Layer 2 Forwarding Protocol

L2F, or Layer 2 Forwarding, is a tunneling protocol developed by Cisco Systems, Inc. to establish virtual private network connections over the Internet. L2F does not provide encryption or confidentiality by itself; It relies on the protocol being tunneled to provide privacy. L2F was specifically designed to tunnel Point-to-Point Protocol (PPP) traffic.^[1]

Use

Virtual dial-up allows many separate and autonomous protocol domains to share common access infrastructure including modems, Access Servers, and ISDN routers. RFCs prior to 2341 have specified protocols for supporting IP dial-up via SLIP and multiprotocol dial-up via PPP.

L2F packet structure

Bits 0-12													13-15	16-23	24-31
F	K	P	S	0	0	0	0	0	0	0	0	C	Ver	Protocol	Sequence (opt)
Multiplex ID														Client ID
Length														Payload offset (opt)
Packet key (optional)
Payload
														L2F Checksum (opt)

Other VPN protocols

References

↑ RFC2341 on IETF.org

External links

Virtual private networking

Communications protocol	SSTP IPsec L2TP L2TPv3 PPTP Split tunneling SSL/TLS (Opportunistic: tcpcrypt)

Free software	Cloudvpn FreeS/WAN Libreswan n2n OpenConnect OpenIKED Openswan OpenVPN Social VPN SoftEther VPN strongSwan tcpcrypt tinc VTun

Vendor-driven protocols	Layer 2 Forwarding Protocol DirectAccess

Proprietary software	Check Point VPN-1 Cisco Systems VPN Client Hamachi Microsoft Forefront Unified Access Gateway

Risk Vectors	Content-control software Deep content inspection Deep packet inspection IP address blocking Network enumeration Stateful firewall TCP Reset VPN blocking

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