linux 网络套接字

Posted 2021-02-07 yooooooo

在内核分析网络分组时，底层协议的数据将传输到跟高的层。而发送数据的时候顺序是相反的。每一层都是通过加（首部+净荷）传向跟底层，直至最终发送。

这些操作决定了网络的的性能。

就如下图所示

linux因此设计了一个结构体

如下代码

/** 
 *  struct sk_buff - socket buffer
 *  @next: Next buffer in list
 *  @prev: Previous buffer in list
 *  @list: List we are on
 *  @sk: Socket we are owned by
 *  @stamp: Time we arrived
 *  @dev: Device we arrived on/are leaving by
 *      @real_dev: The real device we are using
 *  @h: Transport layer header
 *  @nh: Network layer header
 *  @mac: Link layer header
 *  @dst: FIXME: Describe this field
 *  @cb: Control buffer. Free for use by every layer. Put private vars here
 *  @len: Length of actual data
 *  @data_len: Data length
 *  @csum: Checksum
 *  @__unused: Dead field, may be reused
 *  @cloned: Head may be cloned (check refcnt to be sure)
 *  @pkt_type: Packet class
 *  @ip_summed: Driver fed us an IP checksum
 *  @priority: Packet queueing priority
 *  @users: User count - see {datagram,tcp}.c
 *  @protocol: Packet protocol from driver
 *  @security: Security level of packet
 *  @truesize: Buffer size 
 *  @head: Head of buffer
 *  @data: Data head pointer
 *  @tail: Tail pointer
 *  @end: End pointer
 *  @destructor: Destruct function
 *  @nfmark: Can be used for communication between hooks
 *  @nfcache: Cache info
 *  @nfct: Associated connection, if any
 *  @nf_debug: Netfilter debugging
 *  @nf_bridge: Saved data about a bridged frame - see br_netfilter.c
 *      @private: Data which is private to the HIPPI implementation
 *  @tc_index: Traffic control index
 */
 
struct sk_buff {
    /* These two members must be first. */
    struct sk_buff      *next;
    struct sk_buff      *prev;
 
    struct sk_buff_head *list;
    struct sock     *sk;
    struct timeval      stamp;
    struct net_device   *dev;
    struct net_device   *real_dev;
 
    union {
        struct tcphdr   *th;
        struct udphdr   *uh;
        struct icmphdr  *icmph;
        struct igmphdr  *igmph;
        struct iphdr    *ipiph;
        unsigned char   *raw;
    } h;
 
    union {
        struct iphdr    *iph;
        struct ipv6hdr  *ipv6h;
        struct arphdr   *arph;
        unsigned char   *raw;
    } nh;
 
    union {
        struct ethhdr   *ethernet;
        unsigned char   *raw;
    } mac;
 
    struct  dst_entry   *dst;
    struct  sec_path    *sp;
 
    /*
     * 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[48];
 
    unsigned int        len,
                data_len,
                csum;
    unsigned char       local_df,
                cloned,
                pkt_type,
                ip_summed;
    __u32           priority;
    unsigned short      protocol,
                security;
 
    void            (*destructor)(struct sk_buff *skb);
#ifdef CONFIG_NETFILTER
        unsigned long       nfmark;
    __u32           nfcache;
    struct nf_ct_info   *nfct;
#ifdef CONFIG_NETFILTER_DEBUG
        unsigned int        nf_debug;
#endif
#ifdef CONFIG_BRIDGE_NETFILTER
    struct nf_bridge_info   *nf_bridge;
#endif
#endif /* CONFIG_NETFILTER */
#if defined(CONFIG_HIPPI)
    union {
        __u32       ifield;
    } private;
#endif
#ifdef CONFIG_NET_SCHED
       __u32            tc_index;               /* traffic control index */
#endif
 
    /* These elements must be at the end, see alloc_skb() for details.  */
    unsigned int        truesize;
    atomic_t        users;
    unsigned char       *head,
                *data,
                *tail,
                *end;
};

套接字换从区在各个层交换数据，就不用复制数据了。

从以上字段和注释可以看到，head和end字段指向了buf的起始位置和终止位置。然后使用header指针指像各种协议填值。然后data就是实际数据。tail记录了数据的偏移值。

相信大家都能看懂注释，具体的解释就不用介绍了.,

在一个新的分组产生的时候，TCP层首先在用户空间中分配内存来容纳该分组数据。分配的空间大于数据的实际需要长度。因此较低的层可以增加首部，在往下一层走的时候，只需要对字段添值即可。

对接收分组的一样，分组数据复制到内核分配的一个内存区中。并在分析的过程中一直处于内存区中。

skbuf还提供了一个双向链表对这个数据分组进行了管理。

如下代码

struct sk_buff_head {
    /* These two members must be first. */
    struct sk_buff  *next;
    struct sk_buff  *prev;
 
    __u32       qlen;
    spinlock_t  lock;
};

__u32 qlen; 缓冲区中等待队列的长度。就是分组的成员数量。

lock 表示了cpu的互斥。

今天分析到此，跟多源码阅读去看skbuff.h的文件。