问ip_hdr和ipip_hdr有什么区别？

EN

23 static inline struct iphdr *ip_hdr(const struct sk_buff *skb)
24 {
25         return (struct iphdr *)skb_network_header(skb);
26 }
27 
28 static inline struct iphdr *inner_ip_hdr(const struct sk_buff *skb)
29 {
30         return (struct iphdr *)skb_inner_network_header(skb);
31 }
32 
33 static inline struct iphdr *ipip_hdr(const struct sk_buff *skb)
34 {
35         return (struct iphdr *)skb_transport_header(skb);
36 }
37 #endif  /* _LINUX_IP_H */

struct iphdr *ip_hdr ip_hdr_net = ip_hdr(skb);
struct iphdr *ip_hdr ip_hdr_trans = ipip_hdr(skb);