[rbridge] Hop Count processing

[Joe Touch]

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James Carlson wrote:
> Radia Perlman writes:
>> I suspect nobody will care, so we can just go with Dinesh's wording.

If the goal is to make this work like IP, the wording needs correction.

The original proposed wording is:

- ---

The Hop Count field is a 6-bit unsigned integer. It is decremented by 1
by each Rbridge that forwards a TRILL encapsulated frame. The frame is
dropped if either the Hop Count in the received frame is 0 or the Hop
Count is decremented to 0.

- ---

In IP, the following is true:

" A router MUST NOT discard a datagram just because it was received
   with TTL equal to zero or one; if it is to the router and otherwise
   valid, the router MUST attempt to receive it."

I.e., the packet is dropped if the TTL is zero or one on receipt only if
it is forwarded; if it is being 'accepted' (i.e., in this case, if the
destination is the trill node doing the processing), then a TTL of zero
is acceptable.

It seems like the wording needs to include this second case...

Joe
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