On 14/05/2025 13:28, Aijun Wang wrote:
Please overrides the previous unfinished mail.
On 14/05/2025 03:08, Aijun Wang wrote:
I have pointed out these guidelines within 16.2, 16.3 and 16.4 are problematic.
They don't give the reasonable explanations to define LSInfinity.
If you think there is any content within RFC 2328 gives the reason, please
quote them directly.
Let me describe again the scenario that demonstrate these issues that current
RFC 2328 has:
1) Assume the following simple topology:
R1-(10)---R2---(20)--ABR--(30)--R3--(40)--R4--(LSInifinity-20)---P0
With the value in parenthesis indicates the cost the link between
the routers
2) when one set the cost of prefix P0 that attached R4 as "LSInifinity-20",
it is reachable on R4.
3) When the LSA that includes prefix P0, reaches R3, R3 will calculate the
total cost to this prefix exceeds LSInfinity.
Should R3 treat this prefix as reachable or unreachable?
yes, there's nothing wrong with the path cost to be higher than LSInfinity.
It's the cost of LSInfinity advertised inside the LSA which makes the prefix
unreachable.
RFC2328 clearly states it - "If the cost specified by the LSA is LSInfinity".
[WAJ] OK. Then R3 and ABR should treat prefixes P0 as reachable.
Then emerge the contradiction results: in these routers, although the cost of
the prefixes P0 reach to LSInfinity, they are still reachable.
But at the original router, if we set the cost of the prefixes as LSInfinity,
they indicate unreachable.
Don’t you think these contradicting design puzzled?
no, it looks perfectly valid to me.
4) When such LSA reaches ABR, according to current description of section 12.4.3 of
OSPF 2328, there will be no summary LSA generated for prefix P0("Else, if the
routing table cost equals or exceeds the
value LSInfinity, a summary-LSA cannot be generated for this
route.").
Routers R1 and R2, which are in another area, will not have the information
about prefix of P0, the traffic from R1/R0 to prefix P0 will be broken.
Nothing is broken, you reached the limit of what protocol defines as reachable
metric for the prefix.
[WAJ] it is one obvious broken design.
If the ABR treat the prefix P0 as reachable, why don’t advertise them in
summary LSA?
If the ABR doesn’t advertise the prefixes P0, R1 and R2 in other area can’t
reach prefix P0, which is undesirable.
I don't see any problem.
All of the above is completely orthogonal to UPA.
[WAJ] UPA actives the above contradictory design.
no, it does not. UPA originates the prefix with LSInfinity, so it is
unreachable from the very beginning and keeps that property as it is
flooded or propagated.
I'm done with this discussion now.
Peter
regards,
Peter
How can you solve the above dilemma, under the current description of RFC 2328?
Please gives the coherent quotation.
Best Regards
Aijun Wang
China Telecom
-----邮件原件-----
发件人: Acee Lindem [mailto:[email protected]]
发送时间: 2025年5月14日 3:07
收件人: Aijun Wang <[email protected]>
抄送: Peter Psenak <[email protected]>; lsr <[email protected]>
主题: Re: [Lsr] [LSInfinity Features within OSPF is FLAWed, it should be
Abandoned, not Enhanced instead] I-D Action:
draft-ietf-lsr-igp-ureach-prefix-announce-05.txt
On May 13, 2025, at 3:28 AM, Aijun Wang <[email protected]> wrote:
Hi, Acee:
Until now, I haven't found any reasonable explanation that " prefixes with an
infinite metric are unreachable by design " in the existing documents(for example,
https://datatracker.ietf.org/doc/draft-ietf-lsr-ospf-ls-link-infinity/ gives the reason
to introduce the value of LSLinkInfinity).
See sections 16.2, 16.3, and 16.4 or RFC 2328.
There are many possible scenarios that the 'total cost' of one prefix reach the
infinite metric.
Actually, such design can lead the network traffic be broken unintentionally(I
have given you the example, with or without summary LSA).
The proposal within the current UPA will activate such dormant, should be
abandoned feature, and bring more network outages accidently.
The WG should seek other solution to achieve the same aim of UPA proposal.
