| Hi, Ketan:
I reviewed your discussions in detail and also interested that you raised the role of UPA signal originator and UPA signal advertiser in different areas(from access, to aggregate and core etc.)
I know also you are the OSPF experts, and should be aware the description in RFC 2328:
“Else, if the routing table cost equals or exceeds the value LSInfinity, a summary-LSA cannot be generated for this route.”
Then, based on the above multi-areas scenarios, how the ABRs in aggregate or core area can propagate the UPA signal further, via one kind of summary-LSA?
Doesn’t the behavior described in this document conflict with the above design in RFC2328? Hi Peter,
The text looks good to me. I'll clear my DISCUSS position once the updated version is posted.
Thanks, Ketan
Hi Keatn,
please see inline (##PP3):
On 22/09/2025 18:06, Ketan Talaulikar
wrote:
Hi Peter,
Only one point remains. Please check inline below for
KT2.
Hi Ketan,
please see inline (##PP2):
On 22/09/2025 15:43, Ketan Talaulikar wrote:
Hi Peter,
Thanks for your responses. Please check inline
below for follow-ups. Skipping the ones where we
have reached an agreement.
Hi Ketan,
thanks for the comments, please see inline
(##PP):
On 19/09/2025 19:49, Ketan Talaulikar via
Datatracker wrote:
Ketan Talaulikar has entered the following ballot position for
draft-ietf-lsr-igp-ureach-prefix-announce-09: Discuss
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----------------------------------------------------------------------
DISCUSS:
----------------------------------------------------------------------
Thanks to the authors and the WG for their work on this document.
I believe this is a useful feature in specific deployment use cases where
summarization is used for scaling purposes.
I have a few points that I would like to discuss.
discuss#1: Feature Enablement - I believe that UPA is an optional feature
of IGPs and not a core IGP functionality. Therefore, it should be disabled by
default. While there is text in the document about various control knobs and
parameters for implementations, I was not able to find anything about
enablement (at originating, propagating, and receiving routers?) which I
believe is required?
##PP
For originating routers, section 2 says:
"UPA MAY be generated by the ABR or ASBR..".
I added a text in section 2:
"Generation of the UPA at the ABR or ASBR is
optional and SHOULD be controlled by
a configuration knob."
KT> This works for me, but consider
"Generation and propagation of the UPA ..." to
also cover the next part?
##PP2
done
I would leave the default behavior for the
implementations to decide. I see no reason why
an RFC should mandate any specific default
behavior.
For propagation, I would think that if the
ABR supports the UPA, it should propagate it.
Implementations are free to provide control if
they wish to, but I see no reason why an RFC
should mandate that.
KT> Please see previous. I agree it is up to
the implementation - most likely it is the same
knob for UPA enablement as the propagating ABR
might as well be the originating ABR for its local
area.
##PP2
I added "propagation"
For receiving routers, there is a text in
section 7:
"Processing of the received UPAs is optional
and SHOULD be controlled by the configuration
at the receiver."
discuss#2: Limit/control at ABR/ASBR - Just like the ABR/ABSR
that are originating UPAs, is some control and limit expected at an ABR/ASBR
that is propagating UPAs? Is there some check required that those UPAs are
covered by a summary that is being also propagated (or originated) by that
ABR/ASBR?
##PP
Implementations are free to provide all sorts
of control knobs, but from the UPA
specification the only one that are worth of
specifying are the ones at the originating and
processing routers, which has been done.
KT> Section 2 has the following text:
Implementations
MAY limit the UPA generation to specific
prefixes, e.g. host prefixes, SRv6 locators, or
similar. Such filtering is optional and MAY be
controlled via configuration.
It
is also RECOMMENDED that implementations limit
the number of UPA advertisements which can be
originated at a given time.
I assume the reason for this is to ensure that
in some pathological cases, there is not a storm
of UPAs or a large number of UPAs being generated.
If we consider access, aggregation, and core
layers, then at each progressive level the
propagation involves the UPAs of the lower level
of hierarchy being sent towards the core. In this
case, the propagating ABR/ASBRs are also kind of
originating from the UPAs from the lower layer in
its LSAs/LSPs. So, shouldn't the same
controls/limits apply at those routers as well?
