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# Server side: open a socket on a port, listen for
# a message from a client, and send an echo reply; 
# this is a simple one-shot listen/reply per client, 
# but it goes into an infinite loop to listen for 
# more clients as long as this server script runs; 
# notes:
# 1) a host name of '' means the machine that this 
#    server script runs on (any Internet interface);
# 2) servers bind to port numbers, which can be 0-65535, 
#    but ports 0-1023 are reserved for common protocols
#    (ftp, http, smtp, pop, telnet, etc.); clients 
#    connect to both a server name and port to talk; 
# 3) python supports both tcp and udp (connectionless,
#    SOCK_DGRAM) sockets, as well as broadcast mode;
# 4) accept() waits for client; setblocking(0) sets 
#    nonblocking mode for send/recv ops (raises error);
# 5) data is transmitted as strings, but see also the 
#    network byte order converters in the socket module;
# 6) sockobj.makefile() wraps a socket in a file object
# 7) had to ctrl-alt-del to kill the servers on ms-dos,
#    but a simple ctrl-c kills it on the linux port;
# 8) most internet protocols (e.g., ftp, pop mail) use
#    sockets too, but Python libs hide the socket layer;
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