Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-ospf-ospfv3-segment-routing-extensions-20: (with DISCUSS and COMMENT)
Peter Psenak <ppsenak@cisco.com> Wed, 05 December 2018 08:53 UTC
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Date: Wed, 05 Dec 2018 09:53:20 +0100
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Subject: Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-ospf-ospfv3-segment-routing-extensions-20: (with DISCUSS and COMMENT)
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Hi Benjamin, please see inline: On 05/12/18 04:44 , Benjamin Kaduk wrote: > Benjamin Kaduk has entered the following ballot position for > draft-ietf-ospf-ospfv3-segment-routing-extensions-20: Discuss > > When responding, please keep the subject line intact and reply to all > email addresses included in the To and CC lines. (Feel free to cut this > introductory paragraph, however.) > > > Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html > for more information about IESG DISCUSS and COMMENT positions. > > > The document, along with other ballot positions, can be found here: > https://datatracker.ietf.org/doc/draft-ietf-ospf-ospfv3-segment-routing-extensions/ > > > > ---------------------------------------------------------------------- > DISCUSS: > ---------------------------------------------------------------------- > > What is the extensibility model for the "AF" (address family) field in the > OSPFv3 Extended Prefix Range TLV? That is, what do we need to say about > current implementations' behavior to allow future changes? (I also a > little bit wonder if we really need a full eight bits, but that's basically > aesthetic.) I don't think OSPFv3 will ever support other then IPv6 or IPv4 AF. Also the text says: "Prefix encoding for other address families is beyond the scope of this specification." > > Some of the text in Section 8.1 (see the COMMENT section) reads like it > might have an "Updates" relationship with other documents, but I don't know > enough to be sure. Hopefully we can have a conversation to clarify the > situation. please see my comments below. > > > ---------------------------------------------------------------------- > COMMENT: > ---------------------------------------------------------------------- > > Section 1 > > Is there a start of the separate document that covers SR with the IPv6 data > plane that we could reference from here? this document describes OSPFv3 extension for SR with the MPLS data plane, not IPv6 data plane. And rfc8402 is referenced. > > Section 5 > > In some cases it is useful to advertise attributes for a range of > prefixes. The Segment Routing Mapping Server, which is described in > [I-D.ietf-spring-segment-routing-ldp-interop], is an example of where > a single advertisement is needed to advertise SIDs for multiple > prefixes from a contiguous address range. > > I note that the referenced document does not use the word "range" to > describe the prefix being assigned multiple SIDs; it might be helpful to > say a few more words about how the range of prefixes gets mapped to what is > discussed in the linked document. "prefix being assigned multiple SIDs" - that is not what we are doing here. > > I'm also not entirely sure how to construct the prefix range just given > this format description. Suppose I have an IPv4 prefix of 18.18/16 and a > range size of 4; my prefix length is 16 and the address prefix is encoded > as 0x120120000. Am I then representing the four prefixes 18.18/16, > 18.19/16, 18.20/16, and 18.21/16? yes. > Or am I constrained to be a subset of > 18.18/18 (in which case I don't know what the actual distinct prefixes > would be)? The examples in Section 6 suggests the former, but I would suggest > stating this explictly, here. > I would thing that the example in section 16 is clear enough. > Section 6 > > Should there be any discussion of the historical or future reasons why V > and L are separate flag bits, given that the only legal combinations are > currently 00 and 11, i.e., fully redundant? I would rather not get into that discussion here. > > It may not be necessary to expand ASBR on first usage here, since it's in > the terminology section (and marked as "well-known" at > https://www.rfc-editor.org/materials/abbrev.expansion.txt). ASBR is defined in terminology section. > > If the NP-Flag is not set, then any upstream neighbor of the Prefix- > SID originator MUST pop the Prefix-SID. This is equivalent to the > penultimate hop popping mechanism used in the MPLS dataplane. If the > NP-flag is not set, then the received E-flag is ignored. > > Is it going to be clear that "pop" only applies when this Prefix-SID is the > outermost label? (Or am I super-confused about how this is supposed to > work?) you can only POP the outmost label. > > A similar consideration may apply to the discussion of the NP flag as well. > Also some redundantly expanded ABR and ASBR here as well. > > This is useful, e.g., when the originator of the Prefix- > SID is the final destination for the related prefix and the > originator wishes to receive the packet with the original EXP > bits. > > Are we still supposed to call these the EXP bits after RFC 5462? (I had to > look up what they were; not sure if this means that we should put a > reference in for them or not, given that I'm not a practitioner here.) I can rename to "Traffic Class" if you insist. > > When the M-Flag is set, the NP-flag and the E-flag MUST be ignored on > reception. > > Do I understand this correctly that this is because the mapping server may > not