数据包转发详解.docVIP

数据包转发详解 The IP Routing Process 摘自CCNA:Cisco Certified Network Associate Study Guide Fourth Edition第五章 The IP Routing Process The IP routing process is fairly simple and doesn’t change, regardless of the size network you have. For an example, we’ll use Figure 5.1 to describe step by step what happens when Host_ A wants to communicate with Host B on a different network. FIGURE5.1 IP routing example using two hosts and one router In this example, a user on Host_A pings Host_B’s IP address. Routing doesn’t get simpler than this, but it still involves a lot of steps. Let’s work through them: 主机A ping 主机B，没有比这更简单的了，但一样有很多的步骤 1.Internet Control Message Protocol (ICMP) creates an echo request payload (which is just the alphabet in the data field). ICMP生成一个回显请求负载（一些字母） 2.ICMP hands that payload to Internet Protocol (IP), which then creates a packet. At a min- imum, this packet contains an IP source address, an IP destination address, and a protocol field with 01h (remember that Cisco likes to use 0x in front of hex characters, so this could look like 0x01). All of that tells the receiving host to whom it should hand the payload when the destination is reached—in this example, ICMP ICMP把负载交给IP，IP生成数据包，包括源IP地址、目标IP地址、协议类型字段值为0X01。协议类型指明了接收者把负载交给哪个协议，这里是ICMP。 3.Once the packet is created, IP determines whether the destination IP address is on the local network or a remote one 判断目标IP是在局域网还是在外网 4.Since IP determines this is a remote request, the packet needs to be sent to the default gate- way so the packet can be routed to the remote network. The Registry in Windows is parsed to find the configured default gateway. 在外网，包要发给默认网关以便路由到那个网络。从windows注册表得到默认网关。 5.The default gateway of host 172.16.10.2 (Host_A) is configured to 172.16.10.1. To be able to send this packet to the default gateway, the hardware address of the router’s interface Ethernet 0 (configured with the IP address of 172.16.10.1) must be known. Why? So the packet can be handed down to the Da