Comment by sudhirj | Hacker News Reader

Comment by sudhirj | Hacker News Reader

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Let's say you want to send something (say 100KB) to a million listeners, but you don't know which of them is going to be listening when. You'd feed that 100KB into a RaptorQ encoder, configure it to 1KB packets, and it would give you stream of near-infinite 1KB packets that you could broadcast (usually over a satellite, but UDP, multicast, QR codes all work).

Receivers would listen for as many of these packets as they can, as and when they can (the transmission can be "lossy"), and if a receiver pickups up 100 unique packets, any 100 unique packets in any order, it'll have a 99.9% (or something along those lines) chance of decoding the message successfully. Each extra unique packet adds a 9 to the chances.

That's why the "fountain". It's a data fountain, and you can grab any quantity of data off it at any point and still have a good chance of reconstructing the message.

Very interesting. Still struggling to see how this would be more efficient than looping that data for the use case of getting beacon data out. Perhaps it's more consistent/predictable in scenarios of high or targeted loss (ie first X bytes keep getting dropped)?

Not an expert, but if you miss a packet with the looping you have to wait for that particular packet, whereas with the fountain it sounds like you can get that back quicker.

If you’re transferring 5GB over 1KB packets over UDP or a satellite, the benefits are very clear. Without a fountain code, there are 5,000,000 packets you need to receive. Miss one packet, and your have to sit through another 4,999,999 packets and pray very hard that you get your packet reliably the next time.

With a fountain code, it’s not a problem. If you miss one packet, you have a 99% chance of making do with the 5,000,001st packet that comes. 99.9% of making do with the 5,000,002nd packet etc.

In other words, without a fountain code, if you want to reliably receive 1GB, and you miss a packet, your satellite will have to transmit 2GB just for your sake, which is expensive af. With a fountain code, it’ll only need to send 1.01GB.

It vastly reduces the chance of you receiving a packet that contains no new information. If you just rebroadcast the same packets over and over again, a receiver that has 999/1000 packets may have a very low chance of serendipitously receiving the packet they need.

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