437 Allocation Mismatch. RFC 5766. This indicates that the client has picked a 5-tuple that the server sees as already in use. One way this could happen is if an intervening NAT assigned a mapped transport address that was used by another client that recently crashed. The client considers the current transaction as having failed. The client SHOULD pick another client transport address and retry the Allocate request (using a different transaction id). The client SHOULD try three different client transport addresses before giving up on this server.Once the client gives up on the server, it SHOULD NOT try to create another allocation on the server for 2 minutes.