[Nsi-wg] Comments on NSI architecture document.

[John MacAuley]

Thank you Jerry for talking the time to respond.  I have been reading 
the specification with protocol implementation in mind so want to make 
sure I understand the concepts.  I think the Service Definition concept 
is good and have implemented similar mechanisms in other products.  
Where my specific confusion comes in is around the handling of optional 
parameters with defaults where the default values are different per NSA 
domain.  You can reassure me that this is not a valid concern, but let 
me outline the use case that came to mind when I read this section of 
the document.

A User Client (UC) issues a reservation request to a local NSA which 
will assume the role of Requester NSA (RA) for this example.  The RA 
creates a service definition template populated with parameters 
important to the UC.  In this scenario let us assume that the MTU was 
not specified by the UC, and therefore, will utilize the defaults 
supported by the network.

If the RA who receives the UC request then fills in the MTU parameter 
with its default value before issuing the schedule request to 
subordinate Provider NSA (PA), then we are all good and the default 
value chosen by the RA for MTU will be sent down and approved by all 
subordinate PA, or alternatively, rejected if the MTU value is not 
supported.

 From the text written I assumed that the RA would not fill in the MTU 
parameter:

    "If a service parameter is not present in the service request, then
    the provider NSA should “fill in the blanks” from default values in
    the Service Definition."

I may have read too much into this text, and in fact, we really expect 
the RA to fill in the default values, but this was the impression I was 
left with.  So now if we look at the same example, but in this case the 
RA does not fill in the default MTU size, we can have two separate 
subordinate PA select incompatible MTU sizes.  The resulting connection 
service definition information would be returned to the RA and it would 
see that the MTU sizes are incompatible.  I therefore assumed a second 
pass would be required to normalize these values in the subordinate PA.

So am I out in left field on this one?

John.

On 10-06-17 4:53 AM, Jerry Sobieski wrote:
> Here are my thoughts on the Service Definition issues John refers to:
>
>
> John MacAuley wrote:
>>
>> _Section 3.4 NSI Service Definitions_
>>
>> “A service request is fully specified when all parameters associated 
>> with that service have been determined either by explicit user 
>> specification or by implicit default values found in the Service 
>> Definition.”
>>
>> Are service definition defaults a common global configuration or are 
>> these defaults a localized decision? If they are a localized decision 
>> then the requestor NSA should “fill in the blanks” so that all 
>> subsequent provider NSA contacted have the assumed default values 
>> filled in the service request.
>>
> Service Definitions are not expected/required to match one another 
> completely.   Of course, the more similar they are, the better, but I 
> don't think we can assert or assume that every network will  have the  
> exact same service capabilities.    I think "similar" service 
> definitions will have the same service parameters,  and "identical" 
> services would have the same  values for  those parameters.
>
> I think the SD ought to flag parameters that are required (i.e. *must* 
> be specified by the requester), and those that are optional (i.e. the 
> requester *may* specify these parameters).   For optional parameters, 
> the SD *may* specify the default value to be used if the request does 
> not explicitly specify the value.  If a Service Definition specifies a 
> default value for a parameter, it must be used.  If the SD does not 
> specify a default for an optional parameter, then that parameter 
> should not even be considered as a constraint at all when searching 
> the topology for a path (e.g. such a parameter might be the Bit Error 
> Rate...if unspecified, it is not considered as a constraint, and the 
> user gets whatever is associetaed with the path that is found based 
> upon other constraints.)
>
> Implicit here is a notion that when the reservation is confirmed, 
> there exist "settings" for all parameters associated with the service 
> instance.   Some (many?) of these paramters may not have been 
> specified as constraints in the original request, but they 
> nevertheless get resolved as the path is selected.
>
> We (I?) need to consider this default handling issue more thoroughly.  
> But I do not think it is a significant issue for the NSI Architecture 
> doc.  This is part of pathfinding, and as such IMO it is out of scope 
> for the NSI Arch.
>
> However, I do think the Service Definition concept overall should be 
> the subject of its own white paper (its been 5 years since Tom Lehman 
> and I wrote the original concept paper.)   There are many issues for 
> which it appears to be a very elegant approach and so we will want a 
> more complete treatment than a few paragraphs in the NSI Architecture 
> documnet.
>
>> _Section 5.1.2 Service Definitions for Connection Services_
>>
>> “If a service parameter is not present in the service request, then 
>> the provider NSA should “fill in the blanks” from default values in 
>> the Service Definition. As the request is processed down the NSA 
>> service tree, default values adopted in one transit network may 
>> implicitly constrain the request in downstream networks. Therefore, 
>> in general, each NSA should use default values that provide the 
>> greatest leeway to the pathfinder in satisfying the request both 
>> within the local network and in external downstream networks.”
>>
>> This mechanism is rather complex as described. If service parameters 
>> are left open ended by some NSA, then an additional visit to that NSA 
>> must be performed to finalize the actual negotiated parameters. In 
>> the tree model this would require a second pass to commit the final 
>> service definition negotiated across the network. In the chain model 
>> it would require the end terminating NSA in the chain to finalize the 
>> service definition and then every node returning up the chain would 
>> finalize their definition.
>>
> Hmmm... I'm not sure I follow John.
>
> These paramters fill strategies do not affect the tree/chain 
> reservation process.  If an RA does not fill the request, then the PA 
> has two choices: it must make some assumptions and process the 
> request; or not, and reject the request.    If the RA does not 
> specify, say, MTU, should we reject the request until it does?   Why 
> can not we simply apply a default value that offers the best value to 
> the user, and then process the request down the tree normally?    We 
> can adopt different strategies...  The Provider NSA could fill with a 
> value that is most likely to succeed in reserving a path, in this 
> example 1500B MTU might provide the best odds of finding a viable 
> path.   Or an alternative strategy might be to find a value that 
> provides a "better" connection irregardless of how likely it is to be 
> suceesfully found, in this case the MTU might be 9000B MTU.  These 
> default selections strategies are so nuanced that I don't think we 
> want to deal with them at this time.    So I propose we allow the SD 
> to  identify required and optional parametrs, and the SD can specify 
> defaults for optional params.
>
> The pathfinding and confirmation process remains the same regardless 
> of how we set the defaults.   There is no additional cost in 
> complexity for either tree or chain model (remember- the chain is just 
> a degenerate form of the tree model)
>
>
> Hope this helps!
> Jerry

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