昨天更新的是 basic 部分，可以看昨天的文章。
今天更新计算机网络的 Intermediate 部分。昨天的音频有杂音，今天重新尝试一下别的方法。音频放在公众号了，点击此处获取。
Intermediate Interview Questions
Question 12. Describe the OSI Reference Model
Open System Interconnections (OSI) is a network architecture model based on the ISO standards. It is called the OSI model as it deals with connecting the systems that are open for communication with other systems.
The OSI model has seven layers. The principles used to arrive at the seven layers can be summarized briefly as below:
- Create a new layer if a different abstraction is needed.
- Each layer should have a well-defined function.
- The function of each layer is chosen based on internationally standardized protocols.
12. 描述 OSI 参考模型
开放系统互连 (OSI) 是基于 ISO 标准的网络架构模型。它被称为 OSI 模型，因为它处理连接为与其他系统通信而开放的系统。
OSI 模型有七层。用于达到七层的原则可以简要概括如下：
- 如果需要不同的抽象，请创建一个新层。
- 每一层都应该有一个定义明确的功能。
- 每一层的功能是根据国际标准化协议选择的。
Question 13. Define the 7 different layers of the OSI Reference Model
Here the 7 layers of the OSI reference model:
13. 定义 OSI 参考模型的 7 个不同层
这里是 OSI 参考模型的 7 层：
Physical:
- Unit Exchanged: Bit.
- It is concerned with transmitting raw bits over a communication channel.
- Chooses which type of transmission mode is to be selected for the transmission. The available transmission modes are Simplex, Half Duplex and Full Duplex.,
物理层：
- 交换单位：比特。
- 它涉及通过通信信道传输原始比特。
- 选择要为传输选择哪种类型的传输模式。可用的传输模式有单工、半双工和全双工。
Data Link:
- Unit Exchanged: Frame.
- The main task of this layer is to transform a raw transmission facility into a line that appears free of undetected transmission errors.
- It also allows detecting damaged packets using the CRC (Cyclic Redundancy Check) error-detecting, code.
- When more than one node is connected to a shared link, Data Link Layer protocols are required to determine which device has control over the link at a given time.
- It is implemented by protocols like CSMA/CD, CSMA/CA, ALOHA, and Token Passing.
数据链路层：
- 交换单位：帧。
- 该层的主要任务是将原始传输设施转换为看起来没有未检测到的传输错误的线路。
- 它还允许使用 CRC（循环冗余校验）错误检测代码检测损坏的数据包。
- 当多个节点连接到共享链路时，需要数据链路层协议来确定在给定时间哪个设备可以控制该链路。
- 它由 CSMA/CD、CSMA/CA、ALOHA 和令牌传递等协议实现。
Network:
- Unit Exchanged: Packet.
- It controls the operation of the subnet.
- The network layer takes care of feedback messaging through ICMP messages.
网络层：
- 交换单位：包。
- 它控制子网的操作。
- 网络层通过 ICMP 消息处理反馈消息。
Transport:
- Unit Exchanged: TPDU - Transaction Protocol Data Unit (Segment).
- The basic functionality of this layer is to accept data from the above layers, split it up into smaller units if needed, pass these to the network layer, and ensure that all the pieces arrive correctly at the other end.
- The Transport Layer takes care of Segmentation and Reassembly.
传输层：
- 交换单位：数据段。
- 该层的基本功能是接受来自上述层的数据，如果需要，将其拆分为更小的单元，将这些单元传递给网络层，并确保所有片段正确到达另一端。
- 传输层负责分段和重组。
Session:
- Unit Exchanged: SPDU - Session Protocol Data Unit.
- The session layer allows users on different machines to establish sessions between them.
- Dialogue control is using the full-duplex link as half-duplex. It sends out dummy packets from the client to the server when the client is ideal.
会话层：
- 交换单位：SPDU - 会话协议数据单元。
- 会话层允许不同机器上的用户在它们之间建立会话。
- 对话控制使用全双工链路作为半双工。当客户端是理想的时，它会从客户端向服务器发送虚拟数据包。
Presentation:
- Unit Exchanged: PPDU - Presentation Protocol Data Unit.
- The presentation layer is concerned with the syntax and semantics of the information transmitted.
- It translates a message from a common form to the encoded format which will be understood by the receiver.
表示层：
- 交换单位：PPDU - 表示协议数据单元。
- 表示层关注传输信息的语法和语义。
- 它将消息从普通形式转换为接收者可以理解的编码格式。
Application:
- Unit Exchanged: APDU - Application Protocol Data Unit.
- It contains a variety of protocols that are commonly needed by users.
- The application layer sends data of any size to the transport layer.
应用层：
- 交换单位：APDU - 应用协议数据单元。
- 它包含用户通常需要的各种协议。
- 应用层向传输层发送任意大小的数据。
Question 14. Describe the TCP/IP Reference Model
It is a compressed version of the OSI model with only 4 layers. It was developed by the US Department of Defence (DoD) in the 1980s. The name of this model is based on 2 standard protocols used i.e. TCP (Transmission Control Protocol) and IP (Internet Protocol).
