Linux内核--网络栈实现分析（七）--数据包的传递过程（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/7545855

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。

在博文Linux内核--网络栈实现分析（二）--数据包的传递过程（上）中分析了数据包从网卡设备经过驱动链路层，网络层，传输层到应用层的过程。

本文就分析一下本机产生数据是如何通过传输层，网络层到达物理层的。

综述来说，数据流程图如下：

一、应用层

应用层可以通过系统调用或文件操作来调用内核函数，BSD层的sock_write()函数会调用INET层的inet_wirte()函数。

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/*

* Write data to a socket. We verify that the user area ubuf..ubuf+size-1 is

* readable by the user process.

*/

static int sock_write(struct inode *inode, struct file *file, char *ubuf, int size)

{

struct socket *sock;

int err;

if (!(sock = socki_lookup(inode)))

{

printk("NET: sock_write: can't find socket for inode!\n");

return(-EBADF);

}

if (sock->flags & SO_ACCEPTCON)

return(-EINVAL);

if(size<0)

return -EINVAL;

if(size==0)

return 0;

if ((err=verify_area(VERIFY_READ,ubuf,size))<0)

return err;

return(sock->ops->write(sock, ubuf, size,(file->f_flags & O_NONBLOCK)));

}

INET层会调用具体传输层协议的write函数，该函数是通过调用本层的inet_send()函数实现功能的，inet_send()函数的UDP协议对应的函数为udp_write()

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static int inet_send(struct socket *sock, void *ubuf, int size, int noblock,

unsigned flags)

{

struct sock *sk = (struct sock *) sock->data;

if (sk->shutdown & SEND_SHUTDOWN)

{

send_sig(SIGPIPE, current, 1);

return(-EPIPE);

}

if(sk->err)

return inet_error(sk);

/* We may need to bind the socket. */

if(inet_autobind(sk)!=0)

return(-EAGAIN);

return(sk->prot->write(sk, (unsigned char *) ubuf, size, noblock, flags));

}

static int inet_write(struct socket *sock, char *ubuf, int size, int noblock)

{

return inet_send(sock,ubuf,size,noblock,0);

}

二、传输层

在传输层udp_write()函数调用本层的udp_sendto()函数完成功能。

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/*

* In BSD SOCK_DGRAM a write is just like a send.

*/

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));

}

udp_send()函数完成sk_buff结构相应的设置和报头的填写后会调用udp_send()来发送数据。具体的实现过程后面会详细分析。

而在udp_send()函数中，最后会调用ip_queue_xmit()函数，将数据包下放的网络层。

下面是udp_prot定义：

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struct proto udp_prot = {

sock_wmalloc,

sock_rmalloc,

sock_wfree,

sock_rfree,

sock_rspace,

sock_wspace,

udp_close,

udp_read,

udp_write,

udp_sendto,

udp_recvfrom,

ip_build_header,

udp_connect,

NULL,

ip_queue_xmit,

NULL,

NULL,

NULL,

udp_rcv,

datagram_select,

udp_ioctl,

NULL,

NULL,

ip_setsockopt,

ip_getsockopt,

128,

0,

{NULL,},

"UDP",

0, 0

};

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static int udp_send(struct sock *sk, struct sockaddr_in *sin,

unsigned char *from, int len, 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.

*/

........................

/*

* Now build the IP and MAC header.

*/

..........................

/*

* Fill in the UDP header.

*/

..............................

/*

* Copy the user data.

*/

memcpy_fromfs(buff, from, len);

/*

* Set up the UDP checksum.

*/

udp_send_check(uh, saddr, sin->sin_addr.s_addr, skb->len - tmp, sk);

/*

* Send the datagram to the interface.

