计算机网络——ARPANET设计思想

1. 什么是ARPANET

ARPANET是什么？大概大家听到这个词都会感到陌生。但是Internet你一定不陌生。
ARPANET与Internet一样，是计算机网络形成过程中标志性网络，除此之外还有一个NSFNET。ARPANET主要是用于传输军事命令与控制信息的网络。（最先进的技术一定是在国防，奥里给！）

ARPANET具有什么样的特点呢？

2. ARPANET特点

由于在军事应用的需求，ARPANET既需要具有一定的稳定性，同时也要保证通讯的时效性、准确性，确保通讯质量。因此ARPANET具有如下两中特点。

2.1 分布式网络拓扑构型

网络拓扑结构要保证网络的稳定性，当一个结点被破环时，其他结点不能受到影响。而使用星形网络，则网络的连通受几个中心结点的制约。

2.2 分组交换（包交换）

还有一个重要的说明是数据的传输方式，当时数据的传输方式主要有以下几种：

线路交换（Circuit Switching）：面向连接的服务，通信双方要有实际的物理线路连接。
为什么不能用于ARPNET的实现中呢？
线路交换用于传输语音信号，速度慢，误码率也比较高。
报文交换（Message Switching）：把所有数据当作一个逻辑单元发送。
（报文）分组交换（Packet Switching）：限制一次传输数据的最大长度。

在ARPANET中，使用的数据交换方式是分组交换（包交换），那就一起看一下分组交换的神奇之处吧！

分组交换技术有以下3个重要的概念：
1. 分组：数据预分为多个固定格式分组
2. 存储转发：中间节点先存储再转发（发一半了发现分组错了岂不是很尴尬，因此等待所有分组到达再进行转发）
3. 路由选择：转发节点选择合适的传输路径

在明确分组方法之前，需要首先知道报文的数据格式：

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报文号

目的地址

源地址

报文数据

校验

所谓的报文分组，就是将报文的数据分割到多个报文中，报文分组的格式如下:

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报文号

报文分组号

目的地址

源地址

报文数据

校验

在分割报文的同时，在报文中加入报文分组号。为了完成后期的组装。

数据进行分组交换有两种实现方式：

数据报方式：不需要预先建立线路连接，灵活高效、迅速可靠
虚电路方式：在通信双方建立一条逻辑连接的虚电路

虚电路是什么？？？？
虚电路由如下3部分组成：
3. 源和目的主机之间的路径
4. 虚电路号
5. 沿着该路径的每台路由器的转发表项

2.3 ARPANET设计思想

在有了上述知识储备后，可以进一步了解ARPANET的设计思想啦
ARPNET提出了通信子网和资源子网的概念。

通信子网：负责数据通信处理的通信控制处理机与通信线路（IMP）
资源子网：负责数据处理的主计算机与终端（Edge）

ARPANET的研究人员初期将网络协议分为传输控制协议（TCP）和互联网络协议（IP）。

TCP：实现两主机间分布式进程通信的功能
IP：实现分组通过通信子网的路由选择功能