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

Greg Mirsky <gregimirsky@gmail.com> Wed, 22 February 2017 14:22 UTC

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From: Greg Mirsky <gregimirsky@gmail.com>
Date: Wed, 22 Feb 2017 06:22:48 -0800
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To: Robert Sparks <rjsparks@nostrum.com>
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Subject: Re: [mpls] Review of draft-ietf-mpls-residence-time-13
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Hi Robert,
agreed. Thank you for your thorough review and the most helpful comments.
I'll upload the update shortly.

Regards,
Greg

On Wed, Feb 22, 2017 at 1:09 AM, Robert Sparks <rjsparks@nostrum.com>; wrote:

> I would simply delete "To ensure precise accuracy in time determination"
> and start the sentence at "These actions"
>
> RjS
>
> On 2/15/17 9:40 PM, Greg Mirsky wrote:
>
> Hi Robert,
> safe travel and here's another version for your consideration. The point
> I'm trying to convey is that the jitter is real but must be accounted not
> as part of propagation delay but as residence time. Hope I'm getting close.
>
>    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.
>    To ensure precise accuracy in time determination these actions should
>    be done as close to the physical layer as possible at the same point
>    of packet processing striving to avoid introducing the appearance of
>    jitter in propogation delay whereas it should be accounted as
>    residence time.
>
> Regards,
>
> Greg
>
>
> On Wed, Feb 15, 2017 at 11:15 AM, Robert Sparks <rjsparks@nostrum.com>;
> wrote:
>
>>
>>
>> On 2/15/17 11:20 AM, Greg Mirsky wrote:
>>
>> 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.
>>
>> perhaps change "avoid introduction of jitter" to "avoid introducing the
>> appearance of jitter that's not really there"?
>>
>> (I'm about to board a plane, so further responses will be very delayed).
>>
>> Regards,
>>
>> Greg
>>
>>
>> On Wed, Feb 15, 2017 at 7:43 AM, Robert Sparks <rjsparks@nostrum.com>;
>> 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 <rjsparks@nostrum.com>;
>>> 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".)
>>>>
>>>>
>>>
>>>
>>
>>
>
>