[rbridge] draft protocol-10 WGLC Maximum Bridge Transit Delay

[Donald Eastlake]

Hi Jim,

On Mon, Dec 1, 2008 at 1:27 PM, James Carlson <james.d.carlson at sun.com> wrote:
> Donald Eastlake writes:
...
>> The specific provisions in 802.1D-2004 include :
>> "6.3.6 Frame lifetime
>> The MAC Service mandates an upper bound to the transit delay
>> experienced for a particular instance of
>> communication. This maximum frame lifetime is necessary to ensure the
>> correct operation of higher layer
>> protocols. ..."
>
> If there's no limit on the number of bridges that may be encountered
> in transit, and no limit on the speed-of-light delays between nodes,
> then I think there's little that this transit delay limit does to
> protect those upper layer protocols.

Seems like good points. I'm not aware of any hops limit for bridges
like the 7 level limit for hubs.

> It limits variance in some cases, at the cost of higher loss, but
> that's about it; it can't practically set any upper limit.  (For a
> worst-case scenario, consider two adjacent nodes that are unable to
> use a shared link due to redundancy elsewhere in the network.  Failure
> of that redundancy can bring up that low-latency link, and restoring
> the far-away link tears it down again.  The variance in that case may
> be predictable if you know the entire topology, but it's arbitrarily
> large.)
>
...
>> Well, I'm OK with saying MAY. Since this is described in the 802.1
>> standards as being in the port output queue behavior, and since we now
>> incorporate that by reference unless we say otherwise, one could argue
>> that it is mandatory under the current draft. Unless people weigh in
>> with other opinions, I'll change it to MAY.
>
> That seems fine.  (If we're incorporating all of 802.1 by reference,
> do we need to restate much of it ... ?)

Not much of it. But, since we are essentially incorporating a snapshot
of the 802.1Q port processing below the EISS layer, I think we need to
say how we differ from 802.1Q in that area. Currently, the protocol
spec just says we differ for BPDUs and VLAN registration protocols.
Although it seems like a minor difference in practice, if enforcement
of a maximum bridge transit delay is a MAY rather than being mandatory
as it is in 802.1, I think we should say so.

Thanks,
Donald

> James Carlson, Solaris Networking              <james.d.carlson at sun.com>
> Sun Microsystems / 35 Network Drive        71.232W   Vox +1 781 442 2084
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