Perhaps consider tweaking the language in the
above text to cover both origination and
propagation? I am not looking for mention of
specific knobs.
##PP2
added propagation to the above text.
discuss#3: section 4 says:
"UPA in OSPFv3 is supported for Inter-Area-Prefix-LSA [RFC5340],
AS-External-LSA [RFC5340], NSSA-LSA [RFC5340], E-Inter-Area-Prefix-LSA
[RFC8362], E-AS-External-LSA [RFC8362], E-Type-7-LSA [RFC8362], and SRv6
Locator LSA [RFC9513]."
I would like to understand why the base OSPFv3 LSAs are required for UPA and
why it cannot be done with just the extended LSAs (operating in sparse mode)
and the SRv6 Locator LSA. It is likely that I am missing something and hence
asking for clarification.
##PP
I'm not sure I understand the comment. Both
extended LSAs and Locator LSA are mentioned in
the above quoted text.
The base OSPFv3 LSAs are NOT required, but if some deployment uses the base LSAs only, they can be used to signal UPA.
KT> First, if only the base OSPFv3 LSAs are
used, we cannot have the U/UP flags - if that is
the intention, then please specify. I would
assume/expect that we want to use the extended
LSAs so those flags may be included. I also see it
as another motivation for implementing the
extended LSAs ;-) ... since there is no real
reachability, we can safely avoid the duplicate
advertisement of the base LSAs.
##PP2
we can still signal UPA for prefix advertised in legacy
LSAs, if we signal U/UP flag in extended LSAs - e.g.
sparse mode.
KT2> My understanding of sparse mode is that the
extended LSAs are used for new functionality while the base
LSAs are still used for the base OSPFv3 routing. This allows
for a deployment of new features where not all routers need
to support the extended LSAs. I consider UPA as a new
functionality and so it would work with just the extended
LSAs.
So your question is whether we need any base LSA or a
Locator LSA in such case. Well, I guess we don't, but I
would mandate them for consistency reasons - we mandate
the presence of the parent TLV with the NU-bit and
LSInfinity metric for these prefixes in section 5.2.2.
KT2> Let's put aside the Locator LSA - it is
completely a different thing. The base LSAs and extended
LSAs both have the metric (for LSInfinity) and the
PrefixOptions (for NU-bit) fields in them. Only the extended
LSAs can carry the U/UP flags. Therefore, I think it is
unnecessary to carry double the LSAs where the UPA could
just be advertised using the extended LSAs. This is
different from OSPFv2 where the OSPFv2 Extended Prefix LSA
didn't have the metric and hence the base LSAs were
necessary.
##PP3
I have updated the text:
OLD:
UPA in OSPFv3 is supported for Inter-Area-Prefix-LSA [RFC5340], AS-External-LSA [RFC5340], NSSA-LSA [RFC5340], E-Inter-Area-Prefix-LSA
[RFC8362], E-AS-External-LSA [RFC8362], E-Type-7-LSA [RFC8362], and SRv6 Locator LSA [RFC9513].
NEW:
UPA in OSPFv3 is supported for prefix reachability advertised via
OSPFv3 E-Inter-Area-Prefix-LSA [RFC8362], E-AS-External-LSA
[RFC8362], E-Type-7-LSA [RFC8362], and SRv6 Locator LSA [RFC9513].
For prefix reachability advertised via Inter-Area-Prefix-LSA
[RFC5340], AS-External-LSA [RFC5340], NSSA-LSA [RFC5340], UPA is
signaled using their corresponding extended LSAs. This requires
support of the OSPFv3 Extended LSAs in a sparse mode as specified in
section 6.2 of [RFC8362].
thanks,
Peter
Thanks,
Ketan
----------------------------------------------------------------------
COMMENT:
----------------------------------------------------------------------
Please find below some comments provided in the idnits output of the v09 of
the document. Please look for <EoRv09> at the end of the email. If that is not
present then likely the email has been truncated by your email client.
24 This document describes how to use the existing protocol mechanisms
25 in IS-IS and OSPF, together with the two new flags, to advertise such
26 prefix reachability loss.
<minor> Perhaps remove "existing" from the above sentence in view of sections
3.2 and 4.2?
