Diameter
Updated
Diameter is the authentication, authorisation and accounting protocol of 4G networks, successor of RADIUS. Between operators it carries roaming signalling, for example between a visited MME and the home HSS; inside a network it carries charging and policy. The base protocol is defined in RFC 6733.
In practice
When a SIM cannot register in a visited country, the reason is often a Diameter answer between the two operators, such as a refused location update, which only the operators can see.