Re: [Gen-art] Review: draft-ietf-pcn-sm-edge-behaviour-08
Tom Taylor <tom.taylor.stds@gmail.com> Mon, 02 January 2012 14:21 UTC
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Date: Mon, 02 Jan 2012 09:21:12 -0500
From: Tom Taylor <tom.taylor.stds@gmail.com>
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To: "Joel M. Halpern" <jmh@joelhalpern.com>
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Cc: gen-art@ietf.org, David Harrington <ietfdbh@comcast.net>, draft-ietf-pcn-sm-edge-behaviour@tools.ietf.org, Steven Blake <slblake@petri-meat.com>
Subject: Re: [Gen-art] Review: draft-ietf-pcn-sm-edge-behaviour-08
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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. > >>> >>> 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