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-=======================
-Use cases and scenarios
-=======================
-
-Resource reservation is a basic feature in any virtualization-based network
-operation. In order to perform such resource reservation from NFVO to VIM, NFVI
-capacity information is also necessary at the NFVO side. Below, four use cases
-to show typical requirements and solutions for capacity management and resource
-reservation is presented.
-
-#. Resource capacity management
-#. Resource reservation for immediate use
-#. Resource reservation for future use
-#. Co-existence of reservations and allocation requests without reservation
-
-Resource capacity management
-============================
-
-NFVO takes the first decision on in which NFVI it would instantiate a VNF.
-Along with NFVIs resource attributes (e.g. availability of hardware
-accelerators, particular CPU architectures etc.), NFVO needs to know available
-capacity of an NFVI in order to make an informed decision on selecting
-a particular NFVI. Such capacity information shall be in a coarser granularity
-than the respective VIM, as VIM maintains capacity information of its NFVI
-in fine details. However a very coarse granularity, like simply the number of
-available virtual CPU cores, may not be sufficient. In order to allow the NFVO
-to make well founded allocation decisions, an appropriate level to expose the
-available capacity may be per flavor. Capacity information may be required for
-the complete NFVI, or per partition or availability zone, or other
-granularities. Therefore, VIM requires to inform the NFVO about available
-capacity information regarding its NFVI at a pre-determined abstraction, either
-by a query-response, or in an event-based, or in a periodical way.
-
-Resource reservation for immediate use
-======================================
-
-Reservation is inherently for the future. Even if some reserved resources are
-to be consumed instantly, there is a network latency between the issuance of a
-resource reservation request from the NFVO, a response from the VIM, and actual
-allocation of the requested resources to a VNF/VNFM. Within such latency,
-resource capacity in the NFVI in question could change, e.g., due to failure,
-allocation to a different request. Therefore, the response from a VIM to the
-NFVO to a resource reservation request for immediate use should have a validity
-period which shows until when this VIM can hold the requested resources. During
-this time, the NFVO should proceed to allocation if it wishes to consume the
-reserved requested. If allocation is not performed within the validity period,
-the response from VIM for a particular resource reservation request becomes
-invalid and VIM is not liable to provide those resources to NFVO/VNFM anymore.
-Reservations requests for immediate use do not have a start time but may have
-an end time.
-
-Resource reservation for future use
-===================================
-
-Network operators may want to reserve extra resources for future use. Such
-necessity could arise from predicted congestion in telecom nodes e.g. due to
-local traffic spikes for concerts, natural disasters etc. In such a case, the
-NFVO, while sending a resource reservation request to the VIM, shall include a
-start time (and an end time if necessary). The start time indicates at what
-time the reserved resource shall be available to a designated consumer e.g. a
-VNF/VNFM. Here, the requirement is that the reserved resources shall be
-available when the start time arrives. After the start time has arrived, the
-reserved resources are allocated to the designated consumer(s). An explicit
-allocation request is needed. How actually these requested resources are held
-by the VIM for the period in between the arrival of the resource reservation
-request and the actual allocation is outside the scope of this requirement
-project.
-
-Co-existence of reservations and allocation requests without reservation
-========================================================================
-
-In a real environment VIM will have to handle allocation requests without any
-time reference, i.e. time-unbound, together with time-bound reservations and
-allocation requests with an explicitly indicated end-time. A granted
-reservation for the future will effectively reduce the available capacity for
-any new time-unbound allocation request. The consequence is that reservations,
-even those far in the future, may result in denial of service for new
-allocation requests.
-
-To alleviate this problem several approaches can be taken. They imply an
-implicit or explicit priority scheme:
-
-* Allocation requests without reservation and which are time-unbound will be
- granted resources in a best-effort way: if there is instant capacity, but the
- resources may be later withdrawn due to the start time of a previously
- granted reservation
-* Both allocation requests and reservation requests contain a priority which
- may be related to SLAs and contractual conditions between the tenant and the
- NFVI provider. Interactions may look like:
-
- * A reservation request for future use may cancel another, not yet started,
- reservation with lower priority
- * An allocation request without reservations and time-unbound [#unbound]_ may
- be granted resources and prevent a future reservation with lower priority
- from getting resources at start time
- * A reservation request may result in terminating resources allocated to a
- request with no reservation, if the latter has lower priority
-
-.. [#unbound] In this case, the consumer (VNFM or NFVO) requests to immediately
- instantiate and assign virtualized resources without having
- reserved the resources beforehand