An SR inserts the Route-Record AVP to all CCR messages that are passed through it. The Route-Record AVP is not added on the SR originating the message, but instead only on the transit SR, where messages are received from the inter-peer and passed to the network, or when they are received from the network and passed to the inter-peer.
The value inserted in the Route-Record AVP is the origin-host of the peer from which the message is received.
Checks that are performed on the Route-Record AVP are the following:
If the peer’s identity in any of the Route-Record AVPs in the received request messages matches the local host identity, then this means that a routing loop is detected (Figure: Diameter routing loop detection). The SR answers with the Result-Code AVP set to DIAMETER_LOOP_DETECTED.
Route-Record AVPs are examined in received request messages to determine whether the message should or should not be forwarded. Only the next-hop (peers) candidates for the route that are not in any of the Route-Record AVPs of the received message, are allowed to be used. If there are no such next-hop candidates, an error message with the Result-Code AVP set to DIAMETER_UNABLE_TO_DELIVER must be sent back to the peer.
Scenarios in which an SR can receive a request message are:
RAR from the inter-peer
RAR from a network peer
CCR-I/U/T message received from the inter-peer
CCR-I/U/T message is received from a network peer
