Network of Networks

When we're searching for analogies to describe the operation of the internet we often fall back on that of posting a letter. Each packet of data we need to send and receive can be compared to a parcel we need to get from A to B.

In order to achieve that you need to identify the best route to take between those two points. For parcels these routes are relatively static, for the internet that isn't the case with routes being much more dynamic.

Border Gateway Protocol (BGP) is a path vector routing protocol that makes routing decisions on how to get data packets to their intended destination. As internet users we may not directly interact with it but without it the internet wouldn't be functional across the globe as it is today.

Autonomous Systems

The internet as the name suggests is a network of networks, it isn't one cohesive systems, it is a large number of individual networks. These individual networks in the context of routing traffic are referred to as Autonomous Systems (AS).

If we continue our postal analogy then each AS is like an individual postal region covering a town or county.

Each AS consists of routers that know how to route traffic internally but need to rely on connections to neighbouring AS to route traffic outside of its region. Because the internet is a dynamic system with routes appearing and disappearing each AS needs to be kept up to date with routes each other AS can help with.

This is achieved via peering sessions where AS connect to each other in order to build up the full picture of how to route traffic to destinations outside of each others boundary. BGP is the mechanism that allows this routing information to be exchanged.

Not Just The Shortest

In order to become a AS you must register with the Internet Assigned Numbers Authority (IANA) which will assign the AS to a Regional Internet Registry (RIR) as well as allocating a 16 or 32 bit identifier. As of the late 2010s there were around 64,000 AS registered worldwide a number which will have continued to grow.

AS tend to be managed and run by large organisations, typically Internet Service Providers (ISP) but also large tech companies, governments or large institutions such as universities.

Often there are multiple possible routes for a packet to reach its destination. In order to allow the best route to be chosen BGP allows AS to apply attributes to each route that can be factored into the decision on which route to take.

These attributes may indicate hop count (how many steps are involved in getting to the destination) as well as weight where an AS can indicate which route it would prefer traffic to take.

Because some AS are managed by commercial business this creates an interesting quirk in how traffic is routed. It would be assumed that the shortest or fastest route would be chosen. But because some companies will charge for their AS to handle traffic or may not want to help competitors then commercial relationships are sometimes factored into which route to take.

When BGP Goes Wrong

As described earlier the management of routing traffic through the internet is autonomous, it is far too complicated and dynamic to be managed by hand. An AS uses BGP to announce which routes it can provide access to, other AS then use this information to route traffic from within inside their network.

There have been several examples of AS accidentally announcing they can provide access to routes which they can't in fact handle. In 2004 a Turkish ISP accidentally announced it could handle any route on the internet, as this misinformation spread to more and more AS internet access ground to a halt across a large part of the globe. In 2008 a Pakistani ISP which was blocking traffic internally to YouTube accidentally started announcing externally it could handle connections to YouTube and caused an outage in the region to that service.

These are both examples of BGP hijacking, in these cases the announcement of false routes was accidental but this isn't always the case. This can sometimes be done maliciously in order to lure traffic meant for a legitimate site to an imposter, in 2018 hackers announced bad BGP routes for traffic being hosted in Amazon AWS and were able to steal a large amount of cryptocurrency.

In an effort to combat this kind of activity Resource Public Key Infrastructure (RPKI) allows BGP data to be cryptographically signed in order to validate that an AS is authorised to announce a route for a particular resource.

The internet is such an important part of our day to day lives its easy to consider it a slick homogeneous system that always works, and it is true to say that its a miracle of engineering that its been able to scale to the network of networks that we use today. However it is actually a very large collection of individual elements and sometime its fragility is exposed and we see it is all too easy for it to falter.