Re: [Gen-art] Review: draft-ietf-pcn-sm-edge-behaviour-08
"Joel M. Halpern" <jmh@joelhalpern.com> Mon, 02 January 2012 18:04 UTC
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Date: Mon, 02 Jan 2012 13:04:29 -0500
From: "Joel M. Halpern" <jmh@joelhalpern.com>
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To: Michael Menth <menth@informatik.uni-tuebingen.de>
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Cc: draft-ietf-pcn-sm-edge-behaviour@tools.ietf.org, Steven Blake <slblake@petri-meat.com>, gen-art@ietf.org, Bob Briscoe <bob.briscoe@bt.com>, Tom Taylor <tom.taylor.stds@gmail.com>, David Harrington <ietfdbh@comcast.net>
Subject: Re: [Gen-art] Review: draft-ietf-pcn-sm-edge-behaviour-08
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The clarification on U is very helpful. I look forward to comments from others on the routing based behavior / ECMP text removal / replacement question. On 1/2/2012 12:58 PM, Michael Menth wrote: > Hi Joel, hi Tom, > > Am 02.01.2012 18:18, schrieb Joel M. Halpern: >> Michael, I am not sure what to make of your recommended text abut ECMP. >> ECMP is used by almost all operators. It is generally considered a >> necessary tool in the tool-kit. >> More significantly, at least for the egress understanding of the >> ingress, it is not even the single operator's ECMP, but other >> operators selections of paths that produce the issue. So even in the >> unlikely event that this operator does not use ECMP, it still is not >> sufficient. > > Then I better leave the ECMP issue for others to answer. > > The definition of U can be better corrected as follows (improved > rewording of my previous email): > > U represents the average ratio of PCN-supportable-rate to > PCN-admissible-rate over all the links of the PCN-domain. > -> > U is a domain-wide constant which implicitly defines the > PCN-supportable-rate by U*PCN-admissible-rate on all links of the PCN > domain. > > Best wishes, > > Michael > > >> >> Yours, >> Joel >> >> On 1/2/2012 11:54 AM, Michael Menth wrote: >>> Hi Tom, hi Joel, >>> >>> I wish you a happy new year! >>> >>> Here are my comments to address Joel's concerns: >>> >>> ==================================================================== >>> >>> The issue with ECMP: I'd add a comment that CL and SM should not be in >>> the presence of ECMP if routing information is used to determine >>> ingress-egress-aggregates since this seems to be messy and error-prone. >>> >>> ==================================================================== >>> >>> The following text may clarify at the beginning of Section 3.3.2 the >>> relation >>> between admission control and flow termination to address one of Joel's >>> comments (for both SM and CL): >>> >>> In the presence of light pre-congestion, i.e., in the presence of a >>> small, >>> positive ETM-rate (relative to the overall PCN traffic rate), new >>> flows may >>> already be blocked. However, in the presence of heavy pre-congestion, >>> i.e., >>> in the presence of a relatively large ETM-rate, termination of some >>> admitted >>> flows is required. Thus, flow blocking is logical prerequisite for flow >>> termination. >>> >>> ==================================================================== >>> >>> The following sentence in 3.3.2 should be corrected (only SM-specific): >>> >>> U represents the average ratio of PCN-supportable-rate to >>> PCN-admissible-rate >>> over all the links of the PCN-domain. >>> >>> -> >>> >>> U represents the ratio of PCN-supportable-rate to PCN-admissible-rate >>> for all >>> the links of the PCN-domain. >>> >>> ==================================================================== >>> >>> I also recommend to change the following text as I think it may cause >>> misinterpretations (applies both to SM and CL): >>> >>> If the difference calculated in the second step is positive, the >>> Decision >>> Point SHOULD select PCN-flows to terminate, until it determines that the >>> PCN-traffic admission rate will no longer be greater than the estimated >>> sustainable aggregate rate. If the Decision Point knows the bandwidth >>> required by individual PCN-flows (e.g., from resource signalling used to >>> establish the flows), it MAY choose to complete its selection of >>> PCN-flows to >>> terminate in a single round of decisions. >>> >>> Alternatively, the Decision Point MAY spread flow termination over >>> multiple >>> rounds to avoid over-termination. If this is done, it is RECOMMENDED >>> that >>> enough time elapse between successive rounds of termination to allow the >>> effects of previous rounds to be reflected in the measurements upon >>> which the >>> termination decisions are based. (See [IEEE-Satoh] and sections 4.2 >>> and 4.3 >>> of [MeLe10].) >>> >>> -> >>> >>> If the difference calculated in the second step is positive (traffic >>> rate to >>> be terminated), the Decision Point SHOULD select PCN-flows to >>> terminate. To >>> that end, the Decision Point MAY use upper rate limits for individual >>> PCN-flows (e.g., from resource signalling used to establish the >>> flows) and >>> select a set of flows whose sum of upper rate limits is up to the >>> traffic >>> rate to be terminated. Then, these flows are terminated. The use of >>> upper >>> limits on flow rates avoids over-termination. >>> >>> Termination may be continuously needed after consecutive measurement >>> intervals for various >>> reasons, e.g., if the used upper rate limits overestimate the actual >>> flow rates. >>> For such cases it is RECOMMENDED that enough time elapses between >>> successive >>> termination events to allow the effects of previous termination events >>> to be >>> reflected in the measurements upon which the termination decisions are >>> based; >>> otherwise, over-termination may occur. See [IEEE-Satoh] and Sections 4.2 >>> and >>> 4.3 of [MeLe10]. >>> >>> ==================================================================== >>> >>> [IEEE-Satoh] is not a good key for Daisuke's work as the prefix "IEEE" >>> makes it look like a reference to a standards document. >>> You better use [SaUe10] or [Satoh10]. Applies both to CL and SM. >>> >>> >>> >>> Best wishes, >>> >>> Michael >>> >>> >>> Am 02.01.2012 15:21, schrieb Tom Taylor: >>>> It shall be as you say, subject to comment from my co-authors when >>>> they get back from holiday. >>>> >>>> On 01/01/2012 5:43 PM, Joel M. Halpern wrote: >>>>> In-line... >>>>> >>>>> On 1/1/2012 4:06 PM, Tom Taylor wrote: >>>>>> >>>>>> >>>>>> On 01/01/2012 2:58 PM, Joel M. Halpern wrote: >>>>>>> Thank you for responding promptly Tom. Let me try to elaborate on >>>>>>> the >>>>>>> two issues where I was unclear. >>>>>>> >>>>>>> On the ingress-egress-aggregate issue and ECMP, the concern I >>>>>>> have is >>>>>>> relative to the third operational alternative where routing is >>>>>>> used to >>>>>>> determine where the ingress and egress of a flow is. To be blunt, >>>>>>> as far >>>>>>> as I can tell this does not work. >>>>>>> 1) It does not work on the ingress side because traffic from a given >>>>>>> source prefix can come in at multiple places. Some of these >>>>>>> places may >>>>>>> claim reachability to the source prefix. Some may not. While a given >>>>>>> flow will use only one of these paths, there is no way to determine >>>>>>> from >>>>>>> routing information, at the egress, which ingress that flow used. >>>>>>> 2) A site may use multiple exits for a given destination prefix. >>>>>>> Again, >>>>>>> while the site will only use one of these egresses for a given flow, >>>>>>> there is no way for the ingress to know which egress it will be >>>>>>> on the >>>>>>> basis of routing information. >>>>>>> Thus, the text seems to allow for a behavior that simply does not >>>>>>> work. >>>>>> >>>>>> [PTT] I think the disconnect here is that you read the text to say >>>>>> that >>>>>> an individual node uses routing information to determine the IEA. >>>>>> That >>>>>> wasn't the intention. Instead, administrators use routing >>>>>> information to >>>>>> derive filters that are installed at the ingress and egress nodes. >>>>> >>>>> As far as I can tell, your response describes a situation even less >>>>> effective than what I assumed. >>>>> Firstly, it does not matter whether it is the edge node, the decision >>>>> node, or the human administrator. Routing information is not enough to >>>>> determine what the ingress-egress pairing is. The problems I describe >>>>> above apply no matter who is making the decision. >>>>> Secondly, having a human make the decision means that as soon as >>>>> routing >>>>> changes, the configured filters are wrong. >>>>> >>>>> I would suggest that the text in question be removed, and replaced >>>>> with >>>>> a warning against attempting what is currently described. >>>>> >>> My view is also that CL ans SM do not work in the presence of ECMP. This >>> should be indicated as a warning. >>> >>>>>>> >>>>>>> I am still confused about the relationship of section 3.3.2 to the >>>>>>> behavior you describe. 