Introduction to the HA Use Cases

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JIRA: HA-11 HA-14

Change-Id: I04331a4fd25d95a69ac086c962399ce6c49c8c33
Signed-off-by: Maria Toeroe <Maria.Toeroe@ericsson.com>

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+============
+HA Use Cases
+============
+
+**************
+1 Introduction
+**************
+
+This use case document outlines the model and failure modes for NFV systems. Its goal is along
+with the requirements documents and gap analysis help set context for engagement with various
+upstream projects. The OPNFV HA project team continuously evolving these documents, and in
+particular this use case document starting with a set of basic use cases.
+
+*****************
+2 Basic Use Cases
+*****************
+
+
+In this section we review some of the basic use cases related to service high availability,
+that is, the availability of the service or function provided by a VNF. The goal is to
+understand the different scenarios that need to be considered and the specific requirements
+to provide service high availability. More complex use cases will be discussed in
+other sections.
+
+With respect to service high availability we need to consider whether a VNF implementation is
+statefull or stateless and if it includes or not an HA manager which handles redundancy.
+For statefull VNFs we can also distinguish the cases when the state is maintained inside
+of the VNF or it is stored in an external shared storage making the VNF itself virtually
+stateless.
+
+Managing availability usually implies a fault detection mechanism, which triggers the
+actions necessary for fault isolation followed by the recovery from the fault.
+This recovery includes two parts:
+
+* the recovery of the service and
+* the repair of the failed entity.
+
+Very often the recovery of the service and the repair actions are perceived to be the same, for
+example, restarting a failed application repairs the application, which then provides the service again.
+Such a restart may take significant time causing service outage, for which redundancy is the solution.
+In cases when the service is protected by redundancy of the providing entities (e.g. application
+processes), the service is "failed over" to the standby or a spare entity, which replaces the
+failed entity while it is being repaired. E.g. when an application process providing the service fails,
+the standby application process takes over providing the service, while the failed one is restarted.
+Such a failover often allows for faster recovery of the service.
+
+We also need to distinguish between the failed and the faulty entities as a fault may or
+may not manifest in the entity containing the fault. Faults may propagate, i.e. cause other entities
+to fail or misbehave, i.e. an error, which in turn might be detected by a different failure or
+error detector entity each of which has its own scope. Similarly, the managers acting on these
+detected errors may have a limited scope. E.g. an HA manager contained in a VNF can only repair
+entities within the VNF. It cannot repair a failed VM, in  fact due to the layered architecture
+in the VNF it cannot even know whether the VM failed, its hosting hypervisor, or the physical host.
+But its error detection mechanism will detect the result of such failures - a failure in the VNF -
+and the service can be recovered at the VNF level.
+On the other hand, the failure should be detected in the NFVI and the VIM should repair the failed
+entity (e.g. the VM). Accordingly a failure may be detected by different managers in different layers
+of the system, each of which may react to the event. This may cause interference.
+Thus, to resolve the problem in a consistent manner and completely recover from
+a failure the managers may need to collaborate and coordinate their actions.
+
+Considering all these issues the following basic use cases can be identified (see table 1.).
+These use cases assume that the failure is detected in the faulty entity (VNF component
+or the VM).
+
+
+*Table 1: VNF high availability use cases*
+
++---------+-------------------+----------------+-------------------+----------+
+|         | VNF Statefullness | VNF Redundancy | Failure detection | Use Case |
++=========+===================+================+===================+==========+
+| VNF     | yes               | yes            | VNF level only    | UC1      |
+|         |                   |                +-------------------+----------+
+|         |                   |                | VNF & NFVI levels | UC2      |
+|         |                   +----------------+-------------------+----------+
+|         |                   | no             | VNF level only    | UC3      |
+|         |                   |                +-------------------+----------+
+|         |                   |                | VNF & NFVI levels | UC4      |
+|         +-------------------+----------------+-------------------+----------+
+|         | no                | yes            | VNF level only    | UC5      |
+|         |                   |                +-------------------+----------+
+|         |                   |                | VNF & NFVI levels | UC6      |
+|         |                   +----------------+-------------------+----------+
+|         |                   | no             | VNF level only    | UC7      |
+|         |                   |                +-------------------+----------+
+|         |                   |                | VNF & NFVI levels | UC8      |
++---------+-------------------+----------------+-------------------+----------+
+
+As discussed, there is no guarantee that a fault manifests within the faulty entity. For
+example, a memory leak in one process may impact or even crash any other process running in
+the same execution environment. Accordingly, the repair of a failing entity (i.e. the crashed process)
+may not resolve the problem and soon the same or another process may fail within this execution
+environment indicating that the fault has remained in the system.
