Martin hi! Lots of thanks for a prompt response. Please see some comments inline below.
Regards, Sasha Office: +972-39266302 Cell: +972-549266302 Email: [email protected] From: Martin Horneffer [mailto:[email protected]] Sent: Wednesday, January 06, 2016 5:21 PM To: Alexander Vainshtein; [email protected] Cc: [email protected]; [email protected]; [email protected]; [email protected] Subject: Re: [spring] Last Call: <draft-ietf-spring-problem-statement-06.txt> (SPRING Problem Statement and Requirements) to Informational RFC Hello everyone, concerning the first comment: I don't think that a document needs to explicitly specify all out-of-scope topics in order to be self-contained. So not exactly a contradiction, just maybe confusing. [[Sasha]] I fully agree that a document does not have to explicitly specify all out-of-scope issue. But IMHO issues that are not mentioned in the document should not appear in the Abstract. It is up to you and your colleagues to decide whether you remove Multicast from the Abstract or add it to the document:). Concerning the second comment: This is just another example of the frequent misunderstanding, that typical use cases for segment routing would required a lot of explicit path information. [[Sasha]] The typical use cases may or may not require lots of explicit path information. But the draft explicitly mentions support of strict route option for Traffic Engineering (the first bullet in the long list of TE requirements in Section 3.3). And, to the best of my understanding, a strict route option means that all the links (or at least all the nodes) on the path are specified in the right order. Do I miss something? There's more than enough analyses that show that in typical carrier topologies traffic engineering with segment routing can achieve almost everything with just adding one or maybe two segments (labels). [[Sasha]] I have seen several such analyses but they mainly deal with TI-FRR. I have not seen any that say that one or two added segments address the typical traffic engineering needs - and I am not sure we can define what exactly the typical traffic engineering needs are- e.g., can you claim that a typical strict TE route with not exceed 10 hops/segments? However those analyses are not neccessarily public, nor would I think an Internet Draft or RFC would be a good place for this kind of information. [[Sasha]] May I suggest that that you take a look at Section 9 "Operation in Real World Topologies" of RFC 7490<https://www.rfc-editor.org/rfc/rfc7490.txt>? From my POV this is a valid counterexample to your approach. However, I do not expect to see such analysis in the Problem Statement document. I am asking to acknowledge a problem first of all. Once this is done, you can explain (possibly in another document) how it can be addressed. And to make it absolutely clear: from my POV the very fact that somebody has taken care to perform the analyses to which you refer is an indication that the problem exists and that the operators are aware of its existence. Maybe we should add a section to draft-ietf-spring-segment-routing-mpls saying that the operators are responsible for understanding the capabilities of the hardware they use, and to consider those when setting up their specific traffic-engineering tricks. [[Sasha]] Of course the operators are responsible for understanding the capabilities of the HW they use. The really interesting question, from my POV, is, whether the TE entities (e.g., the CSPF algorithms that would be used in conjunction with Segment Routing) should be also aware of these capabilities (or, rather, limitations)? From my POV the answer is "Definitely yes" - I would rather not encounter the situation when the PCE sends to the requesting NE a TE path that meets the operator-specified constraints (e.g., avoid over-utilized links and try to use under-utilized ones) but the network cannot implement "as given". Best regards, Martin Am 05.01.16 um 20:59 schrieb Alexander Vainshtein: Hi all, I have read the Segment Routing Problem Statement and Requirements draft and I have a couple of comments on it. Editorial: The Abstract states that "Multicast use-cases and requirements are out of scope of this document", but this (or equivalent) statement does not appear anywhere in the body of the document. IMHO and FWIW this contradicts the last para in Section 4.3 of RFC 7322 that states that "the RFC should be self-contained as if there were no Abstract". Technical: The draft requires, in Section 2, that "The SPRING architecture SHOULD leverage the existing MPLS dataplane without any modification...". In addition, in Section 3.3 it requires that "The SPRING architecture SHOULD allow incremental and selective deployment without any requirement of flag day or massive upgrade of all network elements" . My reading (FWIW) of these two requirements is that SPRING with MPLS dataplane should work on existing MPLS forwarding HW. If this understanding is correct, it is in potential conflict with another requirement formulated in the Section1 of the draft: "The SPRING architecture SHOULD allow optimal virtualization: put policy state in the packet header and not in the intermediate nodes along the path". This conflict stems from the following (admittedly, naïve) observation: 1. The policy state representing the desired source route must be pushed in its entirety onto the packet by the source (here source is interpreted in the same way as in the draft itself) and must be parsed by all the transit nodes. 