[Nsi-wg] A alternative Modify() proposal - a "shadow" approach

Tue Jul 3 16:13:28 EDT 2012

Four main points I do not want to lose track of:

1. The two phase Modify has nothing to do with the type of modification.  I am still hoping to only support bandwidth and endTime modifications for release 2.0.  The reason we proposed the two phase was because of the complexity of a distributed system and securing resources using the tree model.  If we did a single phase then a modify request would change reservations on the way down the tree, and if a modify failure occurred on a branch, then the only way to restore the schedule to a correct state would be to send down another modify with the original schedule parameters and hope it can be restored.  The only solution to this problem is to do the pre-allocation of resources, and once all participants confirm they can meet the need, then we commit the change.

If we squint our eyes and stand back a bit, we can map two phase modify operations to the existing reservation state machine:

modifyRequest = reserveRequest
modifyCommitRequest = provisionRequest
modifyCancelConfirm = terminateRequest

We had looked at overloading the existing reservation operations but it was quickly dismissed as not feasible.

2. The Modify state machine is separate from the existing reservation and provision lifecycle state machine, and therefore, stands on its own while providing no additional complexity to the existing machine.

3. I was asked to propose a two phase reserve to fix the original release 1.0 deficiency we introduced for simplicity and to "get something out there".  We were lucky with the existing single phase reservation because we get a pseudo commit by having to provision the circuit.  We all agreed to revisit the two phase in release 2.0 so I had no objection to attempt a state machine.  I would rather fix it in 2.0 than try to do it in 3.0 and need to support yet another state machine.

It should also be noted that the improved and simplified provisioning state machine is still there as is.  We just expanded the single reserve state into a two phase commit reservation which can stand on its own.  Very elegant if you ask me :-)

4. Lastly, we need to avoid the pitfall of dropping things just so we can build a demo of release 2.0 for October.  If we can accommodate these features in the specification for release 2.0, then we should and worry about lining up implementations afterwards.  We have done the detailed legwork, and unless there is a fundamental flaw in the concept, I think we need to get it into the specification so we have something useful to put into production.

Of course, if we remove the tree model and use a chain model you get the implicit commit for all your operations.

John.

On 2012-07-03, at 3:09 PM, Jerry Sobieski wrote:

> 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
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