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Best practice for preventing traffic analysis is to send encrypted messages of a constant size, at a constant rate. It's better to pad to a max size, than to randomize the packet length. Sending at a constant rate may not always be feasible or cost-effective, but it can still be useful to send data at a constant rate for a minimum block of time.

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It's just easy to implement constant rate, hence why it's best practice. But adaptive rate might be ok, like, for example, if you only pick from 4 different rates and adapt exactly every second, you only leak 2 bits of information per second. The question then becomes which leaking rate is safe enough in practice?

constant max rate tends not to be practical in remote roaming VPNs for cost and contention reasons. It would blast through typical mobile usage rates, for example, or get you throttled on a shared wifi network.

Ya - the NSA calls this "LINK MASKING". Makes a communication channel look busy 100% of the time at at predefined bit rate. Pretty much worthless to traffic analysis, at the cost of burning extra bits on the wire when not actually sending traffic. Mostly only used in Military applications, and specifically where you think an advanced adversary is going to do things like traffic analysis, etc.

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