file swarming?????????????????

I am just wondering...

How does p2p networks implement multi-sourced download, i.e. downloading the same file from multiple sources.

P2P is something that realy interest me- I am doing a small research about it

Now, I understand UDP would be a very good protocol for implementing this. but still as a programmer I have many questions.

Anyone who knows a good site that explains how does multi-sourced download works - please give me a link.

its the only thing missing for me to completly understand how p2p works.

Many thanks in advance,
SIJP