Re: [mpls] [sfc] The first nibble issue associated with MPLS encapsulation

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

(Removed sfc from the cc-list, this seems out of scope for that WG.)

In designing the BIER header, the BIER WG is free to mandate any value 
it chooses in the first nibble.  These values do not come from a "first 
nibble" registry.

It seems prudent to put a value like 5 for the following reasons:

- If a BIER packet is being parsed by an off-line tool, this is a good 
hint (though just a hint) that the packet is actually a BIER packet;

- If a BIER packet is traveling through an MPLS tunnel, and it traverses 
a node that does its MPLS load splitting by guessing at the type of the 
payload, then this is  a good hint that the MPLS payload is not IPv4, 
IPv6, or PW.

This strategy does incur a risk.  Suppose IPv5 gets designed, 
implemented, and deployed, and folks start to deploy hardware that does 
MPLS load balancing by inspecting the IPv5 headers of the MPLS 
payloads.  If a BIER packet is traversing an MPLS tunnel, inappropriate 
load splitting may occur if the hardware thinks the payload is IPv5 
rather than BIER.

This particular risk doesn't seem very significant to me.

Thus I don't think there's anything here that needs fixing.