L2TP
acronymnetworking
Layer Two Tunneling Protocol: carries PPP frames across an IP network, providing no encryption of its own, which is why it is almost always paired with IPsec.
It came from merging two earlier proposals, Cisco's L2F and Microsoft's PPTP, and its job is narrow: take the point-to-point session a dial-up user would have had with an access server and carry it over an IP network instead, so the user appears to terminate where the network wants rather than where the modem was. On its own it authenticates the tunnel and encrypts nothing, which is the detail practitioners most often get wrong. The pairing written as L2TP/IPsec puts the whole L2TP session inside an IPsec transport-mode association, so IPsec supplies confidentiality and integrity and L2TP supplies the PPP semantics - user authentication, address assignment, multiprotocol carriage. It persists mainly because it is built into consumer operating systems and home routers, which makes it the fallback that works without installing anything, and it is a poor choice where a modern option exists.
Also known as: l2tp, layer 2 tunneling protocol, l2tp/ipsec