Re: [mpls] Review of draft-ietf-mpls-residence-time-13
Robert Sparks <rjsparks@nostrum.com> Wed, 22 February 2017 09:09 UTC
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To: Greg Mirsky <gregimirsky@gmail.com>
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From: Robert Sparks <rjsparks@nostrum.com>
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Date: Wed, 22 Feb 2017 10:09:01 +0100
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Subject: Re: [mpls] Review of draft-ietf-mpls-residence-time-13
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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 > <mailto: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 <mailto: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 <mailto: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 >>> <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".) >>> >>> >> >> > >
- [mpls] Review of draft-ietf-mpls-residence-time-13 Robert Sparks
- Re: [mpls] Review of draft-ietf-mpls-residence-ti… Greg Mirsky
- Re: [mpls] Review of draft-ietf-mpls-residence-ti… Greg Mirsky
- Re: [mpls] Review of draft-ietf-mpls-residence-ti… Robert Sparks
- Re: [mpls] Review of draft-ietf-mpls-residence-ti… Greg Mirsky
- Re: [mpls] Review of draft-ietf-mpls-residence-ti… Robert Sparks
- Re: [mpls] Review of draft-ietf-mpls-residence-ti… Greg Mirsky
- Re: [mpls] Review of draft-ietf-mpls-residence-ti… Robert Sparks
- Re: [mpls] Review of draft-ietf-mpls-residence-ti… Greg Mirsky
- Re: [mpls] Review of draft-ietf-mpls-residence-ti… Alexander Vainshtein