Re: [OSPF] AD review of draft-ietf-ospf-segment-routing-extensions-16

Alia Atlas <akatlas@gmail.com> Wed, 14 June 2017 14:22 UTC

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From: Alia Atlas <akatlas@gmail.com>
Date: Wed, 14 Jun 2017 10:22:32 -0400
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To: "Stefano Previdi (sprevidi)" <sprevidi@cisco.com>
Cc: "Acee Lindem (acee)" <acee@cisco.com>, OSPF List <ospf@ietf.org>, "draft-ietf-ospf-segment-routing-extensions@ietf.org" <draft-ietf-ospf-segment-routing-extensions@ietf.org>, "Alvaro Retana (aretana)" <aretana@cisco.com>, "BRUNGARD, DEBORAH A (ATTSI)" <db3546@att.com>, "spring-chairs@tools.ietf.org" <spring-chairs@tools.ietf.org>
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Subject: Re: [OSPF] AD review of draft-ietf-ospf-segment-routing-extensions-16
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On Wed, Jun 14, 2017 at 9:13 AM, Stefano Previdi (sprevidi) <
sprevidi@cisco.com> wrote:

>
> > On May 31, 2017, at 4:34 PM, Alia Atlas <akatlas@gmail.com> wrote:
> >
> > Hi Acee,
> >
> > On Wed, May 31, 2017 at 10:11 AM, Acee Lindem (acee) <acee@cisco.com>
> wrote:
> > Hi Alia,
> > Thank you for your comments. I certainly don’t agree with all of them
> but will allow the authors to respond. For example, I believe the concept
> of an SRMS to be well-undestood and defined in the SPRING WG. Perhaps we
> just need the right references.
> >
> > I found circular references about an SRMS in the SPRING WG documents but
> nothing that was a clear definition.
>
>
> draft-ietf-spring-segment-routing-ldp-interop is the document where the
> srms is briefly introduced. I agree with you, more details are needed and
> then the sr-igp drafts (ospf,ospfv3,isis) should point to the ldp-interop
> draft.
>
> I will submit a new version of draft-ietf-spring-segment-routing-ldp-interop
> not later than this week.
>

Thank you.

Alia

s.
>
>
> >  I didn't read all the SPRING WG drafts, of course, but I did follow the
> references from this document and from that on - back to the
> isis-segment-routing-extensions draft. Obviously, on the one hand, it
> isn't the job of the OSPF WG to define this - but it does need clear
> references so the technology can be understood in context.
> >
> > The one comment I will respond to is the one regarding the author limit.
> Note that this is covered in the Shepherd’s Write-Up. I’ve excerpted it
> here:
> >
> >       The document does have seven authors. All the authors have
> >       played in active role in the development of the standard including
> >       periodic segment routing design team meetings.  All of the authors
> >       have responded promptly to IPR polls. At least three of the
> >       authors represented independent implementations. There is
> >       absolutely no reason to relegate any of them to contributor status.
> >
> > Then the solution may be to have one or two be editors and on the front
> page.  I am willing to discuss but
> > I am getting quite tired of this consistent issue on almost every draft
> I receive for publication.
> >
> > I’ll be on vacation the remainder of this week but will touch base with
> the authors on Monday.
> >
> > Have a good vacation!
> >
> > Regards,
> > Alia
> >
> >
> > Thanks,
> > Acee
> > From: OSPF <ospf-bounces@ietf.org> on behalf of Alia Atlas <
> akatlas@gmail.com>
> > Date: Tuesday, May 30, 2017 at 10:35 PM
> > To: OSPF WG List <ospf@ietf.org>rg>, "draft-ietf-ospf-segment-
> routing-extensions@ietf.org" <draft-ietf-ospf-segment-
> routing-extensions@ietf.org>gt;, "Alvaro Retana (aretana)" <aretana@cisco.com>om>,
> Deborah Brungard <db3546@att.com>
> > Cc: "spring-chairs@tools.ietf.org" <spring-chairs@tools.ietf.org>
> > Subject: Re: [OSPF] AD review of draft-ietf-ospf-segment-
> routing-extensions-16
> >
> > I forgot to point out that the Security Considerations sections is not
> close to sufficient.
> > At a minimum, it needs to refer to the existing security work for OSPF,
> indicate what new
> > information is being advertised, and discuss if there are any privacy or
> security concerns
> > around them.  I don't personally see any - except for, perhaps, the
> increased ability to fingerprint
> > the type and version of routers with these advertisements.
