It does not provide any encryption or confidentiality by itself. Rather, it relies on an encryption protocol that it passes within the tunnel to provide privacy. L2TP does not provide confidentiality or strong authentication by itself. IPsec is often used to secure L2TP packets by providing confidentiality, authentication and integrity. The LNS waits for new tunnels. Once a tunnel is established, the network traffic between the peers is bidirectional.
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PPP defines an encapsulation mechanism for transporting multiprotocol packets across layer 2 L2 point-to-point links. In such a configuration, the L2 termination point and PPP session endpoint reside on the same physical device i. With L2TP, a user has an L2 connection to an access concentrator e. This allows the actual processing of PPP packets to be divorced from the termination of the L2 circuit.
Level 2 tunneling protocols have seen wide use in both small and large networks. T, Message Type. Specifies if this is a data or control message. L, Length present. Control messages MUST have this bit set. S, Sequence present. If set, the Ns and Nr fields are present. O, Offset present. If set, the Offset Size field is present.
Control messages MUST have this bit cleared to zero. P, Priority. If set, this data message should receive preferential treatment in its local queuing and transmission. LCP echo requests used as a keepalive for the link, for instance, should generally be sent with this bit set. Without it, a temporary interval of local congestion could result in interference with keepalive messages and unnecessary loss of the link. This feature is only for use with data messages. Indicates the L2TP protocol version.
The value 1 is reserved to permit detection of L2F packets should they arrive intermixed with L2TP packets.
Packets received with an unknown value MUST be discarded. Total length of the message in bytes. This field exists only if the L bit is set. Tunnel ID. Indicates the identifier for the control connection. L2TP tunnels are named by identifiers that have local significance only. That is, the same tunnel will be given different Tunnel IDs by each end of the tunnel. Tunnel ID in each message is that of the intended recipient, not the sender.
Session ID. Indicates the identifier for a session within a tunnel. L2TP sessions are named by identifiers that have local significance only. That is, the same session will be given different Session IDs by each end of the session. Session ID in each message is that of the intended recipient, not the sender. Ns, sequence number. Nr, sequence number expected. Indicates the sequence number expected in the next control message to be received.
Offset Size. Specifies the number of bytes past the L2TP header at which the payload data is expected to start. Actual data within the offset padding is undefined. If the offset field is present, the L2TP header ends after the last byte of the offset padding. This field exists if the O bit is set.
RFC The variable length concatenation of a unique Attribute represented by an integer and a Value containing the actual value identified by the attribute. Multiple AVPs make up Control Messages which are used in the establishment, maintenance, and teardown of tunnels. M, Mandatory. If the M bit is set on an unrecognized AVP within a message associated with a particular session, the session associated with this message MUST be terminated. If the M bit is set on an unrecognized AVP within a message associated with the overall tunnel, the entire tunnel and all sessions within MUST be terminated.
The control message must then continue to be processed as if the AVP had not been present. H, Hidden. This capability can be used to avoid the passing of sensitive data, such as user passwords, as cleartext in an AVP. AVP Length. The number of bytes contained in this AVP. If the length is set to 6, then the AVP Value field is absent. Any vendor wishing to implement their own L2TP extensions can use their own Vendor ID along with private Attribute values, guaranteeing that they will not collide with any other vendor's extensions, nor with future IETF extensions.
RFC A device attached to the switched network fabric e. It may tunnel any protocol carried within PPP. L2TP is connection-oriented. ZLB messages are used for explicitly acknowledging packets on the reliable control channel. Working groups :.
Supported L2TP Standards
Layer 2 Tunneling Protocol