Linux内核网络栈实现分析（九）传输层之UDP协议（下）

本文分析基于Linux Kernel 1.2.13

原创作品，转载请标明http://blog.csdn.net/yming0221/article/details/7549340

更多请查看专栏，地址http://blog.csdn.net/column/details/linux-kernel-net.html

作者：闫明

注：标题中的”（上）“，”（下）“表示分析过程基于数据包的传递方向：”（上）“表示分析是从底层向上分析、”（下）“表示分析是从上向下分析。

上篇分析了应用层经过BSD socket层到INET socket层的函数调用关系和数据的处理流程，INET层会调用具体的传输层协议，还是以UDP协议为例

udp_write()函数

static int udp_write(struct sock *sk, unsigned char *buff, int len, int noblock,
      unsigned flags)
{
    return(udp_sendto(sk, buff, len, noblock, flags, NULL, 0));
}

在分析udp_sendto()函数之前，先了解一下sockaddr_in结构，这是标准的网络接口地址结构的定义

struct sockaddr_in {
  short int     sin_family; /* Address family   地址族 */
  unsigned short int    sin_port;   /* Port number  端口号     */
  struct in_addr    sin_addr;   /* Internet address 网络地址    */

  /* Pad to size of `struct sockaddr'. */
  unsigned char     __pad[__SOCK_SIZE__ - sizeof(short int) -
            sizeof(unsigned short int) - sizeof(struct in_addr)];
};
#define sin_zero    __pad       /* for BSD UNIX comp. -FvK  */

udp_sentdto()函数

static int udp_sendto(struct sock *sk, unsigned char *from, int len, int noblock,
       unsigned flags, struct sockaddr_in *usin, int addr_len)
{
    struct sockaddr_in sin;
    int tmp;

    /*
     *  Check the flags. We support no flags for UDP sending
     */
    if (flags&~MSG_DONTROUTE)
        return(-EINVAL);
    /*
     *  Get and verify the address.
     */

    if (usin) //如果usin不是空
    {
        if (addr_len < sizeof(sin))
            return(-EINVAL);
        memcpy(&sin,usin,sizeof(sin));
        if (sin.sin_family && sin.sin_family != AF_INET)
            return(-EINVAL);
        if (sin.sin_port == 0)
            return(-EINVAL);
    }
    else //usin为空
    {
        if (sk->state != TCP_ESTABLISHED)
            return(-EINVAL);
        sin.sin_family = AF_INET;//协议族
        sin.sin_port = sk->dummy_th.dest;//目的端口
        sin.sin_addr.s_addr = sk->daddr;//目的地址
    }

    /*
     *  BSD socket semantics. You must set SO_BROADCAST to permit
     *  broadcasting of data.
     */

    if(sin.sin_addr.s_addr==INADDR_ANY)//目的地址是全0地址，对应当前主机
        sin.sin_addr.s_addr=ip_my_addr();//将目的地址设为当前主机的网络地址

    if(!sk->broadcast && ip_chk_addr(sin.sin_addr.s_addr)==IS_BROADCAST)
            return -EACCES;         /* Must turn broadcast on first */

    sk->inuse = 1;

    /* Send the packet. */
    tmp = udp_send(sk, &sin, from, len, flags);//调用udp_send()真正的发送数据

    /* The datagram has been sent off.  Release the socket. */
    release_sock(sk);
    return(tmp);
}

udp_send()函数

static int udp_send(struct sock *sk, //要发送的数据包使用的协议对用的sock结构
                       struct sockaddr_in *sin,//目的端的标准的网络接口地址
                       unsigned char *from,//要发送的数据所在地址
                       int len,//发送数据的长度
                       int rt)
{
    struct sk_buff *skb;
    struct device *dev;
    struct udphdr *uh;
    unsigned char *buff;
    unsigned long saddr;
    int size, tmp;
    int ttl;

    /*
     *  Allocate an sk_buff copy of the packet.
     */

    size = sk->prot->max_header + len;//计算大小为UDP最长表头+ 数据长度
    skb = sock_alloc_send_skb(sk, size, 0, &tmp);//根据要发送的数据分配sk_buff结构空间并对当前套接字状态检查


    if (skb == NULL)
        return tmp;

    skb->sk       = NULL;    /* to avoid changing sk->saddr */
    skb->free     = 1;
    skb->localroute = sk->localroute|(rt&MSG_DONTROUTE);

    /*
     *  Now build the IP and MAC header.
     */

    buff = skb->data;//将skb中的数据指针赋值给buff指针
    saddr = sk->saddr;//本地地址
    dev = NULL;
    ttl = sk->ip_ttl;//生存时间
#ifdef CONFIG_IP_MULTICAST
    if (MULTICAST(sin->sin_addr.s_addr))
        ttl = sk->ip_mc_ttl;
#endif
    tmp = sk->prot->build_header(skb, saddr, sin->sin_addr.s_addr,
            &dev, IPPROTO_UDP, sk->opt, skb->mem_len,sk->ip_tos,ttl);//调用ip_build_header()创建IP报头和调用ip_send()创建MAC首部

    skb->sk=sk;  /* So memory is freed correctly */

    /*
     *  Unable to put a header on the packet.
     */

    if (tmp < 0 )
    {
        sk->prot->wfree(sk, skb->mem_addr, skb->mem_len);
        return(tmp);
    }

    buff += tmp;
    saddr = skb->saddr; /*dev->pa_addr;*/
    skb->len = tmp + sizeof(struct udphdr) + len;    /* len + UDP + IP + MAC */
    skb->dev = dev;

    /*
     *  Fill in the UDP header. 填写UDP的报头
     */

    uh = (struct udphdr *) buff;
    uh->len = htons(len + sizeof(struct udphdr));//数据包长度
    uh->source = sk->dummy_th.source;//本地端口
    uh->dest = sin->sin_port;//远端端口
    buff = (unsigned char *) (uh + 1);

    /*
     *  Copy the user data.
     */

    memcpy_fromfs(buff, from, len);//复制用户数据

    /*
     *  Set up the UDP checksum.
     */

    udp_send_check(uh, saddr, sin->sin_addr.s_addr, skb->len - tmp, sk);//计算UDP报头的校验和

    /*
     *  Send the datagram to the interface.
     */

    udp_statistics.UdpOutDatagrams++;

    sk->prot->queue_xmit(sk, dev, skb, 1);//调用IP层函数发送数据
    return(len);
}

这样要发送的数据填充的sk_buff结构中之后再对UDP数据包添加IP数据报头和MAC帧的首部。最后调用IP层的发送函数发送数据包。