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Synchronous packet transfer only solves the problem if you build a truly constant rate network. Traffic monitoring works when variances exist; your flow has to be fully homogeneous to provably secure against it. That means in your model your users would need to transmit and receive exactly 96kbps at all times when on net, and your nodes would talk to each other at 1024kbps at all times when on net. Otherwise, consider A->onion1->onion2->B – an attacker could potentially see the flow from onion1->onion2 decrease to 1 PPS sec when A isn't talking, and increase when A is.

Truly constant rate anonymity networks dramatically add resistance to passive traffic analysis, but they move users from a low-latency/high-throughput network to 56k dialup speeds :) Not only does this suck so most people won't use it, but the people who do chose to use it will glow neon bright to adversaries. The use of the system will be a strong indicator that, even if you don't know what the user is doing, the user is doing _something_ interesting.

And even if there was desire, these networks are intrinsically limited in size and scale if they want to maintain constant rate. Herbivore[0] is an interesting proposal in this space - use a DC-net partitioned into smaller cliques to give in-group anonymity but mass participation. And most use chaff packets – A has nothing to send so sends encrypted random data to maintain the constant rate guarantee... I'm trying to find the paper I read that suggests a global passive adversary who goes "hands on" in the network could use a combination of watermarks generated through packet dropping/artificial queues + knowledge of which packets are chaff to build a trace, but I'm struggling. If I do I'll drop it here.

For fun, go check out https://groups.google.com/g/alt.anonymous.messages – this is probably the classic example of a (very) high-latency but very strong anonymizing mix network.

[0] https://www.cs.cornell.edu/people/egs/papers/herbivore-tr.pd...