SKB结构定义 /usr/src/linux/include/linux/skbuff.h
sk_buff_head：
struct sk_buff_head { //SKB的头结点
/* These two members must be first. */
struct sk_buff *next;
struct sk_buff *prev;
__u32 qlen; //队列长度
spinlock_t lock; //自旋锁
};
sk_buff：
/**
* struct sk_buff - socket buffer
* @next: Next buffer in list
* @prev: Previous buffer in list
* @sk: Socket we are owned by
* @tstamp: Time we arrived
* @dev: Device we arrived on/are leaving by
* @input_dev: Device we arrived on
* @h: Transport layer header
* @nh: Network layer header
* @mac: Link layer header
* @dst: destination entry
* @sp: the security path, used for xfrm
* @cb: Control buffer. Free for use by every layer. Put private vars here
* @len: Length of actual data
* @data_len: Data length
* @mac_len: Length of link layer header
* @csum: Checksum
* @local_df: allow local fragmentation
* @cloned: Head may be cloned (check refcnt to be sure)
* @nohdr: Payload reference only, must not modify header
* @pkt_type: Packet class
* @fclone: skbuff clone status
* @ip_summed: Driver fed us an IP checksum
* @priority: Packet queueing priority
* @users: User count - see {datagram,tcp}.c
* @protocol: Packet protocol from driver
* @truesize: Buffer size
* @head: Head of buffer
* @data: Data head pointer
* @tail: Tail pointer
* @end: End pointer
* @destructor: Destruct function
* @mark: Generic packet mark
* @nfct: Associated connection, if any
* @ipvs_property: skbuff is owned by ipvs
* @nfctinfo: Relationship of this skb to the connection
* @nfct_reasm: netfilter conntrack re-assembly pointer
* @nf_bridge: Saved data about a bridged frame - see br_netfilter.c
* @tc_index: Traffic control index
* @tc_verd: traffic control verdict
* @dma_cookie: a cookie to one of several possible DMA operations
* done by skb DMA functions
* @secmark: security marking
*/ struct sk_buff {
/* These two members must be first. */
struct sk_buff *next;
struct sk_buff *prev; struct sock *sk; //宿主传输控制块，本地发送或本地接收时有效，转发时为空
struct skb_timeval tstamp; //时间戳
struct net_device *dev; //网络设备指针
struct net_device *input_dev; //接收报文的原始网络设备，包为本地生成时为空，用于流量控制 union { //指向四层协议首部，包含的数据结构表示在这一层上可以解析的协议
struct tcphdr *th;
struct udphdr *uh;
struct icmphdr *icmph;
struct igmphdr *igmph;
struct iphdr *ipiph;
struct ipv6hdr *ipv6h;
unsigned char *raw; // 用于初始化
} h; union { //指向三层协议首部
struct iphdr *iph;
struct ipv6hdr *ipv6h;
struct arphdr *arph;
unsigned char *raw;
} nh; union { //指向二层协议首部
unsigned char *raw;
} mac; struct dst_entry *dst; //目的路由缓存
struct sec_path *sp; //IPSec协议用来跟踪传输的信息 /*
* 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[]; //信息控制块，存储每层自己的私有信息 unsigned int len, //数据部分长度，包含首部长度
data_len, //SG类型和FRAGLIST类型聚合分散I/O存储区中的数据长度
mac_len; //数据链路层首部长度
union {
__wsum csum; //校验状态为CHECKSUM_NONE时，存放所负载数据报的数据部分校验和
__u32 csum_offset; //校验状态为CHECKSUM_PARTIAL时，记录传输层首部中的校验和字段的偏移
};
__u32 priority; //发送或转发数据包QoS类别，
__u8 local_df:, //表示此SKB在本地允许分片
cloned:, //标记所属SKB是否被克隆
ip_summed:, //标记传输层校验和的状态
nohdr:, //标识payload是否被单独引用
nfctinfo:; //被防火墙使用
__u8 pkt_type:, //帧类型
fclone:, //当前克隆状态
ipvs_property:;
__be16 protocol; //链路层协议类型 void (*destructor)(struct sk_buff *skb); //析构函数指针，释放时调用，没有宿主传输控制块时为空
#ifdef CONFIG_NETFILTER
struct nf_conntrack *nfct; //被防火墙使用
#if defined(CONFIG_NF_CONNTRACK) || defined(CONFIG_NF_CONNTRACK_MODULE)
struct sk_buff *nfct_reasm;
#endif
#ifdef CONFIG_BRIDGE_NETFILTER
struct nf_bridge_info *nf_bridge; //被防火墙使用
#endif
#endif /* CONFIG_NETFILTER */
#ifdef CONFIG_NET_SCHED
__u16 tc_index; /* traffic control index 流量控制*/
#ifdef CONFIG_NET_CLS_ACT
__u16 tc_verd; /* traffic control verdict */
#endif
#endif
#ifdef CONFIG_NET_DMA
dma_cookie_t dma_cookie;
#endif
#ifdef CONFIG_NETWORK_SECMARK
__u32 secmark;
#endif __u32 mark; /* These elements must be at the end, see alloc_skb() for details. */
unsigned int truesize; //缓存区总长度
atomic_t users; //引用计数，标识有多少实体引用SKB，为零时释放
unsigned char *head, //指向缓存区头
*data, //指向数据头
*tail, //指向数据尾
*end; //指向缓存区尾
};
skb_shared_info：
struct skb_shared_info { //保存数据块的附加信息
atomic_t dataref; //引用计数器
unsigned short nr_frags;
unsigned short gso_size;
/* Warning: this field is not always filled in (UFO)! */
unsigned short gso_segs;
unsigned short gso_type;
__be32 ip6_frag_id;
struct sk_buff *frag_list;
skb_frag_t frags[MAX_SKB_FRAGS];
};