Network Engineering English - Assignment 1-Layers in the TCP_IP Model.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。The Link Layer is used to move packets between the Internet Layer interfaces of two different hosts on the same link. The processes of transmitting and receiving packets on a given link can be controlled in the software device driver for the network card, as well as on firmware or specialized chipsets. These will perform data link functions such as adding a packet header to prepare it for transmission, and then actually transmit the frame over a physical medium. The TCP/IP model includes specifications of translating the network addressing methods used in the Internet Protocol to data link addressing, such as Media Access Control (MAC), however all other aspects below that level are implicitly assumed to exist in the Link Layer, but are not explicitly defined. 链路层是用来作为在同一链路上的网络层之间移动数据包的接口。在一个给定的链路上，传输、接收包的进程可以控制在安装有网卡的软件设备驱动设备上，也可在固件或专门的芯片组上。这些将执行数据链路的功能，例如，增加一个数据包的头来准备传输，然后实际上地在物理介质上传输帧。TCP/IP模型包括在网络协议中使用的网络寻址方法的规范，例如介质访存控制协议（MAC），然而其他方面的水平都是完全地假定存在于链路层上，然而这些并没有明确的定义。The Link Layer is also the layer where packets may be selected to be sent over a virtual private network or other networking tunnel. In this scenario, the Link Layer data may be considered application data which traverses another instantiation of the IP stack for transmission or reception over another IP connection. Such a connection, or virtual link, may be established with a transport protocol or even an application scope protocol that serves as a tunnel in the Link Layer of the protocol stack. Thus, the TCP/IP model does not dictate a strict hierarchical encapsulation sequence. 链路层也是在虚拟专用网络或其他网络隧道中可以选择发送数据包的层。在这种情况下，链路层的数据可以被认为是通过遍历另一个IP协议栈的实例化来发送或接受来自另一个IP的连接的应用数据。这样的链接，或者虚电路，可以通过一个传送协议或者一个应用范围协议来建立，作为协议栈在链路层中的通道。因此，TCP/IP模型并不能决定一个严格意义上的分层封装序列。Internet Layer网络层The Internet Layer solves the problem of sending packets across one or more networks. Internetworking requires sending data from the source network to the destination network. This process is called routing. 网络层解决了在一个或多个网络上发送数据包的问题。互联网网络要求从源网络发送数据到目的网络。此进程命名为路径选择。In the Internet Protocol Suite, the Internet Protocol performs two basic functions:在互联网协议套件中，网络协议执行两项基本功能：l Host addressing and identification: This is accomplished with a hierarchical addressing system (see IP address). 主机寻址与识别：这是通过一个分层寻址系统来实现的（如IP地址）。l Packet routing: This is the basic task of getting packets of data (datagrams) from source to destination by sending them to the next network node (router) closer to the final destination. 数据包路径选择：基本任务是从源网络获得的数据包（数据报），通过发送到下一个网络节点（路由器）更接近最终的目的地的方法，发送到目的网络。IP can carry data for a number of different upper layer protocols. These protocols are each identified by a unique protocol number: for example, Internet Control Message Protocol (ICMP) and Internet Group Management Protocol (IGMP) are protocols 1 and 2, respectively.IP可以携带不同数量的上层协议。这些协议分别是由不同的协议号来标识的：例如，控制报文协议（ICMP）和互联网组管理协议（IGMP）分别是1号协议和2号协议。Some of the protocols carried by IP, such as ICMP (used to transmit diagnostic information about IP transmission) and IGMP (used to manage IP Multicast data) are layered on top of IP but perform internetworking functions. This illustrates the differences in the architecture of the TCP/IP stack of the Internet and the OSI model. 部分协议是由IP携带，例如ICMP（用来传输IP传输的诊断信息）和IGMP（用来管理IP组播数据）在IP数据报的头部，但表现出互联网网络的功能。这说明了在互联网和OSI模型中TCP / IP协议栈的结构差异。Transport Layer传输层The Transport Layers responsibilities include end-to-end message transfer capabilities independent of the underlying network, along with error control, segmentation, flow control, congestion control, and application addressing (port numbers). End to end message transmission or connecting applications at the transport layer can be categorized as either connection-oriented, implemented in Transmission Control Protocol (TCP), or connectionless, implemented in User Datagram Protocol (UDP). 传输层的职责包括独立的底层网络上的端对端的消息传输功能，此功能需伴随着差错控制，分割，流量控制，拥塞控制，和应用程序的地址（端口号）。在传输层上，端对端的信息传输或通信设备可以被分类为面向链接的（由传输控制协议TCP实现）和非面向连接的（由用户数据报协议实现UDP）。The Transport Layer can be thought of as a transport mechanism, e.g., a vehicle with the responsibility to make sure that its contents (passengers/goods) reach their destination safely and soundly, unless another protocol layer is responsible for safe delivery. 