[Nsi-wg] Path Object information/function

[Jerry Sobieski]

Hi Jeroen-

Hmm...    I had always assumed that this would be implicit - that a 
strict hop PO and Loose hop PO would not really look any different....  
But honestly I had not considered whther this was necessary or not.     
My thought was that whatever agent was using the Path Object would need 
to check it against the topology anyway - in essence perform a Path 
Computation between Hop(n) and Hop(n+1).  If they are adjacent, this 
would be an almost zero cost check, and if they were not adjacent, a 
Path Comp would be needed anyway.  I don't know that being explicit 
relieves any agent from checking adjacency, but there may be some error 
conditions that are detected if the expectation is explicit in some 
way.   There may be a need to indicate when a hop was/is expected to be 
adjacent (e.g. a strict hop "as-built" PO that describes a provisioned 
path after the fact vs a reserved PO that may or may not be strict)

This also brings up the issue of whether the underlying topology can 
change between when a Path Object is created and when that Path Object 
is referenced/used.   I.e. What happens if a specified hop no longer 
exists?  Or if additional switching points are introduced between say 
when a reservation is created, and when the connection is provisioned?

I don't think I have a position on the question to making strict vs 
loose explicit - it might be useful or necessary.   Or it may be 
superfluous.   Would we specify each particular hop as strict or loose?  
Or simply indicate the entire PO as strict or loose? (I'd say hop by hop 
would be best).  Perhaps we allow for a boolean indicator on each hop 
that says this is "strict hop" to previous hop.   If it is not set it 
could be eiterh loose or strict, but if it *is* set, then the hop must 
be strict. 

Also, as we discuss this, we must formulate what we mean by "adjacent".  
IMO, adjacent means adjacent in terms of provisioning - i.e. nominally, 
a switching point that must be re-configured as part of provisioning is 
a hop that should be presnt in a strict hop PO.   If a switching point 
exists only as part of an underlying tunnel connection, and it is not 
seen as part of the Path Finding process and not reconfigured as part of 
a connection's provisioning process, then it is not part of the PO.  

For example, a Ethernet link established between Cern and Argonne would 
be seen as a single link in the topology when allocating Layer2 
connections.  While there may be lots of switching points that went into 
setting up that express Etehrnet connection, as far as the Path Finder 
is concerend, there is one ethernet STP in Cern and one in Argonne, and 
so provisioning a path between Cern and US over that link would not 
indicate all those lower layer switching SDH and Wave and Fiber 
switching points.  None of them were reconfigured as part of a 
connectionusing that long link.   However, if a connection is built that 
adapts the PDU from an Ethernet VLAN into a GFP payload on a sonet link, 
then that adaptation point is something that is visible to the Path 
Finder in the topology and is something that is reconfigured to 
establsih the connection - the Ethernet egress and the GFP ingress STPs 
are specifically part of the circuit and should be in a strict hop 
PO.    To be complete, that long express Ethernet link would have a 
strict PO, but it would be associated with that a connection request 
from another agent somewhere, not with any particular connection riding 
over the top of it at the time.  

THis is probably clear as mud, (:-) but I hope this is useful.

Jerry


Jeroen van der Ham wrote:
> On 20/01/2010 20:24, Jerry Sobieski wrote:
>   
>> - A "partially specified" path identifies a subset of STPs - in order -
>> that the connection transits - but not necessarily every STP the
>> connection transits.  THis is a "loose hop" path
>>     
>
> Do you propose to make this implicit or explicit? That is, is there a 
> "loose hop" object/marker in the STP?
>
> I would think that some kind of marker is required in order to make this 
> work, and clear to the other parties that part of the STP is hidden.
>
> The loose hop object could also be used to specify whether the "loose" 
> resources have been provisioned, or not, and to specify other kinds of 
> information about the possible path there.
>
> Jeroen.
> _______________________________________________
> nsi-wg mailing list
> nsi-wg at ogf.org
> http://www.ogf.org/mailman/listinfo/nsi-wg
>   
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.ogf.org/pipermail/nsi-wg/attachments/20100125/7e85c879/attachment.html