Rui CarmoLayer 2 Tunneling Protocol (despite the "layer 2" bit, it's basically IP-on-IP in most instances).
Resources:
- l2tpd (SourceForge project)
- Roaring Penguin l2tp (more recent, can act as LAC or LNS, has been ported to OpenWRT)
Layer 2 Tunneling Protocol (despite the "layer 2" bit, it's basically IP-on-IP in most instances).