Re: [Idr] draft-litkowski-idr-flowspec-interfaceset => NLRI vs EXTCT (part 2)

Two flowspec routes with the same NLRI may be originated by different speakers.
The first route will have one group-id. The second route will have another group-id.

Thanks,
Jakob.

From: Idr [mailto:idr-bounces@ietf.org] On Behalf Of Robert Raszuk
Sent: Wednesday, July 26, 2017 4:38 PM
To: Juan Alcaide (jalcaide) <jalcaide@cisco.com>
Cc: idr wg <idr@ietf.org>
Subject: Re: [Idr] draft-litkowski-idr-flowspec-interfaceset => NLRI vs EXTCT (part 2)

Hi Juan,

>  (we assume controller(s) may not want to send multiple ext-communities with same NLRI).

If I recall group-ids are carried in new RT format:

"This new BGP Route Target extended community is encoded as follows :

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Type (TBD)   |      0x02     |    Autonomous System Number   :
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      :     AS Number (cont.)         |O|I|      Group Identifier     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
"

So just like an UPDATE message of SAFI 128 may contain many RTs why would you see any
issue to carry multiple ext communities of the above format here ?

Best,
R.

On Thu, Jul 27, 2017 at 1:01 AM, Juan Alcaide (jalcaide) <jalcaide@cisco.com<mailto:jalcaide@cisco.com>> wrote:

Hi,

From a previous thread, I see from the that using EXTCOMM to carry group-id information the choice. Reason was that every AS could use their own the group-id (perhaps different than another AS). With this choice, ADD-PATHS must be mandatory in order to support multiple group-ids for the same flowspec rule  (we assume controller(s) may not want to send multiple ext-communities with same NLRI).

But the draft really does not describe how to use ADD-PATHS, and it does not discuss its problems:

- Usually, ADD-PATHS is used for path diversity, and implementations typically don't advertise 2 paths with the same next-hop (otherwise, we could have path explosion across multiple levels of RRs)

- If ADD-PATHS has to advertise the same NLRI with different ext-communities, one solution would be for ADD-PAHTS not advertise the same set of ext-communities. Unless, I guess, next-hops are different. Otherwise, we would have path explosion.

- Assuming the above, we should define a particular set of ADD-PATHS rules for flowspec AF. And, of course, leave the door open for future specific ADD-PATHs rules for other AFs (it would not be about path diversity anymore, but about propagating different information for forwarding purposes -imagine what we could have done with an IPv4 prefix: send the same net part as a NLRI and multiple ext-communities representing different prefix-lengths -).

- Since paths in a net are typically implemented as a list, there could be scalability problems if we ever want to support many group-ids.

My solution to simplify all these problems would be to add a discriminator on the NLRI (by defining a new dummy type for flowspec). We could still use ext-communities to actually match the NLRIs to interfaces. Similar to RD and RT usage.

Thoughts?

-J

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