Hi, Peter:
Let's focus on OSPF itself then.
In OSPFv2(RFC2328) and OSPFv3(RFC5340), the metric length for the link or
intra-area prefix is 16 bit; but the metric length for the summary
LSA/inter-area is 24bit.
There will be no problem to define the LSInfinity for the summary LSA as
0xFFFFFF( although the usages of such definition is doubtful and should be
revaluated----for example, is there any real deployment for the mentioned
possible usages?)
But for OSPFv3(RFC8362), if you still define the LSInifiity as 0xFFFFFF, there
is possible the cost to some prefixes advertised by the ABR reach this value,
it is unreasonable to consider such prefixes are unreachable. Then, rely on
such value of the metric to determine the reachability is problematic.
We should decrease or abandon such unsound reliance.
It is easy for the device to implement such special treatment, it is difficult
and complex for the operator to run the network based on such special treatment.
Best Regards
Aijun Wang
China Telecom
-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Peter Psenak
Sent: Monday, October 10, 2022 3:56 PM
To: Aijun Wang <[email protected]>; 'Acee Lindem (acee)'
<[email protected]>; 'Ketan Talaulikar' <[email protected]>; 'Peter
Psenak' <[email protected]>
Cc: [email protected]
Subject: Re: [Lsr] RFC 8362 and LSInfinity
Aijun,
On 09/10/2022 07:44, Aijun Wang wrote:
Hi, Acee, Peter and Ketan:
I propose we limit the usage of LSInfinity within the network. That is
to say, we should depreciate its usages, not enhance it.
As defined in RFC2328, the sole purpose of LSInfinity is to let the
receiver bypass the SPF calculation for the associated LSA:
a)In case the advertisement of LSA for some special aim.
b)Another is for the premature aging the LSA (which is not encouraged).
There is few application for the a) usage until now, same situation
for
b) usage.
definition of LSInfinity is very exact - it means unreachability.
The reason for the above situations may be the definition within the
RFC2328 is counterintuitive----the maximum value of the metric should
be used for the last resort of the reachability, no other more
meanings. Or else, it will complex the implementation and deployment, for
example:
a)For OSPFv2, the LSInfinity is defined as 0xffffff
b)For IS-IS, the equivalent variable is MAX_PATH_METRIC, which is
defined as 0xFE000000
you are comparing apples to oranges. These are two different protocols.
c)For OSPFv3, which value will you be defined, especially for the
Intra-Area-Prefix? Considering the metric for the intra-area and
inter-area are all 24-bit long?
OSPFv3 inherits all the constants defined for OSPFv2, it's explicitly mentioned
in the OSPFv3 RFC. And LSInfinity is 24 bits long.
d)And, for the metric in ”IP Algorithm Prefix Reachability” ,
https://datatracker.ietf.org/doc/html/draft-ietf-lsr-ip-flexalgo#section-6.3
<https://datatracker.ietf.org/doc/html/draft-ietf-lsr-ip-flexalgo#section-6.3>,
its length is again 32-bit, will you define another LSInfinity value later?
0xFFFFFFFF seems like a good candidate for the unreachable metric for the IP
flex-algo.
Won’t you think the above special rule complex the whole situation?
not at all.
I think we should seek other methods to achieve the necessary goals.
I do not see a problem
thanks,
Peter
Best Regards
Aijun Wang
China Telecom
*From:* [email protected] <[email protected]> *On Behalf Of
*Acee Lindem (acee)
*Sent:* Saturday, October 8, 2022 4:03 AM
*To:* Ketan Talaulikar <[email protected]>; Peter Psenak
<[email protected]>
*Cc:* [email protected]
*Subject:* Re: [Lsr] RFC 8362 and LSInfinity
Hi Peter, Ketan,
We’ll do another WG last call on the updated IP Flex Algo document and
it will update RFC 8362. As you probably surmised, this is useful for
OSPFv3 IP Flex Algorithm when you want don’t want to use the prefix
with the base algorithm.
*From: *Lsr <[email protected] <mailto:[email protected]>> on
behalf of Ketan Talaulikar <[email protected]
<mailto:[email protected]>>
*Date: *Thursday, October 6, 2022 at 3:35 AM
*To: *Peter Psenak <[email protected]
<mailto:[email protected]>>
*Cc: *"[email protected] <mailto:[email protected]>" <[email protected]
<mailto:[email protected]>>
*Subject: *Re: [Lsr] RFC 8362 and LSInfinity
Hi Peter,
I support this "update" - not sure if it qualifies as a "clarification".
Also, this obviously is doable only when the network has migrated to
use only Extended LSAs (i.e., legacy LSAs are removed) as indicated in
https://www.rfc-editor.org/rfc/rfc8362.html#section-6.1
<https://www.rfc-editor.org/rfc/rfc8362.html#section-6.1>
In sparse-mode, the legacy LSAs are used. So if you want a prefix to
be unreachable with the base algorithm, simply omit it from the legacy
Intra-Area-Prefix LSA.
Thanks,
Acee
Thanks,
Ketan
On Wed, Oct 5, 2022 at 3:01 PM Peter Psenak
<[email protected]
<mailto:[email protected]>>
wrote:
Hi Folks,
metric of LSInfinity (0xFFFFFF) has been defined in RFC2328:
LSInfinity
The metric value indicating that the destination described
by an
LSA is unreachable. Used in summary-LSAs and
AS-external-LSAs as
an alternative to premature aging (see Section 14.1). It is
defined to be the 24-bit binary value of all ones: 0xffffff.
RFC5340 inherited it from RFC2328:
Appendix B. Architectural Constants
Architectural constants for the OSPF protocol are defined in
Appendix
B of [OSPFV2]. The only difference for OSPF for IPv6 is that
DefaultDestination is encoded as a prefix with length 0 (see
Appendix A.4.1).
Both RFC2328 and RFC5340 used 16 bits metric for intra-area prefix
reachability, so the LSInfinity was not applicable for intra-area
prefixes.
RFC8362 defines 24-bit metric for all prefix reachability TLVs -
Intra-Area-Prefix TLV, Inter-Area-Prefix TLV, External-Prefix TLV.
Although it is silent about the LSInfinity as such, it is assumed that
such metric means unreachability for Inter-Area-Prefix TLV and
External-Prefix TLV. Given that Intra-Area-Prefix TLV now has 24 bits
metric as well, it would make sense to define the LSInfinity as
unreachable for Intra-Area-Prefix TLV as well.
Would anyone object such a clarification in RFC8362?
thanks,
Peter
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