According to the CISSP CBK Official Study Guide1, the protocol that adds end-to-end security inside a Layer
2 Tunneling Protocol (L2TP) Internet Protocol Security (IPSec) connection is Transport Layer Security (TLS).
L2TP is a tunneling protocol that is used or applied to create or establish a virtual private network (VPN) connection between two or more systems or networks, by using or applying the Layer 2 or the data-link layer of the Open System Interconnection (OSI) Reference Model, which is the layer that defines the logical or the intangible aspects of the communication or the networking system, such as the frames, addresses, or protocols that organize or control the data or the information. IPSec is a security protocol that is used or applied to secure or protect the VPN connection between two or more systems or networks, by using or applying the Layer 3 or the network layer of the OSI Reference Model, which is the layer that defines the routing or the forwarding aspects of the communication or the networking system, such as the packets, addresses, or protocols that determine or direct the path or the route of the data or the information. IPSec helps to ensure the security or the integrity of the VPN connection between two or more systems or networks, by providing or supporting the confidentiality, integrity, authentication, or non-repudiation of the data or the information that passes through the VPN connection, by using or applying the appropriate methods or mechanisms, such as the encryption, hashing, digital signature, or key exchange of the data or the information. TLS is a security protocol that is used or applied to secure or protect the application or the service that runs or operates inside the VPN connection between two or more systems or networks, by using or applying the Layer 4 or the transport layer of the OSI Reference Model, which is the layer that defines the reliability or the quality aspects of the communication or the networking system, such as the segments, ports, or protocols that ensure or verify the delivery or the transmission of the data or the information. TLS helps to ensure the security or the integrity of the application or the service that runs or operates inside the VPN connection between two or more systems or networks, by providing or supporting the confidentiality, integrity, authentication, or non-repudiation of the data or the information that passes through the application or the service, by using or applying the appropriate methods or mechanisms, such as the encryption, hashing, digital signature, or key exchange of the data or the information. TLS adds end-to-end security inside the L2TP IPSec connection, as it provides or supports an additional layer or level of security or protection to the data or the information that passes through the VPN connection between two or more systems or networks, as well as the application or the service that runs or operates inside the VPN connection between two or more systems or networks, by securing or protecting the data or the information from the source or the origin to the destination or the end of the VPN connection, as well as the application or the service, regardless or irrespective of the intermediate or the intermediate systems or networks that are involved or included in the VPN connection, as well as the application or the service.
TKIP is not the protocol that adds end-to-end security inside the L2TP IPSec connection, although it may be a benefit or a consequence of using TLS.