CAN201 计网概念收集

Lecture 1 the theoretical basis for networking

Network edge and core

地理覆盖范围：广WAN，城MAN，局LAN，个PAN

交换方式，电路，报文，分组

电路交换vs报文vs分组

Network performance

protocol layers and service models***

OSI七层

TCP/IP四层

···························

e.g 网站访问中不同层所涉及的协议

Application Layer:
- HTTP (Hypertext Transfer Protocol): This protocol is used for transmitting text, images, and other web page components between your browser and the web server. In this case, when you enter the URL, your browser initiates an HTTP request to the server.
Transport Layer:
- TCP (Transmission Control Protocol): HTTP typically uses TCP as its transport layer protocol. TCP ensures reliable and ordered delivery of data between your computer and the web server. It establishes a connection, breaks down large messages into smaller packets, and ensures they are delivered without errors.
Network Layer:
- IP (Internet Protocol): IP is responsible for routing packets between devices across different networks. In this scenario, it helps route the TCP packets containing the HTTP requests and responses between your computer and the university's web server.
Link Layer:
- Ethernet (or other link layer protocols): At the link layer, Ethernet is often used to transmit frames between devices within the same local network. Your router uses Ethernet to send data to and receive data from your computer.
Physical Layer:
- Physical medium (e.g., Ethernet cables, Wi-Fi signals): The physical layer deals with the actual hardware transmission of bits over the physical medium. This could be Ethernet cables, Wi-Fi signals, or other physical means connecting your computer to the router and the router to the wider Internet.

In summary, when you enter the URL in your browser, it initiates an HTTP request. This request is encapsulated in TCP, which is then further encapsulated in IP for routing. The data is transmitted over the physical medium using link layer protocols like Ethernet. Finally, your router facilitates the communication between your computer and the web server.

network security

core - package switching

delay

分组信息的优缺点

优点

1.Error control: Each packet includes a checksum, which can be used to detect and correct errors that may occur during transmission. If an error is detected in a packet, only that packet needs to be retransmitted, rather than the entire message.

2.Flow control: Network devices such as routers can use the sequence numbers in packets to monitor and control the flow of data through the network.

3.Flexibility: By breaking a message into smaller packets, it can be sent over different paths in a network and reassembled at the destination. This is known as packet switching and enables efficient use of network resources.

4.Reliability: Each packet has a header that contains information about the packet's destination, so if a packet is lost or corrupted in transit, it can be retransmitted without having to retransmit the entire message.

5.Scalability: Segmentation allows messages of any size to be sent, regardless of the maximum packet size that a network can handle.

缺点

1. Increased overhead. Each packet must include additional information, such as a sequence number, for reassembling the original information at the destination. This increases the amount of data that needs to be sent and can reduce the overall efficiency of the network.

2. Processing delays. The process of breaking the message into packets and adding the necessary headers introduces additional latency.

3. Complexity. The process of segmentation and reassembly introduces additional complexity into the system, which can make it more difficult to troubleshoot and maintain.

4. Security risks. As packets travel over the network, they may pass through multiple intermediate devices, which increases the risk of unauthorized access or tampering.

5. Delivery errors: Packets are sent independently and may take different paths through the network, so they may arrive out of order or be lost. These errors can occur due to congestion, routing loops, and other problems.

6. Network congestion. When a large number of packets are transmitted in a short period of time, it can cause network congestion, resulting in delays or even loss of packets.

Lecture 2-3 the application layer

Principle of network application

web application

Domain Name System(DNS)

P2P Applications

Socket Programming

HTTP(协议)

为什么browser信息是需要的

The browser is included as a header field in the HTTP request message because it provides the server with important information about the client making the request. Here are a few reasons why the browser is required in the HTTP request message

Device compatibility. Different browsers have different capabilities and support different technologies, so browser information helps the server determine if the client can handle the requested content.

Security. Different browsers have different levels of security, and the server can use this information to determine if the client is using the latest secure browser before serving content.

Analysis and logging. Browser information is used to analyze the performance of the site and to understand how the site is being used, such as browser type, version, etc.

Content delivery. Depending on the browser, the server may choose to deliver different content, such as images or web styles, to optimize the user experience.

Malicious user agents. Some attackers may use malware, scripts, or malware to make requests that provide a fake user agent to evade detection. User agent strings are useful in detecting such cases

Compliance: Certain regulations, such as GDPR, CCPA, etc., may require logging of user agents for compliance purposes.

In summary, browser information in HTTP request messages is used to provide the server with important information about the client that can be used to optimize user experience, security and performance, and compliance.

Lecture 4-5 the transport layer

Transport-layer services

Multiplexing and demultiplexing

Connectionless Transpor: UDP

Principles of reliable data transfer

Piplined communication

TCP: connection-oriented transport

Principles of congestion control

Lecture 6-8 the network layer

Overview

私有地址通常不被因特网上的路由器所转发

子网划分：主机部分不能全为1

Router

Internet Protocol

IPv4 addressing

NAT

IPv6

Generalized Forward and SDN

Routing - Distance vector algorithm

Intra-AS routing in the Internet: OSPF

Routing among the ISPs: BGP

The SDN control plan

ICMP

是互联网控制消息协议，是网络层（第三层）中的一个协议。它用于在IP网络上发送错误消息和操作信息，通常用于网络诊断和错误报告。一些常见的 ICMP 消息类型包括 Echo Request 和 Echo Reply，它们被用于 Ping 工具，以测试网络连接和测量往返时间（RTT）。

SNMP

SNMP（Simple Network Management Protocol）是一种用于管理和监控网络设备的协议。它允许网络管理员远程监视和管理网络中的设备，包括路由器、交换机、服务器等。SNMP使得管理员可以检索设备的信息、监视网络性能并进行配置更改。 SNMP采用客户端-服务器模型，其中网络中的管理系统（通常是NMS，Network Management System）通过SNMP协议与被管理的设备进行通信。