[Bier] Comments on draft-hfa-bier-pim-tunneling

[Eric C Rosen <[e-mail]>]

It seems to me that what this draft really proposes is a way to use BIER 
to create an NBMA (Non-Broadcast Multiple Access) data link on which PIM 
can be run.

If one looks at it that way, there is no interaction with the MVPN layer 
at all.  To MVPN, it looks like the P-tunnels or default-MDTs/data-MDTs 
are created by PIM.  The fact that the network core is acting like an 
NBMA data link is invisible to the application layer.  Thus I don't 
really understand why sections 5 ("ASM") and 6 ("MVPN") are needed.

There are a few questions that might deserve some more attention:

- Part of the proposal is that the Join/Prune messages are unicast, 
using a BIER  header with a single bit set in the BitString.  That will 
certainly work, but it's an unusual way to create a unicast tunnel.  I 
realize it does have the advantage of already including the BFIR-id in 
the encapsulation header.

- Since the BIER edge routers are PIM neighbors of each other, it seems 
like they should be multicasting PIM Hellos to each other. (The draft 
doesn't really say how the BIER edge routers running PIM are expected to 
identify each other; once they do identify each other, multicasting 
Hellos using BIER should be simple.)

- The lack of Join Suppression is worth at least mentioning.

- There are various PIM procedures that may or may not need to be 
modified to deal with an NBMA data link: DR election, Assert, etc.  If 
the usual PIM procedures need to be modified, that has to be discussed.  
(Even if they don't need to be modified, this should be discussed.)

- When a data packet is received, is it necessary to do an RPF check 
against the BFIR-id?  If so, then there is an issue of just what happens 
when a BFER (for a given data flow) changes from one tree to another 
(e.g., changes from the shared tree to the source tree); is the RPF 
check altered when a new Join is sent, or when the first data packet of 
the flow is received from the newly chosen tree, or somewhere in between?

- Will this work with BIDIR-PIM, or is that out of scope?