##PP
Changed to:
"This document specifies protocol mechanisms
in IS-IS and OSPF, together with
the two new flags, to advertise such prefix
reachability loss."
126 IS, or by setting high metric on all-links and prefixes advertised by
127 the node in OSPF. When prefixes from such node are summarized by the
<minor> For OSPF, is the reference here to MaxLinkMetric in RFC6987 and LSInfinity?
Perhaps also the H-bit for v2 [RFC8870] and R-bit for v3 [RFC5340]?
##PP
done.
151 This document defines two new flags in IS-IS, OSPF, and OSPFv3.
152 These flags, together with the existing protocol mechanisms, provide
<minor> Perhaps remove "existing" here as well for the same reasons as
previous comment?
##PP
done
160 2. Generation of the UPA
162 UPA MAY be generated by the ABR or ASBR for a prefix that is
163 summarized by the summary address originated by the ABR or ASBR in
164 the following cases:
<major> Should we also call out that UPA MUST NOT be generated unless it is covered
by a summary?
##PP
I would prefer not to limit the UPA for the
summarization use case, even though that is
the one we are targeting now. Maybe we can use
it for something else in the future.
KT> Sure, perhaps there may be such a use
case in the future. However, having a check for
summary (it can be optional), can help
purge/remove a whole bunch of UPAs when the
summary itself is gone.
##PP
I would prefer not to mention all possible knobs in the
specification. Such a knob is up to the implementation
IMHO.
204 In OSPF and OSPFv3, each inter-area and external prefix is advertised
205 in it's own LSA, so the above optimisation does not apply to OSPF.
<minor> s/optimisation/consideration ? ... or perhaps "constraint" ?
##PP
replaced with "consideration"
207 It is also RECOMMENDED that implementations limit the number of UPA
208 advertisements which can be originated at a given time.
<major> Is the intention here about how many can be originated in one go OR how many
UPAs would be present (active) in that routers LSAs/LSPs at any given point of time? I
am assuming it is the latter and if so please clarify.
##PP
it's latter, done.
210 3. Supporting UPA in IS-IS
212 [RFC5305] defines the encoding for advertising IPv4 prefixes using 4
213 octets of metric information. Section 4 specifies:
<minor> For clarity, suggest:
[RFC5305] defines the encoding for advertising IPv4 prefixes using 4 octets of
metric information and its Section 4 specifies:
##PP
done
234 3.1. Advertisement of UPA in IS-IS
236 Existing nodes in a network that do not suport UPA will not use UPAs
237 during the route calculation, but will continue to flood them. This
238 allows flooding of such advertisements to occur without the need to
239 upgrade all nodes in a network.
<minor> Should "will continue to flood them" be qualified as "will continue to
flood them within the level" or something on similar lines?
##PP
flooding is always limited to the area/level,
so not sure we need to say that.
KT> My concern was with "upgrades all nodes
in a network" - network is broader than area/level
and the ABRs/ASBR need to be updated to go across
them in multi-area/level/domain network.
##PP
ok, added "within the area"
241 Recognition of the advertisement as UPA is only required on routers
242 which have a valid use case for this information. Those ABRs or
243 ASBRs, which are responsible for propagating UPA advertisements into
244 other areas or domains MUST also recognize UPA advertisements.
<major> Perhaps s/domains MUST also recognize/domains are also expected to
recognize ... or word it differently since this is more like an
operational/deployment guideline for UPA feature?
##PP
done
If providing operational or
deployment considerations, then suggest to introduce a new section named as such
and describe which routers are expected to be UPA-aware (or this could be done in
section 2 with a title change that covers not just generation but other aspects
as well).
##PP
I'm not a fan of the deployment considerations in the RFCs, these should be done by the individual vendors outside of the IETF. IETF's role is to guarantee interoperability.
KT> I am not insisting on that. However, I
will take this opportunity to make everyone aware
of the work happening on https://datatracker.ietf.org/doc/draft-opsarea-rfc5706bis/
that will be mandating an operational
consideration section (similar to security
considerations) for all specifications.
