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struct sk_buff {
/* These two members must be first. */
struct sk_buff *next; //这两个变量让sk_buff
struct sk_buff *prev; //构成双向的链表
struct sk_buff_head *list; //指向链表的头
struct sock *sk; //指向创建这个sk_buff的socket
struct timeval stamp; //该packet到达的时间(以jiffies为单位)
struct net_device *dev; //incoming/outcoming 的设备
struct net_device *input_dev; //到达的设备
struct net_device *real_dev; //真实使用的设备
union { //指向传输层
struct tcphdr *th;
struct udphdr *uh;
struct icmphdr *icmph;
struct igmphdr *igmph;
struct iphdr *ipiph;
struct ipv6hdr *ipv6h;
unsigned char *raw;
} h;
union { //指向网络层
struct iphdr *iph;
struct ipv6hdr *ipv6h;
struct arphdr *arph;
unsigned char *raw;
} nh;
union { //指向链路层
unsigned char *raw;
} mac;
struct dst_entry *dst; //目的表项(指向一个路由cache，在后面文章详解)
struct sec_path *sp; //只被xfrm使用(这个讨论基本上没用到)
/*
* This is the control buffer. It is free to use for every
* layer. Please put your private variables there. If you
* want to keep them across layers you have to do a skb_clone()
* first. This is owned by whoever has the skb queued ATM.
*/
char cb[40];
unsigned int len, //真实数据区的长度
data_len, //数据区的长度
mac_len, //mac长度
csum; //checknum
__u32 priority;
//以下都是作为flag用,只用了一个byte的5个bit
__u8 local_df:1, //允许local分片标志
cloned:1, //允许clone操作标志
ip_summed:2,
nohdr:1;
/* 3 bits spare */
__u8 pkt_type; //packet的类型，在if_packet.h中定义取值
__be16 protocol;
void (*destructor)(struct sk_buff *skb);
............
............
/* These elements must be at the end, see alloc_skb() for details. */
unsigned int truesize; //buffer的大小(后面详解)
atomic_t users; //user计数,atomic_t指明该变量只能"原子"操作
//在后面详细介绍
unsigned char *head,
*data,
*tail,
*end;
}; | 
。TCP/IP中的主要协议，可以用下面这个图进行一个简单的说明。(下图没有包含SCTP，它属于Transport Layer，但在2.6的kernel是支持的)
)。下面我将对sk_buff结构进行分析。
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