PathErr (path error) messages report errors in processing Path messages. They are travel upstream towards senders and are routed hop-by-hop using the path state. At each hop, the IP destination address is the unicast address of a previous hop. PathErr messages do not modify the state of any node through which they pass; they are only reported to the sender application.
<PathErr message> ::= <Common Header> [ <INTEGRITY> ] <SESSION> <ERROR_SPEC> [ <POLICY_DATA> ...] [ <sender descriptor> ] <sender descriptor> ::= (see earlier definition)
The ERROR_SPEC object specifies the error and includes the IP address of the node that detected the error (Error Node Address). One or more POLICY_DATA objects may be included message to provide relevant information. The sender descriptor is copied from the message in error. The object order requirements are as given earlier for a Path message, but the above order is recommended.