Re: [OPS-DIR] Opsdir last call review of draft-ietf-tsvwg-sctp-ndata-12
Benoit Claise <bclaise@cisco.com> Thu, 31 August 2017 12:14 UTC
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Subject: Re: [OPS-DIR] Opsdir last call review of draft-ietf-tsvwg-sctp-ndata-12
To: Michael Tuexen <tuexen@fh-muenster.de>
Cc: Stefan Winter <stefan.winter@restena.lu>, ops-dir@ietf.org, draft-ietf-tsvwg-sctp-ndata.all@ietf.org, tsvwg@ietf.org, ietf@ietf.org, IESG IESG <iesg@ietf.org>
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From: Benoit Claise <bclaise@cisco.com>
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Date: Thu, 31 Aug 2017 14:14:04 +0200
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Hi Michael, >> On 30. Aug 2017, at 18:51, Benoit Claise <bclaise@cisco.com> wrote: >> >> Michael, >> >> I stumbled on the same point as Stefan. > Hi Benoit, > > thanks for reviewing the document. See my comments in-line. > > Best regards > Michael >> First of, I had to review RFC4960 for TSN/SSN. I was not sure whether TSN was per stream or not >> It would have helped me to repeat this information in the intro. >> Ok, to be candid, I deduced this information from figure 1 and 2. >>>>>> And the rest are only musings not needing any action if the authors don't want >>>>>> to action on them: >>>>>> >>>>>> The introduction presents a good use case, quoting: "e.g., when the >>>>>> transmission of an urgent >>>>>> message is blocked from transmission because the sender has started >>>>>> the transmission of another, possibly large, message". >>>>>> >>>>>> The later queueing example in figure 1 has three SIDs being queued >>>>>> simultanseously. It is not clear which of those "has already started" and which >>>>> There is actually one message queued for stream 0, three messages for stream 1, >>>>> and one message for stream 2. >>>>>> are the important ones being delayed. For a better understanding of the reader, >>>>> The figure is about in which sequence they are put on the wire. So no >>>>> transmission has startet. This examples also does not deal with the case >>>>> of one stream having a higher priority than another. That would be an >>>>> example using a priority scheduler. This examples illustrates the round >>>>> robin scheduler. >>>>> The point here is that a single scheduler (the round robin scheduler, for >>>>> example) behaves differently when user message interleaving is used or not. >>>>> That is an important point: You can even use the schedulers with regular >>>>> DATA chunk, i.e. with user message interleaving being negotiated. >> This was my source of confusion. >> With figure 1 and figure 2, you want to show the effect of Round Robin Scheduler with and without User Message Interleaving, while we were expecting the figure 2 to be the solution from this spec. Hence Stefan and I were looking for this "urgent message" in figure 2. > I don't know what you mean with "solution from this spec" means. This spec introduces > stream schedulers and a protocol extension to allow interleaving of user messages. > Using this to implement an "urgent" concept is only one possibility: usage of a > strict priority scheduler (SCTP_SS_PRIO). So this "urgent" is misleading. It seems > to ring the wrong bells. That is why I suggested to remove it... >>>>>> it might be useful to revise the figure so that the head-of-line blocking >>>>>> happens for SID 1 because transmission of 0 and 2 has already started (and >>>>>> declare SID 1 to be the "important" message). The ASCII art would just have to >>>>>> indent SID 1 content a bit (placing 0 and 2 earlier in time), and the resulting >>>>>> serialisation would then put all the SID 1 messages at the very end, when 0 and >>>>>> 2 have been completely submitted (right?). >>>>> One could think about adding another example based on the priority scheduler >>>>> and giving stream 1 a higher priority. But for illustrating the point of >>>>> schedulers behaving differently depending on the negotiation of user message >>>>> interleaving this seems not necessary. >>>> That second example might indeed be useful. >>>> >>>> It's simply a bit strange that in the introduction you speak about an >>>> "urgent" message (something deserving a higher-priority sending) and >>>> that another transmission has already started - but then the example has >>>> none of those two. >> Exactly. >> Please add this extra example. >> This would also clarify from the introduction that when you write ... >> >> The sending of such large messages over >> SCTP as specified in [RFC4960] can result in a form of sender side >> head of line blocking (e.g., when the transmission of an urgent >> message is blocked from transmission because the sender has started >> the transmission of another, possibly large, message). >> >> .. the notion of transmission urgency is not within a stream but across streams, and that can be achieved with a priority scheduler. > I