Moaning about "most complete" broadband, someone has, quite sensibly, asked if we can do multicast. We are a very techie ISP, so why not?
So, here is my reasoning why not, at the moment. It is not totally ruled out, but I do think it will disappear to be honest.
First off, I'll try and explain what multicast is. In a packet network you send packets of data, and this is usually a packet going to one end destination. The Internet as a whole is all about routing a packet to its destination.
However, there are two other modes of operation. One is broadcast which is sending a packet everywhere, and one us multicast which is sending to a set of devices. In practice broadcast is "everywhere on this LAN" or some suitable "broadcast network", and not everywhere on the Internet all at once.
On a LAN multicast and broadcast just work. The switch or hub ensure that every device sees the packet, and the devices filter to get those in which they are interested. Multicast and broadcast are effectively the same for most switches as there is no "layer 2 multicast subscription" process to tell the switches which devices want the multicast traffic. It is done by device drivers configuring the individual hardware to filter for what traffic is wanted. Switches can be smarter and snoop on higher layer protocols in some cases, but its not necessarily a problem to solve as switches and LANs are fast enough anyway.
Where it gets interesting is wide area multicast. The logic sounds simple enough, and sounds like a useful thing. For example, the BBC could stream a TV station as multicast, sending it just the once to a peering ISPs. The ISP duplicates the packets to its links, and so on to its customers. Customer routers send the traffic as mulicast packets on the final LAN and multiple devices can see the stream in real time.
The problem, as an ISP, is simple. We would have to copy the packets to each customer. That is technically a pain, and does not save us any money as most of the cost is the link to customers. It does reduce some traffic on peering to the likes of BBC but that is not as costly.
There is, technically, a way to do multicast on L2TP, which would address traffic. I don't think BT support it yet (or anyone else), and I can imagine the fun they would have billing for it. But it would mean a packet to each BRAS, and there are hundreds of them. This would start to have some saving as long as we commonly get two or more people on a BRAS streaming the same traffic at the same time. At present that is simply not going to happen.
But could there be enough demand if ISPS did multicast? I am not convinced. The problem is that there are already very good technical means to distribute the same content to many - using radio and satellite. It works, and is cheap, and already in use.
But could we make multicast better? If the way broadband worked is proper routers and switches at an exchange level, multicast could just be one packet in to BT and that duplicates and spreads at each aggregation point and efficiently gets to every device that wants it. Yay!
But hang on - the big things now are catch-up TV and on-demand TV. The content delivery networks are providing this at an application level. Colocating kit at ISPs and closer to the end users. Eventually you can see how CDNs may have boxes at an exchange level to serve the unicast streamed content. There are already systems to include content at the BRAS level within BT.
If they do that, they can "multicast" efficiently using unicast traffic at local nodes. Once you have that you don't need the layer 2/3 multicast systems.
So, at the moment, it is simply not worth the effort. I think if we get to the stage where it is worth the effort the CDNs will be able to do it way better than we could by some means.
Multicast on the LAN will stay - but basically, I think the idea of wide area multicast being done at layer 2/3 is probably the wrong way to do it, and that is why it is not taking off. The right way to do it is application level live content distribution and recorded content delivery platforms. IMHO
I'll keep an eye on it though, you never know.
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