+Thus, there is a need for extrapolating the failure to a wider scope and perform the
+recovery at that level to get rid of the problem (at least temporarily till a patch is available
+for our leaking process).
+This requires the correlation of repeated failures in a wider scope and the escalation of the
+recovery action to this wider scope. In the layered architecture this means that the manager detecting the
+failure may not be the one in charge of the scope at which it can be resolved, so the escalation needs to
+be forwarded to the manager in charge of that scope, which brings us to an additional use case UC9.
+
+We need to consider for each of these use cases the events detected, their impact on other entities,
+and the actions triggered to recover the service provided by the VNF, and to repair the
+faulty entity.
+
+We are going to describe each of the listed use cases from this perspective to better
+understand how the problem of service high availability can be tackled the best.
+
+Before getting into the details it is worth mentioning the example end-to-end service recovery
+times provided in the ETSI NFV REL document [REL]_ (see table 2.). These values may change over time
+including lowering these thresholds.
+
+*Table 2: Service availability levels (SAL)*
+
++----+---------------+----------------------+------------------------------------+
+|SAL |Service        |Customer Type         | Recommendation                     |
+|    |Recovery       |                      |                                    |
+|    |Time           |                      |                                    |
+|    |Threshold      |                      |                                    |
++====+===============+======================+====================================+
+|1   |5 - 6 seconds  |Network Operator      |Redundant resources to be           |
+|    |               |Control Traffic       |made available on-site to           |
+|    |               |                      |ensure fastrecovery.                |
+|    |               |Government/Regulatory |                                    |
+|    |               |Emergency Services    |                                    |
++----+---------------+----------------------+------------------------------------+
+|2   |10 - 15 seconds|Enterprise and/or     |Redundant resources to be available |
+|    |               |large scale customers |as a mix of on-site and off-site    |
+|    |               |                      |as appropriate: On-site resources to|
+|    |               |Network Operators     |be utilized for recovery of         |
+|    |               |service traffic       |real-time service; Off-site         |
+|    |               |                      |resources to be utilized for        |
+|    |               |                      |recovery of data services           |
++----+---------------+----------------------+------------------------------------+
+|3   |20 - 25 seconds|General Consumer      |Redundant resources to be mostly    |
+|    |               |Public and ISP        |available off-site. Real-time       |
+|    |               |Traffic               |services should be recovered before |
+|    |               |                      |data services                       |
++----+---------------+----------------------+------------------------------------+
+
+Note that even though SAL 1 of [REL]_ allows for 5-6 seconds of service recovery,
+for many services this is too long and such outage causes a service level reset or
+the loss of significant amount of data. Also the end-to-end service or network service
+may be served by multiple VNFs. Therefore for a single VNF the desired
+service recovery time is sub-second.
+
+Note that failing over the service to another provider entity implies the redirection of the traffic
+flow the VNF is handling. This could be achieved in different ways ranging from floating IP addresses
+to load balancers. The topic deserves its own investigation, therefore in these first set of
+use cases we assume that it is part of the solution without going into the details, which
+we will address as a complementary set of use cases.
+
+.. [REL] ETSI GS NFV-REL 001 V1.1.1 (2015-01)
+
+
+2.1 Use Case 1: VNFC failure in a statefull VNF with redundancy
+==============================================================
+
+Use case 1 represents a statefull VNF with redundancy managed by an HA manager,
+which is part of the VNF (Fig 1). The VNF consists of VNFC1, VNFC2 and the HA Manager.
+The latter managing the two VNFCs, e.g. the role they play in providing the service
+named "Provided NF" (Fig 2).
+
+The failure happens in one of the VNFCs and it is detected and handled by the HA manager.
+On practice the HA manager could be part of the VNFC implementations or it could
+be a separate entity in the VNF. The point is that the communication of these
+entities inside the VNF is not visible to the rest of the system. The observable
+events need to cross the boundary represented by the VNF box.
