Re: [Lsr] https://tools.ietf.org/html/draft-wang-lsr-prefix-unreachable-annoucement-05

["Acee Lindem (acee)" <[e-mail]>]

Hi Gyan,

I guess you didn’t understand my first PUA question. See inline.

From: Gyan Mishra <[e-mail]>
Date: Monday, March 8, 2021 at 8:11 PM
To: Acee Lindem <[e-mail]>
Cc: Aijun Wang <[e-mail]>, draft-wang-lsr-prefix-unreachable-annoucement <[e-mail]>, lsr <[e-mail]>
Subject: Re: https://tools.ietf.org/html/draft-wang-lsr-prefix-unreachable-annoucement-05



On Mon, Mar 8, 2021 at 7:37 PM Acee Lindem (acee) <[e-mail]<mailto:[e-mail]>> wrote:
Speaking as a WG member:

Hi Gyan,

The first question is how do you know which prefixes within the summary range to protect? Are these configured? Is this half-assed best-effort protection where you protect prefixes within the range that you’ve installed recently? Just how does this work? It is clearly not specified in the draft.
 Gyan>  All prefixes within the summary range are protected see section 4.


   [RFC7794] and [I-D.ietf-lsr-ospf-prefix-originator<https://tools.ietf.org/html/draft-wang-lsr-prefix-unreachable-annoucement-05#ref-I-D.ietf-lsr-ospf-prefix-originator>] draft both define

   one sub-tlv to announce the originator information of the one prefix

   from a specified node.  This draft utilizes such TLV for both OSPF

   and ISIS to signal the negative prefix in the perspective PUA when a

   link or node goes down.



   ABR detects link or node down and floods PUA negative prefix

   advertisement along with the summary advertisement according to the

   prefix-originator specification.  The ABR or ISIS L1-L2 border node

   has the responsibility to add the prefix originator information when

   it receives the Router LSA from other routers in the same area or

   level.



Acee> So, the ABR will only know about missing prefixes that it has recently received? What if the prefix is already missing when the ABR establishes adjacencies on the path to the PE? What if the prefix is being permanently taken out of service – then this negative advertisement will persist permanently. What if there is an unintentional advertisement in the summary range and it is withdrawn? How do you decide whether or not to protect a prefix with in the range?









When the ABR or ISIS L1-L2 border node generates the summary

   advertisement based on component prefixes, the ABR will announce one

   new summary LSA or LSP which includes the information about this down

   prefix, with the prefix originator set to NULL.  The number of PUAs

   is equivalent to the number of links down or nodes down.  The LSA or

   LSP will be propagated with standard flooding procedures.



   If the nodes in the area receive the PUA flood from all of its ABR

   routers, they will start BGP convergence process if there exist BGP

   session on this PUA prefix.  The PUA creates a forced fail over

   action to initiate immediate control plane convergence switchover to

   alternate egress PE.  Without the PUA forced convergence the down

   prefix will yield black hole routing resulting in loss of

   connectivity.



   When only some of the ABRs can't reach the failure node/link, as that

   described in Section 3.2<https://tools.ietf.org/html/draft-wang-lsr-prefix-unreachable-annoucement-05#section-3.2>, the ABR th.at<http://th.at> can reach the PUA prefix

   should advertise one specific route to this PUA prefix.  The internal

   routers within another area can then bypass the ABRs that can't reach

   the PUA prefix, to reach the PUA prefix.

The second comment is that using the prefix-originator TLV is a terrible choice of encoding. Note that if there is any router in the domain that doesn’t support the extension, you’ll actually attract traffic towards the ABR blackholing it.
 Gyan> I will work with the authors to see if their is any alternative PUA process to signal and detect the failure in case prefix originator TLV is not supported.
Acee> Note that in the case of OSPFv3, the prefix originator TLV is a Sub-TLV of the Inter-Area Prefix TLV advertised in the E-Inter-Area-Prefix-LSA. If there are any OSPFv3 routers in the domain that don’t support this functionality and receive traffic for the protected prefix, they will actually route it towards the blackhole.

Further, I think your example is a bit contrived. I’d hope that an OSPF area with “thousands” of summarized PE addresses wouldn’t be portioned by a single failure as in figure 1 in the draft and your slides. I also that the option of a backbone tunnel between the ABRs was removed from the draft since it diminished the requirement for this functionality.
 Gyan> This is a real world Metro access edge example as the impact is customers that have LSP built to the down egress PE that has not failed over.  In this scenario their is a Primary and Backup PE per Metro edge which is typical for an operator.

The workaround used today is to flood all /32 next hop prefixes and not take advantage of summarization.  This draft makes RFC 5283 inter area FEC binding now viable for operators.
Acee> Or add a reliable intra-area link between your ABRs. Or, as a backup, a tunnel through the backbone area (as was previously in the draft).
Thanks,
Acee


Thanks,
Acee

From: Gyan Mishra <[e-mail]<mailto:[e-mail]>>
Date: Monday, March 8, 2021 at 6:57 PM
To: Acee Lindem <[e-mail]<mailto:[e-mail]>>, Aijun Wang <[e-mail]<mailto:[e-mail]>>, draft-wang-lsr-prefix-unreachable-annoucement <[e-mail]<mailto:[e-mail]>>, lsr <[e-mail]<mailto:[e-mail]>>
Subject: https://tools.ietf.org/html/draft-wang-lsr-prefix-unreachable-annoucement-05


Acee.

Please ask the two questions you raised about the PUA draft so we can address your concerns.

If anyone else has any other outstanding questions or concerns we would like to address as well and resolve.

Once all questions and  concerns are satisfied we would like to ask for WG adoption.

Kind Regards

Gyan
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Gyan Mishra

Network Solutions Architect

M 301 502-1347
13101 Columbia Pike<https://www.google.com/maps/search/13101+Columbia+Pike?entry=gmail&source=g>
Silver Spring, MD

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Gyan Mishra

Network Solutions Architect

M 301 502-1347
13101 Columbia Pike
Silver Spring, MD