/*
* 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, //发送数据的队列所对应的sock结构
struct device *dev,//发送该数据包的网卡设备
struct sk_buff *skb,//封装好的sk_buff结构，要发送的数据在该结构中
int free)//主要配合TCP协议使用，用于数据包的重发，UDP等协议调用是free=1
{
struct iphdr *iph;//IP数据报首部指针
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;//进一步完整sk_buff的相应字段
skb->when = jiffies;//用于TCP协议的超时重传
/*
* 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;//指针指向sk_buff中的数据部分
ptr += dev->hard_header_len;//hard_header_len为硬件首部长度，在net_init.c的函数eth_setup()函数中设置的，dev->hard_header_len = ETH_HLEN; 以太网首部长度为14
iph = (struct iphdr *)ptr;//prt已经指向IP数据包的首部
skb->ip_hdr = iph;
iph->tot_len = ntohs(skb->len-dev->hard_header_len);//计算IP数据报的总长度 #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...
*/ 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;//设置skb的free值，free=1，发送后立即释放；free=2，不但释放缓存，而且不分配新的序列号 /*
* Do we need to fragment. Again this is inefficient.
* We need to somehow lock the original buffer and use
* bits of it.
*/
//数据帧中的数据部分必须小于等于MTU
if(skb->len > dev->mtu + dev->hard_header_len)//发送的数据长度大于数据帧的数据部分和帧首部之和，则需要分片
{
ip_fragment(sk,skb,dev,0);//对数据报分片后继续调用ip _queue_xmit()函数发送数据
IS_SKB(skb);
kfree_skb(skb,FREE_WRITE);
return;
} /*
* Add an IP checksum
*/ ip_send_check(iph);//IP数据报首部检查 /*
* 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)//说明该数据包仍然存在于某个缓存队列
{
printk("ip_queue_xmit: next != NULL\n");
skb_unlink(skb);//将其从缓存链表中删除，否则可能导致内核错误
} /*
* 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 (!free)//free=0
{
unsigned long flags;
/* The socket now has more outstanding blocks */ sk->packets_out++; /* Protect the list for a moment */
save_flags(flags);
cli(); if (skb->link3 != NULL)//link3指向数据报道呃重发队列
{
printk("ip.c: link3 != NULL\n");
skb->link3 = NULL;
}
//sk中send_tail和send_head是用户缓存的单向链表表尾和表头
if (sk->send_head == NULL)
{
sk->send_tail = skb;
sk->send_head = skb;
}
else
{
sk->send_tail->link3 = skb;//link3指针用于数据包的连接
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 //这部分是IP数据报的多播处理 /*
* Multicasts are looped back for other local users
*/
.......................................
#endif
if((dev->flags&IFF_BROADCAST) && iph->daddr==dev->pa_brdaddr && !(dev->flags&IFF_LOOPBACK))//广播数据包的处理
ip_loopback(dev,skb);
if (dev->flags & IFF_UP)//设备状态正常
{
/*
* If we have an owner use its priority setting,
* otherwise use NORMAL
*/
//调用设备接口层函数发送数据: dev_queue_xmit()函数
if (sk != NULL)
{
dev_queue_xmit(skb, dev, sk->priority);
}
else
{
dev_queue_xmit(skb, dev, SOPRI_NORMAL);
}
}
else//设备状态不正常
{
ip_statistics.IpOutDiscards++;
if (free)
kfree_skb(skb, FREE_WRITE);
}
} | 
