On Sia, contracts are direct been renters and hosts. Hosts can collude with themselves to make it look like they are storing lots of data, but to what end? Renters on Sia only pay for the data they are directly storing.
For the Sybil attack, Sia uses proof of burn. Hosts provably burn money to demonstrate legitimacy. A Sybil attacker would have to spend real resources to appear as many hosts, and as the network grows, the required burn grows as well.
Then why use the Sia coin instead of just paying someone to (provably) host your stuff using Bitcoin? Not being snide, I just don't know what the platform offers if you're just doing direct contracts anyway.
In Sia both the renter and the host pay up front (host is putting up collateral). The payment is held for the duration of the contract (typically 12 weeks), and then the host only receives the full payment if they complete a proof of storage on the blockchain.
This means the host is guaranteed to get paid even if the renter disappears forever on week 2.
Sia is quite volatile. If the payment is held for 12 weeks, the host has no idea what that payment will be worth. Do you have a mechanism preventing this issue?
You know what it is worth at the start of the contract, just like when working with foreign currencies. But yes, volatility is an issue that I presume will be less of an issue as it grows.
For the Sybil attack, Sia uses proof of burn. Hosts provably burn money to demonstrate legitimacy. A Sybil attacker would have to spend real resources to appear as many hosts, and as the network grows, the required burn grows as well.