*/

udp_statistics.UdpOutDatagrams++;

sk->prot->queue_xmit(sk, dev, skb, 1);

return(len);

}

三、网络层

在网络层，函数ip_queue_xmit()的功能是将数据包进行一系列复杂的操作，比如是检查数据包是否需要分片，是否是多播等一系列检查，最后调用dev_queue_xmit()函数发送数据。

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/*

* Queues a packet to be sent, and starts the transmitter

* if necessary. if free = 1 then we free the block after

* transmit, otherwise we don't. If free==2 we not only

* free the block but also don't assign a new ip seq number.

* This routine also needs to put in the total length,

* and compute the checksum

*/

void ip_queue_xmit(struct sock *sk, struct device *dev,

struct sk_buff *skb, int free)

{

struct iphdr *iph;

unsigned char *ptr;

/* Sanity check */

............

/*

* Do some book-keeping in the packet for later

*/

...........

/*

* Find the IP header and set the length. This is bad

* but once we get the skb data handling code in the

* hardware will push its header sensibly and we will

* set skb->ip_hdr to avoid this mess and the fixed

* header length problem

*/

..............

/*

* No reassigning numbers to fragments...

*/

if(free!=2)

iph->id = htons(ip_id_count++);

else

free=1;

/* All buffers without an owner socket get freed */

if (sk == NULL)

free = 1;

skb->free = free;

/*

* Do we need to fragment. Again this is inefficient.

* We need to somehow lock the original buffer and use

* bits of it.

*/

................

/*

* Add an IP checksum

*/

ip_send_check(iph);

/*

* Print the frame when debugging

*/

/*

* More debugging. You cannot queue a packet already on a list

* Spot this and moan loudly.

*/

.......................

/*

* If a sender wishes the packet to remain unfreed

* we add it to his send queue. This arguably belongs

* in the TCP level since nobody else uses it. BUT

* remember IPng might change all the rules.

*/

......................

/*

* If the indicated interface is up and running, send the packet.

*/

ip_statistics.IpOutRequests++;

.............................

.............................

if((dev->flags&IFF_BROADCAST) && iph->daddr==dev->pa_brdaddr && !(dev->flags&IFF_LOOPBACK))

ip_loopback(dev,skb);

if (dev->flags & IFF_UP)

{

/*

* If we have an owner use its priority setting,

* otherwise use NORMAL

*/

if (sk != NULL)

{

dev_queue_xmit(skb, dev, sk->priority);

}

else

{

dev_queue_xmit(skb, dev, SOPRI_NORMAL);

}

}

else

{

ip_statistics.IpOutDiscards++;

if (free)

kfree_skb(skb, FREE_WRITE);

}

}

四、驱动层（链路层）

在函数中，函数调用会调用具体设备的发送函数来发送数据包

dev->hard_start_xmit(skb, dev);

具体设备的发送函数在网络初始化的时候已经设置了。

这里以8390网卡为例来说明驱动层的工作原理，在net/drivers/8390.c中函数ethdev_init()函数中设置如下：

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/* Initialize the rest of the 8390 device structure. */

int ethdev_init(struct device *dev)

{

if (ei_debug > 1)

printk(version);

if (dev->priv == NULL) {//申请私有空间

struct ei_device *ei_local;//8390网卡设备的结构体

dev->priv = kmalloc(sizeof(struct ei_device), GFP_KERNEL);//申请内核内存空间

memset(dev->priv, 0, sizeof(struct ei_device));

ei_local = (struct ei_device *)dev->priv;

#ifndef NO_PINGPONG

ei_local->pingpong = 1;

#endif

}

/* The open call may be overridden by the card-specific code. */

if (dev->open == NULL)

dev->open = &ei_open;//设备的打开函数

/* We should have a dev->stop entry also. */

dev->hard_start_xmit = &ei_start_xmit;//设备的发送函数，定义在8390.c中

dev->get_stats = get_stats;

#ifdef HAVE_MULTICAST

dev->set_multicast_list = &set_multicast_list;

#endif

ether_setup(dev);

return 0;

}

驱动中的发送函数比较复杂，和硬件关系紧密，这里不再详细分析。

这样就大体分析了下网络数据从应用层到物理层的数据通路，后面会详细分析。