Inter-Node Communication

VCS uses the cluster interconnect for network communications between cluster nodes. The nodes communicate using the capabilities provided by LLT and GAB.

The LLT module is designed to function as a high performance, low latency replacement for the IP stack and is used for all cluster communications. LLT provides the communications backbone for GAB. LLT distributes, or load balances inter-node communication across up to eight interconnect links. When a link fails, traffic is redirected to remaining links.

The Group Membership Services /Atomic Broadcast module is responsible for reliable cluster communications. GAB provides guaranteed delivery of point-to-point and broadcast messages to all nodes. The Atomic Broadcast functionality is used by HAD to ensure that all systems within the cluster receive all configuration change messages, or are rolled back to the previous state, much like a database atomic commit. If a failure occurs while transmitting a broadcast message, GAB's atomicity ensures that, upon recovery, all systems have the same information. The VCS engine uses a private IOCTL (provided by GAB) to tell GAB that it is alive.

The following diagram illustrates the overall communications paths.

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