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Tuesday, October 21, 2008

The ISP Cat and Mouse game and CDNs finally benefiting out of it.

Politics and policies are everywhere. They say policies are made to govern us and I say many of those are there for inertia - resistance to change. The big and powerful want to be so, always and do not want others to get there. This is well known and very much a cliche. But what has this got to do with ISPs in specific? Here we go:

ISPs are the people who sort of own the "Internet network" physically. It is they who actually connect the various computers by physical cables. And yes, that is why we pay them. For getting us connected to the rest of the world. Now there is no single ISP who has his cables connected to all the computers in the world. In fact there is no one who can even boast about a majority stake in the market. So obviously when the data travels through the internet, it goes through the infrastructure laid and maintained by different ISPs. A simple example will illustrate this :

Let's say a user is connected to the internet via Bharti Airtel connection and he is trying to access, say Indian Railways website, which is hosted on a machine connected to internet, for the sake of illustration, via BSNL connection. So the path of the request from client to the server would involve, both Airtel network path and BSNL network path. The client sends the request to the Airtel server. The Airtel network will route the data in its own network to the extent possible. At one point it needs to request BSNL network to take up the data and then give it to the destination server machine. Now at this cross-over point, Airtel is requesting some service from BSNL. Essentially Airtel is making use of the BSNL's network infrastructure to carry its data. Now there is no point in BSNL giving this for free. So obviously it charges Airtel some amout of money. Airtel does not mind paying it as it mostly gets translated to user charges. This is not really the issue. Problem would arise when BSNL will refuse to take the request and Airtel will have find some other alternate path, which generally ends up to be very very long. Consider this:

Clinet -> Last Airtel machine (router) -- m number of hops
Last Airtel machine -> Destination machine in BSNL network (Direct path) -- 4 hops.
Last Airtel machine -> Destination machine in BSNL network (Indirect path via some other ISP or via some other cross-over point) -- 20 hops

So totally the data has to do m+4 hops if BSNL takes up the request from last Airtel machine. At a time when BSNL is experiencing some heavy traffic in the region where the Airtel-BSNL crossover is happening, it would not be willing to accept more data, that too from a different ISP. So they follow two techniques here:

1. Simply drop the data packets, which will result in bad experience for the end user.
2. As routing happens based on least number of hops, the first BSNL server at the crossover point, will tell the last Airtel machine that the number of hops to the destination machine is actually 25 hops even though it is totally wrong. As a result the last Airtel machine will instead choose the indirect path with 20 hops. This will obviously slows down the internet and again result in bad experience for the end user.

Now you see how policies and profits affect technology. This is, as stated by an electronics professor at my college SJCE, TECHNO-POLITICS.

The solution for this would be to make the data available in every ISPs own network. And that is precisely what the CDN - Content Delivery Network - companies do. These companies have a huge number or servers placed in various parts of the worlds. In most cases they are placed in the data centers of these ISPs. It is symbiosis. With CDNs placing their servers in ISP's data centers, the ISP has a lot of data in its own network, even though the original website (or content owner) might be using a different ISP. This avoids a lot of requests to different ISPs and there by reduce costs significantly. In return the CDN companies get a very sweet deal for the rack space for their machines

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