+
+
+.. figure:: images/Slide4.png
+    :alt: VNFC failure in a statefull VNF
+    :figclass: align-center
+
+    Fig 1. VNFC failure in a statefull VNF with built-in HA manager
+
+
+.. figure:: images/StatefullVNF-VNFCfailure.png
+    :alt: MSC of the VNFC failure in a statefull VNF
+    :figclass: align-center
+
+    Fig 2. Sequence of events for use case 1
+
+
+As shown in Fig 2. initially VNFC2 is active, i.e. provides the Provided NF and VNFC1
+is a standby. It is not shown, but it is expected that VNFC1 has some means to get the update
+of the state of the Provided NF from the active VNFC2, so that it is prepared to continue to
+provide the service in case VNFC2 fails.
+The sequence of events starts with the failure of VNFC2, which also interrupts the
+Provided NF. This failure is detected somehow and/or reported to the HA Manager, which
+in turn may report the failure to the VNFM and simultaneously it tries to isolate the
+fault by cleaning up VNFC2.
+
+Once the cleanup succeeds (i.e. the OK is received) it fails over the active role to
+VNFC1 by setting it active. This recovers the service, the Provided NF is indeed
+provided again. Thus this point marks the end of the outage caused by the failure
+that need to be considered from the perspective of service availability.
+
+The repair of the failed VNFC2, which might have started at the same time
+when VNFC1 was assigned the active state, may take longer but without further impact
+on the availability of the Provided NF service.
+If the HA Manager reported the interruption of the Provided NF to the VNFM, it should
+clear the error condition.
+
+The key points in this scenario are:
+
+* The failure of the VNFC2 is not detectable by any other part of the system except
+  the consumer of the Provided NF. The VNFM only
+  knows about the failure because of the error report, and only the information this
+  report provides. I.e. it may or may not include the information on what failed.
+* The Provided NF is resumed as soon as VNFC1 is assigned active regardless how long
+  it takes to repair VNFC2.
+* The HA manager could be part of the VNFM as well. This requires an interface to
+  detect the failures and to manage the VNFC life-cycle and the role assignments.
+
+2.2 Use Case 2: VM failure in a statefull VNF with redundacy
+============================================================
+
+Use case 2 also represents a statefull VNF with its redundancy managed by an HA manager,
+which is part of the VNF. The VNFCs of the VNF are hosted on the VMs provided by
+the NFVI (Fig 3).
+
+The VNF consists of VNFC1, VNFC2 and the HA Manager (Fig 4). The latter managing
+the role the VNFCs play in providing the service - Provided NF.
+The VMs provided by the NFVI are managed by the VIM.
+
+
+In this use case it is one of the VMs hosting the VNF fails. The failure is detected
+and handled at both the NFVI and the VNF levels simultaneously. The coordination occurs
+between the VIM and the VNFM.
+
+
+.. figure:: images/Slide6.png
+    :alt: VM failure in a statefull VNF
+    :figclass: align-center
+
+    Fig 3. VM failure in a statefull VNF with built-in HA manager
+
+
+.. figure:: images/StatefullVNF-VMfailure.png
+    :alt: MSC of the VM failure in a statefull VNF
+    :figclass: align-center
+
+    Fig 4. Sequence of events for use case 2
+
+
+Again initially VNFC2 is active and provides the Provided NF, while VNFC1 is the standby.
+It is not shown in Fig 4., but it is expected that VNFC1 has some means to learn the state
+of the Provided NF from the active VNFC2, so that it is able to continue providing the
+service if VNFC2 fails. VNFC1 is hosted on VM1, while VNFC2 is hosted on VM2 as indicated by
+the arrows between these objects in Fig 4.
+
+The sequence of events starts with the failure of VM2, which results in VNFC2 failing and
+interrupting the Provided NF. The HA Manager detects the failure of VNFC2 somehow
+and tries to handle it the same way as in use case 1. However because the VM is gone the
+clean up either not initiated at all or interrupted as soon as the failure of the VM is
+identified. In either case the faulty VNFC2 is considered as isolated.
+
+To recover the service the HA Manager fails over the active role to VNFC1 by setting it active.
+This recovers the Provided NF. Thus this point marks again the end of the outage caused
+by the VM failure that need to be considered from the perspective of service availability.
+If the HA Manager reported the interruption of the Provided NF to the VNFM, it should
+clear the error condition.
+
+On the other hand the failure of the VM is also detected in the NFVI and reported to the VIM.
+The VIM reports the VM failure to the VNFM, which passes on this information
+to the HA Manager of the VNF. This confirms for the VNF HA Manager the VM failure and that
+it needs to wait with the repair of the failed VNFC2 until the VM is provided again. The
+VNFM also confirms towards the VIM that it is safe to restart the VM.
