Interdomain Internet Routing

Coming with minimal background in computer networks, each paper is a fascinating study into the inner workings of the Internet. The lecture "Interdomain Internet Routing" attempts to discuss how routing works between autonomous systems (AS) which consist of networks of nodes, but within the context of commercial operation of ASes by ISPs. The 0th order item that the paper points out is the fact that a routing entry equals connectivity, and that in effect an ISP is charging a node for advertising routing information to it (which was non-obvious to me). Related to that are the concepts of transit, where ISP charges for access to its routing table, and peering, where two ASes exchanges a subset of their routing tables (financial or otherwise). Exporting and importing routes (connectivity) costs the ISP money, thus it requires a set of policies that are commercially sound. 

The paper then goes on to describe the Border Gateway Protocol (BGP), which is used primarily by routers of different ASes to exchange route information, its design goals and how it can be used to implement routing policies. The main design goals are scalability, policy (expressiveness) and cooperation under competitive circumstance (imho ensuring things go well in a distributed manner even among selfish ISPs).

The paper also addresses how BGP can be used in internal sessions (iBGP) by routers on the edge (eBGP) within an AS to exchange routing information about the external world. While eBGP routers can be fully connected to exchange such information, this is not scalable. Thus, two techniques, router reflectors and confederations, were discussed. Both however have to be configured manually, which naturally leads to the question if such a process (exchanging routing information among all eBGP routers) can be done autonomously for ease of deployment.

Finally, two open problems are discussed. First is multi-homing, where a customer might want to have access through two ISPs, e.g. for redundancy reasons. The two main issues seem to be scalability, since hacks to make multi-homing work by tweaking the ASPATH seem to require some knowledge of the AS topology, and size constraints on ISP routing tables, which impose a high minimum of nodes the customer pays for. Second is the slow convergence of BGP after the occurrence of a fault.

This piece is obviously meant as a tutorial, and for me, its main value is in shedding light on how ISP actually inter-connect (and the role that routing plays), and in a way that makes sense commercially. For obvious reasons, it assumes that the reader has sufficient exposure to the material, so for me, reading it involved a lot of searching on Wikipedia.