The communication layer in SDN is responsible for the communication between the SDN controller and those controlled network devices (e.g. data plane switches).

The network-wide state-management layer provides up-to-date information about state a network’s hosts, links, switches, and other SDN-controlled devices.

NEXT-HOP attribute in BGP tells the router about the next-hop AS towards the destination.

Echo request/reply used in ping command is based on SNMP protocol.

A "destination port unreachable" ICMP message is used as one stopping criteria in traceroute command.