29Dec/090
l2tp: layer 2 tunneling protocol
In computer networking, Layer 2 Tunneling Protocol (L2TP) is a tunneling protocol used to support virtual private networks (VPNs). It does not provide any encryption or confidentiality by itself; it relies on an encryption protocol that it passes within the tunnel to provide privacy.[1]
Although L2TP acts like a Data Link Layer protocol in the OSI model, L2TP is in fact a Session Layer protocol,[2] and uses the registered UDP port 1701. (see List of TCP and UDP port numbers).
(shamelessly copied from wikipedia)
Additional Reading:
http://en.wikipedia.org/wiki/Layer_2_Tunneling_Protocol
http://www.cisco.com/en/US/docs/ios/12_0t/12_0t1/feature/guide/l2tpT.html
http://www.cisco.com/en/US/tech/tk801/tk703/technologies_tech_note09186a0080094c4f.shtml