Re: [OSPF] Regarding draft-ppsenak-ospf-te-link-attr-reuse-00

<> Fri, 13 November 2015 16:48 UTC

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To: Peter Psenak <>, "Acee Lindem (acee)" <>, Jeff Tantsura <>
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Date: Fri, 13 Nov 2015 17:47:58 +0100
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Subject: Re: [OSPF] Regarding draft-ppsenak-ospf-te-link-attr-reuse-00
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Hi Peter,

See [JM] below.

Nov. 12, 2015 -
> Julien,
> On 11/10/15 17:51 , Julien Meuric wrote:
>> Hi Acee,
>> I think we do not need to agree on the philosophical question whether
>> defining detour path by packet header instead of signaling states brings
>> the feature out of TE...
>> Anyway we agree that consolidating information from 3 separates LSA is
>> not the most efficient processing. My point is that this slight
>> improvement does not balance the risk of inconsistent
>> advertisements/configuration that the current I-D does not (even try to)
>> prevent.
> let me disagree. Current I-D clearly states what TE Opaque LSAs are used
> for.

[JM] I am happy to quote Jeff on this: "thanks to GMPLS IGP extensions 
as per RFC's 4203 & 5307 SRLG info is there, it is up to implementation 
how to use it."

> The risk is when you do what you propose to do as it breaks the existing
> TE

[JM] This is different from Acee's point: "usage of the TE LSAs for 
non-TE purposes was NEVER standardized". It is not about breaking, it is 
about documenting use cases beyond the original one.

  - e.g. you advertise the link in TE Opaque LSA and some remote router
> would try to establish a TE path via such link, even though the link is
> not enabled for that. Result is that the signaling would keep failing or
> in worst case, when signaling is not involved, traffic will be dropped
> when trying to use such link.

[JM] Supposing I am an operator who is playful enough to manage a 
network area using a topology for TE traffic that does not match the 
IP/LDP topology (you may find this realistic, I do not). Then, a router 
ignoring that an SRLG-enabled link has no available bandwidth/a specific 
affinity/a non-PSC switching capability/etc. is misbehaving.

Anyway, this moves beyond the issue at stake here. Acee states that some 
implementations need new definitions to go beyond the original use case. 
I would like to limit the number of fields opening the doors to 
operational inconsistencies. In these regards, an "applicability 
statement of TE LSA parameters beyond MPLS-TE" may be a way to address 
our concerns.

Enjoy the week-end,


> regards,
> Peter
>> Regards,
>> Julien
>> Nov. 07, 2015 -
>>> Hi Julien,
>>> One such non-TE application where there is a clear advantage of
>>> advertising these attributes is segment routing TI-LFA. In addition to
>>> all
>>> the detriments of requiring advertisement of TE LSAs when TE is not
>>> enabled, one would need to consolidate information for a link from 3
>>> separate LSAs (the base Router-LSA, the prefix-list attribute LSA for
>>> the
>>> adjacency SID, and the TE LSA). Clearly, it is better to advertise the
>>> applicable attributes in the Prefix/Link Attribute LSA and reduce this
>>> burden. You will note that this advantage isn’t apparent in IS-IS where
>>> everything is advertised in one monolithic LSP.
>>> Thanks,
>>> Acee
>>> On 11/5/15, 7:03 PM, "OSPF on behalf of Julien Meuric"
>>> < on behalf of> wrote:
>>>> Hello Peter,
>>>> Nov. 05, 2015 -
>>>>> Hi Julien,
>>>>> On 11/5/15 09:12 , Julien Meuric wrote:
>>>>>> Hi Jeff,
>>>>>> Following the WG session yesterday, I'm glad to (lately) join the
>>>>>> thread. Please, see my comments below as [JM].
>>>>>> Oct. 26, 2015 -
>>>>>>> Hi,
>>>>>>> No hats
>>>>>>> I'm familiar with at least 2 implementations which have this issue,
>>>>>>> this draft solves real problem.
>>>>>>> Regards,
>>>>>>> Jeff
>>>>>> [JM] Then you may consider patching them to do parameter
>>>>>> duplication on
>>>>>> the receiver side, not on the wire and/or the emitter
>>>>>> configuration...
>>>>>> Do you imagine operational people tearing hair out while trying to
>>>>>> guess
>>>>>> if they need to configure SRLGs in here, there or both? All the
>>>>>> more as
>>>>>> two places would multiply configuration discrepancies.
>>>>> above is incorrect.
>>>>> Nobody is proposing to configure things like SRLG on multiple places.
>>>> [JM] Actually you do in the I-D: "it is expected that the information
>>>> would be identical. If they are different..."
>>>>> You configure it on a single place, as you do today. If IGP is enabled
>>>>> for global SRLG protection, IGP pulls the SRLGs and advertise them in
>>>>> the Extended Prefix LSA. If TE is enabled and want to use SRLGs, it
>>>>> pulls it from the same place, form the TE Opaque LSA and asks IGP to
>>>>> flood it.
>>>> [JM] This reads to me like "in case both types of LSAs are used, values
>>>> MUST be identical". This is very different from the loose text in your
>>>> I-D.
>>>>>> In the I-D, the beginning and the end of section 3.1 provide a good
>>>>>> summary:
>>>>>> - "One approach for advertising link attributes is to _continue_ to
>>>>>> use
>>>>>> TE Opaque LSA"
>>>>>> - advantages: "no additional standardization requirement", "link
>>>>>> attributes are only advertised once".
>>>>>> I cannot agree more on these.
>>>>> have you read the "disadvantage" section as well?
>>>> [JM] Of course not, since Shraddha already solved them in his original
>>>> e-mail. :-)
>>>>>> In other words, some new use cases, not matching the original one, do
>>>>>> not justify to allocate new code points to the same information (cf.
>>>>>> IS-IS non-issue). In the IETF, uses cases aim at scoping protocol
>>>>>> work,
>>>>>> they aren't made to limit protocol future uses.
>>>>> I;m afraid you are missing the point.
>>>>> TE Opaquer LSA are defined as LSAs that advertise TE topology that is
>>>>> disjoint from the IGP topology (RFC3630). We can NOT make the link
>>>>> part
>>>>> of the TE topology, just because we want to advertise SRLG or some
>>>>> other
>>>>> attribute that is used by IGP for LFA - that would break the RFC3630.
>>>> [JM] Indeed, I am missing the point where a link state protocol is
>>>> forbidden to access the link parameters it is distributing in its link
>>>> state advertisements. Please, point me to the section from RFC 3630 it
>>>> "breaks".
>>>>> thanks,
>>>>> Peter
>>>> [JM] You're welcome,
>>>> Julien
>>>>>> Cheers,
>>>>>> Julien
>>>>>> _______________________________________________
>>>>>> OSPF mailing list
>>>>>> .
>>>> _______________________________________________
>>>> OSPF mailing list
>> .


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