Data Communication Computer Network 简明教程

Transport Layer Introduction

What is Transport Layer?

OSI 模型中的下一层被识别为传输层（层 4）。与数据或数据流传输有关的所有模块和程序都归为此层。与所有其他层一样，此层与其远程主机的对等传输层通信。

Next Layer in OSI Model is recognized as Transport Layer (Layer-4). All modules and procedures pertaining to transportation of data or data stream are categorized into this layer. As all other layers, this layer communicates with its peer Transport layer of the remote host.

传输层提供远程主机上两个进程之间的点对点和端到端连接。传输层从上层（即应用程序层）获取数据，然后将其分解为较小的段，对每个字节进行编号，并交给下层（网络层）进行传递。

Transport layer offers peer-to-peer and end-to-end connection between two processes on remote hosts. Transport layer takes data from upper layer (i.e. Application layer) and then breaks it into smaller size segments, numbers each byte, and hands over to lower layer (Network Layer) for delivery.

Transport Layer Functions

  1. This Layer is the first one which breaks the information data, supplied by Application layer in to smaller units called segments. It numbers every byte in the segment and maintains their accounting.

  2. This layer ensures that data must be received in the same sequence in which it was sent.

  3. This layer provides end-to-end delivery of data between hosts which may or may not belong to the same subnet.

  4. All server processes intend to communicate over the network are equipped with well-known Transport Service Access Points (TSAPs) also known as port numbers.

End-to-End Communication

一个主机上的进程通过 TSAP（也称为端口号）识别远程网络上的对等主机。TSAP 定义非常明确，尝试与对等主机通信的进程会预先知道这一点。

A process on one host identifies its peer host on remote network by means of TSAPs, also known as Port numbers. TSAPs are very well defined and a process which is trying to communicate with its peer knows this in advance.

TSAP

例如，当 DHCP 客户端想要与远程 DHCP 服务器通信时，它总是请求端口号 67。当 DNS 客户端想要与远程 DNS 服务器通信时，它总是请求端口号 53（UDP）。

For example, when a DHCP client wants to communicate with remote DHCP server, it always requests on port number 67. When a DNS client wants to communicate with remote DNS server, it always requests on port number 53 (UDP).

Transport Layer Protocols

两个主要的传输层协议是：

The two main Transport layer protocols are:

  1. Transmission Control Protocol − It provides reliable communication between two hosts.

  2. User Datagram Protocol − It provides unreliable communication between two hosts.