The Hello routing protocol is an Interior Routing Protocol( IRP). It works similarly to the Routing Information Protocol( RIP), but with a different metric: While the RIP protocol uses the number of necessary hops as a metric, the Hello protocol uses the transmission time ( delay) for a connection path.
The Hello protocol's metric gives it a distinct advantage over the RIP protocol: In the RIP protocol, two connection alternatives from "A" to "B" via two intervening routers each are considered to be completely equivalent. In fact, however, they are not, if one connection runs over lines with a 2 Mbit/s data rate, while the other runs over lines with a much higher or lower data rate. The consideration of the transmission time represents an important criterion for path optimization here.
In the Hello protocol, the data packet contains a one-bytedata field for a timestamp, which indicates when the data packet was sent. When the data packet arrives at the destination station, the station calculates the time difference between the sent and received time and thus the transit time of the data packet through the network.
In other protocols such as the OSPF protocol, the Hello packets are used to confirm the neighborly relationships between routers.