TCP and UDP are both communications protocols that is part of your TCP/IP stack (how your computer connects to the Net). In fact, TCP is a protocol on top of UDP.
UDP -- User Datagram Protocol -- handles passing packets back and forth between your computer and a remote computer. It is only concerned with ensuring the integrity of the packets it transfers; it has no provision for ensuring that all the packets reach the destination, or in the right order, or only once.
TCP -- Transmission Control Protocol -- is a reliable packet-control protocol that sits on top of UDP. TCP does this by attaching additional information to the packets, sending management packets, and re-ordering received -- if the network drops a packet on the floor, the sequence number for that packet will be missing, and TCP will request a retransmit of packets starting at that point.
If you have a bad connection, TCP will spend more time asking for and receiving retransmits than it does actually transferring data, which means that your transfer rate goes into the toilet. At a certain quality point, therefore, your FE switches from the more robust but higher-overhead TCP protocol back to UDP, which is less reliable but doesn't waste overhead with retransmits.
When you see the 'session lost TCP swtich to UDP' message, it means that your connection is tanking, and that you're entering the Warp Zone; because UDP is inherently unreliable, you may not get all the position updates for other aircraft, and may see truly incredible warpage.
And there's nothing you can do about it; the switch to UDP is a one-way transition. All you can do is to, when you're shot down or land, quit your login session with the server and go back online again -- which starts up a new connection via TCP.
