Linux内核--网络栈实现分析（五）--传输层之UDP协议（上）

时间：2022-04-16 11:05:27

本文分析基于Linux Kernel 1.2.13

原创作品，转载请标明出处http://blog.csdn.net/yming0221/article/details/7532512

更多请看专栏，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。


这里看看数据包从IP层是如何交给传输层来处理的，为了方便，这里传输层以UDP协议为例来分析。

从ip_rcv()函数中可以看到

       /** Pass on the datagram to each protocol that wants it,
* based on the datagram protocol.  We should really
* check the protocol handler's return values here...
*/
ipprot->handler(skb2, dev, opts_p ? &opt : 0, iph->daddr,
(ntohs(iph->tot_len) - (iph->ihl * 4)),
iph->saddr, 0, ipprot);

这里调用指定协议的handler函数，如果是UDP协议，该函数的定义 udp_protocol如下 

static struct inet_protocol udp_protocol = {  udp_rcv,/* UDP handler*/  NULL,/* Will be UDP fraglist handler */  udp_err,/* UDP error control*/  &tcp_protocol,/* next*/  IPPROTO_UDP,/* protocol ID*/  0,/* copy*/  NULL,/* data*/  "UDP"/* name*/};

先看UDP协议数据报的报头定义如下：比较简单

Linux内核--网络栈实现分析（五）--传输层之UDP协议（上）

struct udphdr {  unsigned shortsource;//源端口  unsigned shortdest;//目的端口  unsigned shortlen;//数据包长度  unsigned shortcheck;//检验和};


下面就分析下udp_rcv()函数，流程图：

Linux内核--网络栈实现分析（五）--传输层之UDP协议（上）


/* *All we need to do is get the socket, and then do a checksum.  */ int udp_rcv(struct sk_buff *skb, struct device *dev, struct options *opt,unsigned long daddr, unsigned short len,unsigned long saddr, int redo, struct inet_protocol *protocol){  struct sock *sk;  struct udphdr *uh;unsigned short ulen;int addr_type = IS_MYADDR;if(!dev || dev->pa_addr!=daddr)//检查这个数据包是不是发送给本地的数据包addr_type=ip_chk_addr(daddr);//该函数定义在devinet.c中，用于检查ip地址是否是本地或多播、广播地址/* *Get the header. */  uh = (struct udphdr *) skb->h.uh;//获得UDP数据报的报头    ip_statistics.IpInDelivers++;/* *Validate the packet and the UDP length. */ ulen = ntohs(uh->len);//参数len表示ip负载长度(IP数据报的数据部分长度)= UDP数据包头+UDP数据包的数据部分+填充部分长度//ulen表示的是UDP数据报首部和负载部分的长度，所以正常情况下len>=ulenif (ulen > len || len < sizeof(*uh) || ulen < sizeof(*uh)) {printk("UDP: short packet: %d/%d\n", ulen, len);udp_statistics.UdpInErrors++;kfree_skb(skb, FREE_WRITE);return(0);}if (uh->check && udp_check(uh, len, saddr, daddr)) //进行UDP数据包校验{/* <mea@utu.fi> wants to know, who sent it, to   go and stomp on the garbage sender... */printk("UDP: bad checksum. From %08lX:%d to %08lX:%d ulen %d\n",       ntohl(saddr),ntohs(uh->source),       ntohl(daddr),ntohs(uh->dest),       ulen);udp_statistics.UdpInErrors++;kfree_skb(skb, FREE_WRITE);return(0);}len=ulen;//对len赋值为实际的UDP数据报长度#ifdef CONFIG_IP_MULTICAST//对多播情况进行处理if (addr_type!=IS_MYADDR){/* *Multicasts and broadcasts go to each listener. */struct sock *sknext=NULL;//next指针/*get_sock_mcast 获取在对应端口的多播套接字队列*下面函数的参数依次表示:sock结构指针，本地端口，远端地址，远端端口，本地地址*/sk=get_sock_mcast(udp_prot.sock_array[ntohs(uh->dest)&(SOCK_ARRAY_SIZE-1)], uh->dest,saddr, uh->source, daddr);if(sk){do{struct sk_buff *skb1;sknext=get_sock_mcast(sk->next, uh->dest, saddr, uh->source, daddr);//下一个满足条件的套接字if(sknext)skb1=skb_clone(skb,GFP_ATOMIC);elseskb1=skb;if(skb1)udp_deliver(sk, uh, skb1, dev,saddr,daddr,len);//对满足条件的套接字调用发送函数发送sk=sknext;}while(sknext!=NULL);}elsekfree_skb(skb, FREE_READ);return 0;}#endif  sk = get_sock(&udp_prot, uh->dest, saddr, uh->source, daddr);if (sk == NULL) //没有找到本地对应的套接字，则进行出错处理  {  udp_statistics.UdpNoPorts++;if (addr_type == IS_MYADDR) {icmp_send(skb, ICMP_DEST_UNREACH, ICMP_PORT_UNREACH, 0, dev);//回复ICMP出错报文，目的主机不可达}/* * Hmm.  We got an UDP broadcast to a port to which we * don't wanna listen.  Ignore it. */skb->sk = NULL;kfree_skb(skb, FREE_WRITE);return(0);  }return udp_deliver(sk,uh,skb,dev, saddr, daddr, len);//调用函数发送套接字}