+
+The repair of the failed VM may take some time, but since the service has been failed over
+to VNFC1 in the VNF, there is no further impact on the availability of Provided NF.
+
+When eventually VM2 is restarted the VIM reports this to the VNFM and
+the VNFC2 can be restored.
+
+The key points in this scenario are:
+
+* The failure of the VM2 is detectable at both levels VNF and NFVI, therefore both the HA
+  manager and the VIM reacts to it. It is essential that these reactions do not interfere,
+  e.g. if the VIM tries to protect the VM state at NFVI level that would conflict with the
+  service failover action at the VNF level.
+* While the failure detection happens at both NFVI and VNF levels, the time frame within
+  which the VIM and the HA manager detect and react may be very different. For service
+  availability the VNF level detection, i.e. by the HA manager is the critical one and expected
+  to be faster.
+* The Provided NF is resumed as soon as VNFC1 is assigned active regardless how long
+  it takes to repair VM2 and VNFC2.
+* The HA manager could be part of the VNFM as well.
+  This requires an interface to detect failures in/of the VNFC and to manage its life-cycle and
+  role assignments.
+* The VNFM may not know for sure that the VM failed until the VIM reports it, i.e. whether
+  the VM failure is due to host, hypervisor, host OS failure. Thus the VIM should report/alarm
+  and log VM, hypervisor, and physical host failures. The use cases for these failures
+  are similar with respect to the Provided NF.
+* The VM repair also should start with the fault isolation as appropriate for the actual
+  failed entity, e.g. if the VM failed due to a host failure a host may be fenced first.
+* The negotiation between the VNFM and the VIM may be replaced by configured repair actions.
+  E.g. on error restart VM in initial state, restart VM from last snapshot, or fail VM over to standby.
+
+
+2.3 Use Case 3: VNFC failure in a statefull VNF with no redundancy
+=================================================================
+
+Use case 3 also represents a statefull VNF, but it stores its state externally on a
+virtual disk provided by the NFVI. It has a single VNFC and it is managed by the VNFM
+(Fig 5).
+
+In this use case the VNFC fails and the failure is detected and handled by the VNFM.
+
+
+.. figure:: images/Slide10.png
+    :alt: VNFC failure in a statefull VNF No-Red
+    :figclass: align-center
+
+    Fig 5. VNFC failure in a statefull VNF with no redundancy
+
+
+.. figure:: images/StatefullVNF-VNFCfailureNoRed.png
+    :alt: MSC of the VNFC failure in a statefull VNF No-Red
+    :figclass: align-center
+
+    Fig 6. Sequence of events for use case 3
+
+
+The VNFC periodically checkpoints the state of the Provided NF to the external storage,
+so that in case of failure the Provided NF can be resumed (Fig 6).
+
+When the VNFC fails the Provided NF is interrupted. The failure is detected by the VNFM
+somehow, which to isolate the fault first cleans up the VNFC, then if the cleanup is
+successful it restarts the VNFC. When the VNFC starts up, first it reads the last checkpoint
+for the Provided NF, then resumes providing it. The service outage lasts from the VNFC failure
+till this moment.
+
+The key points in this scenario are:
+
+* The service state is saved in an external storage which should be highly available too to
+  protect the service.
+* The NFVI should provide this guarantee and also that storage and access network failures
+  are handled seemlessly from the VNF's perspective.
+* The VNFM has means to detect VNFC failures and manage its life-cycle appropriately. This is
+  not required if the VNF also provides its availability management.
+* The Provided NF can be resumed only after the VNFC is restarted and it has restored the
+  service state from the last checkpoint created before the failure.
+* Having a spare VNFC can speed up the service recovery. This requires that the VNFM coordinates
+  the role each VNFC takes with respect to the Provided NF. I.e. the VNFCs do not act on the
+  stored state simultaneously potentially interfering and corrupting it.
+
+
+
+2.4 Use Case 4: VM failure in a statefull VNF with no redundancy
+===============================================================
+
+Use case 4 also represents a statefull VNF without redundancy, which stores its state externally on a
+virtual disk provided by the NFVI. It has a single VNFC managed by the VNFM
+(Fig 7) as in use case 3.
+
+In this use case the VM hosting the VNFC fails and the failure is detected and handled by
+the VNFM and the VIM simultaneously.
