• Networking Basics: Short explanation of networking basics like TCP, UDP and ports.
• Java IO Tutorial: It's absolutely necessary to be familiar with I/O and streams to do sockets operations!
• Java TCP Sockets Tutorial: A very useful introduction to (TCP) sockets programming.
• Java Datagrams Tutorial: Provides all the information you need to send and receive datagram packets using UDP.
• Java Concurrency Tutorial: A tutorial about concurrency that covers threads, thread-pools and synchronization.
• Java Programmierhandbuch und Referenz - Section 13.2.3: German tutorial for using datagram sockets. (optional)

Important Java-API Classes:

• IO: IO Package API
• Concurrency: Thread API, Runnable API, ExecutorService API, Executors API
• Java TCP Sockets: ServerSocket API, Socket API
• Java Datagrams: DatagramSocket API, DatagramPacket API

2. Learning RMI

• Java RMI Tutorial: A short introduction into RMI.
• JGuru RMI Tutorial: A more detailed tutorial about RMI.
• JavaInsel RMI Tutorial: German introduction into RMI programming. (optional)

Important Java-API Classes:

• RMI: Remote API, UnicastRemoteObject API, Registry API, LocateRegistry API
• Properties: Properties API
• IO: IO Package API

3. Security

• Chapter 9 - Security from the book Distributed Systems: Principles and Paradigms (2nd edition)
• Java Cryptography Architecture (JCA) Reference Guide: Tutorial about the Java Cryptography Architecture (JCA)
• Handbook of Applied Cryptography (free version) (optional)

Important Java-API Classes:

• Java SE 6: Cipher, SecureRandom, and Signature classes
• Bouncy Castle: PEMReader and Base64