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The HELLO Protocol (HELLO)
(Page 2 of 2)
Issues with Using Delay as a Metric
In theory, using delay calculations
should result in more efficient route selection than simply using hop
count, but at the cost of more complexity than a hop-count algorithm.
This makes HELLO very interesting indeed, especially for a protocol
that is over 20 years old. However, since the latency of a link is often
unrelated to its bandwidth, using time delay as a link metric may lead
to spurious results.
Furthermore, it is normal for the
delay on any link to vary over time; in the case where there are two
routes that are similar in cost, fluctuations in the delay for each
route could result in rapid changes between routes (a phenomenon sometimes
called route flapping). Adjustments are needed to the basic overview
of the operation of the HELLO protocol above, to avoid these sorts of
Current Role in TCP/IP
Like other early routing protocols,
HELLO includes nothing fancy like authentication and so on; these features
were not really needed in the early days of the Internet, when the internetworks
were small and easily controlled. As the Internet grew, HELLO was eventually
replaced by newer routing protocols such as RIP. It is now considered
a historical protocol (read: obsolete) and is no longer used.
Key Concept: The HELLO Protocol was used on very early routers on the precursors of the Internet to exchange routing information. It is a distance-vector protocol like RIP and GGP but differs in that it uses calculated delay as a metric instead of hop count. Like GGP, it is now considered a historical protocol and is no longer part of TCP/IP.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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