[rbridge] Hop Count processing

[Joe Touch]

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Joe Touch wrote:
> 
> 
> James Carlson wrote:
>> Radia Perlman writes:
>>> Though, rereading that....I think it's even clearer if we remove the 
>>> word "forwarding", so
>>> it would instead be:
>>>
>>> "The Hop Count field is a 6-bit unsigned integer. An Rbridge
>>> drops frames received with TTL=0, otherwise it decrements the TTL."
>> Much better.  The "forwarding" part is the qualifier most likely to
>> confuse, and it doesn't actually change the meaning.
> 
> First, anyone who doesn't know what forwarding means is going to have a
> lot of problems implementing an rbridge. There are things it does with
> packets that terminate, and things it does with packets that forward.
> Forwarding is just a step that happens after you check whether a packet
> matches the addresses of the rbridge (and fails that match).
> 
> Second, there are many more complex things than forwarding in the spec
> as well.
> 
> Third, it not only changes the meaning, it changes the behavior. See the
> walkthrough in my previous mail.

Oh, fourth (and really, wasn't this a goal?) - this would make the
rbridge hopcount completely different from both IPv4 and IPv6 - BOTH of
which define it exactly the way I did, FWIW - only *forwarders* check it
and decrement it.

> Joe
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