Re: [Bier] End-to-end BIER

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

[Caitlin] Each bit position would represent and end-station rather than
a BFER. The application would map a destination IP address to a bit
position (or bit-positions for BIER-TE).

This is really just a control plane issue.

[Caitlin] allowing the actual BFER to use multicast Layer 2 delivery is 
more direct. There is no need for BFERs to inform BFIRs that multicast 
delivery will be used. It makes no difference to the other BFRs. You can 
even make a case that this is already allowed under the "no harm no 
foul" rule.

This seems a bit more problematic to me, as the LAN might attach to 
multiple transit BFRs.  Consider a topology like:

         |----BFR-B---Endpoint-1
BFR-A---|      |
         |----Endpoint-2

That is, BFR-A, BFR-B, and Endpoint-2 are on the LAN, while Endpoint-1 
is downstream of BFR-B.  BFR-B also has another interface that attaches 
to Endpoint-2.  All links are equal cost to the IGP.

Suppose BFR-A receives a packet whose BitString says that the BFERs are 
Endpoint-1 and Endpoint-2.

With existing procedures, BFR-A sends BFR-B the packet with only 
Endpoint-1's bit set, and BFR-A sends Endpoint-2 the packet with only 
Endpoint-2's bit set.

If BFR-A uses layer 2 multicast to send a single packet to both BFR-B 
and Endpoint-2, Endpoint-2's bit will still be set in the packet 
received by BFR-B.  So BFR-B will forward the packet to Endpoint-2, and 
Endpoint-2 will now receive two copies of the packet.

Another problematic scenario:


         |----BFR-B---Endpoint-1
BFR-A---|      |
         |----Endpoint-2
BFR-C---|
         |----Endpoint-3

Here BFR-C and Endpoint-3 are also on the LAN.  Suppose BFR-A receives a 
packet (from one of its other interfaces) whose BitString identifies 
Endpoint-1 and Endpoint-2, while BFR-C receives the same packet (from 
one of its other interfaces), but with a BitString identifying only 
Endpoint-3.  When BFR-A and BFR-C multicast the packet to the LAN, we 
need to make sure that BFR-A and BFR-C don't receive the packet from 
each other, and that BFR-B doesn't receive the packet from BFR-C.  If, 
for example, BFR-C gets the packet from BFR-A, it will conclude from the 
BitString that the packet still needs to be sent to Endpoint-1 and 
Endpoint-2.

It would be an interesting exercise to see if one could state a set of 
enforceable applicability restrictions that eliminate these problems.

The various proposals for MLD/IGMP/PIM overlays address problems like 
this, but the solutions are not at the BIER layer, so I don't think they 
apply directly to the case of end-to-end BIER.