Software packages

• PPP is handled by pppd
• L2TP is handled by l2tpd (www.l2tpd.org)
• IPsec is handled by Linux 2.6 and openswan as the ISAKMP daemon (www.openswan.org)
• Alternatively, you can use racoon as the ISAKMP daemon (www.kame.net/racoon/)
• Certificates can be issued with openssl

Of course, all of the dependencies of these packages have to be met. ISAKMP is necessary to communicate with the kernel, whereas various implementations of l2tp daemons exist. Certificates do not necessarily need to be issued by openssl, and there might be an alternative to pppd. I chose the most supported software packages in this case.