##PP2
wrong move IMHO, but this is not the right place to
debate that :)
thanks,
Peter
Thanks,
Ketan
251 UPA in IS-IS is supported for all IS-IS Sub-TLVs registered in the
252 IS-IS Sub-TLVs Advertising Prefix Reachability registry, which was
253 initially defined in [RFC7370], e.g.,:
<major> For clarity, I would suggest:
[RFC7370] introduced the IS-IS Sub-TLVs for TLVs Advertising Prefix Reachability
registry which lists TLVs for advertising different types of prefix
reachability (that list at the time of publication of this document is below).
UPA in IS-IS is supported for all such TLVs identified by that registry.
##PP
sure, done.
272 level 1 and level 2. Propagation is only done if the prefix is
273 reachable in the source level, e.g., prefix is only propagated from a
<nit> s/e.g.,/i.e.,
##PP
done
315 UPA in OSPFv2 is supported for OSPFv2 Summary-LSA [RFC2328], AS-
316 external-LSAs [RFC2328], NSSA AS-external LSA [RFC3101], and OSPFv2
317 IP Algorithm Prefix Reachability Sub-TLV [RFC9502].
<minor> I think the intention here is to say that "UPA in OSPFv2 is supported
for prefix reachability advertised via ..." ?
##PP
done
333 4.2. Propagation of UPA in OSPF
335 OSPF ABRs or ASBRs, which would be responsible for propagating UPA
336 advertisements into other areas MUST recognize such advertisements.
<major> This is more of a deployment guideline. Please see similar comment in
section 3.1
##PP
Changed MUST to "need to"
352 set in PrefixOptions, for various reasons. Even though in all cases
353 the treatment of such metric, or NU-bit, is specified for IS-IS, OSPF
354 and OSPFv3, having an explicit way to signal that the prefix was
355 advertised in order to signal unreachability is required to
<minor> perhaps s/unreachability/UPA ?
##PP
done
382 5.2. Signaling UPA in OSPF
384 A new Prefix Attributes Sub-TLV has been defined in
385 [I-D.ietf-lsr-ospf-prefix-extended-flags] for advertising additional
386 prefix attribute flags in OSPFv2 and OSPFv3.
<minor> please update reference to RFC9792 and also "OSPFv2 and OSPFv3
Prefix Attributes sub-TLVs have been ..."
##PP
I guess it should be "Prefix
Extended Flags Sub-TLVs"
403 5.2.1. Signaling UPA in OSPFv2
405 In OSPFv2 the Prefix Attributes Sub-TLV is a Sub-TLV of the OSPFv2
406 Extended Prefix TLV [RFC7684].
<minor> The name is "OSPFv2 Prefix Attributes Sub-TLV"
##PP
shoudl be "OSPFv2
Prefix Extended Flags Sub-TLV" I
suppose.
428 metric set to a value LSInfinity. For default algorithm 0 prefixes,
429 the LSInfinity MUST be set in the parent TLV. For IP Algorithm
430 Prefixes [RFC9502], the LSInfinity MUST be set in OSPFv3 IP Algorithm
431 Prefix Reachability sub-TLV. If the prefix metric is not equal to
432 LSInfinity, both of these flags MUST be ignored.
<major> For OSPFv3, RFC9502 is clear about what metric is in operation. Is
this text on default and IP algo needed?
##PP
I feel having it here may be useful for people
implementing it.
444 prefix. As a result, depending on which ABR or ASBR the traffic is
445 using to enter a partitioned area, the traffic could be dropped or be
446 delivered to its final destination. UPA does not make the problem of
<nit> could be either dropped or delivered ...
##PP
done
460 7. Processing of the UPA
462 The setting of the U-Flag signals that the prefix is unreachable. If
463 the U flag is set, the setting of the UP flag signals that the
464 unreachability is due to a planned event.
<minor> Suggest to move the above paragraph at the end of section 5 and just
before section 5.1 where the semantics of the flags would be introduced before
their protocol encodings are specified.
##PP
I feel like this text is redundant. It was
requested by the earlier review comments, but
I feel the meaning of the U/UP flags is well
covered in section 5.
I have removed this text.
496 This document adds two new bits in the "OSPFv2 Prefix Extended Flags"
497 and "OSPFv3 Prefix Extended Flags" registres:
<nit> registries
##PP
done
thanks,
Peter
<EoRv09>
|
