应用层发送一个数据包的时候，是如何到达网卡的（上）

数据包首先从tcp层进行处理，对应的函数是tcp_write

/*
 *	This routine copies from a user buffer into a socket,
 *	and starts the transmit system.
 */

static int tcp_write(struct sock *sk, unsigned char *from,
	  int len, int nonblock, unsigned flags)
{
	int copied = 0;
	int copy;
	int tmp;
	struct sk_buff *skb;
	struct sk_buff *send_tmp;
	unsigned char *buff;
	struct proto *prot;
	struct device *dev = NULL;

	sk->inuse=1;
	prot = sk->prot;
	while(len > 0) 
	{
		if (sk->err) 
		{			/* Stop on an error */
			release_sock(sk);
			if (copied) 
				return(copied);
			tmp = -sk->err;
			sk->err = 0;
			return(tmp);
		}

		/*
		 *	First thing we do is make sure that we are established. 
		 */
		// 关闭了只能读不能写
		if (sk->shutdown & SEND_SHUTDOWN) 
		{
			release_sock(sk);
			sk->err = EPIPE;
			if (copied) 
				return(copied);
			sk->err = 0;
			return(-EPIPE);
		}

		/* 
		 *	Wait for a connection to finish.
		 */
		// 处于不能写状态，close_wait是可写不可读，因为对端已经关闭了写		
		while(sk->state != TCP_ESTABLISHED && sk->state != TCP_CLOSE_WAIT) 
		{
			if (sk->err) 
			{
				release_sock(sk);
				if (copied) 
					return(copied);
				tmp = -sk->err;
				sk->err = 0;
				return(tmp);
			}
			// syn和syn_recv状态的时候可以写，重复发包，否则是出错状态
			if (sk->state != TCP_SYN_SENT && sk->state != TCP_SYN_RECV) 
			{
				release_sock(sk);
				if (copied) 
					return(copied);

				if (sk->err) 
				{
					tmp = -sk->err;
					sk->err = 0;
					return(tmp);
				}
				// 长连接 
				if (sk->keepopen) 
				{
					send_sig(SIGPIPE, current, 0);
				}
				return(-EPIPE);
			}

			if (nonblock || copied) 
			{
				release_sock(sk);
				if (copied) 
					return(copied);
				return(-EAGAIN);
			}

			release_sock(sk);
			cli();
		
			if (sk->state != TCP_ESTABLISHED &&
		    		sk->state != TCP_CLOSE_WAIT && sk->err == 0) 
		    	{
				interruptible_sleep_on(sk->sleep);
				if (current->signal & ~current->blocked) 
				{
					sti();
					if (copied) 
						return(copied);
					return(-ERESTARTSYS);
				}
			}
			sk->inuse = 1;
			sti();
		}

	/*
	 * The following code can result in copy <= if sk->mss is ever
	 * decreased.  It shouldn't be.  sk->mss is min(sk->mtu, sk->max_window).
	 * sk->mtu is constant once SYN processing is finished.  I.e. we
	 * had better not get here until we've seen his SYN and at least one
	 * valid ack.  (The SYN sets sk->mtu and the ack sets sk->max_window.)
	 * But ESTABLISHED should guarantee that.  sk->max_window is by definition
	 * non-decreasing.  Note that any ioctl to set user_mss must be done
	 * before the exchange of SYN's.  If the initial ack from the other
	 * end has a window of 0, max_window and thus mss will both be 0.
	 */

	/* 
	 *	Now we need to check if we have a half built packet. 
	 */
		// 先看是否有小块的数据被缓存起来，是的话先取出skb，不需要立刻发送的话再入队
		if ((skb = tcp_dequeue_partial(sk)) != NULL) 
		{
		        int hdrlen;

		         /* IP header + TCP header */
			hdrlen = ((unsigned long)skb->h.th - (unsigned long)skb->data)
			         + sizeof(struct tcphdr);
	
			/* Add more stuff to the end of skb->len */
			// 不是紧急数据，则把数据追加到缓存的小包数据后面，是紧急数据则先把小包数据发出去，然后下一个循环再发普通数据
			if (!(flags & MSG_OOB)) 
			{	
				// mss-数据长度等于还可以传多少长度的数据
				copy = min(sk->mss - (skb->len - hdrlen), len);
				/* FIXME: this is really a bug. */
				if (copy <= 0) 
				{
			  		printk("TCP: **bug**: \"copy\" <= 0!!\n");
			  		copy = 0;
				}
	  			// 把用户的数据赋值copy长度个字节到数据包的数据部分
				memcpy_fromfs(skb->data + skb->len, from, copy);
				// 更新skb的data字段使用了多少字节
				skb->len += copy;
				// 下次复制的首地址
				from += copy;
				// 已复制的字节长度
				copied += copy;
				// 还有多少字节需要复制
				len -= copy;
				// 下一个发送的字节的***大小
				sk->write_seq += copy;
			}
			// 数据部分大于等于mss或者是带外数据或者还没有发出去一个数据包则直接发送
			if ((skb->len - hdrlen) >= sk->mss ||
				(flags & MSG_OOB) || !sk->packets_out)
				tcp_send_skb(sk, skb);
			else
				// 继续缓存，满足条件后一起发送
				tcp_enqueue_partial(skb, sk);
			continue;
		}

