Which extensions to GRE tunneling provide session tracking and in-order packet delivery in exchange for additional state stored in tunnel endpoints?

RFC 2890 defines the optional GRE Key field to allow a sender to tag packets with a session identifier, enabling the receiver to multiplex and demultiplex multiple logical sessions over a single tunnel. The optional Sequence Number field allows the receiving endpoint to detect out-of-order delivery and reorder packets, providing in-order delivery guarantees. Both features require endpoints to store additional state - the key value for session identity and the last received sequence number for ordering.

The Protocol Type field identifies the encapsulated payload's network-layer protocol, and the Checksum field provides header integrity verification - neither enables session tracking or packet ordering.

The Version and Reserved0 fields are fixed control fields in the standard GRE header and carry no session identity or sequence information.

RFC 2890 explicitly defines the Key and Sequence Number optional extension fields in the GRE header for session tracking and in-order delivery.