Syngress - Eleventh Hour Network+ Exam N10-004 Study Guide (11 ...

128 CHAPTER 8 Security Standards and Services
VPN Client
1 – VPN Client encapsulates packet in regular packet
Internet
2 – Regular packet arrives at VPN server
3 – VPN server de-encapsulates
packet and forwards it
FIGURE 8.2
A VPN in use
VPN server
Internal server
Layer 2 Tunneling Protocol (L2TP) is a VPN protocol which is tunneled over User Datagram
Protocol (UDP). Characteristics of the L2TP are as follows:
■ L2TP uses one data stream only, on UDP port 1701.
■ L2TP packets are divided between control and data by a flag in the header.
■ L2TP operates over UDP, so it has to implement its own acknowledgment and
retransmission mechanisms for the control messages it uses.
■ L2TP uses PPP to encapsulate data traffic that is sent across the tunnel and connections,
or “calls” are created and torn down over the implied circuit created by
the UDP traffic to port 1701 at the server.
■ The server responds to whatever port the client sent its UDP messages from – this
may be port 1701, but is generally a random port number.
■ L2TP’s main usability benefit comes in its use of a single pseudoconnection over a
protocol that is forwarded by most routers, UDP.
■ L2TP’s biggest security benefit comes from the use of a well-defined protocol –
Internet Protocol Security (IPSec).