Overview

Status: Accepted
Author: @ymasmoudi
Last Updated: 09/22

During session establishment, the phone may have an APN configured already. In this case, this APN will be used as it has a higher precedence over the one provided by the SIM card.

This feature enables the overriding of UE requested APN with a network specified APN via IMSI prefix based filtering. Up to 10 IMSI prefix filters and corresponding APNs to overwrite with can be defined.

Note that the APN correction applies to a federated set up only where HSS is used.

Proposition

The PDN connectivity procedure is used by the UE to request the setup of a default EPS bearer to a PDN by sending a PDN Connectivity Request message to the network.

During this phase, the UE may communicate its APN following one of 3 modes:

1/ The UE may set the ESM information transfer flag in the PDN Connectivity Request to indicate that it has ESM information that needs to be transferred to the MME after the NAS signalling security has been activated. In this case, the network initiates the ESM information request procedure in which the UE can provide the MME with APN.

2/ The UE shall include the requested APN in the standalone PDN Connectivity Request when requesting connectivity to an additional PDN.

3/ If no APN is included in the ESM Information Response or PDN Connectivity Request message, then the UE is connected to the default APN.

The APN correction, if enabled by setting enable_apn_correction to true, will overwrite the APN requested by UE for UEs with matching imsi_prefix filter specified as part of apn_correction_map_list with the value of key apn_override.

cat /etc/magma/mme.yml

....
enable_apn_correction: false
apn_correction_map_list:
        - imsi_prefix: "00101"
          apn_override: "magma.ipv4"

The configuration will be limited to a maximum of 10 imsi prefix filters.

How We Will Change Magma

MME Change

Two structures will be added to contain the mme apn correction configuration

typedef struct apn_map_s { bstring imsi_prefix; bstring apn_override; } apn_map_t;

typedef struct apn_map_config_s { int nb; apn_map_t apn_map[MAX_APN_CORRECTION_MAP_LIST]; } apn_map_config_t;

In addition, a new function will be introduced to filter and return the overridden APN based on the MME NAS config.

This function will be called upon reception of the ESM Information Request or PDN Connectivity Request.

Mconfig proto description will be changed to support APN correction configuration pending the support of mme config in swagger and NMS.

ORC8R Change

Exposing MME configuration to swagger will be supported in a later phase.

NMS Change

Exposing APN correction configuration to NMS will be supported in a later phase.