> >
> > Regards,
> > Alia
> >
> > On Tue, May 30, 2017 at 10:05 PM, Alia Atlas <akatlas@gmail.com> wrote:
> > As is customary, I have done my AD review of draft-ietf-ospf-segment-routing-extensions-16
> once publication has been requested.  First, I would like to thank the
> editors & many authors, Peter, Stefano, Clarence, Hannes, Rob, Wim & Jeff,
> for the work that they have put in so far and the remaining work that is
> greatly needed.
> >
> > While there are a great many issues to be handled, they fall primarily
> into three categories.  The first is simply not going through and
> tightening up the details; for example, stating that the length of a TLV is
> variable provides no meaning.  The second is that the technical documents
> from SPRING that this draft depends on do not adequately describe the use
> of the advertised information (SID/Label Binding TLV) or some of the
> concepts (e.g. SR Mapping Server).  The third is a more common set of
> handling error cases and adding clarity to the intended behavior.  I do not
> see issues with the encodings but I do see fragility with the unstated
> assumptions and behaviors.  The draft describes encodings, but very little
> of the handling, behaviors, or meaning - and the references do not provide
> adequate detail.
> >
> > I have spent all day (and evening) doing this review and I am quite
> disappointed and concerned about the document.  I would strongly recommend
> having sharing the next WGLC with the SPRING working group; perhaps more
> eyes will help with the discrepancies.
> >
> > I have not yet decided what to do about the "early" IANA allocation -
> which has now existed for this draft for 3 years.  I do know that there are
> implementations,
> > but I am currently seeing the failure of this work to successfully
> complete as an example of an issue with providing early allocations.
> >
> > MAJOR ISSUES:
> >
> > 1) This draft has 7 authors.  The limit for authors & editors is 5, as
> is clearly stated in RFC 7322 Sec 4.1.1 and has been the case for well over
> a decade, unless there are extraordinary circumstances.  Is there a reason
> to not simply list the active editor and move the others to contributors?
> One of the authors is already listed there.  I regret that failure to deal
> earlier with this long-standing IETF policy will be delaying progressing
> the draft.
> >
> > 2) This expired individual draft(draft-minto-rsvp-lsp-egress-fast-protection-03)
> is listed as Informative - but IS ACTUALLY NORMATIVE since it DEFINES the
> > "M-bit - When the bit is set, the binding represents a mirroring context
> as defined in [I-D.minto-rsvp-lsp-egress-fast-protection]."
> Unfortunately, when I look there for the definition of a mirroring context,
> it doesn't exists.
> >
> > 3) The following Informative references expired several years ago and -
> being individual drafts - do not appear to convey the SPRING or TEAS WG
> consensus.
> >    a)  draft-filsfils-spring-segment-routing-ldp-interop-03 was
> replaced with draft-ietf-spring-segment-routing-ldp-interop-07 and there
> are considerable differences.
> >    b) It is unclear what happened to draft-filsfils-spring-segment-routing-use-cases-01,
> but I do not see any successor - or reason for this individual draft to
> explain the OSPFv2 extensions more than work from the SPRING WG.
> >
> > 4) Sec 3.3: Is it ok to advertise an SRLB TLV without advertising the
> SR-Algorithm TLV?  What is the expected behavior and assumptions by the
> receiver?
> >
> > 5) Sec 3.4:  What happens if an SRMS Preference TLV is advertised
> without an SR-Algorithm TLV in the same scope?  I see that it says "For the
> purpose of the SRMS Preference Sub-TLV advertisement, AS scope flooding is
> required." but also provides for area scope flooding.  Some words
> clarifying the expected behavior would be useful.
> >
> > 6) Sec 5: "In such case, MPLS EXP bits of the Prefix-SID are not
> preserved for
> > the final destination (the Prefix-SID being removed)."   I am quite
> startled to see an assumption that MPLS Pipe mode is being forced as part
> of specifying PHP mode!  This will also break any ECN or 3-color marking
> that has affected the MPLS EXP bits.  I would like to see and understand a
> clear justification for why short-pipe mode is being required instead of
> Uniform (or up to implementation/configuration.).   Basically, this
> sentence means that transport considerations are a necessary section -
> which is completely inappropriate in an IGP draft.
