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+3. Communication Interfaces for VNF HA schemes
+This section will discuss some general issues about communication interfaces
+in the VNF HA schemes. In sections 2, the usecases of both stateful and
+stateless VNFs are discussed. While in this section, we would like to discuss
+some specific issues which are quite general for all the usecases proposed
+in the previous sections.
+3.1. VNF External Interfaces
+Regardless whether the VNF is stateful or stateless, all the VNFCs should act as
+a union from the perspective of the outside world. That means all the VNFCs should
+share a common interface where the outside modules (e.g., the other VNFs) can
+access the service from. There could be multiple solutions for this share of IP
+interface. However, all of this sharing and switching of IP address should be
+ignorant to the outside modules.
+There are several approaches for the VNFs to share the interfaces. A few of them
+are listed as follows and will be discussed in detail.
+1) IP address of VMs for active/stand-by VM.
+2) Load balancers for active/active use cases
+Note that combinition of these two approaches is also feasible.
+For active/standby VNFCs, there is a common IP address shared by the VMs hosting
+the active and standby VNFCs, so that they look as one instance from outside.
+The HA manager will manage the assignment of the IP address to the VMs.
+(The HA manager may not be aware of this, I.e. the address may be configured
+and the active/standby state management is linked to the possession of the IP
+address, i.e. the active VNFC claims it as part of becoming active.) Only the
+active one possesses the IP address. And when failover happens, the standby
+is set to be active and can take possession of the IP address to continue traffic
+For active/active VNFCs, a LB(Load Balancer) could be used. In such scenario, there
+could be two cases for the deployment and usage of LB.
+Case 1: LB used in front of a cluster of VNFCs to distribute the traffic flow.
+In such case, the LB is deployed in front of a cluster of multiple VNFCs. Such
+cluster can be managed by a seperate cluster manager, or can be managed just
+by the LB, which uses heartbeat to monitor each VNFC. When one of VNFCs fails,
+the cluster manager should first exclude the failed VNFC from the cluster so that
+the LB will re-route the traffic to the other VNFCs, and then the failed one should
+be recovered. In the case when the LB is acting as the cluster manager, it is
+the LB's responsibility to inform the VNFM to recover the failed VNFC if possible.
+Case 2: LB used in front of a cluster of VMs to distribute traffic flow.
+In this case, there exists a cluster manager(e.g. Pacemaker) to monitor and manage
+the VMs in the cluster. The LB sits in front of the VM cluster so as to distribute
+the traffic. When one of the VM fails, the cluster manager will detect that and will
+be in charge of the recovery. The cluster manager will also exclude the failed VM
+out of the cluster, so that the LB won't route traffic to the failed one.
+In both two cases, the HA of the LB should also be considered.
+3.2. Intra-VNF Communication
+For stateful VNFs, data synchronization is necessary between the active and standby VMs.
+The HA manager is responsible for handling VNFC failover, and do the assignment of the
+active/standby states between the VNFCs of the VNF. Data synchronization can be handled
+either by the HA manager or by the VNFC itself.
+The state synchronization can happen as
+- direct communication between the active and the standby VNFCs
+- based on the information received from the HA manager on channel or messages using a common queue,
+- it could be through a shared storage assigned to the whole VNF
+- through the checkpointing of state information via underlying memory and/or
+database checkpointing services to a separate VM and storage repository.