3.3.2 says that as long as any excess >>>>>>> traffic is >>>>>>> being reported, teh decision point shall direct the blocking of >>>>>>> additional flows. That does not match 3.3.1, and does not match your >>>>>>> description. >>>>>> >>>>>> [PTT] I can't see the text in section 3.3.2 that says you continue to >>>>>> block as long as any excess traffic is being reported. What I >>>>>> think it >>>>>> says is that as long as excess traffic is reported, the decision >>>>>> point >>>>>> checks to see whether the traffic being admitted to the aggregate >>>>>> exceeds the supportable level. Excess traffic may be non-zero, yet no >>>>>> termination may be required (i.e., traffic is below the second >>>>>> threshold). >>>>> >>>>> I think I see what you are saying. If I am reading this correctly, the >>>>> decision process must re-calculate to determine if there is >>>>> termination >>>>> every time it receives a report with non-zero excess and the port is >>>>> already blocked. But it does not have to actually block anything. >>>>> This however seems to depend upon the correct relative >>>>> configuration of >>>>> the limit that flips it into blocked state, the value of U, and maybe >>>>> some other values. >>>>> Put differently, I understand that the two are not contradictory. >>>>> However, since the two things use different calculations, it is not at >>>>> all clear that they are consistent. This may well be acceptable. >>>>> But the >>>>> difference in methods is likely to lead to confusion. So, as a minor >>>>> (rather than major) comment, I would suggest that you provide >>>>> clarifying >>>>> text explaining why it is okay to use one condition to decide if there >>>>> is blocking, but a different condition (which could produce a lower >>>>> threshold) to decide how much to get rid of. >>>>> >>>>> Yours, >>>>> Joel >>>>> >>>>>>> >>>>>>> Yours, >>>>>>> Joel >>>>>>> >>>>>>> On 1/1/2012 2:48 PM, Tom Taylor wrote: >>>>>>>> Thanks for the review, Joel. Comments below, marked with [PTT]. >>>>>>>> >>>>>>>> On 31/12/2011 4:50 PM, Joel M. Halpern wrote: >>>>>>>>> I am the assigned Gen-ART reviewer for this draft. For >>>>>>>>> background on >>>>>>>>> Gen-ART, please see the FAQ at >>>>>>>>> <http://wiki.tools.ietf.org/area/gen/trac/wiki/GenArtfaq>. >>>>>>>>> >>>>>>>>> Please resolve these comments along with any other Last Call >>>>>>>>> comments >>>>>>>>> you may receive. >>>>>>>>> >>>>>>>>> Document: draft-ietf-pcn-sm-edge-behaviour-08 >>>>>>>>> PCN Boundary Node Behaviour for the Single Marking (SM) Mode of >>>>>>>>> Operation >>>>>>>>> Reviewer: Joel M. Halpern >>>>>>>>> Review Date: 31-Dec-2011 >>>>>>>>> IETF LC End Date: 13-Jan-2012 >>>>>>>>> IESG Telechat date: N/A >>>>>>>>> >>>>>>>>> Summary: This documents is almost ready for publication as an >>>>>>>>> Informational RFC. >>>>>>>>> >>>>>>>>> Question: Given that the document defines a complex set of >>>>>>>>> behaviors, >>>>>>>>> which are mandatory for compliant systems, it seems that this >>>>>>>>> ought to >>>>>>>>> be Experimental rather than Informational. It describes something >>>>>>>>> that >>>>>>>>> could, in theory, later become standards track. >>>>>>>> >>>>>>>> [PTT] OK, we've wobbled on this one, but we can follow your >>>>>>>> suggestion. >>>>>>>>> >>>>>>>>> Major issues: >>>>>>>>> Section 2 on Assumed Core Network Behavior for SM, in the third >>>>>>>>> bullet, >>>>>>>>> states that the PCN-domain satisfies the conditions specified >>>>>>>>> in RFC >>>>>>>>> 5696. Unfortunately, look at RFC 5696 I can not tell what >>>>>>>>> conditions >>>>>>>>> these are. Is this supposed to be a reference to RFC 5559 >>>>>>>>> instead? No >>>>>>>>> matter which document it is referencing, please be more specific >>>>>>>>> about >>>>>>>>> which section / conditions are meant. >>>>>>>> >>>>>>>> [PTT] You are right that RFC 5696 isn't relevant. It's such a long >>>>>>>> time >>>>>>>> since that text was written that I can't recall what the intention >>>>>>>> was. >>>>>>>> My inclination at the moment is simply to delete the bullet. >>>>>>>>> >>>>>>>>> It would have been helpful if the early part of the document >>>>>>>>> indicated >>>>>>>>> that the edge node information about how to determine >>>>>>>>> ingress-egress-aggregates was described in section 5. >>>>>>>>> In conjunction with that, section 5.1.2, third paragraph, seems to >>>>>>>>> describe an option which does not seem to quite work. After >>>>>>>>> describing >>>>>>>>> how to use tunneling, and how to work with signaling, the text >>>>>>>>> refers to >>>>>>>>> inferring the ingress-egress-aggregate from the routing >>>>>>>>> information. In >>>>>>>>> the presence of multiple equal-cost domain exits (which does >>>>>>>>> occur in >>>>>>>>> reality), the routing table is not sufficient information to make >>>>>>>>> this >>>>>>>>> determination. Unless I am very confused (which does happen) this >>>>>>>>> seems >>>>>>>>> to be a serious hole in the specification. >>>>>>>> >>>>>>>> [PTT] I'm not sure what the issue is here. As I understand it, >>>>>>>> operators >>>>>>>> don't assign packets randomly to a given path in the presence of >>>>>>>> alternatives -- they choose one based on values in the packet >>>>>>>> header. >>>>>>>> The basic intent is that packets of a given microflow all follow >>>>>>>> the >>>>>>>> same path, to prevent unnecessary reordering and minimize >>>>>>>> jitter. The >>>>>>>> implication is that filters can be defined at the ingress nodes to >>>>>>>> identify the packets in a given ingress-egress-aggregate (i.e. >>>>>>>> flowing >>>>>>>> from a specific ingress node to a specific egress node) based on >>>>>>>> their >>>>>>>> header contents. The filters to do the same job at egress nodes >>>>>>>> are a >>>>>>>> different problem, but they are not affected by ECMP. >>>>>>>>> >>>>>>>>> Minor issues: >>>>>>>>> Section 3.3.1 states that the "block" decision occurs when the CLE >>>>>>>>> (excess over total) rate exceeds the configured limit. However, >>>>>>>>> section >>>>>>>>> 3.3.2 states that the decision node must take further stapes if >>>>>>>>> the >>>>>>>>> excess rate is non-zero in further reports. Is this inconsistency >>>>>>>>> deliberate? If so, please explain. If not, please fix. (If it is >>>>>>>>> important to drive the excess rate to 0, then why is action only >>>>>>>>> initiated when the ratio is above a configured value, rather than >>>>>>>>> any >>>>>>>>> non-zero value? I can conceive of various reasons. But none are >>>>>>>>> stated.) >>>>>>>> >>>>>>>> [PTT] We aren't driving the excess rate to zero, but to a value >>>>>>>> equal to >>>>>>>> something less than (U - 1)/U. (The "something less" is because of >>>>>>>> packet dropping at interior nodes.) The assumption is that (U - >>>>>>>> 1)/U is >>>>>>>> greater than CLE-limit. Conceptually, PCN uses two thresholds. >>>>>>>> When the >>>>>>>> CLE is below the first threshold, new flows are admitted. Above >>>>>>>> that >>>>>>>> threshold, they are blocked. When the CLE is above the second >>>>>>>> threshold, >>>>>>>> flows are terminated to bring them down to that threshold. In >>>>>>>> the SM >>>>>>>> mode of operation, the first threshold is specified directly on a >>>>>>>> per-link basis by the value CLE-limit. The second threshold is >>>>>>>> specified >>>>>>>> by the same value (U - 1)/U for all links. With the CL mode of >>>>>>>> operation >>>>>>>> the second threshold is also specified directly for each link. >>>>>>>>> >>>>>>>>> >>>>>>>>> Nits/editorial comments: >>>>>>>>> >>>>>>>>> >>>>>>>> >>>>>>> >>>>>>> >>>>>> >>>>> >>> >
- [Gen-art] Review: draft-ietf-pcn-sm-edge-behaviou… Joel M. Halpern
- [Gen-art] A *new* batch of IETF LC reviews - 2011… Mary Barnes
- Re: [Gen-art] Review: draft-ietf-pcn-sm-edge-beha… Tom Taylor
- Re: [Gen-art] Review: draft-ietf-pcn-sm-edge-beha… Joel M. Halpern
- Re: [Gen-art] Review: draft-ietf-pcn-sm-edge-beha… Tom Taylor
- Re: [Gen-art] Review: draft-ietf-pcn-sm-edge-beha… Joel M. Halpern
- Re: [Gen-art] Review: draft-ietf-pcn-sm-edge-beha… Tom Taylor
- Re: [Gen-art] Review: draft-ietf-pcn-sm-edge-beha… Michael Menth
- Re: [Gen-art] Review: draft-ietf-pcn-sm-edge-beha… Joel M. Halpern
- Re: [Gen-art] Review: draft-ietf-pcn-sm-edge-beha… Michael Menth
- Re: [Gen-art] Review: draft-ietf-pcn-sm-edge-beha… Joel M. Halpern
- Re: [Gen-art] Review: draft-ietf-pcn-sm-edge-beha… David Harrington
- [Gen-art] Review: draft-ietf-pcn-sm-edge-behaviou… Joel M. Halpern
- Re: [Gen-art] Review: draft-ietf-pcn-sm-edge-beha… Russ Housley
- Re: [Gen-art] Review: draft-ietf-pcn-sm-edge-beha… Joel M. Halpern
- Re: [Gen-art] Review: draft-ietf-pcn-sm-edge-beha… Tom Taylor
- Re: [Gen-art] Review: draft-ietf-pcn-sm-edge-beha… Tom Taylor
- [Gen-art] Gen-Art review of draft-ietf-sipcore-rf… Alexey Melnikov
- Re: [Gen-art] Gen-Art review of draft-ietf-sipcor… Adam Roach
- Re: [Gen-art] Gen-Art review of draft-ietf-sipcor… Alexey Melnikov