	/*
	 * We also need to worry about the window.
 	 * If window < 1/2 the maximum window we've seen from this
 	 *   host, don't use it.  This is sender side
 	 *   silly window prevention, as specified in RFC1122.
 	 *   (Note that this is different than earlier versions of
 	 *   SWS prevention, e.g. RFC813.).  What we actually do is 
	 *   use the whole MSS.  Since the results in the right
	 *   edge of the packet being outside the window, it will
	 *   be queued for later rather than sent.
	 */
		// 可发送的序列化最大值 - 下一个可写的序列化值等于可以发送的字节数
		copy = sk->window_seq - sk->write_seq;
		if (copy <= 0 || copy < (sk->max_window >> 1) || copy > sk->mss)
			copy = sk->mss;
		// 能发送的比需要发送的大，则取需要发送的
		if (copy > len)
			copy = len;

	/*
	 *	We should really check the window here also. 
	 */
	 
		send_tmp = NULL;
		// 不是紧急数据并且也小于mss，则需要缓存到partial队列，否则直接发送
		if (copy < sk->mss && !(flags & MSG_OOB)) 
		{
			/*
			 *	We will release the socket in case we sleep here. 
			 */
			release_sock(sk);
			/*
			 *	NB: following must be mtu, because mss can be increased.
			 *	mss is always <= mtu 
			 */
			skb = prot->wmalloc(sk, sk->mtu + 128 + prot->max_header, 0, GFP_KERNEL);
			sk->inuse = 1;
			send_tmp = skb;
		} 
		else 
		{
			/*
			 *	We will release the socket in case we sleep here. 
			 */
			release_sock(sk);
			skb = prot->wmalloc(sk, copy + prot->max_header , 0, GFP_KERNEL);
  			sk->inuse = 1;
		}

		/*
		 *	If we didn't get any memory, we need to sleep. 
		 */
		// 没有写空间了
		if (skb == NULL) 
		{
			sk->socket->flags |= SO_NOSPACE;
			// 非阻塞直接返回已经写入的字节
			if (nonblock) 
			{
				release_sock(sk);
				if (copied) 
					return(copied);
				return(-EAGAIN);
			}

			/*
			 *	FIXME: here is another race condition. 
			 */

			tmp = sk->wmem_alloc;
			// 这个函数会处理收到的数据包，如果收到ack包则会腾出写空间
			release_sock(sk);
			cli();
			/*
			 *	Again we will try to avoid it. 
			 */
			// 处于可写状态但是没有写空间，则阻塞
			if (tmp <= sk->wmem_alloc &&
				  (sk->state == TCP_ESTABLISHED||sk->state == TCP_CLOSE_WAIT)
				&& sk->err == 0) 
			{
				sk->socket->flags &= ~SO_NOSPACE;
				interruptible_sleep_on(sk->sleep);
				if (current->signal & ~current->blocked) 
				{
					sti();
					if (copied) 
						return(copied);
					return(-ERESTARTSYS);
				}
			}
			sk->inuse = 1;
			sti();
			continue;
		}

		skb->len = 0;
		skb->sk = sk;
		skb->free = 0;
		skb->localroute = sk->localroute|(flags&MSG_DONTROUTE);
	
		buff = skb->data;
	
		/*
		 * FIXME: we need to optimize this.
		 * Perhaps some hints here would be good.
		 */
		// 构建ip头和mac头，返回ip头+mac头的长度的大小
		tmp = prot->build_header(skb, sk->saddr, sk->daddr, &dev,
				 IPPROTO_TCP, sk->opt, skb->mem_len,sk->ip_tos,sk->ip_ttl);
		if (tmp < 0 ) 
		{
			prot->wfree(sk, skb->mem_addr, skb->mem_len);
			release_sock(sk);
			if (copied) 
				return(copied);
			return(tmp);
		}
		// 更新data中的数据长度
		skb->len += tmp;
		skb->dev = dev;
		// 指向可写地址，准备写入tcp头
		buff += tmp;
		// skb的tcp头指向data字段的tcp头
		skb->h.th =(struct tcphdr *) buff;
		// 构建tcp头，len-copy表示是否已经传输完len字节的数据，用于设置push标记
		tmp = tcp_build_header((struct tcphdr *)buff, sk, len-copy);
		if (tmp < 0) 
		{
			prot->wfree(sk, skb->mem_addr, skb->mem_len);
			release_sock(sk);
			if (copied) 
				return(copied);
			return(tmp);
		}
		// 带外数据
		if (flags & MSG_OOB) 
		{	// 设置urg标记位，设置紧急指针指向紧急数据的后面一个字节
			((struct tcphdr *)buff)->urg = 1;
			((struct tcphdr *)buff)->urg_ptr = ntohs(copy);
		}
		// 更新skb->data中的数据长度
		skb->len += tmp;
		// 复制copy个字节到tcp头后面成为tcp报文的负载
		memcpy_fromfs(buff+tmp, from, copy);
		// 更新需要复制的数据地址
		from += copy;
		// 复制字节数累加
		copied += copy;
		// 还有多少个字节需要复制
		len -= copy;
		// 更新skb->data的数据长度
		skb->len += copy;
		skb->free = 0;
		// 更新下一个tcp报文的序列化
		sk->write_seq += copy;
		// 数据量太少并且不是紧急数据，并且有待确认的包（nagle算法规则），则先缓存
		if (send_tmp != NULL && sk->packets_out) 
		{
			tcp_enqueue_partial(send_tmp, sk);
			continue;
		}
		// 否则直接发送
		tcp_send_skb(sk, skb);
	}
	sk->err = 0;

