Dynamic port allocation is not totally dynamic, however the basis of how this is done is described below.
For the sake of this discussion our Host system will be the RT system and the Client will be your Windows system, however this discussion really applies to any architecture.
1. The Host creates a listener on a defined port and waits. The port# is explicitly defined in the TCP Create listener function, for sake of discussion lets say port 2020.
2. The client opens a TCP connection to the host on port 2020.
3. Upon detecting the new connection, the Host will run TCP Create listener again with no port number wired in (this will cause the port number to be dynamically assigned)
4. This new port number is then transmitted from the Host to client over the socket connection on the known port (2020).
5. Upon receipt of the dynamic port number transmitted over 2020, the client will close the connection on port 2020 above and open a new socket connection to the port number transmitted in #4.
6. When the host sees the client close the socket on the port 2020, it will also close the connection and run the create listener function again on port 2020 (the Host is now ready for another dynamic connection)
7. All further communication between the Host and client is now carried out over the dynamicly assigned socket connection.
The advantage to this approach is that the defined port connection (2020)is only open for a very short period of time while the dynamic port # is obtained and transmitted. After that, all communication is on the dynamically assigned port socket connection. This will preclude a loss of the ability to connect to the system if the socket connection is lost and goes into a TIME-WAIT state. Once in TIME-WAIT you will be unable to reconnect to that port for 2X the segment lifetime (about 2 minutes).
This approach is especially important when dealing with RT systems as VxWorks and PharLap will lock out the port as described above if the connection is broken either by a loss of network communications or if the Host closes the socket first. You can take a look at RFC-793 for a detailed explanation of how the connection is made and broken.
Windows systems implement a feature that allows a connection to transition directly from the TIME-WAIT state back to active so the need for the dynamic port assignment on a Windows host is really not required.
Another feature of the dynamic port assignment is that your host will then have the capability to have multiple clients connecting to it simultaneously.
Greg Sussman
Sr Business Manager A/D/G BU
