求一篇关于计算机的英语文章加翻译，500字左右。

网络层设计问题

在下面的部分里我们将提供一个说明来解释一些网络层设计必须抓住的问题。这些问题包括提供给运输层的服务和内部子网的设计。
5.1.1存储转发分组交换
但是在开始解释网络层详细情况之前，可能值得restate网络成协议运行的环境。这个环境可以在图5-1中看到。这个系统的主要元件是运输工具（被传送线连接的路由器），在阴影椭圆内显示，客户的工具是用椭圆外来表示。主机H1是直接从椭圆外连接到其中的一个路由器A上。相反，H2是在一个带有一个路由器F的网上，它是被客户拥有运行的。这个路由器也有一条线连接传送设备。我们展示了作为椭圆外 的F因为它不属于送信人，而就结构，软件和协议来说，它可能与运输的路由器的算法没什么不同（我们主要关心的就是算法）。
这个设备像下面这样用。一个带有即将要传送到最近的路由器的发送分组的主机，要么在它自己的局域网内，要么通过一个点到点连接到一个发信人。这个分组先储存在那儿直到它都到达并且校验和也通过。然后，它被沿着这个路径转发到下一个路由器直到它到达它所传递的目的主机。这种机制就是我们在前面章节中所提到的分组转发。
5.1 Network Layer Design Issues
In the following sections we will provide an introduction to some of the issues that the designers of the network layer must grapple with. These issues include the service provided to the transport layer and the internal design of the subnet.
5.1.1 Store-and-Forward Packet Switching
But before starting to explain the details of the network layer, it is probably worth restating the context in which the network layer protocols operate. This context can be seen in Fig. 5-1. The major components of the system are the carrier's equipment (routers connected by transmission lines), shown inside the shaded oval, and the customers' equipment, shown outside the oval. Host H1 is directly connected to one of the carrier's routers, A, by a leased line. In contrast, H2 is on a LAN with a router, F, owned and operated by the customer. This router also has a leased line to the carrier's equipment. We have shown F as being outside the oval because it does not belong to the carrier, but in terms of construction, software, and protocols, it is probably no different from the carrier's routers. Whether it belongs to the subnet is arguable, but for the purposes of this chapter, routers on customer premises are considered part of the subnet because they run the same algorithms as the carrier's routers (and our main concern here is algorithms).