> >
> > 7) Sec 6: This section defines the SID/Label Binding sub-TLV - which
> appears to be a way to advertise an explicit path - and has a SID/Label by
> which the path can be entered.   How and what state is set up by the
> sending router to create the indicated segment is completely unclear.   I
> have hunted through draft-ietf-spring-segment-routing,
> draft-ietf-spring-segment-routing-mpls, and draft-ietf-spring-segment-routing-ldp-interop,
> RFC7855, and draft-ietf-isis-segment-routing-extensions.   As far as I
> can tell, NONE of them clearly describe the details of where and why this
> advertising is needed.  Obviously, this mechanism does allow the potential
> shortening of the MPLS label stack at the cost of advertising multi-hop
> explicit path segments across the entire area or AS.  There MUST be a
> normative description of what the sending router will do when a packet is
> received with the specified label.
> >
> > 8) Sec 4: "The Segment Routing Mapping Server, which is described in
> [I-D.filsfils-spring-segment-routing-ldp-interop]"  Where precisely is an
> SRMS and its behavior/role actually defined?  draft-ietf-spring-segment-routing-ldp-interop-07
> claims:"SR to LDP interworking requires a SRMS as defined in
> [I-D.ietf-isis-segment-routing-extensions]." but that wouldn't be
> appropriate, of course, and it isn't there either!
> draft-ietf-spring-conflict-resolution-04 talks about SRMS, but doesn't
> define it.   draft-ietf-spring-segment-routing-11 mentions in Sec 3.5.1
> that "A Remote-Binding SID S advertised by the mapping server M" and refers
> to the ldp-interop draft for further details - but obviously not about an
> SRMS.
> >
> > Minor Issues:
> >
> > 1) In Sec 3.1, it says: "The SR-Algorithm TLV is optional. It MUST only
> be advertised once in the Router Information Opaque LSA.  If the SID/Label
> Range TLV, as defined in Section 3.2, is advertised, then the SR-Algorithm
> TLV MUST
> > also be advertised."  Please provide a pointer in the text to the
> behavior for a receiving router if one or both of these are violated?   For
> the requirement to advertise the SR-Algorithm TLV, please clarify that this
> is in the same RI LSA as the SID/Label Range TLV was advertised & with the
> same scope.  What does it mean, in terms of the receiving router, to
> determine that the sending router supports SR or not - given the
> possibility of receiving other SR-related TLVS in an RI LSA without getting
> an SR-Algorithm TLV?
> >
> > 2) Sec 3.1: The SR-Algorithm TLV simply defines "Length: Variable".
> Given that advertising Algorithm 0 is required, I'm fairly sure that the
> Length has to be a minimum of 1 - and, to prevent overrun & weird issues,
> let's have a reasonable maximum (for instance, 24) too.  It wouldn't hurt
> to remind readers that the length is just that of the value field - though
> experienced OSPF implementers will know that.
> >
> > 3) Sec 3.1 & Sec 3.2 & Sec 3.3: "For the purpose of SR-Algorithm TLV
> advertisement, area scope flooding is required." and "For the purpose of
> SID/Label Range TLV advertisement, area scope flooding is required."  and
> "For the purpose of SR Local Block Sub-TLV TLV advertisement, area scope
> flooding is required." Please capitalize REQUIRED as per RFC 2119.
> Otherwise, please explain behavior when area scope isn't used.
> >
> > 4) Sec 3.2:  The SID/Label Range TLV doesn't indicate that include a
> SID/Label sub-TLV is required - but I don't understand how it could be
> interpreted otherwise; nor does it indicate what to do if there are
> multiple SID/Label sub-TLVs included in a single SID/Label Range TLV. Again
> "Length" is just defined as variable.  In this case, it clearly can't be
> less than 11 (probably 12, assuming padding to the 32-bit boundary).   It
> would be useful to have an upper-bound on length, but at least here I can
> see the argument that meaningful flexibility is provided for.
> >
> > 5) SID index is used without introduction in Sec 3.2.  It isn't defined
> in the terminology of draft-ietf-spring-segment-routing-11 and the other
> uses of it in this document aren't enough to clearly define it.  Please add
> at least a description of its meaning before use - in a terminology
> section, if necessary.
> >
> > 6) Sec 3.2: "The originating router advertises the following ranges:
> >          Range 1: [100, 199]
> >          Range 2: [1000, 1099]
> >          Range 3: [500, 599]"
> > Please turn this into the information actually advertised - i.e.
