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===================================================
1.0 Overall Principle for High Availability in NFV
===================================================
The ultimate goal for the High Availability schema is to provide high
availability to the upper layer services.
High availability is provided by the following steps once a failure happens:
Step 1: failover of services once failure happens and service is out of work
Step 2: Recovery of failed parts in each layer.
******************************************
1.1 Framework for High Availability in NFV
******************************************
Framework for Carrier Grade High availability:
A layered approach to availability is required for the following reasons:
* fault isolation
* fault tolerance
* fault recovery
Among the OPNFV projects the OPNFV-HA project's focus is on requirements related
to service high availability. This is complemented by other projects such as the
OPNFV - Doctor project, whose focus is reporting and management of faults along
with maintenance, the OPNFV-Escalator project that considers the upgrade of the
NFVI and VIM, or the OPNFV-Multisite that adds geographical redundancy to the
picture.
A layered approach allows the definition of failure domains (e.g., the
networking hardware, the distributed storage system, etc.). If possible, a fault
shall be handled at the layer (failure domain) where it occurs. If a failure
cannot be handled at its corresponding layer, the next higher layer needs to be
able to handle it. In no case, shall a failure cause cascading failures at other
layers.
The layers are:
+---------------------------+-------------------------------------+
+ Service + End customer visible service |
+===========================+=====================================+
+ Application + VNF's, VNFC's |
+---------------------------+-------------------------------------+
+ NFVI/VIM + Infrastructure, VIM, VNFM, VM |
+---------------------------+-------------------------------------+
+ Hardware + Servers, COTS platforms |
+---------------------------+-------------------------------------+
The following document describes the various layers and how they need to
address high availability.
**************
1.2 Definitons
**************
Reference from the ETSI NFV doc.
**Availability:** Availability of an item to be in a state to perform a required
function at a given instant of time or at any instant of time within a given
time interval, assuming that the external resources, if required, are provided.
**Accessibility:** It is the ability of a service to access (physical) resources
necessary to provide that service. If the target service satisfies the minimum
level of accessibility, it is possible to provide this service to end users.
**Admission control:** It is the administrative decision (e.g. by operator's
policy) to actually provide a service. In order to provide a more stable and
reliable service, admission control may require better performance and/or
additional resources than the minimum requirement. Failure: deviation of the
delivered service from fulfilling the system function.
**Fault:** adjudged or hypothesized cause of an error
**Service availability:** service availability of <Service X> is the long-term
average of the ratio of aggregate time between interruptions to scheduled
service time of <ServiceX> (expressed as a percentage) on a user-to-user basis.
The time between interruptions is categorized as Available (Up time) using the
availability criteria as defined by the parameter thresholds that are relevant
for <Service X>.
Accoring to the ETSI GS NFV-REL 001 V1.1.1 (2015-01) document service
availability in the context of NFV is defined as End-to-End Service availability
.. (MT) The relevant parts in NFV-REL defines SA as:
Service Availability refers to the End-to-End Service Availability which
includes all the elements in the end-to-end service (VNFs and infrastructure
components) with the exception of the customer terminal. This is a customer
facing (end user) availability definition and it is the result of accessibility
and #admission control (see their respective definitions above).
Service Availability=total service available time/
(total service available time + total restoration time)
**Service continuity:** Continuous delivery of service in conformance with
service's functional and behavioural specification and SLA requirements,
both in the control and data planes, for any initiated transaction or session
until its full completion even in the events of intervening exceptions or
anomalies, whether scheduled or unscheduled, malicious, intentional
or unintentional.
The relevant parts in NFV-REL:
The basic property of service continuity is that the same service is provided
during VNF scaling in/out operations, or when the VNF offering that service
needs to be relocated to another site due to an anomaly event
(e.g. CPU overload, hardware failure or security threat).
**Service failover:** when the instance providing a service/VNF becomes
unavailable due to fault or failure, another instance will (automatically) take
over the service, and this whole process is transparent to the user. It is
possible that an entire VNF instance becomes unavailble while providing its
service.
.. (MT) I think the service or an instance of it is a logical entity on its own and the service availability and continuity is with respect to this logical entity. For examlpe if a HTTP server serves a given URL, the HTTP server is the provider while that URL is the service it is providing. As long as I have an HTTP server running and serving this URL I have the service available. But no matter how many HTTP servers I'm running if they are not assigned to serve the URL, then it is not available. Unfortunately in the ETSI NFV documents there's not a clear distinction between the service and the provider logical entities. The distinction is more on the level of the different incarnations of the provider entity, i.e. VNF and its instances or VNFC and its instances. I don't know if I'm clear enough and to what extent we should go into this, but I tried to modify the definition along these lines. Now regarding the user perception and whether it's automatic I agreed that we want it automatic and seemless for the user, but I don't think that this is part of the failover definition. If it's done manually or if the user detects it it's still a failover. It's just not seemless. Requiring it being automatic and seemless should be in the requirement section as appropriate.
.. (fq) Agree.
**Service failover time:** Service failover is when the instance providing a
service becomes unavailable due to a fault or a failure and another healthy
instance takes over in providing the service. In the HA context this should be
an automatic action and this whole process should be transparent to the user.
It is possible that an entire VNF instance becomes unavailble while providing
its service.
.. (MT) Aligned with the above I would say that the serice failover time is the time from the moment of detecting the failure of the instance providing the service until the service is provided again by a new instance.
.. (fq) So in such definition, the time duration for the failure of the service=failure detection time+service failover time. Am I correct?
.. (bb) I feel, it is; "time duration for failover of the service = failure detection time + service failover time".