would actually prefer to remove the word "urgent" from that sentence. This just rings the wrong bells. That's a missed opportunity in mind, but your call. >> The text might have to be rephrased to explain that this spec provides the ability to create stream scheduler with different relative stream treatments. > What about: > > OLD TEXT > This document also defines several stream schedulers for general SCTP > associations. The stream schedulers may behave differently depending > on whether user message interleaving has been negotiated for the > association or not. > > NEW TEXT > This document also defines several stream schedulers for general SCTP > associations allowing different relative stream treatments. > The stream schedulers may behave differently depending > on whether user message interleaving has been negotiated for the > association or not. > > Would that address this issue? ok. > >> And also that urgent message would be placed in this high priority stream scheduler (reliable, I guess, but that's orthogonal) stream. > I really would like to get rid of this. I don't think giving the strict > priority scheduler as specific role here is a good idea. > > The RR scheduler was chosen as the simplest scheduler to illustrate > that scheduler can behave differently when using interleaving or not. >>> What about removing the word "urgent" from that sentence. >>>> In fact I wonder why you speak about urgent things in the introduction >>>> at all. Neither the scheduler nor the interleaving are designed to help >>>> urgent things to be sent earlier. >>>> >>>> All they do is make the transmission fairer so that *all* chunks get a >>>> more even serialised distribution on the wire. >>> That is not true in general. If you use the priority scheduler, one >>> stream gets preferred treatment. It also makes sure that the time >>> of a message spent in the stream queue is shorter. So these messages >>> get sent earlier. >>> In the case of the round robin scheduler, you are right that this is >>> about getting fairer treatment. If you take the WFQ scheduler, the >>> streams don't get the same treatment, but it depends on the weight >>> given to the stream. >>>> So, rather than saying that the spec helps urgent things to be sent >>>> earlier, it should maybe rather state that the spec helps every message >>>> getting an equal, and equally distributed, share of the medium. >>> It is not meant that the spec does this. It was one example, what >>> a particular stream scheduler does. But there are multiple schedulers >>> defined in the document. >>> >>> The RR scheduler example is not intended to illustrate what you can do >>> with all the schedulers, but that using message interleaving or not >>> has an impact on the scheduler. That is meant by stating: >>> >>> This document also defines several stream schedulers for general SCTP associations. >>> The stream schedulers may behave differently depending on whether user message >>> interleaving has been negotiated for the association or not. >>> >>> This is followed by illustrating this using the RR scheduler. >>> >>> Best regards >> Also, I don't see the value of the last sentence in this paragraph, especially with RFC2119 MAY. >> >> This section defines several stream schedulers. The stream >> schedulers may behave differently depending on whether user message >> interleaving has been negotiated for the association or not. An >> implementation MAY implement any subset of them. > During the review the question was brought up which scheduler have to be implemented > by an implementation. This sentence was add to address it. > If you want, I can remove it. The point is that the MAY is that sentence doesn't mean anything. Maybe you mean: An implementation MUST implement at least one stream scheduler. Regards, B. >> >> Regards, Benoit
- Opsdir last call review of draft-ietf-tsvwg-sctp-… Stefan Winter
- Re: Opsdir last call review of draft-ietf-tsvwg-s… Michael Tuexen
- Re: [OPS-DIR] Opsdir last call review of draft-ie… Stefan Winter
- Re: [OPS-DIR] Opsdir last call review of draft-ie… Michael Tuexen
- Re: [OPS-DIR] Opsdir last call review of draft-ie… Benoit Claise
- Re: [OPS-DIR] Opsdir last call review of draft-ie… Michael Tuexen
- Re: [OPS-DIR] Opsdir last call review of draft-ie… Benoit Claise
- Re: [OPS-DIR] Opsdir last call review of draft-ie… Michael Tuexen
- Re: [OPS-DIR] Opsdir last call review of draft-ie… Spencer Dawkins at IETF
- Re: [OPS-DIR] Opsdir last call review of draft-ie… Benoit Claise
- Re: [OPS-DIR] Opsdir last call review of draft-ie… Michael Tuexen
- Re: [OPS-DIR] Opsdir last call review of draft-ie… Michael Tuexen
- Re: [OPS-DIR] Opsdir last call review of draft-ie… Spencer Dawkins at IETF
- Re: [OPS-DIR] Opsdir last call review of draft-ie… Michael Tuexen
- Re: [OPS-DIR] Opsdir last call review of draft-ie… Spencer Dawkins at IETF
- Re: [OPS-DIR] Opsdir last call review of draft-ie… Benoit Claise