Re: [Lsr] John Scudder's Discuss on draft-ietf-lsr-ospf-prefix-originator-10: (with DISCUSS and COMMENT)

[John Scudder <[e-mail]>]

Hi Ketan,

Thanks for your reply. Some comments in line.

On Apr 7, 2021, at 1:29 AM, Ketan Talaulikar (ketant) <[e-mail]<mailto:[e-mail]>> wrote:
...
2. Section 2.1:

   For intra-area prefix advertisements, the Prefix Source OSPF Router-
   ID Sub-TLV MUST be considered invalid and ignored if the OSPF Router
   ID field is not the same as Advertising Router field in the
   containing LSA.  Similar validation cannot be reliably performed for
   inter-area and external prefix advertisements.

What does it mean for the sub-TLV to be ignored? Since you haven’t specified any processing of the Sub-TLVs, there’s seemingly no ignoring to be done locally
[KT] The ignoring part is for the user of the information. Since the days of RSVP-TE (RFC3630), we've had OSPF flooding information about TE topology opaquely (i.e. not using for OSPF computations) while it is being used by for computation of RSVP-TE tunnel paths/LSPs. The same applies here.

— so does this mean the sub-TLV isn’t even supposed to be stored?
Flooded?
[KT] Per OSPF protocol, we have to store it and flood it - since the information is not "malformed" or "not parsable".

OK. If “ignore” makes it dead obvious to your audience (which might include relative novices as well as seasoned experts) that in context this is what it means, that’s fine. I leave it to your discretion whether it warrants any further clarification such as you’ve provided above.

...
4. Section 3:

   When an ABR generates inter-area prefix advertisements into its non-
   backbone areas corresponding to an inter-area prefix advertisement
   from the backbone area, the only way to determine the originating
   node information is based on the Prefix Source OSPF Router-ID and
   Prefix Source Router Address Sub-TLVs present in the inter-area
   prefix advertisement originated into the backbone area by an ABR from
   another non-backbone area.  The ABR performs its prefix calculation
   to determine the set of nodes that contribute to the best prefix
   reachability.  It MUST use the prefix originator information only
   from this set of nodes.  The ABR MUST NOT include the Prefix Source
   OSPF Router-ID or the Prefix Source Router Address Sub-TLVs when it
   is unable to determine the information of the best originating node.

What is it supposed to do if there are N contributing routes but it can only determine the information for M < N of the contributors?
[KT] Consider that there are N contributing routes at the ABR and B of them were contributing to the "best reachability" (where N >= B). Out of those B routes, if only M advertisements are including the prefix origin info (where B >= M). Then the ABR does a single inter-area prefix advertisement that will include the M prefix origin info. This is conveyed by the two sentences in bold in the text above.

OK. And it’s considered fine that that information for some, but not all, of the contributors is included? It seems potentially problematic that the route only includes partial information, but the consumer of the route has no way to know this. The other obvious choices would have been to omit the information altogether if only partial information was available, or to mark it as partial somehow.

Thanks,

—John