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diff --git a/docs/development/requirements/usecase.rst b/docs/development/requirements/usecase.rst new file mode 100644 index 0000000..bdf8888 --- /dev/null +++ b/docs/development/requirements/usecase.rst @@ -0,0 +1,121 @@ +.. This work is licensed under a Creative Commons Attribution 4.0 International License. +.. http://creativecommons.org/licenses/by/4.0 + +======================= +Use cases and scenarios +======================= + +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 :ref:`uc-brahmaputra`. + +#. 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 + +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. |