+
+
+.. figure:: images/Slide11.png
+    :alt: VM failure in a statefull VNF No-Red
+    :figclass: align-center
+
+    Fig 7. VM failure in a statefull VNF with no redundancy
+
+.. figure:: images/StatefullVNF-VMfailureNoRed.png
+    :alt: MSC of the VM failure in a statefull VNF No-Red
+    :figclass: align-center
+
+    Fig 8. Sequence of events for use case 4
+
+Again, the VNFC regularly checkpoints the state of the Provided NF to the external storage,
+so that it can be resumed in case of a failure (Fig 8).
+
+When the VM hosting the VNFC fails the Provided NF is interrupted.
+
+On the one hand side, the failure is detected by the VNFM somehow, which to isolate the fault tries
+to clean the VNFC up which cannot be done because of the VM failure. When the absence of the VM has been
+determined the VNFM has to wait with restarting the VNFC until the hosting VM is restored. The VNFM
+may report the problem to the VIM, requesting a repair.
+
+On the other hand the failure is detected in the NFVI and reported to the VIM, which reports it
+to the VNFM, if the VNFM hasn't reported it yet.
+If the VNFM has requested the VM repair or if it acknowledges the repair, the VIM restarts the VM.
+Once the VM is up the VIM reports it to the VNFM, which in turn can restart the VNFC.
+
+When the VNFC restarts first it reads the last checkpoint for the Provided NF,
+to be able to resume it.
+The service outage last until this is recovery completed.
+
+The key points in this scenario are:
+
+
+* The service state is saved in external storage which should be highly available to
+  protect the service.
+* The NFVI should provide such a guarantee and also that storage and access network failures
+  are handled seemlessly from the perspective of the VNF.
+* The Provided NF can be resumed only after the VM and the VNFC are restarted and the VNFC
+  has restored the service state from the last checkpoint created before the failure.
+* The VNFM has means to detect VNFC failures and manage its life-cycle appropriately. Alternatively
+  the VNF may also provide its availability management.
+* The VNFM may not know for sure that the VM failed until the VIM reports this. It also cannot
+  distinguish host, hypervisor and host OS failures. Thus the VIM should report/alarm and log
+  VM, hypervisor, and physical host failures. The use cases for these failures are
+  similar with respect to the Provided NF.
+* The VM repair also should start with the fault isolation as appropriate for the actual
+  failed entity, e.g. if the VM failed due to a host failure a host may be fenced first.
+* The negotiation between the VNFM and the VIM may be replaced by configured repair actions.
+* VM level redundancy, i.e. running a standby or spare VM in the NFVI would allow faster service
+  recovery for this use case, but by itself it may not protect against VNFC level failures. I.e.
+  VNFC level error detection is still required.
+
+
+
+2.5 Use Case 5: VNFC failure in a stateless VNF with redundancy
+===============================================================
+
+Use case 5 represents a stateless VNF with redundancy, i.e. it is composed of VNFC1 and VNFC2.
+They are managed by an HA manager within the VNF. The HA manager assigns the active role to provide
+the Provided NF to one of the VNFCs while the other remains a spare meaning that it has no state
+information for the Provided NF (Fig 9) therefore it could replace any other VNFC capable of
+providing the Provided NF service.
+
+In this use case the VNFC fails and the failure is detected and handled by the HA manager.
+
+
+.. figure:: images/Slide13.png
+    :alt: VNFC failure in a stateless VNF with redundancy
+    :figclass: align-center
+
+    Fig 9. VNFC failure in a stateless VNF with redundancy
+
+
+.. figure:: images/StatelessVNF-VNFCfailure.png
+    :alt: MSC of the VNFC failure in a stateless VNF with redundancy
+    :figclass: align-center
+
+    Fig 10. Sequence of events for use case 5
+
+
+Initially VNFC2 provides the Provided NF while VNFC1 is idle or might not even been instantiated
+yet (Fig 10).
+
+When VNFC2 fails the Provided NF is interrupted. This failure is detected by the HA manager,
+which as a first reaction cleans up VNFC2 (fault isolation), then it assigns the active role to
+VNFC1. It may report an error to the VNFM as well.
+
+Since there is no state information to recover, VNFC1 can accept the active role right away
+and resume providing the Provided NF service. Thus the service outage is over. If the HA manager
+reported an error to the VNFM it should clear it at this point.
+
+The key points in this scenario are:
+
+* The spare VNFC may be instantiated only once the failure of active VNFC is detected.