know the needs of the individual routers, and if the routers had > specific needs they should advertise the SIDs directly (which would take > precedence over the mapping server's advertisement)? If so, given the > following discussion, I wouldn't suggest adding any extra text about it, > but I do want to make sure I'm understanding it properly. your understanding is correct. There is also some more details in the next section. > > When a Prefix-SID is advertised in the OSPFv3 Extended Prefix Range > TLV, then the value advertised in the Prefix SID Sub-TLV is > interpreted as a starting SID/Label value. > > Am I remembering correctly that Prefix-SID can appear multiple times within > OSPFv3 Extended Prefix Range? Then each Prefix-SID would be indicating a > distinct range but adhering to the same parameters of the range that are > indicated in the Extended Prefix Range TLV? This seems a little weird on > the face of it (as opposed to a single Prefix-SID sub-TLV per Extended > Prefix Range), but maybe there's a use case that I'm missing on first > glance. the use case is when you need to advertise Prefix-SID for different Algorithms. > > Section 7.1 > > (Probably off-topic: what's the use case for assigning the same Adj-SID to > different adjacencies?) load balancing of traffic over multiple links. > > Section 7.2 > > Perhaps add DR to the terminology section (or expand on first usage)? ok, will do. > > Section 8.1 > > When a Prefix-SID is advertised by the Mapping Server, which is > indicated by the M-flag in the Prefix-SID Sub-TLV (Section 6), the > route type as implied by the LSA type is ignored and the Prefix-SID > is bound to the corresponding prefix independent of the route type. > > Is this considered to be Update-ing the behavior of another RFC? no. All we say is that the LSA type in which the SID from SRMS is advertised does not need to match the route-type of the prefix for which the SID is adverised. > > Advertisement of the Prefix-SID by the Mapping Server using an Inter- > Area Prefix TLV, External-Prefix TLV, or Intra-Area-Prefix TLV > [RFC8362] does not itself contribute to the prefix reachability. The > NU-bit MUST be set in the PrefixOptions field of the LSA which is > used by the Mapping Server to advertise SID or SID Range, which > prevents the advertisement from contributing to prefix reachability. > > This MUST reads like it is restating an existing normative requirement from > elsewhere (in which case we should probably just state it as fact and > provide a reference). Or is it a new requirement (in which case Updates: > might be in order)? not sure I understand. NU-bit is defined in rfc5340. We are just reusing it here. I can add a reference to it. > > Area-scoped OSPFv3 Extended Prefix Range TLVs are propagated between > areas. Similar to propagation of prefixes between areas, an ABR only > propagates the OSPFv3 Extended Prefix Range TLV that it considers to > be the best from the set it received. The rules used to pick the > best OSPFv3 Extended Prefix Range TLV are described in Section 5. > > I don't see any usage of "best" in Section 5; I do see direction to use the > numerically smallest Instance ID when multiple Extended Prefix Range TLVs > advertise *the exact same range*. But this in and of itself does not > safisfy the claim here that there is guidance to pick a single best > Extended Prefix Range TLV, so I'm left confused as to what's supposed to > happen. Perhaps this was intended as a transition to Section 8.2 instead > of referring back to Section 5 (especially considering that Section 8.1 is > supposed to be intra-area but this topic is inter-area)? > (This sort of dangling/unclear internal reference would normally be a > DISCUSS, but it seems very likely this is just a stale section number and > not a real problem, so I'm keeping it in the COMMENT section for now.) right, I will remove the reference to section 5 and correct the text. > > Section 8.4.1 > > Do we need a reference for 2-Way and FULL? these are standard OSPF adjacency states. > > Section 9 > > I would normally expect some text about "IANA has made permanent the > following temporary allocations" or similar, so the reader can quickly tell > that this is not a case of codepoint squatting. well, I guess what is important is that the IANA allocations has been made. thanks, Peter > > > . >
- [Lsr] Benjamin Kaduk's Discuss on draft-ietf-ospf… Benjamin Kaduk
- Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-… Peter Psenak
- Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-… Benjamin Kaduk
- Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-… Benjamin Kaduk
- Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-… Peter Psenak
- Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-… Peter Psenak
- Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-… Benjamin Kaduk
- Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-… Acee Lindem (acee)
- Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-… Peter Psenak
- Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-… Peter Psenak
- Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-… Benjamin Kaduk
- Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-… Benjamin Kaduk
- Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-… Peter Psenak
- Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-… Benjamin Kaduk
- Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-… Peter Psenak
- Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-… Benjamin Kaduk
- Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-… Peter Psenak