Re: [DNA] draft-ietf-dna-link-information-04.txt status

[Greg Daley <Greg.Daley@eng.monash.edu.au>]

Hi Jari, 


> > Section 2.4.2
> >
> > "   Ethernet networks commonly consist of LANs joined together by
> >    transparent bridges (usually implemented as switches).  
> Transparent>    bridges require the active topology to be loop 
> free.  The Spanning
> >    Tree Protocol (STP) achieves this by the exchange of Bridge 
> Protocol>    Data Unit (BPDU), as defined in [IEEE-802.1D], which 
> leads to, where
> >    required, the blocking of ports (i.e., not forwarding)."
> >
> > I think you need to mention RSTP here as well.
> Not in -04. Suggest replacing the last sentence by "This is achieved
> through the Spanning Tree Protocol (STP) or the Rapid Spanning
> Tree Protocol (RSTP). These protocols exchange Bridge Prototocol
> Data Units (BPDUs) [IEEE-802.1D, IEEE802.1W], and leads to, ..."

I believe that 802.1w has been rolled into 802.1D, so apart from the
references needing to be merged this is OK.


> > "   If no bridge configuration messages are received within the
> >    Bridge_Max_Age interval (default 20s), then it is likely that 
> there>    is no visible bridge whose port is enabled for bridging 
> (S8.4.5 of
> >    [IEEE-802.1D]), since at least two BPDU hello messages would have
> >    been lost.  Upon this timeout, a link up notification MUST be
> >    generated.
> >
> >    It is not easy for a non-STP host to distinguish between Disabled
> >    bridge ports and non-bridge ports with no IP nodes on them, as
> >    Disabled ports will have no traffic on them, and incur 100% 
> sender>    loss.
> >
> >    Upon this Bridge_Max_Age timeout, a link up notification must be
> >    generated.  If an earlier link up was generated with the R-
> flag set,
> >    this new one MUST set the A-flag showing that packets sent 
> within the
> >    prior interval were likely to have traversed the forwarding path
> >    (unless the port is disabled)."
> >
> > The next section talks about when to enable the delay, but I 
> think you
> > could be more clear about this here as well.  For example, are you
> > recommending that hosts listen for STP/RSTP messages, or use 802.1AB
> > in order to decide on the appropriate delay?  Delaying for 20 
> seconds> by default probably isn't a good idea now that RSTP is 
> becoming more
> > common.
> There was a long discussion about this in early July.
> Please incorporate the conclusions. This is the only
> item where its unclear to me if the discussion between
> Bernard and Greg converged.

I'm actually not sure it did (Did we still have the flags??) How about:

"
Where it is not known that forwarding operations are available, a
host SHOULD assume that RSTP or STP is being performed. Hosts MAY
listen to STP/RSTP and 802.1AB messages to gain further information
about the timing of full connectivity on the link, for example, to
override an existing indication.

Notably, though, it is not easy for a host to distinguish between
Disabled bridge ports and non-bridge ports with no active transmitters on
them, as Disabled ports will have no traffic on them, and incur 100%
sender loss.

If no bridge configuration messages are received within the
Bridge_Max_Age interval (default 20s), then it is likely that there
is no visible bridge whose port is enabled for bridging 
(S8.4.5 of [IEEE-802.1D]), since at least two BPDU hello messages would
have been lost.  Upon this timeout, a link up notification MUST be
generated, if one has not been already."


I believe this shows that the indications may be sent first when link-layer
connectivity (rather than spanning-tree-connectivity) is available, but 
that hosts need to be wary of sender loss during spanning tree calculation.

Greg
(now employed again).