Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-ospf-ospfv3-segment-routing-extensions-20: (with DISCUSS and COMMENT)

Benjamin Kaduk <kaduk@mit.edu> Mon, 17 December 2018 05:54 UTC

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Date: Sun, 16 Dec 2018 23:53:59 -0600
From: Benjamin Kaduk <kaduk@mit.edu>
To: Peter Psenak <ppsenak@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
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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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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".

> >
> > 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.

> >
> > 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.

> >
> >     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.

> >
> >     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