That can work. Or you can have A use TCP to B, C use TCP to D, and B and C are connected by a very short one-way cable, maybe something optical. The US governm

That can work. Or you can have A use TCP to B, C use TCP to D, and B and C are connected by a very short one-way cable, maybe something optical. The US government term of art for this pattern is a “guard”, often with a regex or manual filter.