Best Regards
Aijun Wang
China Telecom
-----邮件原件-----
发件人: Acee Lindem [mailto:[email protected]]
发送时间: 2025年5月12日 19:12
收件人: Aijun Wang <[email protected]>
抄送: Peter Psenak <[email protected]>; lsr <[email protected]>
主题: Re: [Lsr] [LSInfinity Features within OSPF is FLAWed, it should
be Abandoned, not Enhanced instead] I-D Action:
draft-ietf-lsr-igp-ureach-prefix-announce-05.txt
Speaking as WG member:
Aijun,
On May 11, 2025, at 9:33 PM, Aijun Wang <[email protected]> wrote:
Hi, Acee:
The "area range" is not always be configured at the ABR, or the "area range"
may not cover all of the prefixes within the area.
In such situation, if there is no summary LSA advertised for these prefixes(
with which their total cost exceed LSInifinity), the routers within other area
can't reach these prefixes.
The network traffic will be discarded wrongly.
The UPA is specifically targeted toward prefixes that are subsumed by a shorter
prefix corresponding to a summary.
And, we can image even another scenario, even the routers within the same area
can't reach these prefixes, if they treat the prefixes that the 'total cost'
equal LSinifity as unreachable.
This is one remain bug of RFC 2328. The situation is same for
RFC5305/RFC5308 UPA solution shouldn't depend on such bug base.
There is no bug in RFC 2328/RFC 5305/RFC 5308, prefixes with an infinite metric
are unreachable by design.
I'm not going to debate this.
Acee
Best Regards
Aijun Wang
China Telecom
-----邮件原件-----
发件人: [email protected]
[mailto:[email protected]] 代表 Acee Lindem
发送时间: 2025年5月9日 22:18
收件人: Aijun Wang <[email protected]>
抄送: Peter Psenak <[email protected]>; lsr <[email protected]>
主题: [Lsr] Re: [LSInfinity Features within OSPF is FLAWed, it should
be Abandoned, not Enhanced instead] I-D Action:
draft-ietf-lsr-igp-ureach-prefix-announce-05.txt
On May 9, 2025, at 10:08 AM, Aijun Wang <[email protected]> wrote:
Hi, Acee:
If no summary LSA for these prefixes, then, there will be no LSA for these
prefixes, it leads the same WRONG results—-such prefixes are unreachable in
another area.
As long as there is at least 1 reachable route subsumed by the area range,
there will be a summary-LSA.
Acee
Aijun Wang
China Telecom
On May 9, 2025, at 21:58, Acee Lindem <[email protected]> wrote:
Speaking as WG member,
On May 9, 2025, at 9:28 AM, Peter Psenak <[email protected]>
wrote:
Aijun,
On 09/05/2025 15:22, Aijun Wang wrote:
Hi, Peter:
UPA doesn’t influence the results of the prefixes that are set to be the
LSInfinity at its originator, but it influences the results of the prefixes
whose metrics are lower than LSInfinity.
no UPA does not affect prefixes that are advertised with valid metric.
I have show you the example in the previous mail—-For example, in
https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/47864-ospfdb5.html
, when prefix 4.0.0.0/8 reaches to the ABR, it is reachable(prefix cost is
0xffffff-0x40, lower than LSInfinity).
But after the ABR advertise the prefix in area 1 with the Summary LSA, its
metric will be LSInfinity.
According to the UPA rule, or RFC 2328, every router(includes the final
receiver) within the area 1 will treat this prefix as unreachable, which is NOT
right.
If the prefix metric is LSInfinity, the prefix is unreachable. UPA does not
change any of that.
It’s time to fix RFC2328/RFC5305/RFC5308.
I don't think so.
I agree. There is nothing in the UPA draft, that says the unreachable prefix
will be included in the summary cost calculation. I don't know how one would
come to that conclusion.
Also, for OSFP, in section 12.4.3 of RFC 2328, routes with a metric of
LSInfinity or higher are explicitly disqualified from the summary computation.
o Else, if the routing table cost equals or exceeds the
value LSInfinity, a summary-LSA cannot be generated for
this route.
Thanks,
Acee
Let’s do this together before forwarding the UPA draft?
no, we are not going to modify the LSInfinity in any way inside or outside of
the UPA.
I'm done with this discussion now.
Regards,
Peter
Aijun Wang
China Telecom
On May 9, 2025, at 18:04, Peter Psenak <[email protected]> wrote:
Aijun,
the problem you have described below has no relevance to the UPA. In the UPA
case we are deliberately originating the prefix with the unreachable metric, so
adding anything to it at ABR is not going to make any difference, it will stay
as unreachable.