/*
 *	Nagle's rule. Turn Nagle off with TCP_NODELAY for highly
 *	interactive fast network servers. It's meant to be on and
 *	it really improves the throughput though not the echo time
 *	on my slow slip link - Alan
 */

/*
 *	Avoid possible race on send_tmp - c/o Johannes Stille 
 */
	// 符合nagle算法条件或者没有开启nagle算法且***合法则发送
	if(sk->partial && ((!sk->packets_out) 
     /* If not nagling we can send on the before case too.. */
	      || (sk->nonagle && before(sk->write_seq , sk->window_seq))
      	))
  		tcp_send_partial(sk);

	release_sock(sk);
	return(copied);
}

ip层发送函数，进行了数据包的缓存处理

/*
 * 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 */
	if (dev == NULL)
	{
		printk("IP: ip_queue_xmit dev = NULL\n");
		return;
	}

	IS_SKB(skb);

	/*
	 *	Do some book-keeping in the packet for later
	 */


	skb->dev = dev;
	// 发送时间
	skb->when = jiffies;

	/*
	 *	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
	 */

	ptr = skb->data;
	ptr += dev->hard_header_len;
	iph = (struct iphdr *)ptr;
	skb->ip_hdr = iph;
	// 整个ip头和数据的长度
	iph->tot_len = ntohs(skb->len-dev->hard_header_len);

#ifdef CONFIG_IP_FIREWALL
	if(ip_fw_chk(iph, dev, ip_fw_blk_chain, ip_fw_blk_policy, 0) != 1)
		/* just don't send this packet */
		return;
#endif	

	/*
	 *	No reassigning numbers to fragments...
	 */
	// 用于重组分片的id
	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.
	 */
	// 数据包大小mtu则分片处理
	if(skb->len > dev->mtu + dev->hard_header_len)
	{
		ip_fragment(sk,skb,dev,0);
		IS_SKB(skb);
		kfree_skb(skb,FREE_WRITE);
		return;
	}

	/*
	 *	Add an IP checksum
	 */
	// ip层校验和
	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 (skb->next != NULL)
	{
		printk("ip_queue_xmit: next != NULL\n");
		skb_unlink(skb);
	}

	/*
	 *	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.
	 */
	// free等于0说明这个包要缓存
	if (!free)
	{
		unsigned long flags;
		/* The socket now has more outstanding blocks */
		// 发送但还没收到确认的数据包数量
		sk->packets_out++;

		/* Protect the list for a moment */
		save_flags(flags);
		cli();

		if (skb->link3 != NULL)
		{
			printk("ip.c: link3 != NULL\n");
			skb->link3 = NULL;
		}
		// 插入已发送但未确认队列，用于超时重传
		if (sk->send_head == NULL)
		{
			sk->send_tail = skb;
			sk->send_head = skb;
		}
		else
		{
			sk->send_tail->link3 = skb;
			sk->send_tail = skb;
		}
		/* skb->link3 is NULL */

		/* Interrupt restore */
		restore_flags(flags);
	}
	else
		/* Remember who owns the buffer */
		skb->sk = sk;

	/*
	 *	If the indicated interface is up and running, send the packet.
	 */
	 
	ip_statistics.IpOutRequests++;
#ifdef CONFIG_IP_ACCT
	ip_acct_cnt(iph,dev, ip_acct_chain);
#endif	
	
#ifdef CONFIG_IP_MULTICAST	

	/*
	 *	Multicasts are looped back for other local users
	 */
	 
	if (MULTICAST(iph->daddr) && !(dev->flags&IFF_LOOPBACK))
	{
		if(sk==NULL || sk->ip_mc_loop)
		{
			if(iph->daddr==IGMP_ALL_HOSTS)
				ip_loopback(dev,skb);
			else
			{
				struct ip_mc_list *imc=dev->ip_mc_list;
				while(imc!=NULL)
				{
					if(imc->multiaddr==iph->daddr)
					{
						ip_loopback(dev,skb);
						break;
					}
					imc=imc->next;
				}
			}
		}
		/* Multicasts with ttl 0 must not go beyond the host */
		
		if(skb->ip_hdr->ttl==0)
		{
			kfree_skb(skb, FREE_READ);
			return;
		}
	}
#endif
	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)
		{	
			// 调用mac层发送
			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);
	}
}