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Re: [Bier] BIER-TE related signaling drafts

"Jeffrey (Zhaohui) Zhang" <zzhang@juniper.net> Thu, 30 March 2023 06:58 UTC

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From: "Jeffrey (Zhaohui) Zhang" <zzhang@juniper.net>
To: "'bier@ietf.org'" <bier@ietf.org>
Thread-Topic: BIER-TE related signaling drafts
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Date: Thu, 30 Mar 2023 06:58:23 +0000
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Archived-At: <https://mailarchive.ietf.org/arch/msg/bier/2Ep8a89LYGtJVpe86fg4pDkJni4>
Subject: Re: [Bier] BIER-TE related signaling drafts
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Hi,

Looking at the BIER agenda I noticed the following drafts related to BIER-TE signaling:

5. BIER-TE BP advertisement, Huaimo Chen, 10 minutes
   https://datatracker.ietf.org/doc/draft-ietf-bier-te-isis/
   https://datatracker.ietf.org/doc/draft-ietf-bier-te-ospf/
   https://datatracker.ietf.org/doc/draft-ietf-bier-te-ospfv3/
   https://datatracker.ietf.org/doc/draft-chen-bier-te-lan/

6. BIER-TE PCE&IDR, Ran Chen, 10 minutes
   https://datatracker.ietf.org/doc/draft-chen-pce-bier/
   https://datatracker.ietf.org/doc/draft-chen-bier-idr-bier-te-bgp/

7. BIER-TE BIFT-ID advertisement, Sandy Zhang, 10 minutes
   https://datatracker.ietf.org/doc/draft-zwx-bier-te-extensions/

The drafts in agenda #5 is only about BP advertisement, and there is one draft for each of isis/ospfv2/ospfv3 respectively. The draft in #7 is about BIFT-ID advertisement, with one single draft covering all ISIS/OSPFv2/OSPFv3. On the other hand, draft-chen-bier-idr-bier-te-bgp in #6 covers both BP and BIFT-ID advertisement for BGP.

I vaguely remember I made a comment long time ago that flooding BP from each BFR may not be desired. Those BPs are only used locally for forwarding, and used by either controllers or BFIRs to figure out what BitString should be used for a flow. The assignment of the BPs are done by a central entity (either a controller or a human operator with pen/paper) - so instead of having each BFR learn its own BPs (from the central entity) and then flood, it's better to just have the controller send the each BFR's BP information only to that BFR in the form of BIFT programming (I believe there is a draft on that as well), and optionally send all BP information directly to BFIRs - but only if the BFIRs will figure out what BitString to use by themselves instead of relying on the controller sending them the BitStrings.

Even if we believe it's better to do the indirect way (each BFR learns BP assignments first and then flood), the current approach in the drafts in #5 is not optimal and not enough.
Consider that a BFR has 10 BPs for 10 connected adjacencies. With the approach in those drafts in #5, 10 BIER-TE info TLV will be advertised, each attached to one prefix for the connected adjacency. I don't think it can advertise routed adjacencies.

Besides, there is no need to have two fields to advertise DisEndBitPosition and BitPosition separately (we can leave that discussion aside for now).

For comparison, I believe https://datatracker.ietf.org/doc/draft-chen-bier-idr-bier-te-bgp/ does it a better way and it handles both connected and routed adjacencies.

If my understanding makes sense, I would suggest the WG merges drafts in #5 and #7 into a single bier-te-lsr draft that handles both BP and BIFT-ID signaling, and that draft is the LSR counterpart of the BGP draft.

It is reasonable that in the early days of BIER we had separate ISIS and OSPF specifications. Now with the already laid down foundation, a single IGP draft that covers both ISIS, OSPFv2 and OSPFv3 makes sense. If my understanding is correct, then the existing mechanism in the separate ISIS/OSPF2/OSPF3 drafts would not be used, so it is not unfair to merge then into a single draft with the right mechanism.

Jeffrey


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