How to simulate a direct network connection between 2 pc's over the internet?

Question

I have read about the topic on the wiki articles of Ethernet, OSI model, TCP, UDP, Routing (no links due to new user restrictions) and others, but was not able to get my specific problem answered.

Situation: 4 computers (A-D)

They are connected to each other like this, using ethernet: A <-LAN-> B <-Internet-> C <-LAN-> D (only link instead of picture due to new user restrictions, sorry again)

What i try to accomplish: Computer A should be fooled to think its directly connected to computer D(and vice-versa). Software to accomplish that should only be run on computer B and C.

What is my problem: I cannot really tell what would have to be done in detail to accomplish my goal. Is it at all possible to let A and D think they are connected to each other, by using software that runs on a modern operating System on computers B and C?

What i do understand is that - if i were to really simulate that connection- i would have to transfer the information on the physical layer. How much can be done from the Application layer of the operating system on B/C to accomplish the same? Is it at all possible?

What i could do from my point of knowledge already: Having a client-software on A that sends datagrams or a stream of bytes to B (knowing that B will then send it over the internet to C, which then in turn will send the data to D). Same goes for the direction D to A.

My Problem with that solution: That solution is flawed in the sense that i would need 2 software solutions (one for A/D, one for B/C), plus, it restricts the connection to the TCP and UDP protocols, also it does not really simulate a connection - it is only a workaround to route information from A to D, not the simulation of a peer-to-peer connection.

Which leads me to the question from the title:

How to simulate a direct network connection between two pc's over the internet?

I am also open to any suggestions i might have completely overlooked, or partial solutions and suggestions.

Answer 1

It seems to me that you could use some type of software VPN. I've used LogMeIn Hamachi (which is free) to accomplish this in the past.

For this, the key is going to be to set up a "direct" vpn between B and C, otherwise the Hamachi is pretty slow because it goes through other servers to route your network traffic.

https://secure.logmein.com/products/hamachi/download.aspx