[rbridge] Consensus Check: Point to Point links

[Eastlake III Donald-LDE008]

Hi Eric,

I believe the generally understood meaning of point-to-point is a link
with exactly two devices on it, in this case RBridges.

It is true that one could always do this sort of thing outside the
standards. But in Chicago the working group appeared to believe this was
sufficiently important to say in the standard and, while the consensus
in the room was not that detailed, I got the general impression that
people would be favorable to, in a later management document, having a
MIB variable that would enable one to configure an interface as being
known point-to-point, possibly point-to-point, or prohibited from using
this point-to-point outer address flexibility, or some such management
configuration feature. There also seemed to have been at least some
interest in an automated way of determining that an interface was
point-to-point, perhaps based on the receive of 802.1AB frames all
containing appropriate indications coupled with the receipt of no native
frames.

Point-to-point links are, in fact, very common in modern 802.3 networks.

Donald

-----Original Message-----
From: Eric Gray [mailto:eric.gray at ericsson.com] 
Sent: Wednesday, October 03, 2007 11:11 AM
To: Eastlake III Donald-LDE008; Rbridge at postel.org
Subject: RE: [rbridge] Consensus Check: Point to Point links

Donald,

	Given that it is not crystal clear what you mean by 
"point to point link", I am not sure I agree with this, at
least as you have worded it here.

	If you mean that the link is a full-duplex Ethernet
link and the two end-points have some definitive mechansim
for determining that they are the only entities using the
link between them, there may be issues with doing this.

	For example, if the link is technically an Ethernet 
link, then it is not unlikely that one or the other devices 
may have multiple roles - i.e. - it may be both an RBridge 
for some frames and either a regular bridge or end station
(for example, a router) for others.  It's arguable that, in
this case, the multi-role device is two (or more) separate
entities - thus invalidating a "point to point" definition
for this case (though only two distinct physical devices
are connected via this link).

	Without a clear agreement between involved entities,
this sort of "short-cut" addressing is likely to result in
higher-level (slow path) processing of many (if not all) 
of the frames transiting the link for some implementations.
Moreover, without an unambiguous determination of exactly
when this would apply, it will not be unambiguously clear
when a receiving implementation would have to switch to a
"promiscuous listening" mode.

	I believe omission of the outer VLAN tag suffers from
the same ambiguity.  For instance, it is possible for two
devices to have a "point to point" relationship within a 
VLAN context that would nto be the case without the VLAN
context.

	Hence it appears we would have to be explicit in what
we mean by "point to point" link and how we expect that the
entities (RBridges) involved would be able to disambiguate
this p2p status for any given link.

	If we are saying that two devices - using some means 
out of scope for our specification - are somehow aware of an 
unambiguous point-to-point relationship between them and can
therefore use any MAC DA on transmission, and ignore it on
receipt, we could make the same argument for virtually any 
encapsulation choice we might prefer.  But, it would be as
valid to observe that we don't need to specify what is - in
essence - an out-of-context (mis)behavior between consenting 
implementations...

--
Eric Gray
Principal Engineer
Ericsson  

> -----Original Message-----
> From: rbridge-bounces at postel.org 
> [mailto:rbridge-bounces at postel.org] On Behalf Of Eastlake III 
> Donald-LDE008
> Sent: Tuesday, October 02, 2007 11:27 PM
> To: Rbridge at postel.org
> Subject: [rbridge] Consensus Check: Point to Point links
> 
> 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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