GRE. Объясните пожалуйста что это. [SOLVED]

Объясните что такое GRE.

Networking  --->
  Networking options  --->
    [*] TCP/IP networking
       <*>   IP: GRE tunnels over IP
          IP: broadcast GRE over IP

Зачем оно нужно?

И что это за сообщения:

Feb 29 17:40:08 sux pppd[29662]: Plugin radius.so loaded.
Feb 29 17:40:08 sux pppd[29662]: RADIUS plugin initialized.
Feb 29 17:40:08 sux pppd[29662]: pppd 2.4.4 started by root, uid 0
Feb 29 17:40:08 sux pppd[29662]: Using interface ppp0
Feb 29 17:40:08 sux pppd[29662]: Connect: ppp0 <--> /dev/pts/2
Feb 29 17:40:08 sux pptpd[29651]: GRE: Bad checksum from pppd.

Feb 29 17:40:06 sux pppd[29642]: Plugin radius.so loaded.
Feb 29 17:40:06 sux pppd[29642]: RADIUS plugin initialized.
Feb 29 17:40:06 sux pppd[29642]: pppd 2.4.4 started by root, uid 0
Feb 29 17:40:06 sux udev-net.sh: /etc/init.d/net.ppp0: does not exist or is not executable
Feb 29 17:40:06 sux pppd[29642]: Using interface ppp0
Feb 29 17:40:06 sux pppd[29642]: Connect: ppp0 <--> /dev/pts/2
Feb 29 17:40:06 sux pptpd[29569]: GRE: Bad checksum from pppd.
Feb 29 17:40:06 sux pptpd[29569]: GRE: xmit failed from decaps_hdlc: Operation not permitted
Feb 29 17:40:06 sux pptpd[29569]: CTRL: PTY read or GRE write failed (pty,gre)=(7,8)
Feb 29 17:40:06 sux pptpd[29569]: CTRL: Reaping child PPP[29642]

Те сообщения что про GRE. Это на сервере где pptpd стоит.