Interdomain Internet Routing

H. Balakrishnan, “Interdomain Internet Routing,” MIT Lecture Notes, (January 2009).


Connectivity between different autonomous systems (ASes) with heterogeneous characteristics was the primary objective while designing the Internet. Interdomain routing enables end-to-end communication and global connectivity in a distributed manner through the dissemination of reachability information among the ASes/ISPs in the Internet. This lecture provides an overview of the existing interdomain routing scenario; its components and relationships between them; currently used interdomain routing protocol (BGP4); and underlying mechanisms as well as economic activities that dictate its workflow.

The Internet is a connected graph with ASes (uniquely identifiable and managed by different ISPs) as its nodes and interdomain links (between border gateways of adjacent ASes) as its edges. Each AS has a range of IP addresses that it advertises (after aggregation) to its neighbors along with the reachability information it received from its other neighbors. In steady state, each AS ends up with a global view where it knows which AS a packet to forward to for a particular destination prefix. In practice, however, ISPs have different sizes and reachability, which results in a ISP-hierarchy with different relationships (e.g., peer-to-peer, provider-customer) along the hierarchy. Moreover, there is competitions and economic tensions between connected ISPs. This lecture discusses different ISP relationships (e.g., peering, transits) and related concepts and mechanisms (e.g., ranking, filtering) from an economic standpoint.

The main objective of an interdomain routing protocol is to provide reachability in a scalable manner without violating the autonomy (in terms of policies) of the concerned ISPs. In the existing Internet, BGP (a path-vector protocol) performs the functionalities to provide scalable, policy-based routing under competitive circumstances. As pointed out in the lecture, BGP lacks security and trust and it is prone to misconfigurations, implementation and human errors ; but in terms of connectivity BGP is quite resilient and fault-tolerant. One thing should be noted here is that the first BGP draft came out in 1989, one year after the DARPA design paper, and security was not the biggest concern at that time.

BGP comes in two flavors: eBGP connects border routers from different ISPs, while iBGP internally connects the border routers of a single ISP in a full mesh to share reachability information. In a larger ISP, iBGP faces some scalability issues; techniques to subside them (e.g., route reflecting, hierarchical organization) are addressed in the lecture. In addition, BGP attributes and their functionalities in route selection, filtering, and path ranking are also discussed. Finally, a brief overview of the biggest BGP challenges (e.g., prefix hijacking, convergence and multi-homing issues) concludes the discussion.


Most of the policies in an interdomain routing protocol are driven by economic objectives – be it route selection, exporting reachability information, or traffic engineering. The best thing about this lecture is that it drives the discussion from an economic perspective instead of diving deep into the technical details of BGP internal mechanisms.

One of the fascinating things about BGP is that even though each ISP is a black-box to the rest of the world and there is little trust between the ISPs, BGP is still working and doing pretty well at that. Consequently, a significant literature in game theory and mechanism design have been developed to try to understand and model the behavior and incentives of ISPs in the Internet. I am not really informed of their success though!

Security concerns and ample opportunities for human errors are the two weakest points in BGP design and implementation. Even though S-BGP is out there to add some security, mostly due to bureaucratic reasons its not deployed. Its unfortunate but true for any changes to the core of the Internet. However, why no expressive/declarative, type-safe language for BGP configuration has not yet been created by any vendor is unclear. If they keep the underlying mechanism same but provide a safer human-interface that might be able to cut a large number of misconfiguration errors.

To address various perceived deficiencies of BGP, tens (may be even hundreds) of interdomain routing protocols have appeared over the years without any success. Does it prove BGP’s success or is everyone involved just too reluctant to spent time, money, and effort to replace BGP?

One thought on “Interdomain Internet Routing”

  1. I agree that it is amazing that the routing system works. I think the secret is that most ISPs are very sophisticated and that misconfigurations rarely propagate far enough to hurt a large portion of the net. The Youtube example requires several things to go wrong at the same time, and this happens only rarely. Nevertheless, it would be useful to have more active verification of advertisements as they propagate through the network.

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