Re: [RTG-DIR] Routing Area Directorate Review of draft-ietf-mpls-tp-ring-protection-05
Eric Gray <eric.gray@ericsson.com> Tue, 14 May 2013 16:27 UTC
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From: Eric Gray <eric.gray@ericsson.com>
To: Yaacov Weingarten <wyaacov@gmail.com>
Thread-Topic: Routing Area Directorate Review of draft-ietf-mpls-tp-ring-protection-05
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Subject: Re: [RTG-DIR] Routing Area Directorate Review of draft-ietf-mpls-tp-ring-protection-05
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Yaacov, I didn't see this response until I specifically went looking for it. Sorry to have taken so long to get back to you. For the most part, your counter proposal is good enough. See my replies in line below... Thanks for addressing most of my comments. -- Eric From: Yaacov Weingarten [mailto:wyaacov@gmail.com] Sent: Wednesday, April 17, 2013 8:56 AM To: Eric Gray Cc: draft-ietf-mpls-tp-ring-protection@tools.ietf.org; rtg-ads@tools-ietf.org; rtg-dir@ietf.org Subject: Re: Routing Area Directorate Review of draft-ietf-mpls-tp-ring-protection-05 Importance: High Eric, hi Thank you for your review and very in-depth comments, I will try to address some of them inline below (indicated by "yw>>" before the comment. Hope this helps, yaacov On Tue, Apr 16, 2013 at 5:18 PM, Eric Gray <eric.gray@ericsson.com<mailto:eric.gray@ericsson.com>> wrote: Hello, I have been selected as the Routing Directorate reviewer for this draft. The Routing Directorate seeks to review all routing or routing-related drafts as they pass through IETF last call and IESG review, and sometimes on special request. The purpose of the review is to provide assistance to the Routing ADs. For more information about the Routing Directorate, please see http://www.ietf.org/iesg/directorate/routing.html Although these comments are primarily for the use of the Routing ADs, it would be helpful if you could consider them along with any other comments that you receive, and strive to resolve them through discussion or by updating the draft. Document: http://tools.ietf.org/html/draft-ietf-mpls-tp-ring-protection-05 Reviewer: Eric Gray Review Date: 17 April, 2013 Intended Status: Informational There is at least one major issue with this document, as it is now written. Comments/Questions: ================= The "disclaimer" at the end of the Introduction misses a case related to the cases excluded from the scope of this document - which is itself presumably out of scope. yw>> Assume that you are referring to the disclaimer at the end of section 1.1 (as opposed to the one at the end of section 1) I did not refer to section 1, I referred to the Introduction. Since this was the name of section 1, I thought that referring to it by name would be sufficiently unambiguous, since all of section 1 is the Introduction... The text currently talks to protection of a path prior to entry to the ring, or after exit from the ring. The missing case is when the path is part of a protected service that involves one or more alternative paths external to the ring. It is not difficult to think of a scenario in which a protection event outside of the ring will require that data forwarded across the ring will need to enter the ring at a different point, leave the ring at a different point, or both. While this would seem to impact the protection of the ring itself (possibly requiring setup of similar protection for the new path traversing the ring), it should be quite easy to represent such an occurrence as a set of discrete event/operations that would not be significantly different when compared to initial setup of protection for the previous path in the ring. If for any reason folks feel that this may not be the case under some circumstances this document should either address those cases or explicitly state that they will not be addressed by this document. Alternatively, the issue may be addressed by simply removing the 2nd sentence of the "disclaimer" paragraph, or the cases explicitly stated should be expressed as "examples." yw>> How about if I change the text of the second sentence to read: "Protection triggers on the transport path prior to the ring ingress node or beyond the egress nodes may be protected by some other mechanism."? Fine. ------------------------------------------------------------------------------------------------------------- Bullet 4 of section 1.3 is confusing as written. The meaning of "LI" is clear enough, but "LE" presents some problems. Suppose that PHP is used; in this case, there is no label "expected" at the egress point from the ring (although a label may be present and will be used by the LSR at that point, that label may not always be the same label). Is PHP never permitted? yw>> regarding this, I seem to recall such a discussion regarding MPLS-TP and PHP. However, in any case if there is no such label, why couldn't LE=null? It is just being presented for discussion, not as an actual label. Okay. Even as an item for discussion, the text seems to imply that one would always find a label. With PHP, the egress LSR can signal that PHP is to e used by using the appropriate NULL label. However, this label never actually shows up on the wire, so the egress LSR does not expect to see it. My concern is that someone reading this may look to see this as an object to be managed. I suppose that it could be read as NULL, so that may not be an issue. And any LSR that signals PHP and then expects to see a specific label is a darwin candidate. So, fine. Also, under what circumstances would the ring egress point SPME exit LSR not be the same? Based on subsequent reading, I believe this would be the case for use of an SPME to provide "Wrapping" protection - it would probably therefore be a good idea to either say something to this effect (if that is the case) or provide a forward reference (to section 2.3.2, 2.3.3 and 2.3.4, for example, or possibly just to section 2.3). I would suggest adding a sentence to this bullet, along the lines of: "See section 2.3 for examples of where the SPME egress and ring exit might not be the same." yw>> How about if I add ", for example as described in Section 2.3" to the parenthetical phrase? Fine. ------------------------------------------------------------------------------------------------------------- Section 2 appears to ignore 1+1 protection, which would (similar to "Steering") use pre-established protection paths from an ingress LSR/node to each egress LSR/node but would send that traffic both ways at all times. The main differences between this approach and Steering are: o the need to notify the ingress node is eliminated as a consideration for protection switching latency o there is (therefore) no reliance on the existence of a failure detection method that is able to notify the ingress of a failure in the ring o double bandwidth is required This is carried forward in all subsequent sections/subsections. yw>> 1+1 protection is a linear protection methodology, and Section 2 is describing basic Transport Network ring protection as defined in the ITU - which include Steering and Wrapping. 