> >    Range 1: Range Size: 100   SID/Label sub-TLV: 100  => meaning [100,
> 199]
> > etc.
> >
> > 7) 3.2. SID/Label Range TLV:  Please specify that the sender MUST NOT
> advertise overlapping ranges & how to handle the case when it does.  This
> is required by draft-ietf-spring-conflict-resolution.
> >
> > 8) Sec 3.3  SR Local Block (SRLB) Sub-TLV: The document doesn't specify
> that the SR Local Block TLV MUST include a SID/Label sub-TLV nor indicate
> what to do if multiple are included.  The Length, again, isn't specified at
> all and clearly has at least a minimum.   I don't see a reference to an SR
> Local Block or the need to advertise it in draft-ietf-spring-segment-routing-11;
> perhaps I missed where the requirement and usage are defined?
> >
> > 9) Sec 3.3: "Each time a SID from the SRLB is allocated, it SHOULD also
> be
> >    reported to all components..."  Presumably, this is subjected to the
> normal OSPF dampening - it'd be nice to note that somewhere - since rapid
> sequential allocation may not provide the reporting speed anticipated.
> >
> > 10) Sec 4: "AF: Address family for the prefix. Currently, the only
> supported
> >       value is 0 for IPv4 unicast.  The inclusion of address family in
> >       this TLV allows for future extension."  Could you please clarify
> if this is to reuse the same TLV for OSPFv3 so IPv6 can be supported, are
> you thinking of extending OSPFv2 for IPv6 prefixes for some cases or
> something else? I think the current phrasing is likely to raise questions.
> Similarly, please define "Prefix length: Length of the prefix" clearly.  I
> really don't understand what the benefit of having a TLV that pretends to
> support multiple AFs but can't is versus the clarity of specifying the
> prefix lengths.
> >
> > 11) Sec 4:  Again "Length: Variable" - It should have a minimum and
> preferable describe a function for how it is computed.  A maximum is
> probably unlikely  with sub-TLVs.
> >
> > 12) Sec 4: OSPF Extended Prefix Range TLV:  Does this TLV has any
> meaning or action associated with it without including sub-TLVs?  Are there
> mandatory sub-TLVs?  What is a receiving router to do with it?
> >
> > 13) Sec 5: "If multiple Prefix-SIDs are advertised for the same prefix,
> the
> >   receiving router MUST use the first encoded SID and MAY use
> >   subsequent SIDs."  What does this even mean?  A receiving router when
> making the decision to use a subsequent SID is making a decision to not use
> the first encoded SID; it's not like the router is going to stick both
> SID/Labels onto the stack.   Please describe this in meaningful normative
> terms.
> >
> > 14) Sec 5:" When calculating the outgoing label for the prefix, the
> router MUST
> >    take into account the E and P flags advertised by the next-hop router
> >    if that router advertised the SID for the prefix.  This MUST be done
> >    regardless of whether the next-hop router contributes to the best
> >    path to the prefix."  First, I assume this is "NP flag" because there
> is no P flag.
> >    Second - please clarify to "take into account, as described below,
> the E and NP flags...".  Third, the M flag must also be taken into account
> - given the text later in the section.
> >
> > 15) Sec 5: "When a Prefix-SID is advertised in an Extended Prefix Range
> TLV, then the value advertised in the Prefix SID Sub-TLV is interpreted as a
> >    starting SID value."   This appears to contradict "SID/Index/Label:
> According to the V and L flags, it contains either:
> >
> >          A 32-bit index defining the offset in the SID/Label space
> >          advertised by this router.
> >
> >          A 24-bit label where the 20 rightmost bits are used for
> >          encoding the label value."
> >   I assume that what is meant by the first quote is "...is interpreted,
> if the V flag is clear, as a starting SID value, and if the V flag is set,
> as a starting Label value."  Otherwise, it looks like the Prefix-SID
> sub-TLV couldn't be included in the Extended Prefix Range TLV if a label
> value would be used.
> >
> > It would be helpful for Example 2 to show the label case.
> >
> > 16) Sec 6.1: "aggregate IGP or TE path cost."  Given that this is an
> OSPF draft, it'd be helpful to indicate whether there are challenges with
> non-comparable OSPF metrics (I'm thinking about AS-external type 2 costs)
> or if the path will never include such costs.
