Gx Usage Monitoring (UM) relies on the Credit Control Instance (CCI) for the collection of byte counters. CCI is instantiated through a credit-category map (monitoring-key) and associated with the SLA profile instance (SPI) of the DHCP/PPPoE session. The bytes collected through CCI are periodically synchronized between the nodes. The synchronization of UM is automatically enabled for Gx/UM enabled subscribers in a dual-homed environment.
The direction of synchronization depends on the activity of a Gx sessions in ESM because UM is an integral part of Gx. Gx sessions in a dual-homed ESM environment follow the active or standby models, where the activity of Gx sessions can be distributed between the nodes, according to SRRP activity. SRRP as a basis of dual-homing in subscriber management is used to determine which node owns a Gx session. A Gx session maps to a IPoE/PPPoE session in a 1:1 manner. If multiple IPoE/PPPoE sessions share the same SLA profile instance (SPI), and with this the CCI, then the corresponding Gx sessions all share the same CCI. A Gx session under the SRRP instance in a master state is used to communicate with the PCRF servers, while the mirrored Gx session on the peering node with SRRP instance in the standby mode remains silent. In this context, Gx session can be considered as active or standby.
Although it is possible to have multiple Gx sessions per SPI, only one of the sessions can control UM at any time. In terms of UM, this Gx session can be referred to as a UM controlling Gx session. This is possible when there are multiple hosts or IPoE/PPPoE sessions per SPI, and each of them is associated with its own Gx session. Only when the UM is explicitly stopped via a session (RAR/CCA, or the UM controlling Gx session is terminated via CCR-T) can another Gx session take control over UM.
The initial quota for UM sent by the PCRF is always received by the active Gx session and the quota is periodically synchronized to the standby Gx session on the peering node. This is achieved with cooperation between ESM and Diameter in a dual-homed environment. When the quota is exhausted, only the active Gx session reports the exhausted quota to the server.
Gx communication with the DRA/PCRF is performed through Diameter Base and this functionality is described in Diameter base and Diameter multi-chassis redundancy.