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

Mirjam Kuehne

As described earlier in AS Path Lengths Over Time, you can determine how interconnected the Internet is by looking 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 choose the path with the least number of ASes to get to the receiving AS.

With the number of ASes connected to the Internet constantly increasing, one could expect that the length of the AS paths would also increase as the network as a whole gets wider. However, this doesn't seem to be the case. Also, with IPv6 being more widely deployed, how does the interconnectedness of the IPv6 portion of the Internet compare to IPv4?

We analysed data collected at several of the RIPE NCC route collectors located at various places around the globe, starting in 2005. We extracted the average length of all the AS paths found and separated the IPv4 from the IPv6 routes1.

Figure 1: Average AS path length for IPv4 and IPv6 excluding AS prepending

This update shows that during the last two years, even though the overall number of ASes on the Internet is steadily increasing, the average AS path length for IPv4 networks not getting longer. In fact, it has been decreasing slightly. That means, the Internet is not becoming wider per se, but more dense, and therefore more interconnected.

For IPv6 networks, the paths are generally still shorter than in IPv4 networks by more than .5 hops, but this is increasing slightly. As IPv6 deployment progressively reaches the edges of the Internet it might stabilise and reach the same density level as IPv4.
The practice of AS prepending seems to be increasing progressively, even though it is still rarer in IPv6 than in IPv4.

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

1AS 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.

Related topics: Internet Protocol, IPv6
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