Re: [mpls] Review of draft-ietf-mpls-residence-time-13

[Greg Mirsky <[e-mail]>]

Hi Robert,
yes, you've absolutely correct in your example. The importance of RTM is to
exclude and quantify, as much as possible, jitter from propagation delay.
I've updated the wording to reflect that. Below is new text I propose,
please let me know if it makes it clearer.

   Each RTM-capable
   node on the explicit path receives an RTM packet and records the time
   at which it receives that packet at its ingress interface as well as
   the time at which it transmits that packet from its egress interface.
   These actions should be done as close to the physical layer as
   possible at the same point of packet processing striving to avoid
   introduction of jitter in propogation delay to ensure precise
   accuracy in time determination.

Regards,

Greg


On Wed, Feb 15, 2017 at 7:43 AM, Robert Sparks <[e-mail]> wrote:

>
>
> On 2/14/17 10:06 PM, Greg Mirsky wrote:
>
> Hi Robert,
> Section 5 Data Plane Theory of Operation has the following recommendation
> on reading time value at ingress and egress:
>
>    Each RTM-capable
>    node on the explicit path receives an RTM packet and records the time
>    at which it receives that packet at its ingress interface as well as
>    the time at which it transmits that packet from its egress interface;
>    this should be done as close to the physical layer as possible to
>    ensure precise accuracy in time determination.
>
> Do you find the text sufficient in providing guidance to an implementer of RTM?
>
> Well, no - that was point in the text from my review of -12 that caused me
> to make comment in the first place.
>
> What does "precise accuracy" even _MEAN_? You're waving your hands with
> words that don't help the reader guess what you mean at the moment. The
> subsequent email exchange said the important part was that you measure
> _precisely_ (in that the measurements are always taken the same way), but
> accuracy isn't that important (you've (and here by "you" I mean the set of
> people that have responded to the review") told me it's not important
> whether the reading is taken at the beginning of the bit-stream, the end,
> or several 1000 nanoseconds after the last bit came off the physical media
> as long as it's done the same way for each packet). The fact that you're
> carrying a measurement that can talk about times smaller than the
> interarrival time for individual bits for some real world line speeds says
> that different implementations taking the measurement different ways is
> going to result in different residence time measurements, even if the
> residence time was really identical. You've told me that's not important to
> the protocol using the measurement as long as the an individual
> implementation doesn't introduce something that will look to the using
> protocol like jitter that's not really there.
>
> The text does not say that to an implementer right now.
>
> To restate that long paragraph with maybe a longer one,  assume a
> simplified world where everything is perfectly constant. Packets are all
> flowing at the same size and same rate. For reference, I assert that the
> time between the first bit of a packet entering the system taking the
> measurement and the first bit of that packet leaving the system is exactly
> T (every time). You are telling me that if the system returns cT for some
> constant T other than 1 (and not necessarily  close to 1), everything is
> fine. You're also telling me that if you replace the system with another
> that preserves T, but measures differently so that it returns c'T where c'
> != c, everything is fine. The important part to you is that c and c' don't
> change for their respective systems. (That surprises me, I can see how the
> way the clocks are going to use this will do the right thing, but I can't
> help but think someone later will look at the difference between the two
> systems and say something is wrong with one of them). What you've told me
> is not fine is that if you drop i a third system that also preserves T but
> reports aT where a _varies_ over some range.
>
> Have I heard you correctly?
>
> Regards,
>
> Greg
>
>
> On Mon, Feb 13, 2017 at 1:51 PM, Robert Sparks <[e-mail]>
> wrote:
>
>> Reviewer: Robert Sparks
>> Review result: Ready
>>
>> I am the assigned Gen-ART reviewer for this draft. The General Area
>> Review Team (Gen-ART) reviews all IETF documents being processed
>> by the IESG for the IETF Chair. Please wait for direction from your
>> document shepherd or AD before posting a new version of the draft.
>>
>> For more information, please see the FAQ at
>>
>> <https://trac.ietf.org/trac/gen/wiki/GenArtfaq>.
>>
>> Document: draft-ietf-mpls-residence-time-13
>> Reviewer: Robert Sparks
>> Review Date: 2017-02-13
>> IETF LC End Date: 2017-01-17
>> IESG Telechat date: 2017-03-02
>>
>> Summary: Ready for publication as PS
>>
>> Thanks for addressing my comments.
>> (I still think some discussion about taking arrival and departure time
>> measurements would be helpful, even if it only said "do it
>> consistently so you don't indicate jitter that's not really there".)
>>
>>
>
>