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A tunnel mode SA is essentially an SA applied to an IP tunnel, with the access controls applied to the headers of the traffic inside the tunnel. Two hosts MAY establish a tunnel mode SA between themselves.
Aside from the two exceptions below, whenever either end of a security association is a security gateway, the SA MUST be tunnel mode. Thus, an SA between two security gateways is typically a tunnel mode SA, as is an SA between a host and a security gateway. The two exceptions are as follows.
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