2017-heydari-scalable
findings extracted from this paper
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A CAPTCHA-gated registration scheme with sequences of reCAPTCHAs at random intervals and short solve windows limits automated censor deployment. With 5 minutes spent per registration, a human adversary working non-stop for 24 hours can create at most 288 censors; combined with a 12-hour registration reset cycle, this bounds the adversary's censor accumulation rate.
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For complete blockage (>99%) over 10 hours, the adversary requires a swarming ratio of 12.8, translating to 128,000 censors against a single server with 10,000 CoAs. Scaling to a 10-server, 10-interface deployment forces the adversary to operate 106,700 humans in parallel; with a 5-minute CAPTCHA registration and a 12-hour reset cycle, achieving complete blockage within 10 hours requires 1,067 non-stop human operators in the first two hours.
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A credit-based accounting method dynamically assigns users to larger groups as their trust score accumulates (credit increases by G−1 per unblocked interval), requiring a user's credit to be twice the group's risk before joining. This reduces the total number of CoAs needed while making it costly for censor agents to infiltrate large groups, since they must wait through many clean intervals before the group reaches exploitable size.
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A proof-of-concept Linux prototype using UMIP (open-source MIPv6) with three routers and five commodity machines (2.4GHz Intel Core 2 Duo, 4GB RAM) demonstrated correct CoA rotation every 10 seconds. Signaling overhead was reduced to one-third of standard MIPv6 by eliminating return routability messages; per-packet transmission overhead was 24 bytes (IPsec ESP), identical to the baseline secure-channel cost, yielding zero net overhead attributable to the MTD mechanism.
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The MI-MTD framework uses Mobile IPv6 Care-of Addresses (CoAs) rotated among randomized user groups every shuffling interval. With 1,000,000 users, 5,000 censors, and 10,000 CoAs (swarming ratio φ=0.5), per-interval access probability is 60.88%; over one minute with 10-second shuffling intervals, blocking probability drops to approximately 0.358%, meaning users retain ~99.6% chance of access.