Peer and server failover mechanisms in the SR OS are concerned with retransmission of Diameter request messages and selection of alternative network paths toward the destinations during network failures. Both mechanisms (peer and server failover) assume that there are multiple peers, paths, and possibly redundant servers available in the network that can serve requests redundantly.
Peer failover involves the selection of the next best peer for a Diameter request message that have failed to be delivered over the current peer because of an explicit error notification or simply because of a peer failure. The selection of the next best peer in the SR node is based on forwarding and routing lookups performed in the Diameter base.
However, server failover is concerned with Diameter application servers (NASREQ, Gx, Gy) that may or may not be directly connected to an SR node. From an SR node viewpoint, the server failover procedure involves retransmission of request messages that are not successfully acknowledged by the Diameter application servers (NASREQ, Gx, Gy). Such unacknowledged request messages can be attributed to the loss of a specific Diameter application server and as a result be optionally retransmitted in a way that allows delivery of the retransmitted message to an alternate and possibly redundant Diameter application server.
Application answer messages do not rely on peer or server failover procedures because their forwarding is governed on a hop-by-hop basis (the exact reverse path of the request message).