Data Communication Computer Network 简明教程
Internetworking in Computer Network
在实际情况下，处于同一管理下的网络通常在地理上分散。可能存在将相同类型的两个不同网络以及不同类型的两个不同网络连接起来的需求。两个网络之间的路由被称为互联网络。
In real world scenario, networks under same administration are generally scattered geographically. There may exist requirement of connecting two different networks of same kind as well as of different kinds. Routing between two networks is called internetworking.
可以根据各种参数（例如，协议、拓扑、第 2 层网络和寻址方案）将网络视为不同。
Networks can be considered different based on various parameters such as, Protocol, topology, Layer-2 network and addressing scheme.
在互联网络中，路由器相互了解对方的地址以及超出它们的其他地址。它们可以被静态地配置到不同的网络上，也可以通过使用互联网络路由协议进行学习。
In internetworking, routers have knowledge of each other’s address and addresses beyond them. They can be statically configured go on different network or they can learn by using internetworking routing protocol.
在组织或管理中使用的路由协议称为内部网关协议或 IGP。RIP、OSPF 是 IGP 的示例。不同组织或管理之间的路由可能具有外部网关协议，并且只有一种 EGP，即边界网关协议。
Routing protocols which are used within an organization or administration are called Interior Gateway Protocols or IGP. RIP, OSPF are examples of IGP. Routing between different organizations or administrations may have Exterior Gateway Protocol, and there is only one EGP i.e. Border Gateway Protocol.
Tunneling
如果两个在地理上分离的网络想要相互通信，它们可以在它们之间部署一条专线，或者它们必须通过中间网络传输数据。
If they are two geographically separate networks, which want to communicate with each other, they may deploy a dedicated line between or they have to pass their data through intermediate networks.
隧道是一种机制，两个或更多相同的网络通过传递中间网络复杂性相互通信。隧道配置在两端。
Tunneling is a mechanism by which two or more same networks communicate with each other, by passing intermediate networking complexities. Tunneling is configured at both ends.
当数据从隧道的其中一端输入时，它被标记。然后将此标记数据在中间或中转网络内路由到隧道的另一端。当数据退出隧道时，其标记将被移除并传递到网络的另一部分。
When the data enters from one end of Tunnel, it is tagged. This tagged data is then routed inside the intermediate or transit network to reach the other end of Tunnel. When data exists the Tunnel its tag is removed and delivered to the other part of the network.
两端似乎直接连接，并且标记使数据在中转网络中传输而不会进行任何修改。
Both ends seem as if they are directly connected and tagging makes data travel through transit network without any modifications.
Packet Fragmentation
大多数以太网段的最大传输单元 (MTU) 固定为 1500 字节。数据包的长度可以根据应用程序而多或少。中转路径中的设备还具有硬件和软件功能，这些功能指示该设备可以处理多少数据以及可以处理多大的数据包。
Most Ethernet segments have their maximum transmission unit (MTU) fixed to 1500 bytes. A data packet can have more or less packet length depending upon the application. Devices in the transit path also have their hardware and software capabilities which tell what amount of data that device can handle and what size of packet it can process.
如果数据包大小小于或等于中转网络可以处理的数据包大小，则该数据包以中立的方式进行处理。如果数据包较大，则将其分解为较小的片段，然后转发。这称为数据包分段。每个片段包含相同的目标和源地址，并且很容易通过中转路径路由。在接收端将其再次组装。
If the data packet size is less than or equal to the size of packet the transit network can handle, it is processed neutrally. If the packet is larger, it is broken into smaller pieces and then forwarded. This is called packet fragmentation. Each fragment contains the same destination and source address and routed through transit path easily. At the receiving end it is assembled again.
如果带有 DF（请勿分段）比特设置设为 1 的数据包到达因其长度而无法处理该数据包的路由器，则丢弃该数据包。
If a packet with DF (don’t fragment) bit set to 1 comes to a router which can not handle the packet because of its length, the packet is dropped.
当路由器收到 MF（更多片段）比特设为 1 的数据包时，路由器知道这是一个分段数据包，原始数据包的部分正在传送中。
When a packet is received by a router has its MF (more fragments) bit set to 1, the router then knows that it is a fragmented packet and parts of the original packet is on the way.
如果数据包分段太小，则会增加开销。如果数据包分段太大，则中间路由器可能无法处理它，并且可能会被丢弃。
If packet is fragmented too small, the overhead is increases. If the packet is fragmented too large, intermediate router may not be able to process it and it might get dropped.