Add requirement doc

Change-Id: Ibfd83f84ae47291ece09150af9f35d38ef3a1288
JIRA: PROMISE-1
Signed-off-by: Ryota MIBU <r-mibu@cq.jp.nec.com>

1 files changed, 97 insertions, 0 deletions

diff --git a/requirements/resource_management/02-usecase.rst b/requirements/resource_management/02-usecase.rst
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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  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