Re: [Bier] [babel] About bierin6 and signalling of encapsulation

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

I think the model is that a BFR knows by virtue of provisioning what 
encapsulation to use by default when transmitting a BIER packet.  If a 
BFR has certain interfaces that lead, e.g., from an MPLS network to an 
IPv6-only network, those interfaces can be provisioned so that the BFR 
knows to use a specified non-default encapsulation when transmitting a 
BIER packet over those interfaces.

In that model, there is no real need for every BFR to know the set of 
encapsulations that every other BFR can receive.

To transition a backbone from one encapsulation to another, one would 
have to first provision all the BFRs to be able to receive both 
encapsulations.  Then one could change the default transmission 
encapsulation on the BFRs one by one.

This model doesn't really support hop-by-hop changing of the 
encapsulation very well, since there is no signaling of the supported 
encapsulation.  But is there really a practical need to optimize for the 
situation in which every hop uses a different encapsulation than the one 
before?

> Juliusz> I'm probably missing something, but I only see three solutions:
>
>    1. statically configured (e.g. a given BIER domain is statically
>       configured to use a given encapsulation);

As suggested above.

>
>    2. carried by a specific signalling protocol (call it the BIER Hello
>       Protocol for now);

I would avoid this.  Hello protocols always seem like they're going to 
be simple, and then turn up being nightmares.  Note that BIER neighbors 
are not necessarily layer 2  neighbors, and once we start sending Hellos 
that are not link-local, the Hellos are easy to attack and we end up 
with some thorny security issues.  I'd certainly want to avoid creating 
new soft state flooding mechanisms for passing BIER control information, 
as we're supposed to be simplifying things rather than making them more 
complicated.

>
>    3. carried by the routing protocol (e.g. carried by a sub-TLV of the IHU
>       TLV in Babel, or using RFC 5613 signalling in OSPF).

IF there is really a practical need for dynamic signaling of "here is 
the list of BIER encapsulations I can parse", adding that information to 
the existing BIER control signaling mechanisms makes sense.  (It's 
analogous to the distribution of information about the supported 
Disposition BSLs).