HELLO packet

In the Open Shortest Path First (OSPF) communications protocol - which enables network routers to share information with each other, a HELLO packet is a special packet (message) that is sent out periodically from a router to establish and confirm network adjacency relationships. On networks capable of broadcast or multicast transmission, a HELLO packet can be sent from a router simultaneously to other routers to discover neighboring routers.

A BYE packet is a packet transmitted by a multicast participant to signify that the router is leaving the session.