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generic routing encapsulation
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... Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic Routing Encapsulation (GRE)", RFC 1701, October 1994. ...
... Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic Routing Encapsulation (GRE) over IPv4 Networks", RFC 1702 ...