other important agreements

ICMP协议(网际控制报文协议)

ICMP协议的作用：Improved forwardingIPDatagrams and Delivery Chances of Success.（Mainly by sending error report messages and inquiry messages）

1.ICMP报文被封装在IP数据报中发送.

其中：
①：ICMPThere are the following five types of error checking messages：

• 终点不可达：If the host or router cannot deliver the datagram to the destination address,An endpoint unreachable message will be sent to the host that sent the packet.（具体可以根据ICMPThe field is divided into destination network unreachable、目的主机不可达、目的协议不可达、目的端口不可达、目的网络未知、Destination host unknown, etc3种错误）（The router that is helping to forward does not have any information about the destination address of the packet to be forwarded,The unreachable message will be sent）
• 源点抑制：Source Suppression message sent when a router or host drops a packet due to congestion.（Let the source point to slow down the rate at which datagrams are sent）
• 时间超过：The datagram to be sent has not yet reached the destination host,此时如果说TTL的值为0了,Then send the message.（Or when the destination host does not receive all the fragments of a datagram within the specified time,Received fragments are discarded,Then return the message）
• 参数问题：The content of the sent datagram is wrong,send the message.
• 改变路由（重定向）：Choose the best path to proceed.（That is, the router sends the message to change the route to the host,This time when the host receives it,You will know the best path to take next time）

②：When an error message cannot be sent：

• 对ICMPError messages cannot be sentICMP差错报文.
• All subsequent datagram fragments for the first fragmented datagram fragment are not sentICMP差错报文.
• 对具有多播地址的数据报不发送ICMP差错报文.
• Datagrams with special addresses are not sentICMP差错报文.

2.常用的ICMP询问报文：

①回送请求和回答：测试目的站是否可达.（The source host sends to the destination hostICMP请求报文进行询问,After the destination host receives it,It must be sent to the source hostICMPAnswer the message,At this point, the source host knows that the destination host can connect successfully）

②时间戳请求和回答：Do always sync and measure time.（sent to a hostICMP时间戳请求报文,Have a host answer the current date and time）

3.Two specific commands：

①：ping命令（分组网间探测）

• Test connectivity between hosts or routers.
• The Internet layer is used between the application layersICMP报文.（not through the transport layerTCP或者UDP）
• 使用ICMP回送请求和回答报文.

如图：

在虚拟机上pingA Baidu server.

②：traceroute（跟踪路由）

• 用来测试IPDatagrams go through those routes from the source host to the destination host.
• Windows版本：
  ①：tracert命令.
  ②：The Internet layer is used between the application layersICMP协议.
  ③：使用ICMP回送请求和回答报文以及差错报告报文.
• Unix版本：
  ①：traceroute命令.
  ②：在运输层使用UDP协议.
  ③：仅使用ICMP差错报告报文.

如图：

如图,is on a virtual machinetracerouteBaidu servers go through those routes,有很多.

原理：By echoing the request message.（One router by one router,And each router sendsICMP报文的TTLThe messages follow in sequence1增加,然后依次增加TTL）

DNS服务(域名系统服务)

1.域名服务器DNS的作用

①：首先我们先使用ping命令,ping一下百度,如下图：

我们ping的是www.baidu.com,But what we get is from14.215.177.38IPSome connection relationship of the address.

②作用：由于在TCP/IP中使用IP地址和端口号来确定网络上的一台主机的一个程序. 但是IP地址不方便记忆.于是人们发明了一种叫主机名的东西, 是一个字符串, 并且使用hosts文件来描述主机名和IP地址的关系.（easy to be remembered）

2.DNS域名结构：

①：The Internet adopts a hierarchical tree-like domain name structure.
②：The domain name structure consists of several components,各分量之间用“点”隔开,分别代表不同级别的域名.

• Each level of domain name is composed of English letters and numbers,不超过63个字符,不区分大小写字母.
• Lower-level domain names are written on the far left,Higher-level top-level domains are written on the far right.
• 完整的域名不超过255个字符.

③：域名系统不规定一个域名需要包含多少个下级域名,It also does not specify what each level of domain name represents.
④：Domain names at all levels are managed by their higher-level domain name management agencies,而最高的顶级域名则由因特网名称与数字地址分配机构ICANN进行管理.

