Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-ospf-ospfv3-segment-routing-extensions-20: (with DISCUSS and COMMENT)
Peter Psenak <ppsenak@cisco.com> Thu, 20 December 2018 12:08 UTC
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To: Benjamin Kaduk <kaduk@mit.edu>
References: <154398144445.4943.7198735398003216566.idtracker@ietfa.amsl.com> <5C079200.1030701@cisco.com> <20181217055358.GC94620@kduck.kaduk.org> <69190220-4994-f9c9-4adf-5016abf3a39b@cisco.com>
Cc: The IESG <iesg@ietf.org>, draft-ietf-ospf-ospfv3-segment-routing-extensions@ietf.org, Acee Lindem <acee@cisco.com>, aretana.ietf@gmail.com, lsr-chairs@ietf.org, lsr@ietf.org
From: Peter Psenak <ppsenak@cisco.com>
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Date: Thu, 20 Dec 2018 13:07:59 +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, are you ok with my responses and proposed changed text for the range? thanks, Peter On 17/12/2018 12:32 , Peter Psenak wrote: > Hi Benjamin, > > please see inline (##PP): > > On 17/12/2018 06:53 , Benjamin Kaduk wrote: >> Sorry for the slow reply -- you caught me right as I was leaving for >> vacation. >> >> On Wed, Dec 05, 2018 at 09:53:20AM +0100, Peter Psenak wrote: >>> 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." >> >> Perhaps it would be better encoded in a 1-bit field (rather than an 8-bit >> one), then? That would at least make the (lack of) semantics of the >> other >> 7 bits more clear, as the usual "MUST set to zero on transmit and >> ignore on >> receipt". > > ##PP > it's too late now to change the encoding. This draft has several years > of history and there are implementation shipping. Changing the encoding > would cause the backward compatibility issues. > >> >>>> >>>> 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. >> >> Okay. >> >>>> >>>> ---------------------------------------------------------------------- >>>> 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. >> >> I understand the difference between OSPFv3 SR with MPLS vs. IPv6 data >> plane >> (well, at least that there is a difference). My point is that you say it >> "will be specified in a separate document". If there's an existing I-D >> that is the start of this work, listing it as an informative reference >> seems helpful to me. (If there's not, perhaps "at a later date" would >> work >> instead of "in a separate document".) >> >> But of course this is a non-blocking comment, so feel free to ignore -- I >> really don't mind. >> >>>> >>>> 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. >> >> Hmm, I must have misspoke; sorry. My point remains, though, that if I >> go to >> I-D.ietf-spring-segment-routing-ldp-interop and search for "range", I >> will >> not find anything to help me know which part of that document you are >> talking about. I would encourage some additional text to clarify how the >> terminology used in this document relates to the terminology and work >> used >> in the referenced document. > > ##PP > range is not defined in I-D.ietf-spring-segment-routing-ldp-interop, > it's the SRMS functionality that is defined there. > The range was defined for IGPs to optimize the encoding for SRMS > advertisement - with thousands of prefixes the encoding would not scale > if we advertise the individual SID for every prefix independently. > > What about the following updated text in the OSPFv3 draft: > > "In some cases it is useful to advertise attributes for a range of > prefixes. The Segment Routing Mapping Server, which is described in > <xref target="I-D.ietf-spring-segment-routing-ldp-interop"/>, is an > example of where SIDs for multiple prefixes can be advertised. To > optimize such advertisement in case of multiple prefixes from a > contiguous address range, OSPFv3 Extended Prefix Range TLV is defined." > >> >>>> >>>> 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. >> >> I generally prefer to describe the normative behavior in actual text >> description instead of relying on examples to clarify the expected >> behavior. That said, this is a non-blocking comment, so feel free to >> retain the current text. If you did want to add something, I would >> propose the strawman: >> >> OLD: >> The range represents the contiguous set of prefixes with the same >> prefix length as specified by the Prefix Length field. The set >> starts with the prefix that is specified by the Address Prefix field. >> The number of prefixes in the range is equal to the Range size. >> >> NEW: >> The range represents the contiguous set of prefixes with the same >> prefix length as specified by the Prefix Length field. The set >> starts with the prefix that is specified by the Address Prefix >> field and >> continues with the subsequent prefixes of the same length, forming a >> contiguous block of addresses. Since the Range Size is not >> restricted to a >> power of two, this new block of addresses may not be describable >> using a >> single address prefix/length. The number of prefixes in the range is >> equal to the Range size. >> >> >> >>> >>>> 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. >> >> That's fine, though even just noting the redundancy and that it exists >> for >> [historical/...] reasons might help some readers understand more easily. >> >>>> >>>> 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. >> >> Precisely; you can just use the abbreviation if you want and make the >> text >> here shorter. > > ##PP > done. > >> >>>> >>>> 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. >> >> Okay, thanks for confirming. >> >>>> >>>> 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. >> >> I do not insist; I'm just trying to understand the common >> usage/conventions. > > ##PP > it has been updated. > > thanks, > Peter > >> >>>> >>>> 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. >> >> Thanks for helping me understand better (here and above). >> >>>> >>>> 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. >> >> Okay, thanks. >> >>>> >>>> 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. >> >> Thanks for the pointer. I was wondering whether RFC5340 itself would >> require the NU bit to be set in this situation -- from a quick skim, it >> seems that it does not, so there's nothing to do here (other than add >> that >> reference, if you want.) >> >>>> >>>> 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. >> >> Okay. Sorry for my ignorance here (and throughout), and thank you again >> for your patient explanations of the "basic concepts". >> >>>> >>>> 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. >> >> Indeed. >> >> -Benjamin >> . >> > > . >
- [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