2.5.3 Previous Location Check (PreLocChk)

This countermeasure screens S6a/d PUR and NOR messages of Outbound Roaming Subscribers to check if the MME from which the PUR/NOR message is received is the same MME on which the subscriber is currently registered.

The Outbound Roaming Subscriber is considered successfully registered to a Foreign network MME when an ingress S6a/d ULR/A (ULA with Result-Code as 2xxx) is processed by DSA.

The Outbound Roaming Subscriber is considered de-registered from the Foreign network MME when:

• An egress S6a/d CLR is processed by DSA, or
• An egress S6a/d RSR is processed by DSA, or
• A non-vulnerable ingress PUR message is processed by DSA.

This countermeasure considers the ingress S6a/d PUR and NOR message as vulnerable if any of these conditions are true:

• The subscriber has not registered to any MME.
• The MME from which the PUR/NOR message is received is different from the MME on which the subscriber is registered.

Appropriate ART configuration needs to be done for routing the egress request messages (only towards foreign networks) to DSA so that the egress CLR can be processed by this countermeasure. See ART Configuration for DSA for more details.