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Assume there is a P2P file sharing system which has no trackers but only a DHT.

How to know the number of all active peers uploading/downloading a specific file?

Is it just keep querying the DHT by get_peers to get new peers? Are there any better solution?

Encombe
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s9527
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1 Answers1

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The distributed part in the DHT makes it hard to get the exact number of peers in a swarm from it. Technically it's also unneeded and not a very useful number, as its only necessary to get in contact with only one other peer in the swarm and then the PeerEXchange extension will give plenty more peers in a more efficient way than the DHT.

Some clients also support the BEP33 DHT scrape extension that can give a approximate number of peers registered in the DHT with a max capacity of ca 6000.
Unfortunately it's badly designed and has a vulnerability making it the currently most potent vector for UDP amplification attacks using the BitTorrent protocol. It has a BAF (Bandwidth Amplification Factor) of 13.4 The attack is called Distributed Reflective Denial of Service (DRDoS) and is decribed in this paper. If this vector starts to get used it may be necessary to speedly remove this extension from the protocol.

Encombe
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  • The number of peers is a very good indicator of the popularity of the file being shared. This is why I want to know how to measure it in DHT. – s9527 Aug 02 '17 at 19:51
  • Yes, but statistics are *nice to haves* and not necessary for the protocol to work. It's also very easy to fake stats in a distributed system like the DHT by doing a eclipse attack on the info_hash. This could be mitigated somewhat if BEP42 was enforced, unfortunately it's not. – Encombe Aug 02 '17 at 20:08
  • I agree that in hindsight I could have done much better with BEP33. But if issues become troubling we can always improve it instead of removing it. It *is* a commonly requested feature after all. – the8472 Aug 03 '17 at 10:45
  • So are there any alternative ways to quantify the popularity of contents if the number of peers is hard to acquire? – s9527 Aug 03 '17 at 18:05
  • He said it would be hard to get an *exact number*. Do you actually need exact numbers for a popularity assessment? – the8472 Aug 05 '17 at 12:09
  • @the8472 Not necessarily, as long as I can know the relative size of different swarms – s9527 Aug 31 '17 at 13:22