3.Domain names are classified as follows：

①顶级域名TLD：

• 国家顶级域名nTLD：如cn表示中国,usIdentifies the United States and more.
• 通用顶级域名gTLD：常见的有7个如：com（公司企业）、net（网络服务机构）、org（非营利性组织）、int（国际组织）、edu（美国教育结构）、gov（美国政府部门）、mil（美国军事部门）.
• direction fieldarpa：用于反向域名解析,即IPAddress reverse domain name resolution to domain name.

②：The second-level domain names registered under the national top-level domain name are determined by the country itself.

③：The second-level domain names in my country are as follows：

• 类别域名：7个：ac（科研机构）、com（工、business and other enterprises）、edu（教育机构）、gov（政府部门）、net（提供网络服务的机构）、mil（军事机构）和org（非营利性组织）.
• 行政区域域名：According to our country、自治区、Municipalities and other divisions.

4.域名的IP地址的映射关系必须保存在域名服务器中,供所有其他应用查询.（Because one domain name server is simply not enough,所以DNS使用分布在各地的域名服务器来实现域名到IP地址的转换）

域名服务器的划分：

• 根域名服务器：最高层次的域名服务器.（知道所有顶级域名服务器的域名和IP地址）（Domain names are usually not resolved directly,而是返回该域名所属顶级域名的顶级域名服务器的IP地址）
• 顶级域名服务器：管理在该顶级域名服务器注册的所有二级域名.（When a request is received, it will return to the next-level authority domain name serverIPaddress or final result）
• 权限域名服务器：管理某个区的域名.（Each host's domain name must be registered with an authoritative name server,And it also knows the server address of the subordinate domain name）
• 本地域名服务器：代理作用,主机发送的DNS请求报文时,The message will be sent to the above-mentioned hierarchical structure of the domain name server.（也成为默认域名服务器）

5.域名解析的过程

①递归查询：All the way from the local name server up through the root name server recursively、顶级域名服务器、权限域名服务器,然后返回.（Whichever gives the result returns the host from the local name server,If no result is given,Then go to the authority domain name server in turn）
②迭代查询：The root name server is accessed by the local name server one by one、顶级域名服务器、权限域名服务器,然后返回.（Whichever gives the result returns the host from the local name server,If no result is given,Then go to the authority domain name server in turn）

③：为了提高DNSThe query efficiency and reduce the load on the root name server and reduce the InternetDNS查询报文数量,Widely used in domain name servers高速缓存.（A record used to store the recently queried domain name and the mapping information obtained from the initial domain name）

注意：

• The domain name in the cache toIPThe mapping relationship of addresses is not permanent,In order to preserve the correctness of the content in the cache,Nameservers should set timers for unexpected content.（Delete items older than a reasonable time）
• There is also a cache in the user host.

6.问题：浏览器中输入URL后,会发生什么？

Please see this article：浏览器中输入url后发生的事情

应用层

应用层的作用:

• 满足我们日常需求的网络程序, 都是在应用层.
• 能够根据自己的需求, 设计应用层协议.
• 了解HTTP协议.
• 理解DNS的原理和工作流程.

传输层

传输层的作用:

• 负责数据能够从发送端传输接收端.
• 理解端口号的概念.
• 认识UDP协议, 了解UDP协议的特点.
• 认识TCP协议, 理解TCP协议的可靠性;理解TCP协议的状态转化.
• 掌握TCP的连接管理, 确认应答, 超时重传, 滑动窗口, 流量控制, 拥塞控制, 延迟应答, 捎带应答特性.
• 理解TCP面向字节流, 理解粘包问题和解决方案.
• 能够基于UDP实现可靠传输.
• 理解MTU对UDP/TCP的影响.

网络层

网络层的作用:

• 在复杂的网络环境中确定一个合适的路径.
• 理解IP地址, 理解IP地址和MAC地址的区别.
• 理解IP协议格式.
• 了解网段划分方法.
• 理解如何解决IP数目不足的问题, 掌握网段划分的两种方案. 理解私有IP和公网IP.
• 理解网络层的IP地址路由过程. 理解一个数据包如何跨越网段到达最终目的地.
• 理解IP数据包分包的原因.
• 了解NAT设备的工作原理.

数据链路层

数据链路层的作用:

• 两个设备(同一种数据链路节点)之间进行传递数据.
• 以太网是一种技术标准; 既包含了数据链路层的内容, 也包含了一些物理层的内容.例如: 规定了网络拓扑结构, 访问控制方式, 传输速率等.
• 以太网帧格式.
• 理解mac地址.
• 理解arp协议.
• 理解MTU.