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diff --git a/UseCases/UseCases.rst b/UseCases/UseCases.rst new file mode 100644 index 0000000..57dfbc0 --- /dev/null +++ b/UseCases/UseCases.rst @@ -0,0 +1,731 @@ +============ +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. |