RFC Layer Two Tunneling Protocol “L2TP” (Q). request for comments publication. RFC; Layer Two Tunneling Protocol. Layer Two Tunneling Protocol “L2TP”, August Canonical URL: https://www. ; File formats: Plain Text PDF; Status: PROPOSED. In computer networking, Layer 2 Tunneling Protocol (L2TP) is a tunneling protocol used to Published in as proposed standard RFC , L2TP has its origins primarily in two older tunneling protocols for point-to-point communication.

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L2TP tunnels are named by identifiers that have local significance only. Control messages MUST have this bit cleared to zero. Published in as proposed standard RFCL2TP has its origins primarily in two older tunneling protocols for point-to-point communication: RFC A device attached to the switched network fabric e.

Session ID in each message is that of the rdc recipient, not the sender. To facilitate this, an L2TP session or ‘ call’ is established within the tunnel for each higher-level protocol such as PPP. Nr, sequence number expected. The term tunnel-mode refers to a channel which allows untouched packets of one network to be transported over another network. Indicates the sequence number expected in the next control message to be received.

One of the aforementioned difficulties is that it is not compatible with NAT.


LCP echo requests used as a keepalive for the link, for instance, should generally be sent with 22661 bit set. Layer 2 Forwarding Protocol DirectAccess.

Supported L2TP Standards

If the length is set to 6, then the AVP Value field is absent. That is, the same session will be given different Session IDs by each end of the session. Since the L2TP packet itself is wrapped and hidden within the IPsec packet, the original source and destination IP address is encrypted within the packet. Once a tunnel is established, the network traffic between the peers is bidirectional. Internet protocols Internet Standards Tunneling protocols Virtual private networks.

Supported L2TP Standards – TechLibrary – Juniper Networks

Then using this tunnel and session id, data packets are exchanged with the compressed PPP frames as payload. With L2TP, a user has an L2 connection to an access concentrator e.

If the M bit is set on an unrecognized AVP within a message associated with the overall tunnel, the entire tunnel and all l2fp within MUST be terminated. ZLB messages are used for explicitly acknowledging packets on the reliable control channel. If set, this data message should receive preferential treatment in its local queuing and transmission.

ATM access network extensions. Another problem rf that servers must be specified only by IP address in the new Vista configuration utilities; the hostname of the server cannot be used, so if the IP address of the IPsec server changes, all clients will have to be informed of this new IP address which also rules out servers that addressed rfcc utilities such as DynDNS. Windows Vista provides two new configuration utilities that attempt to make using L2TP without IPsec easier, both described in sections that follow below:.


Tunnel ID in each message is that of the intended recipient, not the sender.

Specifies the number of bytes past the L2TP header at which the payload data is expected to start. From Wikipedia, the ,2tp encyclopedia. The LNS waits for new tunnels. From the end user, packets travel over a wholesale network service provider’s network to a server called a Broadband Remote Access Server BRASa protocol converter and router combined.

If set, the Offset Size field is present.

It may tunnel any protocol carried within PPP. 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 l2yp IETF extensions. Views Read Edit View history. The number of bytes contained in this AVP.

IPsec is often used to secure L2TP packets by providing confidentiality, authentication and integrity.