1. Layer 2 Tunneling Protocol (L2TP)
An example of network layer VPN: use IP packets to encapsulate Layer 2 frames
Previous RFC (v2)
RFC2661 Layer Two Tunneling Protocol L2TP W. Townsley, A. Valencia, A. Rubens, G. Pall, G. Zorn, B. Palter. August 1999 (PROPOSED STANDARD)
A standard method for tunneling Point-to-Point Protocol (PPP) [RFC1661] sessions
Note: L2TP has since been adopted for tunneling a number of other L2 protocols (e.g., Ethernet, Frame Relay, etc).  L2TPv3 [RFC3931]
T. A. Yang
Network Security

2. Point-to-Point Protocol (PPP [RFC1661])
PPP defines an encapsulation mechanism for transporting multiprotocol packets across layer 2 (L2) point-to-point links.
PPP relies on the Link Control Protocol (LCP) for establishing, configuring, and testing the data-link connection.
It has a family of Network Control Protocols (NCPs) for establishing and configuring different network-layer protocols.
Typically, a user obtains a L2 connection to a Network Access Server (NAS) using one of a number of techniques (e.g., dialup POTS, ISDN, ADSL, etc.) and then runs PPP over that connection.
Example: A customer uses a dialup modem or a DSL line to connect to the ISP or the company’s modem pool.
Dial client (PPP peer)  PPP  NAS (e.g., ISP)
In such a configuration, the L2 termination point and PPP session endpoint reside on the same physical device (i.e., the NAS).
T. A. Yang
Network Security

3. Layer 2 Tunneling Protocol
Types of L2TP Tunnels
Compulsory L2TP Tunneling
The client is completely unaware of the presence of an L2TP connection.
The L2TP Access Concentrator (LAC) is aware of L2TP.
Figure 12-3: (client)  PPP + Data  (LAC)  L2TP + Data  (LNS)
LAC (L2TP Access Concentrator) & LNS (L2TP Network Server): The LAC is the initiator of the tunnel while the LNS is the server, which waits for new tunnels. (
T. A. Yang
Network Security

4. Layer 2 Tunneling Protocol
Types of L2TP Tunnels (cont.)
Voluntary L2TP Tunneling
The client is aware of the presence of an L2TP connection.
The LAC is unaware of L2TP.
Figure 12-4: (client)  PPP + L2TP + Data  (LAC)  L2TP + Data  (LNS)
LAC (L2TP Access Concentrator) & LNS (L2TP Network Server): The LAC is the initiator of the tunnel while the LNS is the server, which waits for new tunnels. (
T. A. Yang
Network Security

5. Layer 2 Tunneling Protocol (cont.)
L2TP
L2TP extends the PPP model by allowing the L2 and PPP endpoints to reside on different devices interconnected by a packet-switched network (PSN).
With L2TP, a user has an L2 connection to an L2TP access concentrator (LAC, e.g., modem bank, ADSL DSLAM, etc.), and the concentrator then tunnels individual PPP frames to the L2TP Network Server (LNS).
Dial client (PPP peer)  PPP  LAC  L2TP tunnel  LNS
This allows the actual processing of PPP packets to be separated from the termination of the L2 circuit.
T. A. Yang
Network Security

6. Layer 2 Tunneling Protocol (cont.)
A typical L2TP scenario (from RFC2661)
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Network Security

7. Layer 2 Tunneling Protocol (cont.)
RFC3931 Layer Two Tunneling Protocol - Version 3 (L2TPv3) J. Lau, Ed., M. Townsley, Ed., I. Goyret, Ed. March 2005 (PROPOSED STANDARD)
L2TPv3 defines the base control protocol and encapsulation for tunneling multiple Layer 2 connections between two IP nodes.
L2TPv3 consists of
the control protocol for dynamic creation, maintenance, and teardown of L2TP sessions, and
the L2TP data encapsulation to multiplex and demultiplex L2 data streams between two L2TP nodes across an IP network.
T. A. Yang
Network Security

8. Layer 2 Tunneling Protocol (cont.)
L2TP (according to TheFreeDictionary,
A protocol from the IETF that allows a PPP session to travel over multiple links and networks. (Note: a limitation of L2TPv2)
L2TP is used to allow remote users access to the corporate network.
PPP is used to encapsulate IP packets from the user's PC to the ISP, and L2TP extends that session across the Internet.
L2TP was derived from Microsoft's Point-to-Point Tunneling Protocol (PPTP) and Cisco's Layer 2 Forwarding (L2F) technology.
T. A. Yang
Network Security

