Re: [mpls] [Bier] Encapsulation first nibble

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

On 3/17/2015 7:17 AM, Xuxiaohu wrote:
> Another way is to add a protocol identifier field after the bottom of
> the label stack
> (https://tools.ietf.org/html/draft-xu-mpls-payload-protocol-identifier-00).
> In this way, we would not be bothered about the nibble issue anymore
> when proposing any new encapsulation header which may be encapsulated
> within an MPLS packet.

When a BIER packet is traveling from BFIR to BFER via a sequence of 
directly connected BFRs, each BFR already knows, via the bottom label, 
that the packet is a BIER packet.  The nibble issue really only matters 
when a BIER packet is traveling through a non-BIER tunnel.

When a BIER packet is traveling through a non-BIER tunnel, some of the 
transit routers may be routers that are running older software.  They 
will base their ECMP treatment of the packet on the first nibble.  A 
protocol identifier field won't help, because these old routers won't 
understand it.  For newer routers, I think the proper direction is to 
use the MPLS entropy label, rather than to have each router compute the 
entropy based on an analysis of the next encapsulation header.

So I don't think BIER justifies the use of a new MPLS special purpose 
payload-protocol-identifier label.

There is one case though in which it could potentially be useful to 
enable the transit routers of a unicast MPLS tunnel to determine that a 
particular packet has a BIER payload.  Suppose thtat the MPLS TTL of the 
unicast tunnel expires while the packet is still in the tunnel.  It 
might be useful for the transit router to generate an ICMP response to 
the TTL expiration, and for this ICMP response to be BIER-aware. 
However, this is only likely to happen in a traceroute-like application, 
and I don't think that is sufficient justification for having all BIER 
packets carry the two or three extra label stack entries it would take 
to provide the special purpose payload-protocol-identifier label.  On 
the other hand, a nibble value specific to BIER might be useful, as it 
provides the same information without requiring additional overhead.