1. gre protocol
In the most general case, a system has a packet that needs to be encapsulated and delivered to some destination. We will call this the payload packet. The payload is first encapsulated in a GRE packet. The resulting GRE packet can then be encapsulated in some other protocol and then forwarded. We will call this outer protocol the delivery protocol.
base header
struct gre_base_hdr {
__be16 flags;
__be16 protocol;
};
flags:
flags bit 0: with checksum
flags bit 1: with key
flags bit 2: with sequence
protocol: the protocol type of higher level packet
