Linux内核网络栈实现分析（十）网络层之IP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/7552455

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。

上篇博文分析传输层最终会调用函数ip_queue_xmit()函数，将发送数据的任务交给网络层，下面就分析了下该函数：

该函数的主要函数调用关系图如下：

/*
 * 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);
    }
}

这个函数中对长度过长的数据包进行了分片，ip_fragment()函数，该函数没有详细分析。

void ip_fragment(struct sock *sk, struct sk_buff *skb, struct device *dev, int is_frag)
{
    struct iphdr *iph;
    unsigned char *raw;
    unsigned char *ptr;
    struct sk_buff *skb2;
    int left, mtu, hlen, len;
    int offset;
    unsigned long flags;

    /*
     *  Point into the IP datagram header.
     */

    raw = skb->data;
    iph = (struct iphdr *) (raw + dev->hard_header_len);

    skb->ip_hdr = iph;

    /*
     *  Setup starting values.
     */

    hlen = (iph->ihl * sizeof(unsigned long));
    left = ntohs(iph->tot_len) - hlen;   /* Space per frame */
    hlen += dev->hard_header_len;        /* Total header size */
    mtu = (dev->mtu - hlen);     /* Size of data space */
    ptr = (raw + hlen);         /* Where to start from */

    /*
     *  Check for any "DF" flag. [DF means do not fragment]
     */

    if (ntohs(iph->frag_off) & IP_DF)
    {
        /*
         *  Reply giving the MTU of the failed hop.
         */
        ip_statistics.IpFragFails++;
        icmp_send(skb,ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED, dev->mtu, dev);
        return;
    }

    /*
     *  The protocol doesn't seem to say what to do in the case that the
     *  frame + options doesn't fit the mtu. As it used to fall down dead
     *  in this case we were fortunate it didn't happen
     */

    if(mtu<8)
    {
        /* It's wrong but it's better than nothing */
        icmp_send(skb,ICMP_DEST_UNREACH,ICMP_FRAG_NEEDED,dev->mtu, dev);
        ip_statistics.IpFragFails++;
        return;
    }

    /*
     *  Fragment the datagram.
     */

    /*
     *  The initial offset is 0 for a complete frame. When
     *  fragmenting fragments it's wherever this one starts.
     */

    if (is_frag & 2)
        offset = (ntohs(iph->frag_off) & 0x1fff) << 3;
    else
        offset = 0;


    /*
     *  Keep copying data until we run out.
     */

    while(left > 0)
    {
        len = left;
        /* IF: it doesn't fit, use 'mtu' - the data space left */
        if (len > mtu)
            len = mtu;
        /* IF: we are not sending upto and including the packet end
           then align the next start on an eight byte boundary */
        if (len < left)
        {
            len/=8;
            len*=8;
        }
        /*
         *  Allocate buffer.
         */

        if ((skb2 = alloc_skb(len + hlen,GFP_ATOMIC)) == NULL)
        {
            printk("IP: frag: no memory for new fragment!\n");
            ip_statistics.IpFragFails++;
            return;
        }

        /*
         *  Set up data on packet
         */

        skb2->arp = skb->arp;
        if(skb->free==0)
            printk("IP fragmenter: BUG free!=1 in fragmenter\n");
        skb2->free = 1;
        skb2->len = len + hlen;
        skb2->h.raw=(char *) skb2->data;
        /*
         *  Charge the memory for the fragment to any owner
         *  it might possess
         */

        save_flags(flags);
        if (sk)
        {
            cli();
            sk->wmem_alloc += skb2->mem_len;
            skb2->sk=sk;
        }
        restore_flags(flags);
        skb2->raddr = skb->raddr; /* For rebuild_header - must be here */

        /*
         *  Copy the packet header into the new buffer.
         */

        memcpy(skb2->h.raw, raw, hlen);

        /*
         *  Copy a block of the IP datagram.
         */
        memcpy(skb2->h.raw + hlen, ptr, len);
        left -= len;

        skb2->h.raw+=dev->hard_header_len;

        /*
         *  Fill in the new header fields.
         */
        iph = (struct iphdr *)(skb2->h.raw/*+dev->hard_header_len*/);
        iph->frag_off = htons((offset >> 3));
        /*
         *  Added AC : If we are fragmenting a fragment thats not the
         *         last fragment then keep MF on each bit
         */
        if (left > 0 || (is_frag & 1))
            iph->frag_off |= htons(IP_MF);
        ptr += len;
        offset += len;

        /*
         *  Put this fragment into the sending queue.
         */

        ip_statistics.IpFragCreates++;

        ip_queue_xmit(sk, dev, skb2, 2);//还是调用ip_queue_xmit()函数来发送分片后的数据
    }
    ip_statistics.IpFragOKs++;
}

网络层的发送函数调用了设备接口层，相当于网络模型的链路层的发送函数dev_queue_xmit()

