http://en.wikipedia.org/wiki/Layer_2_Tunneling_Protocol

http://www.networksorcery.com/enp/protocol/l2tp.htm

http://www.faqs.org/rfcs/rfc2661.html

http://www.networkdictionary.com/protocols/l2tp.php

http://www.cisco.com/warp/public/cc/pd/iosw/tech/l2pro_tc.htm

http://www.cisco.com/warp/public/cc/pd/iosw/prodlit/l2tun_ds.htm

http://www.ietf.org/rfc/rfc2661.txt

http://www.javvin.com/protocolL2TP.html

http://www.linktionary.com/l/l2tp.html

http://www.answers.com/topic/layer-2-tunneling-protocol

http://www.ietf.org/rfc/rfc3193.txt

http://www.faqs.org/rfcs/rfc3355.html