3. Use cases and scenarios

3.1. Use cases

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. A typical use case as considered for the Brahmaputra release is described in ANNEX A: Use case for OPNFV Brahmaputra.

  1. Resource capacity management
  2. Resource reservation for immediate use
  3. Resource reservation for future use
  4. Co-existence of reservations and allocation requests without reservation

3.1.1. 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.

3.1.2. 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.

3.1.3. 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.

3.1.4. 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 [1] 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
[1]In this case, the consumer (VNFM or NFVO) requests to immediately instantiate and assign virtualized resources without having reserved the resources beforehand

3.2. Scenarios

This section describes the expected behavior of the system in different scenarios.

As we are targeting a cloud platform with the above use cases, it is crucial to keep the flexibility in the system. By this means it is hard to explicitely block hardware resources for the future and ensure their availablility for allocation therefore it can happen that the reservation plan cannot be fulfilled due to certain changes in the underlying environment. We need to ensure that we are prepared for error and edge cases during the design period.