+* As a result the HA manager's role might be limited to life-cycle management, i.e. no role
+  assignment is needed if the VNFCs provide the service as soon as they are started up.
+* Accordingly the HA management could be part of a generic VNFM provided it is capable of detecting
+  the VNFC failures. Besides the service users, the VNFC failure may not be detectable at any other
+  part of the system.
+* Also there could be multiple active VNFCs sharing the load of Provided NF and the spare/standby
+  may protect all of them.
+* Reporting the service failure to the VNFM is optional as the HA manager is in charge of recovering
+  the service and it is aware of the redundancy needed to do so.
+
+
+2.6 Use Case 6: VM failure in a stateless VNF with redundancy
+============================================================
+
+
+Similarly to use case 5, use case 6 represents a stateless VNF composed of VNFC1 and VNFC2,
+which are managed by an HA manager within the VNF. The HA manager assigns the active role to
+provide the Provided NF to one of the VNFCs while the other remains a spare meaning that it has
+no state information for the Provided NF (Fig 11) and it could replace any other VNFC capable
+of providing the Provided NF service.
+
+As opposed to use case 5 in this use case the VM hosting one of the VNFCs fails. This failure is
+detected and handled by the HA manager as well as the VIM.
+
+
+.. figure:: images/Slide14.png
+    :alt: VM failure in a stateless VNF with redundancy
+    :figclass: align-center
+
+    Fig 11. VM failure in a stateless VNF with redundancy
+
+
+.. figure:: images/StatelessVNF-VMfailure.png
+    :alt: MSC of the VM failure in a stateless VNF with redundancy
+    :figclass: align-center
+
+    Fig 12. Sequence of events for use case 6
+
+
+Initially VNFC2 provides the Provided NF while VNFC1 is idle or might not have been instantiated
+yet (Fig 12) as in use case 5.
+
+When VM2 fails VNFC2 fails with it and the Provided NF is interrupted. The failure is detected by
+the HA manager and by the VIM simultaneously and independently.
+
+The HA manager's first reaction is trying to clean up VNFC2 to isolate the fault. This is considered to
+be successful as soon as the disappearance of the VM is confirmed.
+After this the HA manager assigns the active role to VNFC1. It may report the error to the VNFM as well
+requesting a VM repair.
+
+Since there is no state information to recover, VNFC1 can accept the assignment right away
+and resume the Provided NF service. Thus the service outage is over. If the HA manager reported
+an error to the VNFM for the service it should clear it at this point.
+
+Simultaneously the VM failure is detected in the NFVI and reported to the VIM, which reports it
+to the VNFM, if the VNFM hasn't requested a repair yet. If the VNFM requested the VM repair or if
+it acknowledges the repair, the VIM restarts the VM.
+
+Once the VM is up the VIM reports it to the VNFM, which in turn may restart the VNFC if needed.
+
+
+The key points in this scenario are:
+
+* The spare VNFC may be instantiated only after the detection of the failure of the active VNFC.
+* As a result the HA manager's role might be limited to life-cycle management, i.e. no role
+  assignment is needed if the VNFC provides the service as soon as it is started up.
+* Accordingly the HA management could be part of a generic VNFM provided if it is capable of detecting
+  failures in/of the VNFC and managing its life-cycle.
+* Also there could be multiple active VNFCs sharing the load of Provided NF and the spare/standby
+  may protect all of them.
+* The VNFM may not know for sure that the VM failed until the VIM reports this. It also cannot
+  distinguish host, hypervisor and host OS failures. Thus the VIM should report/alarm and log
+  VM, hypervisor, and physical host failures. The use cases for these failures are
+  similar with respect to each Provided NF.
+* The VM repair also should start with the fault isolation as appropriate for the actual
+  failed entity, e.g. if the VM failed due to a host failure a host needs to be fenced first.
+* The negotiation between the VNFM and the VIM may be replaced by configured repair actions.
+* Reporting the service failure to the VNFM is optional as the HA manager is in charge recovering
+  the service and it is aware of the redundancy needed to do so.
+
+
+
+2.7 Use Case 7: VNFC failure in a stateless VNF with no redundancy
+==================================================================
+
+Use case 7 represents a stateless VNF composed of a single VNFC, i.e. with no redundancy.
+The VNF and in particular its VNFC is managed by the VNFM through managing its life-cycle (Fig 13).
+
+In this use case the VNFC fails. This failure is detected and handled by the VNFM. This use case
+requires that the VNFM can detect the failures in the VNF or they are reported to the VNFM.
