[Nsi-wg] A alternative Modify() proposal - a "shadow" approach
Jerry Sobieski
jerry at nordu.net
Tue Jul 3 15:09:55 EDT 2012
Hi everyone-
The connection modification capability for version 2.0 was initially
presented as a simple enhancement to extend the scheduled end time. Or
perhaps to increase the bandwidth, on an existing reservation. This was
supposed to be a very limited functional tweek for v2.0.
But then we decide "hitless" was a requirement; And then we added "path
preservation" as a requirement. It was *assumed* that we needed a
unique Modify() primitive to do this... probably because prior tools
have them... Suddenly, _/we are re-defining the entire state
machine/_ (yet again), and making it still more complex, in order to
make this "simple" enhancement.
This increasing complexity is actually counter to what we were trying to
do in Oxford: to /simplify/ the state machine. And in general, counter
to good protocol design. I think the existing state machine has been
thoroughly vetted and is adequate for the protocol, and that we should
consider functions like "Modify" as higher layer constructs that should
be implemented using the existing atomic primitives we already have.
Things like protection circuits, and diversity attributes, and the like
will all pose similar challenges - and we cant keep changing the state
machine everytime someone has a "simple" feature they can't live without...
Given the developing complexity, we should step back and re-evaluate a)
the urgency for Modify(), b) the means/scope of implementing it, and
c) the timeline it will require to "do it properly".
I would like to also propose an alternative "shadow" approach to provide
a modify capability in version 2.0:
In a shadow approach, we build a simple second "shadow" connection
reservation, and then perform a Release()-Provision() sequence to cut
over to the modified service instance when ready. This shadow approach
uses only existing protocol primitives and existing state machine.
(This is similar to John's talk about "bridge and roll"... but without a
bridge:-)
Currently, a separate circuit approach like this would require separate
STPs as endpoints for the modified connection reservation. However,
given virtual STPs (e.g. VLANs), a shadow connection would not *really*
need to terminate at the same source or destination STP to be useful -
i.e. the A and Z endpoints of a modified connection could be different
VLANs without imposing any detectable performance hit on end-to-end data
flow (!) - the sending system simply begins using a new tag when the
shadow provisioning is completed. (This requires the end systems
agents to know this will occur, but, strictly speaking, this is entirely
feasible.) The shadow path would likely even be along the same
geographic route - i.e. the packets would transit all the same network
infrastructure, just with different tags. Given this situation, the
need to "modify" an *existing* connection, particularly with ethernet
based STPs, seems somewhat unnecessary if you can simply request another
connection with the desired new attributes along the same path and start
using it whenever you please...
Being pragmatic though, there are many applications that will not be
able to change their termination point, thus the source/destination STPs
should be simultaneously acceptable for both the shadow connection as
well as the working connection. Likewise, other resources (say
bandwidth) may not be sufficient to reserve a completely separate
upgraded Connection, and so the path finders ought to be able to
"double-book" resources assigned to the working connection to be used by
the shadow connection. Since the working conenction and the shadow
connection should never both be active, this double booking will never
cause a conflict. This ability for shadows to double-book resources of
their working counterpart provides the functionality we initially
wanted: simply upgrading the existing path.
We can easily indicate when we wish to create a shadow Reservation
within the existing protocol: We simply specify an existing
ConnectionID in a Reservation Request. If the ReservationRequest
specifies an existing Reservation rather than a new Reservation, then a
[new] shadow Reservation/Connection is to be created and linked to the
original "working" reservation. Thus, an otherwise normal Connection
is identified as a "shadow" connection solely by the link to a working
Connection. When a reservation is confirmed, if it links to a working
connection, the RA will immediately replace the working with the shadow
and Terminate the working reservation. In the one case where the
working connection is Active, the shadow will remain in its Reserved
state as if it had passed the start time and was awaiting a provision
request. When a Release occurs for the working connection, a check is
made to see if a shadow is linked to it. If so, the shadow will then
replace the working, and the working connection is Terminated.
This process does not change the NSI-CS protocol or the state machine.
It incur [minor] code additions to the existing primitives, but does not
change the event driven state transitions. Pathfinders should to also
be enhanced to double-book shadow resources.
This "shadow" approach has this major advantage: Since it is
essentially just building a second reservation, it does not require
changing the fundamental NSI-CS protocol or the state machine. All the
"modification" processing is implemented using existing primitives and
state transitions. The cost to the user is minimal: a single
*potential* brief hit as the A and Z endpoints are switched to the
[new/modified] connection. And since the user initiated the modify() in
the first place, and will need to adjust the behaviour of the
application to take advantage of the new characteristics, it does not
seem unreasonable to expect the user to be able to deal with a hiccup -
if it occurs.
Finally, as a general recommendation: Modifying the existing primitives
and the associated state machine should be a /last/ resort. Any new
feature should have a very strong case for modifying the NSI-CS state
machine, and alternatives that do not do so should be strongly
encouraged. We should only modify the NSI core protocols in order to
simplify them, delivering additional features through higher level
service constructs wherever possible.
Thoughts?
Jerry
On 7/2/12 11:06 PM, John MacAuley wrote:
> Peoples,
>
> Here is the new and improved NSI CS state machine fresh off the presses and ready for your viewing pleasure. Please study it and prepare questions for the Wednesday call. We would like to close on this action ASAP.
>
> Thank you,
> John.
>
>
> _______________________________________________
> nsi-wg mailing list
> nsi-wg at ogf.org
> https://www.ogf.org/mailman/listinfo/nsi-wg
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