This document describes a network diagnostic tool called PROBE. PROBE is similar to PING in that it can be used to query the status of a probed interface@ but it differs from PING in that it does not require bidirectional connectivity between the probing and probed interfaces. Instead@ PROBE requires bidirectional connectivity between the probing interface and a proxy interface. The proxy interface can reside on the same node as the probed interface@ or it can reside on a node to which the probed interface is directly connected. This document updates RFC 4884.