Interdomain Internet Routing

Summary

This paper focuses on BGP (Border Gateway Protocol) - both the technical details of the protocol as well as the policy that controls the flow of traffic.

BGP is the interdomain routing protocol, used by routers at the boundaries of ISP's to share routing information. What actually controls the flow of traffic are the policies put in place by AS's. An AS is owned by a commercial entity and determines its policies based largely on financial reasons as well as its customers' needs. Each AS uses its own Internal Gateway Protocol.

There are 2 types of inter-AS relationships: transit and peering. A transit relationship usually involves a financial settlement from one AS (the customer) to the other (the provider). A peering relationship, on the other hand, does not include a financial settlement. Instead, they are usually between business competitors and involve reciprocal agreements allowing each AS to access a subset of the other's routing tables.

An AS must decide which routes it will export to its neighboring AS's. Exporting a route means that it agrees to take all traffic to that destination IP address. Because an ISP agrees to provide its customers access to anywhere on the web, it will prioritize the exporting of routes to customers first. In addition, it will export all paths to its customers to the other AS's. An AS will also export some peering routes.

When deciding which routes to import, an AS will always prioritize a customer over a peer over a provider. All the import and export rules are stored in the LOCAL PREF.

BGP itself is a simple protocol, but is designed so that it can also implement policy. BGP was designed to meet 3 goals: scalability, policy and cooperation under competitive circumstances. A BGP router initially shares the subset of routes it wants to share with another BGP router. After that, it simply sends "update" and "withdraw" messages.

There are 2 types of BPG sessions: iBGP and eBPG. eBPG sessions are used between BPG routers in different ASes while iBPG sessions are between routers within the same AS. iBGP is used to share externally learned routes within the AS.

Then the paper provides the details for the various attributes in a BGP route announcement. It also goes into some of the security problems with BGP, giving a real-world example involving Pakistan Telecom and Youtube.

Criticisms & Questions

One thing that was unclear to me throughout the paper was the relationship between an AS and an ISP. I couldn't figure out if an ISP was made up several ASes, if an AS can cover several ISP's, if an ISP is an AS, etc. I would have liked it if the paper had made that point clear when it introduced ASes.

The paper went into detail about the "full-mesh" implementation of iBGP. It also offered route reflectors and confederation of BGP routers as 2 alternative implementations. I would like to know which of those implementations is most commonly used in practice? It also wasn't clear to me if the iBGP implementation is something that each AS chooses and so would have to be independent of any other AS's iBGP.

The problem with the lack of origin authentication really surprised me. It seems to me that "hijacking" routes is not very hard to do. And, if that is the case, why is it that more attackers are not taking advantage of it (possibly to launch a DOS attack)? And, are there any other measures being taken to protect against this vulnerability aside from the poorly-maintained registry?

Feedback

I would say definitely keep this paper in the syllabus. It helped me to understand a lot more about the reasoning behind the inter-AS policies. It's also nice to see the clear contrast between what's technically possible and ideal and what actually happens in practice.