uip代码分析-带书签.pdf

uIP的ARP协议代码分析之一 ARP请求 对于一个设备用的ARP协议而言,重要的东西包括三方面: 1. 一个本地的IP与MAC地址的缓存表.以有对应的更新和查询操作. 2. 一个发送ARP请求的函数. 3. 一个处理ARP 回复的函数. 下面我们来看uIP 中是如何实现的(代码见uip_arp.c: 首先,定义一个缓存表的数据结构,99行起: structarp_entry { u16_tipaddr[2]; structuip_eth_addrethaddr; u8_ttime; }; 只有三个项,很简单 第一项是ip地址,16*2=4*8位的,保存四个八位组. 第二项是MAC地址. 第三项是缓存更新时间. 下来是ARP请求发送函数: /* */ /** * Prepend Ethernet header to an outbound IP packet and see if we need * to send out an ARP request. *为传出的IP包添加以太网头并看是否需要发送ARP请求. * This function should be called before sending out an IP packet. The * function checks the destination IP address of the IP packet to see * what Ethernet MAC address that should be used as a destination MAC * address on the Ethernet. *此函数应该在发送IP包时调用,它会检查IP包的目的IP地址,看看以太网应该使用什么目 的MAC地址. * If the destination IP address is in the local network (determined * by logical ANDing of netmask and our IP address), the function * checks the ARP cache to see if an entry for the destination IP * address is found. If so, an Ethernet header is prepended and the * function returns. If no ARP cache entry is found for the * destination IP address, the packet in the uip_buf[] is replaced by * an ARP request packet for the IP address. The IP packet is dropped * and it is assumed that they higher level protocols (e.g.,TCP) * eventually will retransmit the dropped packet. *如果目的IP地址是在局域网中(由IP地址与子网掩码的与逻辑决定),函数就会从ARP缓存 表中查找有 *无对应项.若有,就取对应的MAC地址,加上以太网头,并返回,否则uip_buf[]中的数据包会 被替换成一个 *目的IP 址的ARP请求.原来的IP包会被简单的仍掉,此函数假设高层协议(如TCP)会最 终重传扔掉的包. * If the destination IP address is not on the local network, the IP * address of the default router is used instead. *如果目标IP地址并非一个局域网IP,则会使用默认路由的IP地址. * When the function returns, a packet is present in the uip_buf[] * buffer, and the length of the packet is in the global variable * uip_len.函数返回时,uip_buf[]中已经有了一个包,其长度由uip_len指定. */ /* */ void uip_arp_out(void) { struct arp_entry *tabptr; /* Find the destination IP address in the ARP table and