduce the domain sharding practice. I think the only
> thing that can do is likely a new HTTP transport strata that shows that
> significantly improved performance for multiple objects from the same
> domain over fewer transport flows. Maybe SPDY + IW10 can provide such
> incentive, but I don't think IW10 alone will do much.
>
> Cheers
>
> Magnus
>
> On 2012-11-26 22:29, The IESG wrote:
> >
> > The IESG has received a request from the TCP Maintenance and Minor
> > Extensions WG (tcpm) to consider the following document:
> > - 'Increasing TCP's Initial Window'
> >   <draft-ietf-tcpm-initcwnd-06.txt> as Experimental RFC
> >
> > The IESG plans to make a decision in the next few weeks, and solicits
> > final comments on this action. Please send substantive comments to the
> > [e-mail] mailing lists by 2012-12-10. Exceptionally, comments may
> be
> > sent to [e-mail] instead. In either case, please retain the
> > beginning of the Subject line to allow automated sorting.
> >
> > Abstract
> >
> >
> >    This document proposes an experiment to increase the permitted TCP
> >    initial window (IW) from between 2 and 4 segments, as specified in
> >    RFC 3390, to 10 segments, with a fallback to the existing
> >    recommendation when performance issues are detected. It discusses the
> >    motivation behind the increase, the advantages and disadvantages of
> >    the higher initial window, and presents results from several large
> >    scale experiments showing that the higher initial window improves the
> >    overall performance of many web services without resulting in a
> >    congestion collapse. The document closes with a discussion of usage
> >    and deployment for further experimental purpose recommended by the
> >    IETF TCP Maintenance and Minor Extensions (TCPM) working group.
> >
> >
> >
> >
> > The file can be obtained via
> > http://datatracker.ietf.org/doc/draft-ietf-tcpm-initcwnd/
> >
> > IESG discussion can be tracked via
> > http://datatracker.ietf.org/doc/draft-ietf-tcpm-initcwnd/ballot/
> >
> >
> > No IPR declarations have been submitted directly on this I-D.
> >
> >
> >
> >
>
>
> --
>
> Magnus Westerlund
>
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