what's the gre mode IPTunnel?

[dream_seeker <dream_seeker@sina.com>]

hello,I'm a newcomer to IPv6.
I want to know what's the gre mode IPTunnel.and I find a interface sit0
on my linux,what' this ?

[root@dream_seeker include]# ifconfig sit0
sit0      Link encap:IPv6-in-IPv4
          NOARP  MTU:1480  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0                                                  

thanks!
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