2. The amount of the policy state grows (linearly?) as the number of elements in the source route selected by the packet. In particular, the policy representing a strict route could be potentially quite large. 3. In the nodes that use hardware-based forwarding, the size of the policy state that can be pushed and parsed with the expected throughput is inherently limited. These limits differ for different implementations but they usually cannot be exceeded without replacing the forwarding hardware. 4. Passing "offending" packets for software handling could result in dramatic decrease of throughput. S In the case of the MPLS dataplane, the policy state is expressed as the MPLS label stack where each segment is represented by a label stack entry. AFAIK, existing (and probably future) forwarding HW that supports MPLS is inherently limited (the limits differ for different implementations) both regarding the number of labels that could be pushed on the packet, and regarding the total depth of the label stack that it can parse. Note: The limit on the number of labels that can be pushed on a packet by forwarding HW is obvious. The limit on that can be parsed becomes essential in the scenarios when ECMP is used, because: �. As per RFC 7325, Section 2.4.5.1., "The most entropy is generally found in the label stack entries near the bottom of the label stack (innermost label, closest to S=1 bit)" �. As per Section 2.4.5.2 of the same RFC, "Inspecting the IP payload provides the most entropy in provider networks. The practice of looking past the bottom of stack label for an IP payload is well accepted and documented in [RFC4928] and in other RFCs". �. Both these methods (hashing the label stack and hashing IP header) obviously require parsing the entire label stack. The limits of forwarding HW could be considered an implementation problem, were it not for the draft requiring (and I fully agree with validity of this requirement) that SPRING could be used on existing MPLS-capable HW. >From my POV the document should at least explicitly acknowledge this conflict >as part of the SPRING problem statement. Preferably it should also include >some guidelines for its resolution: �. One possibility that comes to mind could be a requirement to provide the information about hardware-specific limitations to traffic-engineering entities in order to avoid computation of paths that do not meet HW-imposed constraints. �. Another possibility is to clearly indicate that loose route options are preferable for using with SPRING. To the best of my understanding this could be translated into a requirement for a new type of constrained path computation algorithms that yield loose (rather than strict) routes Of course there may be other (and, possibly, better) ways to resolve this conflict. But, from my POV, if it is not acknowledged explicitly, its resolution becomes much more problematic. Hopefully, these LC comments would be useful. Regards, Sasha Office: +972-39266302 Cell: +972-549266302 Email: [email protected]<mailto:[email protected]> -----Original Message----- From: spring [mailto:[email protected]] On Behalf Of The IESG Sent: Tuesday, December 15, 2015 8:11 PM To: IETF-Announce Cc: [email protected]<mailto:[email protected]>; [email protected]<mailto:[email protected]>; [email protected]<mailto:[email protected]>; [email protected]<mailto:[email protected]>; [email protected]<mailto:[email protected]> Subject: [spring] Last Call: <draft-ietf-spring-problem-statement-06.txt> (SPRING Problem Statement and Requirements) to Informational RFC The IESG has received a request from the Source Packet Routing in Networking WG (spring) to consider the following document: - 'SPRING Problem Statement and Requirements' <draft-ietf-spring-problem-statement-06.txt> as Informational RFC The IESG plans to make a decision in the next few weeks, and solicits final comments on this action. Please send substantive comments to the [email protected]<mailto:[email protected]> mailing lists by 2016-01-05. Exceptionally, comments may be sent to [email protected]<mailto:[email protected]> instead. In either case, please retain the beginning of the Subject line to allow automated sorting. Abstract The ability for a node to specify a forwarding path, other than the normal shortest path, that a particular packet will traverse, benefits a number of network functions. Source-based routing mechanisms have previously been specified for network protocols, but have not seen widespread adoption. In this context, the term 'source' means 'the point at which the explicit route is imposed' and therefore it is not limited to the originator of the packet (i.e.: the node imposing the explicit route may be the ingress node of an operator's network). This document outlines various use cases, with their requirements, that need to be taken into account by the Source Packet Routing in Networking (SPRING) architecture for unicast traffic. Multicast use- cases and requirements are out of scope of this document. The file can be obtained via https://datatracker.ietf.org/doc/draft-ietf-spring-problem-statement/ IESG discussion can be tracked via https://datatracker.ietf.org/doc/draft-ietf-spring-problem-statement/ballot/ No IPR declarations have been submitted directly on this I-D. _______________________________________________ spring mailing list [email protected]<mailto:[email protected]> https://www.ietf.org/mailman/listinfo/spring _______________________________________________ spring mailing list [email protected]<mailto:[email protected]> https://www.ietf.org/mailman/listinfo/spring
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