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Why last ARPANET approach for setting link weights abandoned in the Internet

Posted on 2010-08-24
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 In the ARPANET, link weights used to change dynamically. They represented the average delay over a period of 10 seconds. Whenever a link delay changed considerably, the router would issue a routing update to report the link weight change. Why was this approach for setting link weights abandoned in the Internet?
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Question by:mrkidhiworld
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The current routing protocol, BGP does still use a metric weighting system that is affected by link speed and other factors like link stability.

Each AS participating in Internet routing sort of keeps its own view of the world and every route possible relative to itself, based on information from its neighbors that keep their own view based on ITS neighbors and so on.

For issues of scaling - directly notifying a backbone router in Brazil that a link in Nebraska many hops away is running a bit slow would not serve any meaningful purpose.

Out of curiosity, which specific version of ARPANET routing are you referring to? The system evolved over time and adjustments/changes happened many times in the evolutions that lead to the modern internet.
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Small excerpt taken from
http://www.isoc.org/internet/history/brief.shtml


The increase in the size of the Internet also challenged the capabilities of the routers. Originally, there was a single distributed algorithm for routing that was implemented uniformly by all the routers in the Internet. As the number of networks in the Internet exploded, this initial design could not expand as necessary, so it was replaced by a hierarchical model of routing, with an Interior Gateway Protocol (IGP) used inside each region of the Internet, and an Exterior Gateway Protocol (EGP) used to tie the regions together. This design permitted different regions to use a different IGP, so that different requirements for cost, rapid reconfiguration, robustness and scale could be accommodated. Not only the routing algorithm, but the size of the addressing tables, stressed the capacity of the routers. New approaches for address aggregation, in particular classless inter-domain routing (CIDR), have recently been introduced to control the size of router tables.
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And I neglected to mention before that alot of the metric re-jiggering is done by the network operators and less by the protocol itself
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by:mikebernhardt
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BGP does not, and has never done, any automatic changes due to link speeds or other dynamic factors! A link is either up or it isn't. After that everything is determined by policies that have been configured by humans.

Dynamic changes are a function of IGPs like OSPF or EIGRP. EIGRP especially has the capability to change metrics based on factors like congestion (but no one uses it). The Internet is far too big, and therefore has far too many links, for tens of thousands of routers to be constantly updating routing tables due to metric changes somewhere. The whole Internet would come to a halt!
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