NetworkEngineeringEnglish-Assignment1-LayersintheTCP_IPModel.doc

Layers in the TCP/IP Model TCP/IP模型中的分层 The layers near the top are logically closer to the user application, while those near the bottom are logically closer to the physical transmission of the data. Viewing layers as providing or consuming a service is a method of abstraction to isolate upper layer protocols from the detail of transmitting bits over, for example, Ethernet and collision detection, while the lower layers avoid having to know the details of each and every application and its protocol. 在逻辑上，接近于顶部的层次对用户更加亲和，而接近于底部的层次对于数据的物理传输更加亲和。有一种方法是将层次视为可以提供并使用服务的，这样就可以抽象地隔离传输比特细节的上层协议，例如，以太网与碰撞检测，此时低分层不必须了解任何设备及其协议。 This abstraction also allows upper layers to provide services that the lower layers cannot, or choose not to, provide. Again, the original OSI Reference Model was extended to include connectionless services. For example, IP is not designed to be reliable and is a best effort delivery protocol. This means that all transport layer implementations must choose whether or not to provide reliability and to what degree. UDP provides data integrity (via a checksum) but does not guarantee delivery; TCP provides both data integrity and delivery guarantee (by retransmitting until the receiver acknowledges the reception of the packet). 此抽象概念也允许上层提供服务，然而低层不可提供服务，或选择型不可提供服务。此外，原始的OSI参照模型可被扩展用于包含非面向连接的服务。例如，IP协议并非设计为可以靠的，但却是一种最有效的传输协议。那意味着所有的传输层的实现必须选择是否拥有可依靠性及可依靠性的级别。UDP协议提供数据的保存（通过校检和），但并不保证传输；TCP供数据的保存，也保证数据的传输（通过中继站的转发，直至接收方承认包的获取时中继站进行转发）。 Link Layer 链路层 The Link Layer is the networking scope of the local network connection to which a host is attached. This regime is called the link in Internet literature. This is the lowest component layer of the Internet protocols, as TCP/IP is designed to be hardware independent. As a result TCP/IP has been implemented on top of virtually any hardware networking technology in existence. 链路层是指某个主机所依附的局域网的网络范围。这个制度被称为是网络的连接。这是互联网协议的最低组成部分，例如TCP/IP协议是独立于硬件的。结果就是TCP/IP协议可以在现今几乎所有的硬件网络技术上实现