1+1 does not seem to be popular in rings, since historically SDH seemed to shun this alternative. We do reference 1+1 transmission in section 3.2 and in the conclusions. Your recollection of ring protection schemes is probably far fresher than mine. I distinctly recall ring protection methodologies that provide hitless protection by always sending traffic both ways around the ring. In fact, it is hard for me to imagine how one could get hitless protection without this sort of scheme. Because this requires sending traffic on both the working and protection path, it is 1+1 and it requires double bandwidth. I am not certain that ignoring 1+1 protection in section 2 and lightly glossing it over in later sections addresses my comment, but - if it is too late at this point to fix it - then I imagine we will just have to live with it. ------------------------------------------------------------------------------------------------------------- Sections 2.1 and 2.2 do not exactly provide an "apples-to-apples" comparison basis in the bullets relating to "considerations" in each subsection. In order to allow this comparison, both sections would need to provide an indication of the order of protection paths required in a protection domain. Because - in the "Wrapping" case - it is necessary to provide protection SPME for each node and link (in a ring the number of links is exactly the same as the number of nodes), the number of protection SPME is O(2N). For the "Steering" case, however, it is necessary to provide protection SPME for each ingress node to each egress node (except for the last one) - which is O(N^2). This statement is actually made in the introductory text in section 2.4. I would suggest either adding a bullet in section 2.2 and including forward references in these bullets to the analysis in section 2.4, or removing the bullet in section 2.1. yw>> Could you please specify which bullet you are suggesting to delete? Thanx I hope you worked this out on your own. The target bullet is both obvious and unambiguous, given the first paragraph of this comment. I don't even have to look back at the draft to see it in context. Ths two section compare aspects of protection schemes. One apparently includes a statement about the order of the number of required protection paths, while the other does not. The comment suggests either adding the missing information as a bullet in section 2.2, or removing the corresponding bullet in section 2.1. Are you saying that you cannot determine from my comment which bullet that is? ... now looking at the latest draft ... I see (based on the -06 version) that you did not figure this out. Look at the bullets in section 2.1. Then look at the bullets in section 2.2. What you should see is that the 2nd bullet in section 2.2 has no counterpart in section 2.2. While this is true of other bullets as well, the comment I made shows that it is relatively easy to determine the (order of the) number of protection paths required to use steering as the protection scheme. For other bullets, I can see why they were omitted. A bullet that talks about the order of the number of protection paths required for steering is simply not there, for no good reason that I can determine. ------------------------------------------------------------------------------------------------------------- Section 2.3 appears to assume that the desired protection scheme will be based on "Steering" rather than "Wrapping." This assumption is made without any effort/analysis to justify why this assumption is made (at least as far as I can see). If that is the intention, minimally, section 2.3 should start with a statement that this is the assumption made. Reading further, however, subsequent subsection 2.3.2 describes how the SPME concept - as outlined in the introductory text of section 2.3 - may be used for the "Wrapping" approach. The confusion arises as a result of figure 3 and the text that describes it in the preceding paragraph, as this figure/text is what most contributes to an impression that an SPME would start at the ingress and end at an egress. I would suggest that this figure and the preceding paragraph would find a better home on section 2.3.1. This would also align subsections of section 2.3 better as there is a similar figure in section 2.3.2, 2.3.3 and 2.3.4. yw>> Will move the paragraph and figure, as you suggest. ------------------------------------------------------------------------------------------------------------- The last paragraph in introductory text in section 2.3 (immediately prior to the start of section 2.3.1) appears to ignore the case where the egress LSR would use labels to "steer" delivery of labeled packets it receives over the SPME to specific egress ports (of the egress LSR). These labels would most likely not be a part of the label stack received by the ingress LSR (there is no need for these labels to be known outside of the ring) and would have to be pushed onto the stack before pushing on the label for an SPME to the egress LSR/node. yw>> Do you have a suggestion on how better to include this consideration? For thatI need to re-read the draft. ... now looking at the latest draft ... Okay, the last paragraph actually ignores a few things: 1) If the ingress and egress LSRs on the ring are adjacent nodes on the ring, no swapping occurs. 