[rbridge] Consensus Check: Point to Point links

[Eastlake III Donald-LDE008]

Hi Radia,

See below at @@@

-----Original Message-----
From: Radia Perlman [mailto:Radia.Perlman at sun.com] 
Sent: Thursday, October 04, 2007 11:51 AM
To: Eastlake III Donald-LDE008
Cc: Rbridge at postel.org
Subject: Re: [rbridge] Consensus Check: Point to Point links

Personally, I need a reminder of what we are trying to accomplish with 
this before I can have
any opinion.

a) Is omitting the outer VLAN tag to save space?

@@@ Yes. The outer VLAN tag does nothing for you on a point-to-point
link.

b) Why put in anything for destination address other than the MAC 
address of the next hop
RBridge, or put in anything into the source address other than your own 
MAC address?
It won't save space. So what does it gain?

@@@ While no one has given a really crisp response to that question, it
is my impression that some believe it will make it possible to produce
simpler, less expensive, or more efficient hardware for this case.

c) Is there any danger if an RBridge is confused about whether this is a

pt-to-pt link or not?

@@@ I think there might be. And because of this and the extreme
commonness of the point-to-point case, it may be reasonable to consider
this in designing TRILL. For example, if a fixed MAC address were used
(such as the unicast version of the All-Rbridges multicast address (just
turn off the group bit)), then an interface receiving a frame with that
source address would know there was a sender on the link who believes
the link was point-to-point. If the receiver knows it is not
point-to-point or is unwilling to handle such frames, it could take
appropriate action. Also, Rbridge would know to never bother "learning"
the location of that MAC address.

@@@ Thanks,
@@@ Donald

I can see the advantage of omitting the entire outer header if it is 
somehow absolutely
known this is a pt-to-pt link, and both ends of the link understand 
this. But that isn't what's
being proposed here. It seems to be only omitting the VLAN tag, and 
allowing insertion of
random addresses into the source and destination fields in the outer 
header, if I'm reading
it correctly.

So anyway, clarification at this point would certainly help me.


Eastlake III Donald-LDE008 wrote:
> This is a check via the mailing list to confirm or refute an apparent
> consensus from the minutes of the Chicago meeting for a change from
> protocol draft -05:
>
>    If it is known that a link is a point to point link between two
>    RBridges, then the outer header, if it is an Ethernet header, can
>    have any source and/or destination addresses, those addresses will
>    be ignored on receipt, and the outer VLAN tag can be omitted.
>
> If no particular controversy arises over this in the next two weeks,
we
> will declare it to be the working group consensus.
>
> Thanks,
> Donald & Erik
>
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