As I have replied to you many times the meaning of the LSInfinity was defined
in the base protocol specification and we are not altering it in any way. We
are using it the way it was defined.
Regards, Peter
On 09/05/2025 10:55, Aijun Wang wrote:
Hi, Peter:
I noticed the updated draft includes the new contributors to respect their
previous efforts, this should be encouraged within IETF.
But, I must point out that, the direction that Reusing the LSInfinity to
advertise the unreachable information should be discarded.
The LSInfinity feature that is defined in RFC 2328 is FLAWED, we should try to
fix it, not exploit it again.
Let's give you the simple example, that described in "OSPF
Inter-Area Routing" [1] This is one 20 years ago article, it states clearly
that when ABR do the summary action, it will add the cost of the prefix itself and
the cost of the path between the prefix originator and the ABR together, as the
newly cost of the summary LSA for the prefix:
In the example, the original cost of 4.0.0.0/8 is 10, the link
cost between Router 1.1.1.1 and Router 2.2.2.2 is 64, the
ABR(router 2.2.2.2) will advertise the summary LSA for
4.0.0.0/8 to Area 1, with the cost set to 10+64=74 (please see
the output of "r2.2.2.2#show ip ospf database summary
4.0.0.0")
Then coming the question(let's take the same example):
If the cost of prefix 4.0.0.0/8 is set to 0xffffff-0x40(64), on ABR(router
2.2.2.2), the cost of summary LSA for prefix 4.0.0.0/8 will reach 0xfffff.
If the ABR(router 2.2.2.2) follow the guideline of RFC 2328, the prefix
4.0.0.0/8 will be unreachable, and will be not advertised to area 1, router in
area 1 can't reach the 4.0.0.0/8.
But actually, 4.0.0.0/8 is reachable via the ABR(router 2.2.2.2).
If we consider there may be several hops between the prefix originator and the
ABR, then the cost of the prefix can't exceed 【0xffffff-(several
hops)*(possible link metric)】, which will be varied with different network
topology, and can't be considered as one universal value, even a definite range.
Then, such flaw in OSPF 2328, and also the similar mechanism in RFC
5305/RFC5308 for IS-IS should be fixed.
The reason that there is no emerged network outrage in these years is that the
operator configure seldom the cost of the prefix directly.
But if we expand the LSInfinity feature as described in this WG document, more
chaos, and network outrages will be emerged.
Let's stop forwarding to this direction.
[1]:
https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-pa
t
h
-
first-ospf/47864-ospfdb5.html
Best Regards
Aijun Wang
China Telecom
-----邮件原件-----
发件人: [email protected]
[mailto:[email protected]] 代表
[email protected]
发送时间: 2025年5月9日 2:21
收件人: [email protected]
抄送: [email protected]
主题: [Lsr] I-D Action:
draft-ietf-lsr-igp-ureach-prefix-announce-05.txt
Internet-Draft draft-ietf-lsr-igp-ureach-prefix-announce-05.txt is now
available. It is a work item of the Link State Routing (LSR) WG of the IETF.
Title: IGP Unreachable Prefix Announcement
Authors: Peter Psenak
Clarence Filsfils
Daniel Voyer
Shraddha Hegde
Gyan Mishra
Name: draft-ietf-lsr-igp-ureach-prefix-announce-05.txt
Pages: 15
Dates: 2025-05-08
Abstract:
In the presence of summarization, there is a need to signal
loss of reachability to an individual prefix covered by the summary.
This enables fast convergence by steering traffic away from
the node which owns the prefix and is no longer reachable.
This document describes how to use the existing protocol
mechanisms in IS-IS and OSPF, together with the two new flags,
to advertise such prefix reachability loss.
The IETF datatracker status page for this Internet-Draft is:
https://datatracker.ietf.org/doc/draft-ietf-lsr-igp-ureach-pre
f
i
x
-announce/
There is also an HTMLized version available at:
https://datatracker.ietf.org/doc/html/draft-ietf-lsr-igp-ureac
h
-
p
refix-announce-05
A diff from the previous version is available at:
https://author-tools.ietf.org/iddiff?url2=draft-ietf-lsr-igp-u
r
e
a
ch-prefix-announce-05
Internet-Drafts are also available by rsync at:
rsync.ietf.org::internet-drafts
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