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
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 tohttps://www.ietf.org/about/groups/iesg/statements/handling-ballot-positions/
for more information about how to handle DISCUSS and COMMENT positions.
The document, along with other ballot positions, can be found here:
https://datatracker.ietf.org/doc/draft-ietf-lsr-igp-ureach-prefix-announce/
----------------------------------------------------------------------
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."
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.
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.
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.
----------------------------------------------------------------------
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.
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.
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.
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-TLV
<https://www.rfc-editor.org/rfc/rfc9792.html#name-ospfv2-prefix-extended-flag>s"
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
<https://www.rfc-editor.org/rfc/rfc9792.html#name-ospfv2-prefix-extended-flag>"
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>
_______________________________________________
Lsr mailing list -- [email protected]
To unsubscribe send an email to [email protected]