+
+The failure is only detectable at the VNFM level and it is handled by the VNFM restarting the VNFC.
+
+
+.. figure:: images/Slide16.png
+    :alt: VNFC failure in a stateless VNF with no redundancy
+    :figclass: align-center
+
+    Fig 13. VNFC failure in a stateless VNF with no redundancy
+
+
+.. figure:: images/StatelessVNF-VNFCfailureNoRed.png
+    :alt: MSC of the VNFC failure in a stateless VNF with no redundancy
+    :figclass: align-center
+
+    Fig 14. Sequence of events for use case 7
+
+The VNFC is providing the Provided NF when it fails (Fig 14). This failure is detected or reported to
+the VNFM, which has to clean up the VNFC to isolate the fault. After cleanup success it can proceed
+with restarting the VNFC, which as soon as it is up it starts to provide the Provided NF
+as there is no state to recover.
+
+Thus the service outage is over, but it has included the entire time needed to restart the VNFC.
+Considering that the VNF is stateless this may not be significant still.
+
+
+The key points in this scenario are:
+
+* The VNFM has to have the means to detect VNFC failures and manage its life-cycle appropriately.
+  This is not required if the VNF comes with its availability management, but this is very unlikely
+  for such stateless VNFs.
+* The Provided NF can be resumed as soon as the VNFC is restarted, i.e. the restart time determines
+  the outage.
+* In case multiple VNFCs are used they should not interfere with one another, they should
+  operate independently.
+
+
+2.8 Use Case 8: VM failure in a stateless VNF with no redundancy
+================================================================
+
+Use case 8 represents the same stateless VNF composed of a single VNFC as use case 7, i.e. with
+no redundancy. The VNF and in particular its VNFC is managed by the VNFM through managing its
+life-cycle (Fig 15).
+
+In this use case the VM hosting the VNFC fails. This failure is detected and handled by the VNFM
+as well as by the VIM.
+
+
+.. figure:: images/Slide17.png
+    :alt: VM failure in a stateless VNF with no redundancy
+    :figclass: align-center
+
+    Fig 15. VM failure in a stateless VNF with no redundancy
+
+
+.. figure:: images/StatelessVNF-VMfailureNoRed.png
+    :alt: MSC of the VM failure in a stateless VNF with no redundancy
+    :figclass: align-center
+
+    Fig 16. Sequence of events for use case 8
+
+The VNFC is providing the Provided NF when the VM hosting the VNFC fails (Fig 16).
+
+This failure may be detected or reported to the VNFM as a failure of the VNFC. The VNFM may
+not be aware at this point that it is a VM failure. Accordingly its first reaction as in use case 7
+is to clean up the VNFC to isolate the fault. Since the VM is gone, this cannot succeed and the VNFM
+becomes aware of the VM failure through this or it is reported by the VIM. In either case it has to wait
+with the repair of the VMFC until the VM becomes available again.
+
+Meanwhile the VIM also detects the VM failure and reports it to the VNFM unless the VNFM has already
+requested the VM repair. After the VNFM confirming the VM repair the VIM restarts the VM and reports
+the successful repair to the VNFM, which in turn can start the VNFC hosted on it.
+
+
+Thus the recovery of the Provided NF includes the restart time of the VM and of the VNFC.
+
+The key points in this scenario are:
+
+* The VNFM has to have the means to detect VNFC failures and manage its life-cycle appropriately.
+  This is not required if the VNF comes with its availability management, but this is very unlikely
+  for such stateless VNFs.
+* The Provided NF can be resumed only after the VNFC is restarted on the repaired VM, i.e. the
+  restart time of the VM and the VNFC determines the outage.
+* In case multiple VNFCs are used they should not interfere with one another, they should
+  operate independently.
+* The VNFM may not know for sure that the VM failed until the VIM reports this. It also cannot
+  distinguish host, hypervisor and host OS failures. Thus the VIM should report/alarm and log
+  VM, hypervisor, and physical host failures. The use cases for these failures are
+  similar with respect to each Provided NF.
+* The VM repair also should start with the fault isolation as appropriate for the actual
+  failed entity, e.g. if the VM failed due to a host failure the host needs to be fenced first.
+* The repair negotiation between the VNFM and the VIM may be replaced by configured repair actions.
+* VM level redundancy, i.e. running a standby or spare VM in the NFVI would allow faster service
+  recovery for this use case, but by itself it may not protect against VNFC level failures. I.e.
