Actually just a few minutes ago, I found the source code of the package sun.xxx at the following link.
http://java.sun.com/j2se/jrl_download.html
To be more specific, if you download the zip file and unzip to a directory, sun.rmi.transport.tcp.TCPTransport is located at:
j2se/src/share/classes/sun/rmi/transport/tcp/TCPTransport.java
Thanks
SCJP, SCJD, SCWCD, SCBCD, SCEA