Re: [Ntp] Drafts on interleaved modes and correction field

[Tal Mizrahi <[e-mail]>]

Hi Miroslav,

> Do you know if there are any switches that actually support it?

Yes, some switches actually support it fast-to-slow cut-through.

>If a switch cut through from a faster link to a slower link, it would
>know the length of the packet and could calculate the (negative)
>correction before the correction field was transmitted, right?

Hmmm... So the switch would have to evaluate the packet length based on the
Length field in the UDP header, and use that in the correction field
computation.
It would be significantly easier to implement a correction field
computation based on first-bit-in-to-first-bit-out.


I may be missing something here, but I would argue that if we consistently
compute the correction based on first-bit-in-to-first-bit-out, the packet
transmission times on both directions would cancel out, and would not
affect the symmetry of the computation.
Can you provide an example in which this hypothesis is wrong?

Thanks,
Tal.



On Mon, Nov 6, 2017 at 2:36 PM, Miroslav Lichvar <[e-mail]>
wrote:

> On Wed, Nov 01, 2017 at 11:41:26AM +0200, Tal Mizrahi wrote:
> > Small correction:
> > Slow-to-fast cut-through is *not* possible.
> > Fast-to-slow cut-through *is* possible.
>
> Do you know if there are any switches that actually support it? I
> suspect the buffering would be a major complication for a relatively
> small improvement in the overall latency.
>
> If a switch cut through from a faster link to a slower link, it would
> know the length of the packet and could calculate the (negative)
> correction before the correction field was transmitted, right? I
> assume that the field is in the second half of the packet and the
> ratio between the speeds is at least 2:1.
>
> I think the draft can specify that the correction needs to be
> calculated from the end of the reception and beginning of the
> transmission if the link speeds are different. If the speeds are
> equal, it can be any combination of beginning/end of the
> reception/transmission, because a symmetric correction will be made in
> the opposite direction. The delay will be different, but there won't
> be any asymmetry.
>
> Does that make sense?
>
> > > On Tue, Oct 31, 2017 at 5:57 PM, Miroslav Lichvar <[e-mail]
> >
> > > wrote:
> > >
> > >> On Tue, Oct 31, 2017 at 04:59:52PM +0200, Tal Mizrahi wrote:
> > >> > Moreover, we never know which nodes along the path use
> > >> store-and-forward,
> > >> > and which nodes use cut-through. In cut-through forwarding the
> packet
> > >> > transmission may start before the last bit is received, so it
> certainly
> > >> > makes more sense to measure the first-bit-in-to-first-bit-out.
> > >>
> > >> That's a good point. With cut-through switching the correction would
> > >> be negative and generally couldn't be determined in time to be put in
> > >> the packet as the length is unknown. Are there any switches that can
> > >> cut-through from a faster link to slower link?
> > >>
> > >> Maybe it would make sense to allow corrections from preamble RX
> > >> timestamps when the link speeds are equal.
>
> --
> Miroslav Lichvar
>