Entropy labels are generated by an ingress LSR, based entirely on
load balancing information. However, they MUST not have values in
the reserved label space (0-15). Entropy labels MUST be at the
bottom of the label stack, and thus the "end-of-stack" bit in the
label should be set. To ensure that they are not used inadvertently
SB> MUST be set
for forwarding, entropy labels SHOULD have a TTL of 0.
SB> That is a good idea, we will put it in the PW draft.
WH> If we use TTL=0 and we use PHP on the egress LER with BGP shortcuts,
the packet on the egress LER will be received with TTL=0. The egress LER
will drop this packet normally.
I see 3 options for this:
1. Either use TTL=1 for the entropy label
2. Do something different for BGP shortcuts, introduce a service label
like is done for 6PE e.g., but this requires changes from today's
implementations and is always hard.
3. Avoid PHP with entropy label , but this changes again current
deployment models which is always though.
For me option 1 seems the best, unless I miss something.
Since entropy labels are generated by the ingress LSR, an egress LSR
MUST be able to tell unambiguously that a given label is an entropy
label. This of course depends on the underlying application. If any
ambiguity is possible, the label above the entropy label MUST be an
"entropy label indicator" (ELI), which says that the following label
is an entropy label. The ELI may be signaled, or may be a reserved
label reserved specifically for this purpose. Fortunately, for many
applications, the use of entropy labels is unambiguous, and does not
need an ELI.
