In next generation VoIP and IP multimedia networks, individual control plane components, (e.g. Session Border Controllers (SBC), Media Gateway Controllers (MGC) and Call Servers (CS)) usually have some inbuilt overload self-protection mechanisms intended to prevent their collapse of processing under overload by discarding messages that request a new session set-up. Most mechanisms have little or no discrimination on the type of calls that are being affected, with emergency calls often receiving the only discrimination in the load shedding process. Whilst protecting itself, this does nothing to maximise the successful establishment of new sessions.
In networks implementing VoIP technology the call control and the media transport are separated into independent media and session control planes. This enables the design of a flat control architecture that allows any node to have a signalling relationship with any other node. With the addition of a centralised routing function this allows for highly scalable and distributed networks but it also leads to scaling issues in large networks when it comes to signalling the appropriate load information of all destination nodes to all the nodes generating the new session set-ups.
For such flat control plane architectures, some schemes have been proposed where an overloaded node responds to requests for new session set-up by signalling that all sending nodes to discard a proportion of its new requests. This scheme has a number of problems.
It either has to hold a configuration of all the sending nodes to which to send the overload control information or respond just to the nodes making the load requests. This means that any new sending nodes will not initially have the restriction information and may start sending at full rate. Also as traffic demand varies, with a very fast rise time, it becomes impractical to match and feedback the proportional discard rate that will produce the optimal traffic demand offered to the overloaded node. If for example a node can receive no more than 100 new session set-ups per second and it anticipates a traffic demand of 200 new session set-ups per second, requesting a 50% discard of traffic will not help if the real traffic rate rises to 1000 new session set-ups per second before the overloaded node can re-evaluate and signal the change in discard rate. Therefore, for nodes to protect themselves within such schemes, they tend to use overly severe restriction predictions which will reduce customer experience through unsuccessful sessions and reduce network revenue below that which could have been achieved with the available resources.
Another technique that is used for overload control is for the node under stress to signal to its neighbour nodes the absolute rate of session set-ups it requires instead a proportional discard as described above. For example, send me no more than 100 new session set-ups per second rather than discard 30% of your new call requests. The advantage of this scheme is that the demand sent can be matched to the available capacity of the overloaded node and be can safely protected regardless of how rapidly the traffic rate may rise. If the sending nodes are well behaved, the demand sent will not exceed the demand requested. The problem with current implementations of this scheme is that the node in question needs to know how to apportion its optimal total target rate for offered traffic amongst all the nodes that can send it traffic. This means that not only does it has to have preconfigured information of the other nodes so it can send each the restriction information of that specific node, but it must also know something of the relative size of the nodes so that some apportionment of fairness can be built in. If 10 source nodes can send traffic to a destination node that decides it needs to limit demand to 100 new session set-ups per second, dividing the traffic limit to 10 new session set-ups per second for each connecting node may suffer from the following problems:                One or more of the nodes may only be able to generate less than 10 new session set-ups per second in total so the total demand at the destination can only be sub-optimal, i.e. the destination node will be underutilised.        One or more of the nodes may be able to send much more than 10 new session set-ups per second so the disproportional restriction will be ‘unfair’ on the user on the node in terms of customer experience.        In mass media events, such as television gaming applications, where ‘winners’ are selected from the calls arriving at the destination; callers from the disproportionately restricted nodes are disadvantaged.        
US patent application 20100220587 discloses a method for implementing a two throttle overload control architecture in a node, one throttle on the input and one on the output side of the node. The in-throttle filters the input traffic according to the perceived limits of the target nodes' processing capacities. This allows requests that would be dropped by the target nodes due to overload to be filtered prior to being processed by the node. In addition, filter weights for the in-throttle is set such that the possibility of admitting to-be-dropped requests is minimized. The purpose of the method is thus to protect the node itself from having to process traffic that will anyway later be dropped. The disadvantage of this method is that each target node has to report its capacity to every possible source node and a source node cannot filter requests to targets it is not in direct contact with.
Patent application EP2445164 describes a method for load balancing between call session control functions (CSCF) in order to overcome the problem of a registration storm when a large number of user terminals try to reconnect for a service. Each CSCF reports its load, or rather the number of users currently registered to it, to a centralised DNS server which then uses the information for load balancing SIP registration requests between available CSCFs. This system is, however, only relevant for the process of SIP registering, that is user equipment must register with a CSCF upon connecting to a network. However, once the user equipment is registered to one of the CSCFs it remains associated with this node. Thus, this method does not provide a solution to the problem of avoiding call setups to overloaded session control edge nodes on a call by call basis.
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