GRE, Generic Routing EncapsulationC, Checksum Present. 1 bit. The Checksum field is present and contains valid information if set. If either the Checksum Present bit or the Routing Present bit are set, the Checksum and Offset fields are both present. R, Routing Present. 1 bit. If set then the Offset field is present and contains valid information. If either the Checksum Present bit or the Routing Present bit are set, the Checksum
IP in IP encapsulation
Minimal Encapsulation Protocol
ICMP type 3, Destination unreachable message
