Multicast for File Delivery

Multicast will soon be available in the UK for video distribution, does it have a role in distributing large files, like the recent OS X update Lion.

James Cridland wrote about how broadcast was a potential way to distribute Lion, the new version of OS X. While digital broadcasting is compelling as a mass-distribution media, the logistics don’t stack up to my mind, and while tuners are getting quite cheap, the hassle of trying to get aerial signal to your computer is still there.

Multicast was briefly mentioned in the comments, this provides a much more realistic alternative, and is something that BT, the main ISP Wholesale provider, is implementing. This post covers ISPs that use BT to provide connections to their subscribers, the method most ADSL customers are provided by. Customers with Sky (who are Local-Loop Unbundled) or Virgin (who run a cable network) are on different network topologies.

What is multicast? Basic machine to machine communication is unicast. Broadcast allows one machine to ‘talk’ to all others. Multicast is somewhere between, where other machines can choose to listen to stream on the network, but without the “speaking” machine having to transmit at additional copy. It’s perfect for things like live Radio or TV viewing, where many hundreds of users are viewing/listening to the same thing.

Where are we today?

Most ISPs don’t support multicast, so every download or action on the internet is a separate stream:


Content delivery with with unicast, without any multicasting.


The ISPs that do support multicast have to split it before it goes to BT. The BBC multicast some content out as a trial, and the traffic looks something like this:

The ISPs save on their ‘transit’ bandwidth, but still have to inject a separate copy of the video for each person watching when it goes to BT. Transit costs are unfortunately the cheaper part of an ISP budget, which means that multicast doesn’t help ISPs that much at the moment.

BT are introducing multicast into their network, which means that the “splitting” of the multi-cast to uni-cast occurs much deeper in the BT network, which means the result looks something like:

Much more efficient, only one copy of Radio1 goes from the ISP to BT (and some older BT documentation implies this may even go directly to BT bypassing the ISP).

But how does this apply to a ‘file’ like Lion? 

If you were to treat the 4 gig Lion update as a broadcast you could transmit it as a “carousel”, where it loops around. Run multiple carousels at different positions in the file and a client would join 1 or more streams based on the amount of bandwidth available, and how much of the file was downloaded

For efficiency, you might also want to offer streams at higher bit-rates so that a client could join fewer streams.

This traffic would need to be flagged as lower priority. TV multicast will be the opposite, and have Quality of Service to make sure video plays back smoothly, here though you would want the download to be dropped if other traffic was present. This will leave you with “holes” in the download.

The client could wait for the broadcast to “come around” on one of the streams and join that appropriately, or it could also make direct connections back to the CDN, or use Peer-to-peer networking, to “mop up” the specific bits of the download it had missed.

Would it make everything faster? 

Yes, and no. It would definitely be more efficient, however the limiting factor will still be the copper at the end of the journey, between the exchange and the customer. However, by reducing the demand to both the CDNs, and the ISPs bandwidth, that last mile should be used more fully.

Will it happen?

For things like downloads using CDNs that are embedded deeper in the network would be simpler for the client, and ease the load on ISPs. After the initial download “flurry” the multicast approaches efficiency reduces.

Two things make it more sense for video content like the BBC iPlayer: you have many people downloading content as it is being released, and you have a custom download manager in place.

Multicast will definitely happen for live-content. For other content it probably remains an interesting thought-experiment, unless the economics proved compelling enough. That is less likely since those impacted (the ISPs) are not those able to make the changes (Content Distributors).