[转]linux网络协议栈（1）——socket buffer

  1 //套接字缓存
  2 struct sk_buff {
  3     /* These two members must be first. */
  4     struct sk_buff        *next;  
  5     struct sk_buff        *prev;
  6 
  7     struct sk_buff_head    *list;  
  8     struct sock        *sk;          //指向创建报文的socket
  9     struct timeval        stamp;  //此报文收到时的时间
 10     struct net_device    *dev;          //收到此报文的网络设备 
 11     struct net_device    *input_dev;
 12     struct net_device    *real_dev;
 13 
 14     //TCP报头
 15     union {  
 16         struct tcphdr    *th;   //tcp头
 17         struct udphdr    *uh;  //udp头
 18         struct icmphdr    *icmph;
 19         struct igmphdr    *igmph;
 20         struct iphdr    *ipiph;
 21         struct ipv6hdr    *ipv6h;
 22         unsigned char    *raw;
 23     } h;
 24     //IP报头
 25     union {
 26         struct iphdr    *iph;
 27         struct ipv6hdr    *ipv6h;
 28         struct arphdr    *arph;
 29         unsigned char    *raw;
 30     } nh;
 31     //链路层帧头
 32     union {
 33           unsigned char     *raw;
 34     } mac;
 35 
 36     struct  dst_entry    *dst;  //此报文的路由，路由确定后赋此值
 37     struct    sec_path    *sp;
 38 
 39     /*
 40      * This is the control buffer. It is free to use for every
 41      * layer. Please put your private variables there. If you
 42      * want to keep them across layers you have to do a skb_clone()
 43      * first. This is owned by whoever has the skb queued ATM.
 44      */
 45     char            cb[40];
 46     
 47     //此报文的长度，这是指网络报文在不同协议层中的长度，包括头部和数据。在协议栈的不同层，这个长度是不同的。 
 48     unsigned int        len,
 49                 data_len,
 50                 mac_len,
 51                 csum;
 52     unsigned char        local_df,
 53                 cloned,
 54                 pkt_type, //网络报文的类型，常见的有PACKET_HOST，代表发给本机的报文；还有PACKET_OUTGOING，代表本机发出的报文。 
 55                 ip_summed;
 56     __u32            priority;
 57      
 58     unsigned short        protocol,//链路层协议
 59                 security;
 60 
 61     void            (*destructor)(struct sk_buff *skb);
 62 #ifdef CONFIG_NETFILTER
 63         unsigned long        nfmark;
 64     __u32            nfcache;
 65     __u32            nfctinfo;
 66     struct nf_conntrack    *nfct;
 67 #ifdef CONFIG_NETFILTER_DEBUG
 68         unsigned int        nf_debug;
 69 #endif
 70 #ifdef CONFIG_BRIDGE_NETFILTER
 71     struct nf_bridge_info    *nf_bridge;
 72 #endif
 73 #endif /* CONFIG_NETFILTER */
 74 #if defined(CONFIG_HIPPI)
 75     union {
 76         __u32        ifield;
 77     } private;
 78 #endif
 79 #ifdef CONFIG_NET_SCHED
 80        __u32            tc_index;        /* traffic control index */
 81 #ifdef CONFIG_NET_CLS_ACT
 82     __u32           tc_verd;               /* traffic control verdict */
 83     __u32           tc_classid;            /* traffic control classid */
 84 #endif
 85 
 86 #endif
 87 
 88 
 89     /* These elements must be at the end, see alloc_skb() for details.  */
 90 
 91        //此报文存储区的长度，这个长度是16字节对齐的，一般要比报文的长度大
 92     unsigned int        truesize;
 93     atomic_t        users;
 94     /*head和end指向报文数据的整个单元.head与data之间的空间称为headroom,tail与end之间的空间称为tailroom.
 95     */
 96     unsigned char        *head,
 97                 *data,
 98                 *tail,
 99                 *end;
100 };