lecture16: tree-like networks
http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman/cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm
http://www.javvin.com/protocolSTP.html
http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci214602,00.html
http://www.ieee802.org/1/pages/802.1D-2003.html
http://standards.ieee.org/getieee802/download/802.1D-2004.pdf
http://www.ics.uci.edu/~eppstein/161/960206.html
http://www.csse.monash.edu.au/~lloyd/tildeAlgDS/Graph/Undirected/
http://www-b2.is.tokushima-u.ac.jp/~ikeda/suuri/kruskal/Kruskal.shtml
http://portal.acm.org/citation.cfm?id=52357
http://www.cs.sunysb.edu/~algorith/files/minimum-spanning-tree.shtml