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Minitel - Layer 5

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Node: a device on a minitel network
Client: node establishing a connection
Server: node accepting a connection

Establishing a connection

Client sends "openstream" to the server on the port it would like to connect to
Server can refuse, and respond with anything but a number
Server, upon accepting, sends a string representation of another port, chosen by the server, on the same port, to the client
Server sends a random string of its chosing to the client, which will be used to close the session
Client and server can now exchange messages

Ending a connection

Upon either side deciding to close, it sends the previously generated random string
Neither side should send packets now

Example exchange:

Acknowledgements are implied.

Client bob sends request to open a connection on port 40:
"packet ID 1", 1, "alice", "bob", 40, "openstream"

Server alice accepts and sends a port to client bob for further communications:
"packet ID 3", 1, "bob", "alice", 40, "32800"

Server alice then sends a string on that port as a close session marker:
"packet ID 4", 1, "bob", "alice", 32800, "asdfghjkl"

Nodes alice and bob can now freely exchange data on that port: "packet ID 5", 1, "alice", "bob", 32800, "random data and stuff"
"packet ID 6", 1, "bob", "alice", 32800, "response data"

Client bob decides to close the connection, and sends the close session marker:
"packet ID 7", 1, "alice", "bob", 32800, "asdfghjkl"