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

2017-02-15 14:41:06
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(_at_)nostrum(_dot_)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(_at_)nostrum(_dot_)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(_at_)nostrum(_dot_)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".)