上面函数中调用了get_sock_mcast()函数，下面具体分析一下该函数的功能，该函数定义的位置在文件af_inet.c中 

/* *Deliver a datagram to broadcast/multicast sockets. */ struct sock *get_sock_mcast(struct sock *sk, //套接字指针unsigned short num,//本地端口unsigned long raddr,//远端地址unsigned short rnum,//远端端口unsigned long laddr)//本地地址 {struct sock *s;unsigned short hnum;hnum = ntohs(num);/* * SOCK_ARRAY_SIZE must be a power of two.  This will work better * than a prime unless 3 or more sockets end up using the same * array entry.  This should not be a problem because most * well known sockets don't overlap that much, and for * the other ones, we can just be careful about picking our * socket number when we choose an arbitrary one. */s=sk;for(; s != NULL; s = s->next) {if (s->num != hnum) //本地端口不符合，跳过continue;if(s->dead && (s->state == TCP_CLOSE))//dead=1表示该sock结构已经处于释放状态continue;if(s->daddr && s->daddr!=raddr)//sock的远端地址不等于条件中的远端地址continue;if (s->dummy_th.dest != rnum && s->dummy_th.dest != 0)continue; if(s->saddr  && s->saddr!=laddr)//sock的本地地址不等于条件的本地地址continue;return(s);  }  return(NULL);}

下面是udp_rcv调用的udp_deliver()函数 

static int udp_deliver(struct sock *sk,//sock结构指针struct udphdr *uh,//UDP头指针struct sk_buff *skb,//sk_buffstruct device *dev,//接收的网络设备long saddr,//本地地址long daddr,//远端地址int len)//数据包的长度{//对skb结构相应字段赋值skb->sk = sk;skb->dev = dev;//skb->len = len;/* *These are supposed to be switched.  */ skb->daddr = saddr;//设置目的地址为本地地址skb->saddr = daddr;//设置源地址为远端地址/* *Charge it to the socket, dropping if the queue is full. */skb->len = len - sizeof(*uh);   if (sock_queue_rcv_skb(sk,skb)<0) //调用sock_queu_rcv_skb()函数，将skb挂到sk接构中的接收队列中{udp_statistics.UdpInErrors++;ip_statistics.IpInDiscards++;ip_statistics.IpInDelivers--;skb->sk = NULL;kfree_skb(skb, FREE_WRITE);release_sock(sk);return(0);}  udp_statistics.UdpInDatagrams++;release_sock(sk);return(0);}
sock_queu_rcv_skb()函数的实现如下： 

/* *Queue a received datagram if it will fit. Stream and sequenced protocols *can't normally use this as they need to fit buffers in and play with them. */int sock_queue_rcv_skb(struct sock *sk, struct sk_buff *skb){unsigned long flags;if(sk->rmem_alloc + skb->mem_len >= sk->rcvbuf)return -ENOMEM;save_flags(flags);cli();sk->rmem_alloc+=skb->mem_len;skb->sk=sk;restore_flags(flags);skb_queue_tail(&sk->receive_queue,skb);if(!sk->dead)sk->data_ready(sk,skb->len);return 0;}

这里就完成了数据包从网络层到传输层的传输。下面的博文将会分析数据包的从上到下的传输过程。


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