Problems with gif tunnels

[Giorgos Keramidas]

On 2005-06-08 12:40, Jeremie Le Hen <jeremie at le-hen.org> wrote:
>
> IIRC,
>     - Linux uses the ipip module to do IP-over-IP tunnel
>     - FreeBSD uses the gre(4) interface to do GRE tunnels

>     - GRE is a Cisco product and means ``Generic Routing
>       Encapsulation''.  I don't know what they mean with the term
>       "Generic" because I have only seen IP encapsulated tunnel so
>       far.

It's not mandatory to tunnel IP over GRE.  Other, non IPv4 protocols can
be encapsulated in the GRE payload.

>       According to the GRE header, I guess GRE is far more powerful
>       than a simple IP-over-IP encapsulation, and I would be glad if
>       someone could explain us what are the benefits of this protocol.

GRE is not a simple encapsulation of an IP packet as the payload of an
IP packet.  It also includes fields in the outter IP header that may
support, among other things:

	- Source routing
	- A crude form of authentication (key & sequence number pair)
	- Separation of tunelled "flows" without the payload inspection
	  IP-IP tunnels may require