+  VNFC level error detection is still required.
+
+2.9 Use Case 9: Repeated VNFC failure in a stateless VNF with no redundancy
+===========================================================================
+
+Finally use case 9 represents again a stateless VNF composed of a single VNFC as in use case 7, i.e.
+with no redundancy. The VNF and in particular its VNFC is managed by the VNFM through managing its
+life-cycle.
+
+In this use case the VNFC fails repeatedly. This failure is detected and handled by the VNFM,
+but results in no resolution of the fault (Fig 17) because the VNFC is manifesting a fault,
+which is not in its scope. I.e. the fault is propagating to the VNFC from a faulty VM or host,
+for example. Thus the VNFM cannot resolve the problem by itself.
+
+
+.. figure:: images/Slide19.png
+    :alt: Repeated VNFC failure in a stateless VNF with no redundancy
+    :figclass: align-center
+
+    Fig 17. VM failure in a stateless VNF with no redundancy
+
+
+To handle this case the failure handling needs to be escalated to the a bigger fault zone
+(or fault domain), i.e. a scope within which the faults may propagate and manifest. In case of the
+VNF the bigger fault zone is the VM and the facilities hosting it, all managed by the VIM.
+
+Thus the VNFM should request the repair from the VIM (Fig 18).
+
+Since the VNFM is only aware of the VM, it needs to report an error on the VM and it is the
+VIM's responsibility to sort out what might be the scope of the actual fault depending on other
+failures and error reports in its scope.
+
+
+.. figure:: images/Slide20.png
+    :alt: Escalation of repeated VNFC failure in a stateless VNF with no redundancy
+    :figclass: align-center
+
+    Fig 18. VM failure in a stateless VNF with no redundancy
+
+
+.. figure:: images/StatelessVNF-VNFCfailureNoRed-Escalation.png
+    :alt: MSC of the VM failure in a stateless VNF with no redundancy
+    :figclass: align-center
+
+    Fig 19. Sequence of events for use case 9
+
+
+This use case starts similarly to use case 7, i.e. the VNFC is providing the Provided NF when it fails
+(Fig 17).
+This failure is detected or reported to the VNFM, which cleans up the VNFC to isolate the fault.
+After successful cleanup the VNFM proceeds with restarting the VNFC, which as soon as it is up
+starts to provide the Provided NF again as in use case 7.
+
+However the VNFC failure occurs N times repeatedly within some Probation time for which the VNFM starts
+the timer when it detects the first failure of the VNFC. When the VNFC fails once more still within the
+probation time the Escalation counter maximum is exceeded and the VNFM reports an error to the VIM on
+the VM hosting the VNFC as obviously cleaning up and restarting the VNFC did not solve the problem.
+
+When the VIM receives the error report for the VM it has to isolate the fault by cleaning up at least
+the VM. After successful cleanup it can restart the VM and once it is up report the VM repair to the VNFM.
+At this point the VNFM can restart the VNFC, which in turn resumes the Provided VM.
+
+In this scenario the VIM needs to evaluate what may be the scope of the fault to determine what entity
+needs a repair. For example, if it has detected VM failures on that same host, or other VNFMs
+reported errors on VMs hosted on the same host, it should consider that the entire host needs a repair.
+
+
+The key points in this scenario are:
+
+* The VNFM has to have the means to detect VNFC failures and manage its life-cycle appropriately.
+  This is not required if the VNF comes with its availability management, but this is very unlikely
+  for such stateless VNFs.
+* The VNFM needs to correlate VNFC failures over time to be able to detect failure of a bigger fault zone.
+  One way to do so is through counting the failures within a probation time.
+* The VIM cannot detect all failures caused by faults in the entities under its control. It should be
+  able to receive error reports and correlate these error reports based on the dependencies
+  of the different entities.
+* The VNFM does not know the source of the failure, i.e. the faulty entity.
+* The VM repair should start with the fault isolation as appropriate for the actual
+  failed entity, e.g. if the VM failed due to a host failure the host needs to be fenced first.
+
+********************
+3 Concluding remarks
+********************
+
+This use case document outlined the model and some failure modes for NFV systems. These are an
+initial list.  The OPNFV HA project team is continuing to grow the list of use cases and will
+issue additional documents going forward.  The basic use cases and service availability considerations
+help define the key considerations for each use case taking into account the impact on the end service.
+The use case document along with the requirements documents and gap analysis help set context for
+engagement with various upstream projects.