[rbridge] Critical bits for options

[James Carlson]

Russ White writes:
> So, the problem is--how do you signal, without looking for the options,
> whether or not the faster switching path the packet comes in on should
> punt the packet to some slower path because of the presence of some
> option or another.

Yes.  In a particular degenerate case that appears to be of interest
to several posters here, systems that implement _no_ options at all
would like to know if any "critical" (must-comply) ones are present.
If they are, then the only thing the node can do is drop.

> 1. What are these options for? I'm a bit confused as to why there are
> such things as options (?)....

Good question.  Presumably, they'd be there to handle special
features, such as encryption in transit and labeling for security
purposes, and perhaps for debug options (trill-traceroute).  It's hard
to imagine too many _other_ purposes here, but I suppose others are
possible.

> 2. Will there be a consistent list, through time, of which options a
> given platform will switch in some faster path, and which they will
> switch in some slower path?

Almost certainly not.  The only assertion I've seen here is that some
implementations will be completely unable to support options at all.
They need to know whether to drop or pass a packet with options.

> 1. It seems like if 2 is false, then there's no point in trying to put
> the options in any sort of order (?).... How can the sender know what
> some processing node in the middle can switch faster or slower?

There's a difference between critical and non-critical hop-by-hop
options.

If there are any critical options, then all nodes on the path must
process them.  Any nodes that don't understand the option must drop
the packet.  That's the reason some are requesting a "summary" bit.

One way to resolve the whole problem is to outlaw critical hop-by-hop
options.  Maybe not the _best_ way, though.

> Overall, I think it might be useful to have a bit saying: "This packet
> has options you really need to process, so don't just switch it without
> paying attention." A bit that says: "I have no options" might also be
> useful.

The latter we have -- Op-Length will be zero when there are no
options.

The former we can get in one of at least two ways:

  - Summary bits (some like this, I don't)

  - Option ordering such that must-parse options are always first.

-- 
James Carlson, Solaris Networking              <james.d.carlson at sun.com>
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