L2TP

L2TP tunnels layer 2 (PPP) frames between LAC and LNS:

  Successor to Cisco L2F (SLIP, PPP)
  Reliable control, unreliable data
  Relatively lightweight for LAC
  Commonly used for:
     Wholesale dialup/ADSL
     L2TP/IPSEC VPN
  Implementations:  RP-L2TP, OpenL2TP, L2TPD

Data packets carrying PPPIP frames are trivial:

L2TP Data Packet