Source : Free On-Line Dictionary of Computing

hello packet
     
         An {OSPF} {packet} sent
        periodically on each {network interface}, real or {virtual},
        to discover and test connections to neighbours.  Hello packets
        are multicast on physical networks capable of {multicasting}
        or {broadcasting} to enable dynamic {router} discovery.  They
        include the parameters that routers connected to a common
        network must agree on.
     
        Hello packets increase network resilience by, e.g., allowing a
        router to establish a secondary connection when a primary
        connection fails.
     
        (1999-11-02)