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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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

>
>
> .
>