2) in any case where there are more than 3 LSRs involved in the LSP at the hierarchical layer of the SPME, there will be N-2 label swaps that ultimately result in the LE label attached to the PDU arriving at the egress LSR. 3) the point I was trying to make - which was that the ingress LSR may have been given a label stack (which would not be what you refer to as "LI", and would consist of more than one label where only the top level label would be seen anywhere in the LSP but at the ingress, where it is pushed on, and the egress LSR). In re-looking at this, it seems to me that you are trying to avoid this level of detail, so it would be sufficient to "qualify" this description by inserting "in the simplest case" before "packets that arrive..." ------------------------------------------------------------------------------------------------------------- I believe the text in the last paragraph of section 2.3.4 is incorrect in stating that protected traffic might not share the same protection path in both directions. If both LSR-B and LSR-F detect that LSR-A is not available, then both will use the protection SPME between LSR-B and LSR-F. If both LSR-A and LSR-E detect that LSR-F is not available, then both will use the protection SPME between LSR-A and LSR-E. If both LSR-A and LSR-F detect that the link between them is not available, then both will use the protection SPME between them. All that is required is that both LSRs in each case agree as to the nature of the failure (node or link) - which may be easily and consistently determined by monitoring the status of the working and protection SPME. While it is possible for the end-point LSRs in each case to have an inconsistent view of the nature of the failure, this inconsistency should be short lived (based on the periodicity of OAM monitoring of SPMEs). Depending on the OAM monitoring periodicity used - and the length of links in the ring - this inconsistency may exist for a period of time that is less than the latency to be expected for "Steering" based protection. Note that this is a major issue with the current text as it impacts on analysis in subsequent section 2.4 and the conclusion/recommendations of this document. yw>> This statement is the result of review comments that we received during an earlier review of the document, and I am wary of removing it from the document at this late stage in the review process. In any case, there are other considerations for limiting both types of protection. I am sure that your analysis is correct although I am not sure that I understand your emphatic objection to the text and how it is related to the analysis in section 2.4. The analysis appears to be missing information. Yet the analysis is presumably the basis on which the conclusions of the draft are based. However, if the WG feels that the conclusions of this draft are correct, then there may not be much point in addressing this comment. ------------------------------------------------------------------------------------------------------------- The analysis in section 2.4 is incomplete or based on an incorrect assertion made in section 2.3.4. If the alternative suggested in section 2.3.4 were used, the number of SPME that will be required would be O(4N) - which is still O(N) and would be less than O(N^2) for fewer than the 16-node limit assumption made for the analysis. yw>> This is correct and is certainly correct when the comparison is between O(2N) and O(N^2). Therefore whether the statement (reiterated in parenthesis here) is correct or not does not really affect the conclusions. This is based on the simplicity of Steering and the avoidance of wasted resources by transmitting the traffic twice over certain links. Fine ------------------------------------------------------------------------------------------------------------- In the 1st sentence of the last paragraph in the introductory text of section 2.4, the sentence mentions the use of additional resources but does not mention the additional latency. Is there any reason for the omission? yw>> not really I suggest removing the text in this sentence that starts with "even though" as it might otherwise be necessary to attempt an exhaustive list of detractions. yw>> I have no problem with this suggestion ------------------------------------------------------------------------------------------------------------- Also in the last paragraph of the introductory text in section 2.4, I imagine that the case where one or more pairs of SPME may be eliminated because of LSRs that are not ingress-egress pairs is most likely of trivial value in any deployment involving a ring topology. Perhaps the last sentence in the last paragraph of this text in section 2.4 could be removed? yw>> Again, I have no problem with this NITs: ==== In the Introduction section, 5th bullet in (or after) the 2nd paragraph - " Impact of signaling and routing information exchanged during protection, in presence of control plane." - the phrase "in presence of control plane" should probably be "in the presence of a control plane." yw>> will fix ------------------------------------------------------------------------------------------------------------- In the 2nd bullet in section 2.1, I believe it is the case that O(2N) = O(N). yw>> while this is true, I think it is significant to mention the 2N ------------------------------------------------------------------------------------------------------------- In the 1st paragraph of section 2.3.1, where it says "there is always an alternative path ..." - at the SPME level (as described in this document), there's always exactly one alternative path. yw>> This is not 100% true. What is true is that there is exactly on alternative path within the ring (there may be additional alternatives that go outside the ring). This is why I did not originally add the "exactly" in the first place. Paths outside of the ring are explicitly out-of-scope. However, this is fine. ------------------------------------------------------------------------------------------------------------- In the 1st bullet in section 2.4, I believe it is the case that O(2N^2) = O(N^2). yw>> see above, same applies to next statement. ------------------------------------------------------------------------------------------------------------- In the 1st and 2nd bullets in section 2.4, I believe it is the case that O(2N) = O(N). ------------------------------------------------------------------------------------------------------------- -- Thanx and BR, yaacov Still looking for new opportunity
- [RTG-DIR] Routing Area Directorate Review of draf… Eric Gray
- Re: [RTG-DIR] Routing Area Directorate Review of … Yaacov Weingarten
- Re: [RTG-DIR] Routing Area Directorate Review of … Eric Gray
- Re: [RTG-DIR] Routing Area Directorate Review of … Yaacov Weingarten
- Re: [RTG-DIR] Routing Area Directorate Review of … Eric Gray