From owner-freebsd-net@FreeBSD.ORG  Wed Jun  8 12:20:54 2005
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Date: Wed, 8 Jun 2005 15:20:51 +0300
From: Giorgos Keramidas <keramida@FreeBSD.org>
To: Jeremie Le Hen <jeremie@le-hen.org>, freebsd-net@FreeBSD.org
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Subject: Re: Problems with gif tunnels
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On 2005-06-08 12:40, Jeremie Le Hen <jeremie@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