传输层可以被认为是一种传输机制，就像是一辆有着确保其内容（乘客/货物）到达目的地的责任的交通工具一般，除非另一个协议层负责安全地送达。The Transport Layer provides this service of connecting applications through the use of service ports. Since IP provides only a best effort delivery, the Transport Layer is the first layer of the TCP/IP stack to offer reliability. IP can run over a reliable data link protocol such as the High-Level Data Link Control (HDLC). Protocols above transport, such as RPC, also can provide reliability. 传输层通过服务端口来提供应用程序间连接的服务。鉴于IP协议只提供尽力而为的服务，传输层的TCP / IP协议栈是提供可靠服务的第一层。IP协议可以运行在一个可靠的数据链路协议之上，如高级数据链路控制（HDLC）。传输层以上的协议也可以提供可靠性，例如，远程过程调用协议RPC。For example, the Transmission Control Protocol (TCP) is a connection-oriented protocol that addresses numerous reliability issues to provide a reliable byte stream:例如，传输控制协议（TCP）是面向连接的协议，解决了许多可靠性问题提供一个可靠的字节流：l data arrives in-order 数据按序到达l data has minimal error 最小错误数据l duplicate data is discarded 丢弃重复数据l lost/discarded packets are resent 丢失/丢弃的包可被重复发送l includes traffic congestion control 包括交通拥塞控制User Datagram Protocol is a connectionless datagram protocol. Like IP, it is a best effort, unreliable protocol. Reliability is addressed through error detection using a weak checksum algorithm. UDP is typically used for applications such as streaming media (audio, video, Voice over IP etc) where on-time arrival is more important than reliability, or for simple query/response applications like DNS lookups, where the overhead of setting up a reliable connection is disproportionately large. Real-time Transport Protocol (RTP) is a datagram protocol that is designed for real-time data such as streaming audio and video.用户数据报协议是非面向连接的数据报协议。例如IP协议就是一个提供尽力而为的协议，“并不可靠的”。可靠性是通过弱校验算法来进行错误检测从而处理。UDP通常应用于传输流媒体（音频，视频，IP网络电话等）等准时到达比可靠性更重要的数据，或建立可靠连接成本的比例过大的简单的查询/响应申请，如DNS查找。TCP and UDP are used to carry an assortment of higher-level applications. The appropriate transport protocol is chosen based on the higher-layer protocol application. For example, the File Transfer Protocol expects a reliable connection, but the Network File System (NFS) assumes that the subordinate Remote Procedure Call protocol, not transport, will guarantee reliable transfer. Other applications, such as VoIP, can tolerate some loss of packets, but not the reordering or delay that could be caused by retransmission. TCP和UDP经常携带各种各样的更高级别的应用程序。适当的传输协议的选择基于更高层次的协议应用程序。例如，文件传输协议需要可靠的连接，但网络文件系统（NFS）假定下属的远程过程调用协议，没有运输，保证可靠传输。The applications at any given network address are distinguished by their TCP or UDP port. By convention certain well known ports are associated with specific applications. 在给定网络地址的应用程序的区别在于它们的TCP或UDP端口。按惯例，某些著名的端口与特定的应用程序关联。Application Layer应用层The Application Layer refers to the higher-level protocols used by most applications for network communication. Examples of application layer protocols include the File Transfer Protocol (FTP) and the Simple Mail Transfer Protocol (SMTP). Data coded according to application layer protocols are then encapsulated into one or (occasionally) more transport layer protocols (such as the Transmission Control Protocol (TCP) or User Datagram Protocol (UDP), which in turn use lower layer protocols to effect actual data transfer. 应用层是最高级的协议，用于网络通信的大多数应用程序。应用层协议的例子有文件传输协议（FTP）和简单邮件传输协议（SMTP）。根据应用层协议编码的数据进行封装成一个或（偶尔）更多的传输层协议（如传输控制协议（TCP）和用户数据报协议（UDP）在使用较低层协议的影响实际数据传输。Since the IP stack defines no layers between the application and transport layers, the application layer must include any protocols that act like the OSIs presentation and session layer protocols. This is usually done through libraries. 由于IP协议栈定义了应用层和传输层之间的层，应用层必须包括任何协议，像OSI的表示层和会话层协议。这经常是通过数据库。Application Layer protocols generally treat the transport layer (and lower) protocols as black boxes which provide a stable network connection across which to communicate, although the applications are usually aware of key qualities of the transport layer connection such as the end point IP addresses and port numbers. As noted above, layers are not necessarily clearly defined in the Internet protocol suite. Application layer protocols are most often associated with client-server applications, and the commoner servers have specific ports assigned to them by the IANA: HTTP has port 80; Telnet has port 23; etc. Clients, on the other hand, tend to use ephemeral ports, i.e. port numbers assigned at random from a range set aside for the purpose.应用层协议通常把传输层（和较低的）协议视为“黑箱”，这些黑箱可以提供了