Which field of the IP header is modified on a payload packet when using GRE?
A.
time to live
B.
source address
C.
protocol
D.
fragment offset
Explanation:
GRE: Generic Routing EncapsulationGeneric Routing Encapsulation is a protocol for encapsulation of an arbitrary network layer protocol over another arbitrary network layer protocol.
In the most general case, a system has a packet that needs to be encapsulated and delivered to some destination, which is called payload . The payload is first encapsulated in a GRE packet. The resulting GRE packet can then be encapsulated in some other protocol and then forwarded. This outer protocol is called the delivery protocol.
When IPv4 is being carried as the GRE payload, the Protocol Type field MUST be set to 0x800. When a tunnel endpoint decapsulates a GRE packet which has an IPv4 packet as the payload, the destination address in the IPv4 payload packet header must be used to forward the packet and the TTL of the payload packet MUST be decremented. Care should be taken when forwarding such a packet, since if the destination address of the payload packet is the encapsulator of the packet (i.e., the other end of the tunnel), looping can occur. In this case, the packet must be discarded. The IPv4 protocol 47 [is used when GRE packets are encapsulated in IPv4.