RFC 4891:Using IPsec to Secure IPv6-in-IPv4 Tunnel...
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L2TP


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... [RFC4023] and Layer 2 Tunneling Protocol (L2TP) [RFC3193].) ...


... Transport mode has typically been applied to L2TP, GRE, and other tunneling ...


... Patel, B., Aboba, B., Dixon, W., Zorn, G., and S. Booth, "Securing L2TP using IPsec", RFC 3193prop, November 2001. ...



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