rconnects of a SP edge SBC are shown in Figure 5-1. Edge SBC Customer A Terminate customer IP addresses (VRF) Validate SIP Traffic (including security functions such as REGISTRATION throttling) Normalizes messages with any changes Call Agent required Provide resources for transcoding if Validated SIP Traffic required on SP Routeable Route traffic toward SP Call Agent address-Targeted Customer B toward SP Call Agent Raw SIP Traffic from Customers. This traffic often comes in on multiple different VRFs with different signaling parameters. Figure 5-1 Features and Interconnects of a SP Edge SBC SP Network-Call Agent The central call agent in a SP network is the component that maintains the list of IP address or Fully Qualified Domain Names (FQDN) and the list of Direct Inward Dial (DID) numbers that those FQDN or IP Addresses correspond to. All call agents must have some type of table, as shown in Table 5-1, to determine the destination of SIP messages that it received. The mapping database is by no means the complete set of functionality that the call agent needs to accomplish the task of routing SIP messages. It also needs an extensive set of routing functionality and to maintain the state of the calls that are in progress at any 60 SIP Trunking given time. The most complex requirement of the call agent is this maintenance of indi- vidual call states. One of the reasons that this is difficult is because maintaining state in a highly scalable system is inherently complex. Because many different calls occur simulta- neously, the call agent requires a software design that locks the most scarce resource to ensure that contention does not occur. This scarce resource can be any number of things such as the bandwidth to particular locations, or it might be the number of PSTN offnet media gateway ports available. Regardless of the item that is the constraint with the par- ticular SP, a system designed to handle and process new calls while maintaining an accu- rate count of any constrained resources is an essential requirem