gre_hdr
nhoff + offsetof(struct gre_hdr, flags));
nhoff + offsetof(struct gre_hdr, proto));
nhoff + offsetof(struct gre_hdr, flags));
nhoff + offsetof(struct gre_hdr, proto));
.gre_hdr = {
struct gre_base_hdr gre_hdr;
struct gre_hdr *gre, _gre;
struct gre_hdr gre;
struct gre_hdr greh;
olen += sizeof(struct gre_hdr);