Please Whitelist This Site? I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :) If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads. If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK. Thanks for your understanding! Sincerely, Charles Kozierok Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited. If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  TCP/IP Routing Protocols (Gateway Protocols)                 9  TCP/IP Interior Routing Protocols (RIP, OSPF, GGP, HELLO, IGRP, EIGRP)                      9  Other Interior Routing Protocols

TCP/IP Gateway-to-Gateway Protocol (GGP) 1 2 Interior Gateway Routing Protocol (IGRP)

The TCP/IP Internet as we know it today evolved over the course of decades. It began as an experimental research project started by the United States Defense Advanced Research Projects Agency (DARPA or ARPA), called the ARPAnet. The ARPAnet eventually grew through the addition of other networks, such as the important NSFnet developed by the National Science Foundation (NSF). The NSFnet backbone grew over the course of many years and was instrumental to the eventual creation of the modern Internet.

The original NSFnet backbone consisted of six Digital Equipment Corporation (DEC, absorbed by Compaq years ago) LSI-11 computers located across the United States. These computers ran special software colloquially called “fuzzball” that enabled them to function as routers. These “fuzzball routers” connected various networks to the NSFnet and the ARPAnet.

The six NSFnet routers worked as an autonomous system (AS) and like any AS, used an interior routing protocol to exchange routing information. The routing protocol used in these early routers was called the HELLO protocol. It was developed in the early 1980s and documented in RFC 891, DCN Local-Network Protocols, published December 1983. The name “HELLO” is capitalized, but is not an acronym; it simply refers to the word hello, since the protocol uses messages that are sort of analogous to the routers talking to each other.

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, router 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.

TCP/IP Gateway-to-Gateway Protocol (GGP) 1 2 Interior Gateway Routing Protocol (IGRP)