该函数的调用关系如下：

/*
 *  Send (or queue for sending) a packet.
 *
 *  IMPORTANT: When this is called to resend frames. The caller MUST
 *  already have locked the sk_buff. Apart from that we do the
 *  rest of the magic.
 */

void dev_queue_xmit(struct sk_buff *skb, struct device *dev, int pri)
{
    unsigned long flags;
    int nitcount;
    struct packet_type *ptype;
    int where = 0;      /* used to say if the packet should go  */
                /* at the front or the back of the  */
                /* queue - front is a retransmit try    */
                /* where=0 表示是刚从上层传递的新数据包；where=1 表示从硬件队列中取出的数据包*/

    if (dev == NULL)
    {
        printk("dev.c: dev_queue_xmit: dev = NULL\n");
        return;
    }

    if(pri>=0 && !skb_device_locked(skb))//锁定该skb再进行操作，避免造成内核的不一致情况
        skb_device_lock(skb);   /* Shove a lock on the frame */
#ifdef CONFIG_SLAVE_BALANCING
    save_flags(flags);
    cli();
    if(dev->slave!=NULL && dev->slave->pkt_queue < dev->pkt_queue &&
                (dev->slave->flags & IFF_UP))
        dev=dev->slave;
    restore_flags(flags);
#endif
#ifdef CONFIG_SKB_CHECK
    IS_SKB(skb);
#endif
    skb->dev = dev;

    /*
     *  This just eliminates some race conditions, but not all...
     */

    if (skb->next != NULL) //这种条件似乎永远不能成立，因为发送数据包前，数据包已经从缓存队列摘下
    {//以防内核代码有BUG
        /*
         *  Make sure we haven't missed an interrupt.
         */
        printk("dev_queue_xmit: worked around a missed interrupt\n");
        start_bh_atomic();
        dev->hard_start_xmit(NULL, dev);
        end_bh_atomic();
        return;
    }

    /*
     *  Negative priority is used to flag a frame that is being pulled from the
     *  queue front as a retransmit attempt. It therefore goes back on the queue
     *  start on a failure.
     */

    if (pri < 0) //优先级小于0表示是从硬件队列中取出的数据包
    {
        pri = -pri-1;
        where = 1;
    }

    if (pri >= DEV_NUMBUFFS)
    {
        printk("bad priority in dev_queue_xmit.\n");
        pri = 1;
    }

    /*
     *  If the address has not been resolved. Call the device header rebuilder.
     *  This can cover all protocols and technically not just ARP either.
     */

    if (!skb->arp && dev->rebuild_header(skb->data, dev, skb->raddr, skb)) {//用于ARP协议，并重建MAC帧首部
        return;
    }

    save_flags(flags);
    cli();
    if (!where) {//表示是新数据包，需要将其加入设备队列中
#ifdef CONFIG_SLAVE_BALANCING
        skb->in_dev_queue=1;//该数据包在设备队列
#endif
        skb_queue_tail(dev->buffs + pri,skb);//将发送数据包加入硬件队列
        skb_device_unlock(skb);     /* Buffer is on the device queue and can be freed safely */
        skb = skb_dequeue(dev->buffs + pri);//从硬件队列中取出一个数据包
        skb_device_lock(skb);       /* New buffer needs locking down */
#ifdef CONFIG_SLAVE_BALANCING
        skb->in_dev_queue=0;
#endif
    }
    restore_flags(flags);

    /* copy outgoing packets to any sniffer packet handlers */
    if(!where)//对于新的数据包，则遍历网络层协议队列，内核支持混杂模式
    {
        for (nitcount= dev_nit, ptype = ptype_base; nitcount > 0 && ptype != NULL; ptype = ptype->next)
        {
            /* Never send packets back to the socket
             * they originated from - MvS (miquels@drinkel.ow.org)
             */
            if (ptype->type == htons(ETH_P_ALL) &&
               (ptype->dev == dev || !ptype->dev) &&
               ((struct sock *)ptype->data != skb->sk))
            {
                struct sk_buff *skb2;
                if ((skb2 = skb_clone(skb, GFP_ATOMIC)) == NULL)
                    break;
                /*
                 *  The protocol knows this has (for other paths) been taken off
                 *  and adds it back.
                 */
                skb2->len-=skb->dev->hard_header_len;
                ptype->func(skb2, skb->dev, ptype);//IP层函数对应func为ip_rcv()，将发送的数据回送一份给对应的网络层协议
                nitcount--;//用于及时退出循环
            }
        }
    }
    start_bh_atomic();//开始原子操作
    if (dev->hard_start_xmit(skb, dev) == 0) {//调用硬件的发送函数发送数据
        end_bh_atomic();//结束原子操作
        /*
         *  Packet is now solely the responsibility of the driver
         */
        return;//到这里说明数据包成功发送
    }
    //数据包没有成功发送，进行处理，将数据包从新加入硬件队列
    end_bh_atomic();

    /*
     *  Transmission failed, put skb back into a list. Once on the list it's safe and
     *  no longer device locked (it can be freed safely from the device queue)
     */
    cli();
#ifdef CONFIG_SLAVE_BALANCING
    skb->in_dev_queue=1;
    dev->pkt_queue++;
#endif
    skb_device_unlock(skb);//对SKB解锁
    skb_queue_head(dev->buffs + pri,skb);//这次采用头插法插入硬件发送队列
    restore_flags(flags);
}

具体的硬件发送函数dev->hard_start_xmit的实现将做下篇博文中分析。