Networking - What is the HELLO protocol used for?

[hasini]

What is the HELLO protocol used for?

[muralipn]

Hi

The Hello Protocol is responsible for establishing and maintaining neighbor relationships. It also ensures that communication between neighbors is bidirectional. Hello packets are sent periodically out all router interfaces. Bidirectional communication is indicated when the router sees itself listed in the neighbor's Hello Packet.

The HELLO protocol uses a distance-vector algorithm, like the Routing Information Protocol (RIP) and the Gateway-to-Gateway Protocol (GGP). What's interesting about it, however, is that unlike RIP and GGP, HELLO does not use hop count as a metric. Instead, it attempts to select the best route by assessing network delays and choosing the path with the shortest delay.
One of the key jobs of routers using HELLO is to compute the time delay to send and receive datagrams to and from its neighbors. On a regular basis, routers exchange HELLO messages that contain clock and timestamp information. By comparing the clock value and timestamp in the message to its own clock using a special algorithm, a receiving device can compute an estimate for the amount of time it takes to send a datagram over the link.

Like RIP and GGP, HELLO messages also contain routing information in the form of a set of destinations that the sending router is able to reach and a metric for each. However in this case, the metric is an estimate of the round-trip delay cost for each destination. This information is added to the computed round-trip delay time for the link over which the message was received, and used to update the receiving router's own routing table.
This seems a bit confusing, but is really similar to the way a hop-count distance-vector protocol like RIP works. Router A using RIP receiving an RIP Response message from Router B knows it can reach every destination Router B can, but at a cost of one extra hop (the hop to go from Router A to Router B). Similarly, A receiving a HELLO message from Router B knows it can reach every destination that Router B can, but at an additional approximate cost of the computed delay for the link between Router A and Router B.

Regards,
Murali