.. (MT) I would say that the "failure detection time" + "service failover time" = "service outage time" or actually we defined it below as the "service recovery time" . To reduce the outage we probably can't do much with the "service failover time", it is whatever is needed to perform the failover procedure, so it's tied to the implementation. It's somewhat "given". We may have more control over the detection time as that depends on the frequency of the health-check/heartbeat as this is often configurable.
.. (fq) Got it. Agree.
**Failure detection:** If a failure is detected, the failure must be identified
to the component responsible for correction.
.. (MT) I would rather say "failure detection" as the fault is not detectable until it becomes a failure, even then we may not know where the actual fault is. We only know what failed due to the fault. E.g. we can detect the memory leak, something may crash due to it, but it's much more difficult to figure out where the fault is, i.e. the bug in the software.
.. (MT) Also I think failures may be detected by different entities in the system, e.g. it could be a monitoring entity, a watchdog, the hypervisor, the VNF itself or a VNF tryng to use the services of a failed VNF. For me all these are failure detections regardless whether they are reported to the VNF. I think from an HA perspective what's important is the error report API(s) that entities should use if they detect a failure they are not in charge of correcting.
.. (fq) Agree. I modify the definition.
**Failure detection time:** Failure detection time is the time interval from the
moment the failure occurs till it is reported as a detected failure.
**Alarm:** Alarms are notifications (not queried) that are activated in response
to an event, a set of conditions, or the state of an inventory object. They
also require attention from an entity external to the reporting entity (if not
then the entity should cope with it and not raise the alarm).
.. (MT) According to NFV-INF 004: Alarms are notifications (not queried) that are activated in response to an event, a set of conditions, or the state of an inventory object. I would add also that they also require attention from an entity external to the reporting entity (if not then the entity should cope with it and not raise the alarm).
**Alarm threshold condition detection:** Alarm threshold condition is detected
by the component responsible for it. The component periodically evaluates the
condition associated with the alarm and if the threshold is reached, it
generates an alarm on the approprite channel, which in turn delivers it to the
entity(ies) responsible, such as the VIM.
.. (fq) I don't think the VNF need to know all the alarm. so I use VIM as the terminal point for the alarm detection
.. (MT) The same way as for the faults/failures, I don't think it's the receiving end that is important but the generatitng end and that it has the right and appropriate channel to communicate the alarm. But I have the impression that you are focusing on a particular type of alarm (i.e. threshold alarm) and not alarms in general.
.. (fq) Yes, I actully have the threshold alarm in my mind when I wrote this. So I think VIM might be the right receiving end for these alarm. I agree with your ideas about the right channel. I am just not sure whether we should put this part in a high lever perspective or we should define some details. After all OPNFV is an opensource project and we don't want it to be like standarization projects in ETSI. But I agree for the definition part we should have a high level and abstract definition for these, and then we can specify the detail channels in the API definition.
.. (MT) I tried to modify accordingly. Pls check. I think when it comes to the receiver we don't need to be specific from the detection perspective as usually there is a well-known notification channel that the management entity if it exists would listen to. The alarm detection does not require this entity, it just states that something is wrong and someone should deal with it hence the alarm.
**Alarm threshold detection time:** the threshold time interval between the
metrics exceeding the threshold and the alarm been detected.
.. (MT) I assume you are focusing on these threshold alarms, and not alarms in general.
.. (MT) Here similar to the failover time, we may have some control over the detection time (i.e. shorten the evaluation period), but may not on the delivery time.
.. (MT2) I changed "condition" to "threshold" to make it clearer as failure is a "condition" too :-)
**Service recovery:** The restoration of the service state after the instance of
a service/VNF is unavailable due to fault or failure or manual interuption.
.. (MT) I think the service recovery is the restoration of the state in which the required function is provided
**Service recovery time:** Service recovery time is the time interval from the
occurrence of an abnormal event (e.g. failure, manual interruption of service,
etc.) until recovery of the service.
.. (MT) in NFV-REL: Service recovery time is the time interval from the occurrence of an abnormal event (e.g. failure, manual interruption of service, etc.) until recovery of the service.
**SAL:** Service Availability Level
************************
1.3 Overall requirements
************************
Service availability shall be considered with respect to the delivery of end to
end services.
* There should be no single point of failure in the NFV framework
* All resiliency mechanisms shall be designed for a multi-vendor environment,
where for example the NFVI, NFV-MANO, and VNFs may be supplied by different
vendors.
* Resiliency related information shall always be explicitly specified and
communicated using the reference interfaces (including policies/templates) of
the NFV framework.
*********************
1.4 Time requirements
*********************
The time requirements below are examples in order to break out of the failure
detection times considering the service recovery times presented as examples for
the different service availability levels in the ETSI GS NFV-REL 001 V1.1.1
(2015-01) document.
The table below maps failure modes to example failure detection times.
+------------------------------------------------------------+---------------+
|Failure Mode | Time |
+============================================================+===============+
|Failure detection of HW | <1s |
+------------------------------------------------------------+---------------+
|Failure detection of virtual resource | <1s |
+------------------------------------------------------------+---------------+
|Alarm threshold detection | <1min |
+------------------------------------------------------------+---------------+
|Failure detection over of SAL 1 | <1s |
+------------------------------------------------------------+---------------+
|Recovery of SAL 1 | 5-6s |
+------------------------------------------------------------+---------------+
|Failure detectionover of SAL 2 | <5s |
+------------------------------------------------------------+---------------+
|Recovery of SAL 2 | 10-15s |
+------------------------------------------------------------+---------------+
|Failure detectionover of SAL 3 | <10s |
+------------------------------------------------------------+---------------+
|Recovery of SAL 3 | 20-25s |
+------------------------------------------------------------+---------------+
|
