In recent years, the resiliency of mobility gateways became a very important requirement for practically every operator. The main reason is that broadband gateways in mobile networks are usually responsible for the connectivity of a very large number of terminals—from hundreds of thousands to millions. Failure of such a gateway causes all those subscribers to disconnect and in the best case to try to reconnect quickly. In the worst case, they will be waiting for an application to timeout before that.
Apparently, such behavior of the network is highly undesirable. There are multiple ways to handle resiliency. An immediate one is to protect some vital hardware resources. It starts with fans and power supplies, continues to external ports protection, and some complex implementation provides the entire computing platform protection either internally with e.g. blades protecting other blades in the chassis, or externally with one network element protecting one or more other network elements.
In many cases it is expensive and complex from both software and hardware perspective to include a built-in redundancy for all components. Hence, usually only a redundancy for power supply and fans is provided which are components with a lowest mean time between failure. However, this does not provide a solution against the most common network element failure which is caused by software.
With respect thereto, there are multiple implementations known in different fields for stateful redundancy, all of them are very complex. Some are placed in the field of routing space with non-stop routing and non-stop forwarding, thus achieving a full state recovery after a failure. Others are strictly application-dependent, like a stateful database replication.
One well-known example of a network-based redundancy is the virtual router redundancy protocol (VRRP) to be implemented in routers. Unfortunately, VRRP was not designed to protect against a failure in the reachability of internal network element addresses, but only for addresses external to redundant devices. A description of the VRRP can be found e.g. in the RFC3768 of the Internet Engineering Task Force.