VPN PLUS GSM PLUS
Nr is set to the Ns of the last in-order message received plus one (modulo 2 16). Nr (optional) sequence number for expected message to be received. Ns (optional) sequence number for this data or control message, beginning at zero and incrementing by one (modulo 2 16) for each message sent.
Session ID Indicates the identifier for a session within a tunnel. Tunnel ID Indicates the identifier for the control connection. Length (optional) Total length of the message in bytes, present only when length flag is set. This can be represented by four different tunneling models, namely:įlags and version control flags indicating data/control packet and presence of length, sequence, and offset fields. L2TP allows the creation of a virtual private dialup network (VPDN) to connect a remote client to its corporate network by using a shared infrastructure, which could be the Internet or a service provider's network.Īn L2TP tunnel can extend across an entire PPP session or only across one segment of a two-segment session. Reliability, if desired, must be provided by the nested protocols running within each session of the L2TP tunnel. L2TP provides reliability features for the control packets, but no reliability for data packets. The packets exchanged within an L2TP tunnel are categorized as either control packets or data packets. The traffic for each session is isolated by L2TP, so it is possible to set up multiple virtual networks across a single tunnel. Either the LAC or LNS may initiate sessions. To facilitate this, an L2TP session is established within the tunnel for each higher-level protocol such as PPP. To be useful for networking, higher-level protocols are then run through the L2TP tunnel. Once a tunnel is established, the network traffic between the peers is bidirectional. The two endpoints of an L2TP tunnel are called the L2TP access concentrator (LAC) and the L2TP network server (LNS). The combination of these two protocols is generally known as L2TP/IPsec (discussed below).
IPsec is often used to secure L2TP packets by providing confidentiality, authentication and integrity. L2TP does not provide confidentiality or strong authentication by itself. It is common to carry PPP sessions within an L2TP tunnel.
A virtue of transmission over UDP (rather than TCP) is that it avoids the "TCP meltdown problem". The entire L2TP packet, including payload and L2TP header, is sent within a User Datagram Protocol (UDP) datagram. L2TPv3 provides additional security features, improved encapsulation, and the ability to carry data links other than simply Point-to-Point Protocol (PPP) over an IP network (for example: Frame Relay, Ethernet, ATM, etc.). A new version of this protocol, L2TPv3, appeared as proposed standard RFC 3931 in 2005. Published in 2000 as proposed standard RFC 2661, L2TP has its origins primarily in two older tunneling protocols for point-to-point communication: Cisco's Layer 2 Forwarding Protocol (L2F) and Microsoft's Point-to-Point Tunneling Protocol (PPTP).
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