Early Review of draft-ietf-opsawg-ipfix-bgp-community-04
review-ietf-opsawg-ipfix-bgp-community-04-genart-early-halpern-2018-02-09-00

This is an early gen-art review of draft-ietf-opsawg-ipfix-bgp-04.

The document is clear about what it is trying to do, and readable.  It is not
clear about how it expects this to actually work.

However, I find the underlying concept confusing.
1) BGP Communities may sometimes represent subsets of traffic.  But usually
they represent tagging intended to influence routing which is only indirectly
related to meaningful subsets of traffic for TE purposes.  One may be able to
make an argument that this could better enable monitoring the effects of some
BGP communities.  But the draft does not make that argument. 2) It is unclear
what this actually expects the router to do in generating this information. 
Reading between the lines, it seems that what is desired is for the router
control process to go through the IPFIX collected information before it is
exported, and add BGP community tags to the export information.  (Generating
such information directly from the forwarding plane would place significant
load on the forwarding representation and processing, and on the control logic
to generate FIB information.)  Given that off-line BGP information collection
is a common practice, and that such information is common across the AS, it
would actually seem simpler to perform such processing and aggregation offline
rather than in the router.

If the IDR working group has not been consulted about this, I would strongly
recommend working with them as to whether this is actually useful information
to collect, and how and where to collect it. If the IDR working group does not
consider important to work on this, then that gives you useful information in
and of itself.