L2TP
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In computer networking, the Layer 2 Tunneling Protocol (L2TP) is a tunneling protocol used to support virtual private networks (VPNs). L2TP can crudely be described as "PPP over IP", although it has many more features than this simple description would imply.
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History and future
Published in 1999 as a proposed standard, L2TP has its origins primarily in two older tunneling protocols: Cisco's Layer 2 Forwarding (L2F) and Microsoft's Point-to-Point Tunneling Protocol (PPTP). A new version of this protocol, L2TPv3, is currently in draft form. In addition to the L2TP features, it is engineered to be a possible alternative to Multiprotocol Label Switching (MPLS).
Description
L2TP acts as a data link layer (layer 2 of the OSI model) protocol for tunneling network traffic between two peers over an existing network (usually the Internet). It is an extension of the Point-to-Point Protocol (PPP). It is still common to use PPP sessions within an L2TP tunnel. L2TP does not provide confidentiality or strong authentication itself. IPsec is often used to tunnel L2TP packets and provide confidentiality and authentication. The combination of these two protocols is generally known as L2TP/IPsec. It is standardized in RFC3193 [1] (http://www.ietf.org/rfc/rfc3193.txt).
The two endpoints of an L2TP tunnel are called the LAC (L2TP Access Concentrator) and the LNS (L2TP Network Server). The LAC is the initiator of the tunnel while the LNS is the server, which waits for new tunnels. Once a tunnel is established, the network traffic between the peers is bidirectional. To be useful for computing, higher-level protocols are then run through the L2TP tunnel. To facilite this an L2TP session (or call) is established within the tunnel for each higher-level protocol such as PPP. Either the LAC or LNS may initiate sessions. The traffic for each session is isolated by L2TP, so it is possible to set up multiple virtual networks across a single tunnel.
The packets exchanged within an L2TP tunnel are either categorised as control packets or data packets. L2TP provides reliability features for the control packets, but no reliability for data packets. Reliability, if desired, must be provided by the nested protocols running within each session of the L2TP tunnel.
Cisco claims to have a software patent related to L2F and L2TP [2] (http://www.ietf.org/ietf/IPR/CISCO-L2TP). It has been assigned patent number 5,918,019 in the United States. [3] (http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=5,918,019.WKU.&OS=PN/5,918,019&RS=PN/5,918,019)
External links
Implementations
- Cisco: Cisco L2TP documentations (http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/120newft/120t/120t1/l2tpt.htm#wp5939), also read Technology brief from Cisco (http://www.cisco.com/warp/public/cc/pd/iosw/tech/l2pro_tc.htm)
- Open source and Linux: L2TPD project (http://sourceforge.net/projects/l2tpd/), Linux RP-L2TP project (http://sourceforge.net/projects/rp-l2tp/), OpenL2TP project (http://sourceforge.net/projects/openl2tp/), L2tpns project (http://l2tpns.sourceforge.net/)
- Microsoft: Microsoft L2TP/IPSec VPN Client (http://www.microsoft.com/windows2000/server/evaluation/news/bulletins/l2tpclient.asp)
Internet standards and extensions
- RFC 2341 Cisco Layer Two Forwarding (Protocol) "L2F". (A predecessor to L2TP)
- RFC 2637 Point-to-Point Tunneling Protocol (PPTP). (A predecessor to L2TP)
- RFC 2661 Layer Two Tunneling Protocol "L2TP"
- RFC 2809 Implementation of L2TP Compulsory Tunneling via RADIUS
- RFC 2888 Secure Remote Access with L2TP
- RFC 3070 Layer Two Tunneling Protocol (L2TP) over Frame Relay
- RFC 3145 L2TP Disconnect Cause Information
- RFC 3193 Securing L2TP using IPsec
- RFC 3301 Layer Two Tunnelling Protocol (L2TP): ATM access network
- RFC 3308 Layer Two Tunneling Protocol (L2TP) Differentiated Services
- RFC 3355 Layer Two Tunnelling Protocol (L2TP) Over ATM Adaptation Layer 5 (AAL5)
- RFC 3371 Layer Two Tunneling Protocol "L2TP" Management Information Base
- RFC 3437 Layer Two Tunneling Protocol Extensions for PPP Link Control Protocol Negotiation
- RFC 3438 Layer Two Tunneling Protocol (L2TP) Internet Assigned Numbers: Internet Assigned Numbers Authority (IANA) Considerations Update
- RFC 3573 Signaling of Modem-On-Hold status in Layer 2 Tunneling Protocol (L2TP)
- RFC 3817 Layer 2 Tunneling Protocol (L2TP) Active Discovery Relay for PPP over Ethernet (PPPoE)
- IANA assigned numbers for L2TP (http://www.iana.org/assignments/l2tp-parameters)
- L2TP Extensions Working Group (l2tpext) (http://www.ietf.org/html.charters/l2tpext-charter.html) - (where future standardization work is being coordinated)fr:Layer 2 Tunnelling Protocol