本文分析基于Linux Kernel 1.2.13
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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。
上篇分析了应用层经过BSD socket层到INET socket层的函数调用关系和数据的处理流程，INET层会调用具体的传输层协议，还是以UDP协议为例
udp_write()函数
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));
}
在分析udp_sendto()函数之前，先了解一下sockaddr_in结构，这是标准的网络接口地址结构的定义
struct sockaddr_in { short intsin_family;/* Address family地址族*/ unsigned short intsin_port;/* Port number端口号*/ struct in_addrsin_addr;/* Internet address网络地址*/ /* Pad to size of `struct sockaddr'. */ unsigned char__pad[__SOCK_SIZE__ - sizeof(short int) -sizeof(unsigned short int) - sizeof(struct in_addr)];};#define sin_zero__pad/* for BSD UNIX comp. -FvK*/
udp_sentdto()函数
static int udp_sendto(struct sock *sk, unsigned char *from, int len, int noblock, unsigned flags, struct sockaddr_in *usin, int addr_len){struct sockaddr_in sin;int tmp;/* *Check the flags. We support no flags for UDP sending */if (flags&~MSG_DONTROUTE) return(-EINVAL);/* *Get and verify the address. */ if (usin) //如果usin不是空{if (addr_len < sizeof(sin)) return(-EINVAL);memcpy(&sin,usin,sizeof(sin));if (sin.sin_family && sin.sin_family != AF_INET)return(-EINVAL);if (sin.sin_port == 0) return(-EINVAL);} else //usin为空{if (sk->state != TCP_ESTABLISHED) return(-EINVAL);sin.sin_family = AF_INET;//协议族sin.sin_port = sk->dummy_th.dest;//目的端口sin.sin_addr.s_addr = sk->daddr;//目的地址 } /* *BSD socket semantics. You must set SO_BROADCAST to permit *broadcasting of data. */ if(sin.sin_addr.s_addr==INADDR_ANY)//目的地址是全0地址，对应当前主机 sin.sin_addr.s_addr=ip_my_addr();//将目的地址设为当前主机的网络地址 if(!sk->broadcast && ip_chk_addr(sin.sin_addr.s_addr)==IS_BROADCAST) return -EACCES;/* Must turn broadcast on first */sk->inuse = 1;/* Send the packet. */tmp = udp_send(sk, &sin, from, len, flags);//调用udp_send()真正的发送数据/* The datagram has been sent off. Release the socket. */release_sock(sk);return(tmp);}
udp_send()函数
static int udp_send(struct sock *sk, //要发送的数据包使用的协议对用的sock结构 struct sockaddr_in *sin,//目的端的标准的网络接口地址 unsigned char *from,//要发送的数据所在地址 int len,//发送数据的长度 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. */ size = sk->prot->max_header + len;//计算大小为UDP最长表头+ 数据长度skb = sock_alloc_send_skb(sk, size, 0, &tmp);//根据要发送的数据分配sk_buff结构空间并对当前套接字状态检查if (skb == NULL) return tmp;skb->sk = NULL;/* to avoid changing sk->saddr */skb->free = 1;skb->localroute = sk->localroute|(rt&MSG_DONTROUTE);/* *Now build the IP and MAC header. */ buff = skb->data;//将skb中的数据指针赋值给buff指针saddr = sk->saddr;//本地地址dev = NULL;ttl = sk->ip_ttl;//生存时间#ifdef CONFIG_IP_MULTICASTif (MULTICAST(sin->sin_addr.s_addr))ttl = sk->ip_mc_ttl;#endiftmp = sk->prot->build_header(skb, saddr, sin->sin_addr.s_addr,&dev, IPPROTO_UDP, sk->opt, skb->mem_len,sk->ip_tos,ttl);//调用ip_build_header()创建IP报头和调用ip_send()创建MAC首部skb->sk=sk;/* So memory is freed correctly *//* *Unable to put a header on the packet. */ if (tmp < 0 ) {sk->prot->wfree(sk, skb->mem_addr, skb->mem_len);return(tmp); } buff += tmp;saddr = skb->saddr; /*dev->pa_addr;*/skb->len = tmp + sizeof(struct udphdr) + len;/* len + UDP + IP + MAC */skb->dev = dev;/* *Fill in the UDP header. 填写UDP的报头 */ uh = (struct udphdr *) buff;uh->len = htons(len + sizeof(struct udphdr));//数据包长度uh->source = sk->dummy_th.source;//本地端口uh->dest = sin->sin_port;//远端端口buff = (unsigned char *) (uh + 1);/* *Copy the user data. */ memcpy_fromfs(buff, from, len);//复制用户数据 /* *Set up the UDP checksum. */ udp_send_check(uh, saddr, sin->sin_addr.s_addr, skb->len - tmp, sk);//计算UDP报头的校验和/* *Send the datagram to the interface. */ udp_statistics.UdpOutDatagrams++; sk->prot->queue_xmit(sk, dev, skb, 1);//调用IP层函数发送数据return(len);}
这样要发送的数据填充的sk_buff结构中之后再对UDP数据包添加IP数据报头和MAC帧的首部。最后调用IP层的发送函数发送数据包。