Subject: [OPSAWG] SAP at IETF113: Scott and Rob clarification questions

From: <[email-masked]>
Date: Tue, 29 Mar 2022 06:28:24 +0000

Hi all,

Please find below a follow-up to some questions raised in the meeting:

(1) Scott asked during the meeting whether the service type is a type or a pointer to an instance of a service. I confirm that this is a type, not a service instance. Service types are typically taken from RFC9181. That part is covered in the I-D:

   A node in the topology can support one or multiple service types
   ('service-type') among those listed under the 'sap-network'
   container.  A list of SAPs are then bound to each service type that
   is supported by a given node.

Service instances of given service may be bound to the same or separate SAPs having the same service type. For typical services such LxVPN, dedicated sub-interfaces are used to demux these instances but the parent interface is also exposed as a SAP as per the following:

      SAPs that are associated with the interfaces that are directly

      hosting services, interfaces that are ready to host per-service

      sub-interfaces (but not yet activated), or service that are

      already instantiated on sub-interfaces are listed as SAPs.

Note that a SAP may even host services of #types as per the following from the draft:

      The same SAP may appear under distinct service types.  In such a

      case, the same identifier is used for these service types in

      association.

That's the reason why we do separate sap-status vs. service-status:

   'service-status':  Reports the operational status of service for a

      given SAP.  This information is particularly useful when many

      services are enabled for the same SAP, but only a subset of them

      are activated.

(2) Then, followed this comment from Rob:

==
Rob Wilton: Wouldn't you end up binding the sap against an sub-interface?
Med: not necessarily-please clarify
Rob: If you have multiple saps how are they modeled?
Med: I will check against sub-interfaces offline.
==

I confirm may first answer: not necessarily. For typical services such as LxVPN, sub-interfaces are likely to be created for each service instance. All these are exposed as separate SAPs, which may have their own characteristics, especially these leafs:

                    +--rw attachment-interface?       string

                    +--rw interface-type?             identityref

                    +--rw encapsulation-type?         identityref

                    +--rw peer-sap-id?                string
However, the parent termination point that hosts these sub-interfaces is also exposed as a SAP. That SAP does not have a sub-interface. We do cover that aspect in the draft:

      SAPs that are associated with the interfaces that are directly

      hosting services, interfaces that are ready to host per-service

      sub-interfaces (but not yet activated), or service that are

      already instantiated on sub-interfaces are listed as SAPs.

The example depicted in Appendix B is useful to digest this aspect of SAP modelling.

Cheers,
Med

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