9. Layer 2 Tunneling Protocol (cont.)
From Access Concentrator to Network Server
The "L2TP Access Concentrator" (LAC) encapsulates PPP frames with L2TP headers and sends them over the Internet as UDP packets (or over an ATM, frame relay or X.25 network).
At the other end, the "L2TP Network Server" (LNS) terminates the PPP session and hands the IP packets to the LAN. L2TP software can also be run in the user's PC.
Carriers also use L2TP to offer remote points of presence (POPs) to smaller ISPs. Users in remote locations dial into the carrier's local modem pool, and the carrier's LAC forwards L2TP traffic to the ISP's LNS.
user  original IP packet (p)  PPP+p  LAC  L2TP+PPP+p  LNS
L2TP and IPsec
L2TP does not include encryption (as does PPTP), but is often used with IPsec in order to provide virtual private network (VPN) connections from remote users to the corporate LAN.
T. A. Yang
Network Security

10. L2TP Operations Assumptions: Compulsory tunneling The Procedure:
The Client initiates a PPP connection to the LAC.
The LAC does LCP negotiation with the client, and challenges the client for authentication credentials.
The client supplies the credentials (such as user name, domain name, password).
The LAC uses the domain name to ascertain which LNS it needs to contact (in the case of multiple domains).
The LAC begins establishing an L2TP tunnel with the LNS.
Two Stages of L2TP Tunnel Setup:
Set up a control session between the LAC and the LNS.
Set up the actual L2TP tunnel for passing the data (aka. ‘creating the session’)
Notes:
Between a pair of LAC and LNS, there may exist multiple tunnels.
Across a single L2TP tunnel, there may exist multiple sessions.
* LCP negotiation:
Short for Link Control Protocol, a protocol that is part of the PPP. In PPP communications, both the sending and receiving devices send out LCP packets to determine specific information that will be required for the data transmission. The LCP checks the identity of the linked device and either accepts or rejects the peer device, determines the acceptable packet size for transmission, searches for errors in configuration and can terminate the link if the parameters are not satisfied. Data cannot be transmitted over the network until the LCP packet determines that the link is acceptable.
T. A. Yang
Network Security

11. L2TP Tunnel Setup (from RFC2661)
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Network Security

12. L2TP Operations Control Connection Establishment
Securing the peer’s identity, identifying the peer’s L2TP version, framing, etc.
LAC  SCCRQ (start-control-connection-request)  LNS
LAC  SCCRP (start-control-connection-reply  LNS
LAC  SCCN (start-control-connection-connected  LNS
LAC  ZLB ACK  LNS
The ZLB ACK is sent if there are no further messages waiting in queue for that peer.
Zero-Length Body Acknowledgement (ZLB ACK)
T. A. Yang
Network Security

13. L2TP Operations Session Establishment
A session may be created after successful control connection is established.
Each session corresponds to a single PPP stream between the LAC and the LNS.
Session establishment is directional:
Incoming call: The LAC asks the LNS to accept a session;
Outgoing call: The LNS asks the LAC to accept a session
Incoming Call Establishment:
LAC  ICRQ (Incoming-Call-Request)  LNS
LAC  ICRP (Incoming-Call-Reply  LNS
LAC  ICCN (Incoming-Call-Connected  LNS
LAC  ZLB ACK  LNS
The ZLB ACK is sent if there are no further messages waiting in queue for that peer.
T. A. Yang
Network Security

14. T. A. Yang Network Security
Challenge-Handshake Authentication Protocol (CHAP)
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Network Security

15. L2TP Message Header T. A. Yang Network Security
Ns (optional) sequence number for this data or control message
Nr (optional) sequence number for expected message to be received.
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Network Security

16. L2TP Control Messages (from RFC2661)
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Network Security

17. L2TP Authentication (from RFC2661)
Authentication, Authorization and Accounting may be provided by the Home LAN's Management Domain, which is behind the LNS.
In that case, the LAC performs proxy authentication, by passing authentication information back and forth between the user and the LNS.
Although deprecated, Password authentication protocol (PAP) is still sometimes used.
(source: Advantages of Proxy Authentication
In multi-tier environments, proxy authentication allows you to control the security of middle-tier applications by preserving client identities and privileges through all tiers, and auditing actions taken on behalf of clients. For example, this feature allows the identity of a user using a web application (also known as a "proxy") to be passed through the application to the database server.
T. A. Yang
Network Security

18. L2TP Operations Case Studies: Setting up compulsory L2TP Tunneling
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Network Security

19. L2TP Operations Case Studies (cont.)
Protecting L2TP Traffic using IPsec in a compulsory tunneling setup
NOTE: L2TP encapsulation occurs before IPSec processing.
T. A. Yang
Network Security

20. L2TPv3 Topology (from RFC3931)
L2TP operates between two L2TP Control Connection Endpoints (LCCEs), tunneling traffic across a packet network.
There are three predominant tunneling models in which L2TP operates:
LAC-LNS (or vice versa),
LAC-LAC, and
LNS-LNS.
T. A. Yang
Network Security

21. L2TPv3 Topology (from RFC3931)
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Network Security

22. L2TPv3 Topology (from RFC3931)
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Network Security

23. L2TPv3 Topology (from RFC3931)
T. A. Yang
Network Security