> >
> > 17) Sec 6.2: "a domain and hence need to be disambiguated using a
> domain-unique Router-ID."  Given that the Prefix-SIDs and sub-TLVs can be
> distributed between areas and even redistributed between protocols, please
> clearly define what is meant by a "domain" or point to the appropriate
> definition.
> >
> > 18) Sec 4, 5, 6:  Is it possible to have an OSPF Extended Prefix Range
> TLV that includes both a Prefix SID Sub-TLV and a SID/Label Binding
> Sub-TLV?   What does that mean?
> >
> > What does it mean if there are multiple prefixes described in the OSPF
> Extended Prefix Range TLV that includes a SID/Label Binding Sub-TLV?  Does
> the SID/Label sub-sub-TLV indicate a single SID Index or Label that is used
> for the single path to all those prefixes?  Is it the start of a list of
> SID Indices or Labels?
> > I see that the SID/Label Binding sub-TLV can be in both the OSPF
> Extended Prefx Range TLV as well as the OSPF Extended Prefix TLV - but
> there is no text on differences in interpretation.
> >
> > 19) Sec 7.1 & 7.2: Another  couple "Length: Variable."  Please actually
> specify the value. I think that, given the padding to 32-bit alignment,
> there is a single correct value.
> >
> > 20) Sec 7.1 and 7.2: Given that the Flag bits have exactly the same
> meaning - it'd be clearer to have them defined once.
> >
> > 21) Sec 8.1: "An SR Mapping Server MUST use the OSPF Extended Prefix
> Range TLV when advertising SIDs for prefixes.  Prefixes of different
> route-types can be combined in a single OSPF Extended Prefix Range TLV
> advertised by an SR Mapping Server."    So - I can't find a normative
> definition of an SRMS to determine why it is always necessary to use an
> OSPF Extended Prefix Range TLV instead of an OSPF Extended Prefix TLV.   I
> don't see how advertising prefixes from different route-types can work
> unless the prefixes are adjacent, which seems likely to be uncommon.
> Perhaps what is meant is "Because the OSPF Extended Prefix Range TLV
> doesn't include a Route-Type field, as in the OSPF Extended Prefix TLV, it
> is possible to include adjacent prefixes from different Route-Types in the
> OSPF Extended Prefix Range TLV."
> >
> > 22) Sec 8.1: "If multiple routers advertise a Prefix-SID for the same
> prefix, then
> > the Prefix-SID MUST be the same.  This is required in order to allow
> traffic load-balancing when multiple equal cost paths to the destination
> exist in the OSPFv2 routing domain."  How is this enforced?  What are the
> consequences of it not being conformed to?  This is NOT a protocol
> implementation requirement.  This should really be called out in a
> Manageability Considerations with warnings.
> >
> > 23) Sec 8.2:"If no Prefix-SID was advertised for the prefix in the
> source area
> >       by the router that contributes to the best path to the prefix, the
> >       originating ABR will use the Prefix-SID advertised by any other
> >       router when propagating the Prefix-SID for the prefix to other
> >       areas."  I believe that this depends on the assumption that if a
> Prefix-SID is advertised by any router, the Prefix-SID will be the same.
> Please be explicit in this assumption, since the requirement on the network
> operator should be clear as well as the consequences of not conforming.
> >
> > 24) Sec 10:  The Implementation Status section should indicate that it
> is to be removed before publication as an RFC.   Also, the complete
> implementation part seems a bit dated - given the draft's technical changes
> in the last 2 years.
> >
> >
> > NITS:
> >
> > 1) Sec 2.1: s/"SID/Label TLV"/"SID/Label sub-TLV"
> >
> > 2) Sec 3.2:"Initially, the only supported Sub-TLV is the SID/Label TLV
> as defined
> >    in Section 2.1.  The SID/Label advertised in the SID/Label TLV
> >    represents the first SID/Label in the advertised range."
> >    replace SID/Label TLV with SID/Label sub-TLV.
> >
> > 3) Sec 3.3 & Sec 3.4: " The SR Local Block (SRLB) Sub-TLV is a top-level
> TLV of the Router Information Opaque LSA (defined in [RFC7770])."   Please
> correct the descriptions (many) to SR Local Block (SRLB) Sub-TLV to SR
> Local Block SRLB TLV.   The same issue exists for "SRMS Preference Sub-TLV".
> >
> > Regards,
> > Alia
> >
> >
> >
> >
>
>