#ifndef __LINUX_GRE_H
  #define __LINUX_GRE_H
  
  #include <linux/skbuff.h>

  #include <net/ip_tunnels.h>

  struct gre_base_hdr {
  	__be16 flags;
  	__be16 protocol;

  } __packed;
  
  struct gre_full_hdr {
  	struct gre_base_hdr fixed_header;
  	__be16 csum;
  	__be16 reserved1;
  	__be32 key;
  	__be32 seq;
  } __packed;

  #define GRE_HEADER_SECTION 4

  #define GREPROTO_CISCO		0
  #define GREPROTO_PPTP		1
  #define GREPROTO_MAX		2

  #define GRE_IP_PROTO_MAX	2

  
  struct gre_protocol {
  	int  (*handler)(struct sk_buff *skb);
  	void (*err_handler)(struct sk_buff *skb, u32 info);
  };
  
  int gre_add_protocol(const struct gre_protocol *proto, u8 version);
  int gre_del_protocol(const struct gre_protocol *proto, u8 version);

  struct net_device *gretap_fb_dev_create(struct net *net, const char *name,
  				       u8 name_assign_type);

  int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,

  		     bool *csum_err, __be16 proto, int nhs);

  
  static inline int gre_calc_hlen(__be16 o_flags)
  {
  	int addend = 4;
  
  	if (o_flags & TUNNEL_CSUM)
  		addend += 4;
  	if (o_flags & TUNNEL_KEY)
  		addend += 4;
  	if (o_flags & TUNNEL_SEQ)
  		addend += 4;
  	return addend;
  }
  
  static inline __be16 gre_flags_to_tnl_flags(__be16 flags)
  {
  	__be16 tflags = 0;
  
  	if (flags & GRE_CSUM)
  		tflags |= TUNNEL_CSUM;
  	if (flags & GRE_ROUTING)
  		tflags |= TUNNEL_ROUTING;
  	if (flags & GRE_KEY)
  		tflags |= TUNNEL_KEY;
  	if (flags & GRE_SEQ)
  		tflags |= TUNNEL_SEQ;
  	if (flags & GRE_STRICT)
  		tflags |= TUNNEL_STRICT;
  	if (flags & GRE_REC)
  		tflags |= TUNNEL_REC;
  	if (flags & GRE_VERSION)
  		tflags |= TUNNEL_VERSION;
  
  	return tflags;
  }
  
  static inline __be16 gre_tnl_flags_to_gre_flags(__be16 tflags)
  {
  	__be16 flags = 0;
  
  	if (tflags & TUNNEL_CSUM)
  		flags |= GRE_CSUM;
  	if (tflags & TUNNEL_ROUTING)
  		flags |= GRE_ROUTING;
  	if (tflags & TUNNEL_KEY)
  		flags |= GRE_KEY;
  	if (tflags & TUNNEL_SEQ)
  		flags |= GRE_SEQ;
  	if (tflags & TUNNEL_STRICT)
  		flags |= GRE_STRICT;
  	if (tflags & TUNNEL_REC)
  		flags |= GRE_REC;
  	if (tflags & TUNNEL_VERSION)
  		flags |= GRE_VERSION;
  
  	return flags;
  }

  static inline __sum16 gre_checksum(struct sk_buff *skb)
  {
  	__wsum csum;
  
  	if (skb->ip_summed == CHECKSUM_PARTIAL)
  		csum = lco_csum(skb);
  	else
  		csum = skb_checksum(skb, 0, skb->len, 0);
  	return csum_fold(csum);
  }
  
  static inline void gre_build_header(struct sk_buff *skb, int hdr_len,
  				    __be16 flags, __be16 proto,
  				    __be32 key, __be32 seq)
  {
  	struct gre_base_hdr *greh;
  
  	skb_push(skb, hdr_len);

  	skb_set_inner_protocol(skb, proto);

  	skb_reset_transport_header(skb);
  	greh = (struct gre_base_hdr *)skb->data;
  	greh->flags = gre_tnl_flags_to_gre_flags(flags);
  	greh->protocol = proto;
  
  	if (flags & (TUNNEL_KEY | TUNNEL_CSUM | TUNNEL_SEQ)) {
  		__be32 *ptr = (__be32 *)(((u8 *)greh) + hdr_len - 4);
  
  		if (flags & TUNNEL_SEQ) {
  			*ptr = seq;
  			ptr--;
  		}
  		if (flags & TUNNEL_KEY) {
  			*ptr = key;
  			ptr--;
  		}
  		if (flags & TUNNEL_CSUM &&
  		    !(skb_shinfo(skb)->gso_type &
  		      (SKB_GSO_GRE | SKB_GSO_GRE_CSUM))) {
  			*ptr = 0;
  			*(__sum16 *)ptr = gre_checksum(skb);
  		}
  	}
  }

  #endif