[AusNOG] ADSL2+ line sync data

[Joshua D'Alton]

That RFC isn't very accurate/applicable any more, the bandwidths we're
talking about now throws most of those calculations and factors etc out of
the window. The maths is still solid, but the results are ... not. Stream
delay (buffer size, mean RTT for a single TCP packet etc) now being 100th
of what it was with dialup, and so on.

Google might have that Q and A, and they are correct in theory that it will
let you upload alot as well (whatever that means *caugh P2P caugh*), but
they needn't have gone anywhere near symmetrical to rule out 99.999% risk
of downloads being slowed due to upload congestion. Put another way,
looking at that 18/1 vs 11/11 the majority of users will be experiencing
downstream congestion before they see upstream congestion.

I don't know exactly what ratio is 'ideal', especially given a swath of
difference user types, but if we were to maximise bandwidth usage and
'altruistically' determine the rates, it definitely wouldn't be 1:1
(symmetrical), perhaps not 18:1, but definitely somewhere inbetween given
total bandwidth > 10Mbps or so (haven't sat down and figured it out
exactly, but that is probably ballpark given tcp overhead figures, ie for
the download stream, and then whatever extra upload data is being sent).

And then that's not to mention fiber typically 'has to be' symmetrical so
theres really no administrative difference, or in fact its less burden
leaving as is vs upload throttling (for whatever reason one might want to
do that, and if that was even an issue).




On Fri, Sep 13, 2013 at 9:47 PM, Mark ZZZ Smith
<markzzzsmith at yahoo.com.au>wrote:

>
>
>
>
> ----- Original Message -----
> > From: Mark ZZZ Smith <markzzzsmith at yahoo.com.au>
> > To: Tony <td_miles at yahoo.com>; "Beeson, Ayden" <ABeeson at csu.edu.au>;
> Guy Ellis <guy at traverse.com.au>
> > Cc: "ausnog at lists.ausnog.net" <ausnog at lists.ausnog.net>
> > Sent: Friday, 13 September 2013 9:41 PM
> > Subject: Re: [AusNOG] ADSL2+ line sync data
> >
> >  From: Tony <td_miles at yahoo.com>
> >> To: "Beeson, Ayden" <ABeeson at csu.edu.au>; Guy Ellis
> > <guy at traverse.com.au>
> >> Cc: "ausnog at lists.ausnog.net" <ausnog at lists.ausnog.net>
> >> Sent: Friday, 13 September 2013 9:55 AM
>
> <snip>
>
> >> If at 2000m you get either 18/1Mbps ADSL2+ or 11/11Mbps VDSL2 then thats
> > fairly close to the same amount of total bandwidth (19 v's 22). This
> > probably doesn't please the user though as they
> > want faster downloads (to "obtain" the latest TV eps) and hence an
> > option to choose which "DSL" you want might be of benefit depending on
> > your intended usage of the link.
> >>
> >
> > Sad thing about this is that doing that appears to be a good idea, but
> that
> > actual best thing for performance is symmetry, due to how TCP uses ACK
> feedback
> > to determine it's send rate. Congestion on the uplink, which is more
> likely
> > to occur when the uplink bandwidth is lower than downlink bandwidth,
> will slow
> > down downloads:
> >
> > BCP69/RFC3449, "TCP Performance Implications of Network Path
> > Asymmetry"
> > http://tools.ietf.org/html/rfc3449
> >
> >
> >
>
>
> I should add that Google gets it, which is why their FTTP is symmetric.
>
> http://googlefiberblog.blogspot.com.au/2012/04/construction-update.html
>
>
> Q: I see a label on your diagram, “Gigabit Symmetric Fiber Connectivity.”
> What does that mean?
>
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