AS Path Lengths Over Time - How Interconnected is the Internet?

By Mirjam Kuehne
Mirjam Kuehne

One way to determine the denseness of the Internet, or its "interconnectedness", is to look at the path length between Autonomous Systems (ASes). The "shortest AS path" is a route selection rule in the Border Gateway Protocol (BGP) that means traffic from one AS will chose the path with the least number of ASes to get to the receiving AS. With IPv6 being deployed in parts of the Internet, we looked at the AS path length to see if the IPv6 portion of the Internet is more or less interconnected than the IPv4 Internet.

To do this, we analysed data collected at several of the RIPE NCC route collectors, located at various places around the globe, starting in 2005. We then extracted the average length of all the AS paths found and separated the IPv4 from the IPv6 routes*.

For IPv4, the trend shows stability over time of about 4 hops for the average AS path length. That trend indicates that, even though the Internet has been growing progressively since 2005 in terms of active AS numbers, it is not becoming wider per se, but more dense and interconnected, and is therefore not generating longer paths between ASes. Therefore, the ratio of global interconnectedness remains stable over time.

For IPv6, the AS path length was initially longer than for IPv4 networks, because there were not as many IPv6 networks others could connect to. Then, between 2007 and 2008, one can see a drop in the AS path length. This indicates an increase in the numbers of IPv6 networks and is consistent with the data Daniel Karrenberg showed in an earlier article, 'A Look at IPv6 Allocations Since 1999', on CircleID where one can clearly see a steep increase in the number of IPv6 allocations made by the RIPE NCC around the same time.

The shorter path length in the IPv6 graph (compared to IPv4) could be explained by the fact that there is a higher concentration of IPv6 networks at the core of the Internet than at its edges. Recently, we have observed stabilisation of the AS path length in IPv6, but as IPv6 becomes more deployed at the edges of the Internet, it might converge into the same trends of interconnections as IPv4.

For more analysis and the methodology used to produce these graphs, please refer to the background article on RIPE Labs: Interesting Graph - AS Path Lengths Over Time.

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* AS prepending is a method of making routes less attractive by artificially lengthening the AS path. Because the most common routing protocol tends to send traffic over the shortest path, an artificially prepended path is taken less often. For the purpose of these statistics, we removed the prepended ASes from the count.

By Mirjam Kuehne

Related topics: IPv6, Networks

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