I wonder if Earth hosts will be restricted from accessing Mars hosts.

At first, it will be a very strict filter. It'll get more lax with time. In time you'll probably have to apply for a permit to initiate transmissions and adhere to bandwidth restrictions.

Any ideas on how such DTN would work exactly aside from the fact that it turns into a man in the middle attack between the two hosts?

The DTN broadcaster would be on-planet and would communicate with a satellite mesh (providing full-planet coverage and backup communication for laser-prohibitive weather). A GPS-like system would be used for the telemetrics necessary for laser-based communication. Standard routing would be used to find a path through satellites (both in orbit around mars, those in solar orbit, and those in Earth orbit) to the Earth ground-station.

At first this would be incredibly slow - 256kb/s would be expected from modern setups. But laser-based communication is viable, tested, and won't take too long to setup, and that's on the order of 500Mb/s. If multiple, exclusive paths are found it could get even higher.

Broadcasters would have intimate knowledge of the network and, at least at first, would be tightly controlled. As such congestion control is pretty easy to achieve - we know the state of "our side" of the entire network at any given time - just don't transmit if the network is full. We also avoid transmitting if the satellites are going to lose communiSo packets in the network aren't lost.

Forward error correction would be built in via something like LDPC. Unrecoverable errors would simply be dropped.