Host names and realm names are essential in Diameter routing. They are conveyed in Diameter messages through the following AVPs:
Origin-Host
Origin-Realm
Destination-Host
Destination-Realm
Origin-host and origin-realm AVPs are used to identify nodes in Diameter networks. Every Diameter message must carry these two AVPs. In SR OS, the origin hostname must be statically configured through CLI. The configuration of the origin realm is optional. If the origin realm is not configured, the origin realm is extracted from the origin hostname. The first ‟.” in the configured origin hostname is used as a delimiter between the host portion and the realm portion. If there is no ‟.” in the configured origin hostname, then the origin hostname is used as both, the origin hostname and the origin realm name. Origin host and realm names do not support online changes (once they are configured, because they cannot be changed without first deleting the Diameter node hierarchy).
The origin-host and origin-realm names received from peers, as a result of successful Capability Exchange negotiation, are used to populate peer and realm tables in SR nodes.
While origin-host and origin realms are used for identification of Diameter nodes within realms, the destination-hosts and destination-realms are used in Diameter request messages for forwarding and routing purposes. They steer the message through the Diameter network toward the destination.
The destination hostname that is used by applications (NASREQ, Gx, Gy) cannot be configured in an SR node and is only learned from the incoming application level messages (the origin-host AVP in the incoming request or answer messages). within each Diameter session.
On the other hand, the destination realm that is used in application request messages is a mandatory configuration parameter in an SR node. However, its value can optionally be learned from the incoming application messages and any newly learned value overwrites the previous one. This learning is performed on a per-message basis. The destination realm value (configured or learned) is not used to populate the realm table but is only used on the application level to populate the destination-realm AVP in every application level request message.
Because the destination-host AVP is optional and the destination-realm is mandatory in application request messages, the destination-realm can be used without the destination-host in forwarding and routing decisions to guide the request message to its destination. The destination-realm used in initial request message (CCR-I) is a configured one, while the destination-host is not used in the same message because it has not been learned yet.
Destination-host or destination-realm AVPs are never present in:
Diameter messages that are not to be proxied or relayed (such as CER)
Diameter answer messages