The value is set to 4. Most cases the protocol will be IPv4 unless there is some additional headers for encapsulated packet. Header Checksum: 16 bits This field is the IP checksum of outer header. Source IP Address: 32 bits This field is the IP address of the encapsulator  Destination IP Address: 32 bits This field is the IP address of the decapsulator  Options: Variable length This field in general is NOT copied from the inner IP header. New options can be added. Padding. Variable length. This field is used to fill the datagram so that IP Payload starts on a 32 bit boundary. Internet Control Message Protocol, 6in4, 4in6