Hitchhiker's Guide to Streaming Media: Multicast

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In 1985 Steve Deering published a Request for Comments titled “Host Groups: A Multicast Extension to the internet Protocol.” It was here that IP multicast was born as far as the internet is concerned.

It contains the following definition of multicast: “ Multicast is the transmission of a datagram packet to a set of zero or more destination hosts in a network or internetwork, with a single address specifying the set of destination hosts. For example, hosts A, B, C and D may be associated with multicast address X. On transmission, a packet with destination address X is delivered with datagram reliability to hosts A, B, C and D.”
{The word “multicast” is also used in the marketing community as a catch-all to describe the delivery of multiple versions of a video stream (for example delivering TV, web and mobile). This is only relevant at an application layer, and is not an engineering term. The two terms have often been used at cross purposes by marketing people and engineers—causing a good deal of confusion for both parties and all those around, including many analysts and leaders of multinational telcos! }

Multicast in its network engineering sense is also known as “selective broadcast.” Deering”s model above is a bit mathematical, so just to be clear what multicast does: Multicast enables a packet network to deliver a replica packet to more than one client at the same time. All the clients that want a copy let the network know using a multicast protocol. It means you really can “broadcast” on the internet but JUST to an audience who have signed up and let you know they are “tuned in”

The great thing about multicast is that you don’t need servers in the network for live streaming. You can just plug in a single source, and the routers split and copy the packets for you to all the intended recipients. The internet essentially becomes your CDN. And the best thing is that it makes no difference to the router if there is one host on the receiving network or 6 billion. Its almost like broadcasting from a radio mast but on the internet.
The worst thing about multicast is the myth that it is not supported on the internet. While it is true to say that it is not widely supported, there is a mistaken belief that it is impossible. Far from it. Many of the networks that make up the internet (loosely “ISPs”) are multicast enabled internally. They certainly have the kit, since the standards for multicast have been around since the mid 90s and are part of their design. Those ISPs and networks that don’t use multicast simply haven’t yet taken advantage of functionality on kit they already have. They are gradually waking up to the fact that many of their neighbors are enabled and the form connections between them. This creates a multicast internet. If a user is on an access network that is also on the multicast internet then they can reach broadcasts directly in their house.

That said, multicast availability over the typical internet service provider link is limited. The effort of learning how to turn it on and a general sense of “if it ain”t broke don’t fix it” pervades within engineering communities who don’t see the advantages of putting down their Xbox for an hour or two and doing something on their routers.

So what would those advantages be? One server can deliver the live news to every desk in your office with no more overhead on your internet connection than the first user’s 500Kbps. This compares with the common unicast streaming method of 500Kbps x N users. In an office of even 20 lunchtime news viewers, that quickly creates a demand at 10Mbps—all of the same data—and if you want to meet that demand on DSL you need overhead to deal with your neighbour’s office too.

Now think of all the offices with N users who are all watching that news stream. By the time you get to the trunk pipe to the nearest big city with national and international internet connections there’s a lot of copies of the same stream. And so the cost of delivering the media goes up, and the cost of running an ISP goes up, and those costs have to be met somewhere or the network has to be made more efficient. This can only be achieved with multicast.

Multicast is the only real option for proper TV over the internet. Not to offer it will soon and suddenly become a real minus point for an ISP’s service.

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