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L2TP OVERVIEW
18-May-05
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Agenda VPN Tunneling PPTP L2F LT2P
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VPN Virtual Private Network is a private
network that uses a public network (usually the Internet) to connect remote sites or users together. Instead of using a dedicated connection such as leased line, a VPN uses “virtual” connections routed though the internet.
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Tunneling Tunneling is the transmission of
data intended for use only within a private, usually corporate network through a public network in such a way that the routing nodes in the public network are unaware that the transmission is part of a private network.
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Tunneling illustrated
Router A
Workstation X
Router BWorkstation
Y
Original IPpacket dest Y
Step 1.Original, unroutable
IP Packet sent to router
Step 2Original IP
packetencapsulatedin another IP
packetOriginal IP
packetNew IPPacket
Tunnel
Step 3Original packetextracted, sentto destination
Original IPpacket dest Y
Tunnel
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Types of Tunneling Two basic types of tunnels
Voluntary tunnels Tunneling initiated by the end-user
(Requires client software on remote computer) Compulsory tunnels
Tunnel is created by NAS or router(Tunneling support required on NAS or Router)
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Voluntary Tunnels
Dial IP Access
PPP access protocol
Dial Access Provider VPN Service
Dial AccessServer
PPTP AccessServerClient Host Serial Interface
PPTP Virtual Interface
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Voluntary Tunnels (Cont.) Will work with any network device
Tunneling transparent to leaf and intermediate devices
But user must have a tunneling client compatible with tunnel server
PPTP, L2TP, L2F, IPSEC, IP-IP, etc. Simultaneous access to Intranet (via tunnel)
and Internet possible Employees can use personal accounts for corporate
access Remote office applications
Dial-up VPN’s for low traffic volumes
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Compulsory Tunnels
L2TPV.x modem protocol
PPP access protocol
Dial Access Provider Internet or VPN Service
Non-routedforwarding path
Dial AccessServer
L2TP AccessServerClient Host
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Compulsory Tunnels (Cont.) Will work with any client But NAS must support same tunnel method
But… Tunneling transparent to intermediate routers Network access controlled by tunnel server
User traffic can only travel through tunnel Internet access possible
Must be by pre-defined facilities Greater control Can be monitored
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Compulsory Tunnels (Cont.) Static Tunnels
All calls from a given NAS/Router tunneled to a given server
Realm-based tunnels Each tunnel based on information in NAI
(I.e. user@realm) User-based tunnels
Calls tunneled based on userID data stored in authentication system
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PPTP Point-to-point tunneling protocol
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PPTP (Cont.)
PPP access by remote computers to a private network through the Internet
1. Remote user dials in to the local ISP network access server using PPP.
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PPTP (Cont.)
2. The PAC establishes a control channel (TCP) across the PPP connection and through the internet to the PNS attached to the home network.
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PPTP (Cont.)
3. Parameters for the PPTP channel are negotiated over the control channel, and the PPTP tunnel is established.
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PPTP (Cont.)
4. A second PPP connection is made from the remote user, through the PPTP tunnel between the PAC and the PNS, and into the private networks NAS.
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PPTP (Cont.)
5. IP datagrams or any other protocol’s datagrams are sent inside the PPP frames
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L2F Layer 2 Forwarding
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L2F
Tunnel is constructed from the service provider.1. Remote user dials in to the local ISP network
access server using PPP/SLIP.
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L2F (Cont.)
2. L2F builds a tunnel from the NAS to the private network.
Uses packet-oriented protocol that provides end-to-end connectivity, such as UDP, frame relay, etc. as the encapsulating protocol.
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L2F (Cont.)
3. L2F establishes PPP connection between NAS and home gateway.
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L2F (Cont.)
4. IP packets are sent over the PPP.
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L2TP Layer 2 Tunneling protocol
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L2TP