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

[Robert Sparks <[e-mail]>]

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] 
> <mailto:[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
>     <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".)
>
>