14. 描述 TCP/IP 参考模型
它是 OSI 模型的压缩版本，只有 4 层。它是由美国国防部 (DoD) 在 1980 年代开发的。该模型的名称基于使用的 2 个标准协议，即 TCP（传输控制协议）和 IP（互联网协议）。
Question 15. Define the 4 different layers of the TCP/IP Reference Model
15. 定义 TCP/IP 参考模型的 4 个不同层
Link:
- Decides which links such as serial lines or classic Ethernet must be used to meet the needs of the connectionless internet layer.
链路层：
- 决定必须使用哪些链路（如串行线路或经典以太网）来满足无连接互联网层的需求。
Internet:
- The internet layer is the most important layer which holds the whole architecture together.
- It delivers the IP packets where they are supposed to be delivered.
网际层：
- 网际层是将整个架构结合在一起的最重要的层。
- 它将 IP 数据包传送到应该传送的位置。
Transport:
- Its functionality is almost the same as the OSI transport layer. It enables peer entities on the network to carry on a conversation.
传输层：
- 它的功能与 OSI 传输层几乎相同。它使网络上的对等实体能够进行对话。
Application:
- It contains all the higher-level protocols.
应用层：
- 它包含所有更高级别的协议。
Question 16. Differentiate OSI Reference Model with TCP/IP Reference Model
16. OSI 参考模型和 TCP/IP 参考模型的区别
| OSI Reference Model | TCP/IP Reference Model |
|---|---|
| 7 layered architecture | 4 layered architecture |
| Fixed boundaries and functionality for each layer | Flexible architecture with no strict boundaries between layers |
| Low Reliability | High Reliability |
| Vertical Layer Approach | Horizontal Layer Approach |
| OSI 参考模型 | TCP/IP 参考模型 |
|---|---|
| 7 层架构 | 4 层架构 |
| 每层的固定边界和功能 | 层与层之间没有严格界限的灵活架构 |
| 低可靠性 | 高可靠性 |
| 垂直层法 | 水平层法 |
Question 17. What are the HTTP and the HTTPS protocol?
HTTP is the HyperText Transfer Protocol which defines the set of rules and standards on how the information can be transmitted on the World Wide Web (WWW). It helps the web browsers and web servers for communication. It is a ‘stateless protocol’ where each command is independent with respect to the previous command. HTTP is an application layer protocol built upon the TCP. It uses port 80 by default.
HTTPS is the HyperText Transfer Protocol Secure or Secure HTTP. It is an advanced and secured version of HTTP. On top of HTTP, SSL/TLS protocol is used to provide security. It enables secure transactions by encrypting the communication and also helps identify network servers securely. It uses port 443 by default.
17、什么是 HTTP 和 HTTPS 协议？
HTTP 是超文本传输协议，它定义了有关如何在万维网 (WWW) 上传输信息的规则和标准。它有助于网络浏览器和网络服务器进行通信。这是一个“无状态协议”，其中每个命令相对于前一个命令是独立的。HTTP 是建立在 TCP 之上的应用层协议。它默认使用端口 80。
HTTPS 是超文本传输协议安全或安全 HTTP。它是 HTTP 的高级和安全版本。在 HTTP 之上，SSL/TLS 协议用于提供安全性。它通过加密通信来实现安全交易，还有助于安全地识别网络服务器。它默认使用端口 443。
Question 18. What is the SMTP protocol?
SMTP is the Simple Mail Transfer Protocol. SMTP sets the rule for communication between servers. This set of rules helps the software to transmit emails over the internet. It supports both End-to-End and Store-and-Forward methods. It is in always-listening mode on port 25.
18. 什么是 SMTP 协议？
SMTP 是简单邮件传输协议。SMTP 设置服务器之间通信的规则。这套规则有助于软件通过 Internet 传输电子邮件。它支持端到端和存储转发方法。它在端口 25 上处于始终侦听模式。
Question 19. What is the DNS?
DNS is the Domain Name System. It is considered as the devices/services directory of the Internet. It is a decentralized and hierarchical naming system for devices/services connected to the Internet. It translates the domain names to their corresponding IPs. It uses port 53 by default.
19.什么是 DNS？
DNS 是域名系统。它被认为是 Internet 的设备/服务目录。它是连接到 Internet 的设备/服务的分散和分层命名系统。它将域名转换为相应的 IP。它默认使用端口 53。
Question 20. What is the use of a router and how is it different from a gateway?
The router is a networking device used for connecting two or more network segments. It directs the traffic in the network. It transfers information and data like web pages, emails, images, videos, etc. from source to destination in the form of packets. It operates at the network layer. The gateways are also used to route and regulate the network traffic but, they can also send data between two dissimilar networks while a router can only send data to similar networks.
20. 路由器有什么用，与网关有什么不同？
路由器是用于连接两个或多个网段的网络设备。它引导网络中的流量。它以数据包的形式将信息和数据（如网页、电子邮件、图像、视频等）从源传输到目的地。它在网络层运行。网关还用于路由和调节网络流量，但它们也可以在两个不同的网络之间发送数据，而路由器只能将数据发送到相似的网络。
音频放在公众号了，点击此处获取。
