1. Network of networks

  • The network connects hosts, while the Internet connects many different networks, so the Internet is a network of networks
  • The Internet is a global Internet

2.ISP

  • Internet service providers (ISPs) can obtain many IP addresses from Internet management agencies, and have communication lines and routers and other networking equipment. Individuals or institutions can access the Internet by paying a certain fee to ISPs

  • The current Internet is a multi-level ISP structure, which is divided into tier 1 ISP, regional ISP and access ISP according to the size of the coverage area. The Internet exchange point IXP allows two ISPs to connect directly without going through a third ISP

3. Communication mode between hosts

  • CS: The client is the requester of the service and the server is the provider of the service

  • Peer-to-peer: No distinction between client and server

3. Circuit switching and packet switching

3.1 Circuit Switching

  • Circuit switching is used in telephone communication system. Before two users communicate, they need to establish a dedicated physical link and occupy this link throughout the communication process
  • Because the communication process is impossible to transmit the line all the time, the circuit switching on the line is very low, often less than 10 percent

3.2 Packet Switching

  • Each packet has a head and tail, containing control information such as source address and destination address. The simultaneous transmission of multiple packets on the same transmission line does not affect each other. Therefore, the simultaneous transmission of multiple packets on the same transmission line is allowed, that is, packet switching does not need to occupy the transmission line
  • In a post office communication system, after receiving a mail, the post office stores it and then forwards the mail to the next destination together. This process is the store-and-forward process, which is also used in packet switching

4. The time delay

Total delay = queuing delay + processing delay + transmission delay + propagation delay

4.1 Queuing delay

  • The amount of time a packet waits in a router’s input queue and output queue, depending on the current traffic on the network

4.2 Processing delay

  • The time it takes for a host or router to process a packet when it receives it, such as analyzing headers, extracting data from the packet, checking for errors, or finding the appropriate route

4.3 Transmission delay

  • The time required for a host or router to transmit a data frame lock

  • L represents the data frame length and V represents the transmission rate

4.4 Propagation delay

  • The time it takes for an electromagnetic wave to travel through a channel at speeds close to the speed of light

  • L represents the channel length, and V represents the propagation speed of electromagnetic waves in the channel

5. Computer network architecture

5.1 Layer 5 Protocol

  • Application layer: Provides data transmission services for specific applications, such as HTTP and DNS protocols. The unit of data is packet.

  • Transport layer: Provides common data transfer services for processes

    • Since there are many application layer protocols, defining common transport layer protocols can support the increasing number of application layer protocols
    • The transport layer consists of two protocols: transmission control protocol (TCP), which provides connection-oriented and reliable data services. The data unit is packet segment. User datagram protocol, UDP, provides connectionless, best – effort data transfer service, data unit is user datagram
    • TCP mainly provides integrity service,UDP mainly provides timeliness service

  • Network layer: Provides data transfer services for hosts, and transport layer protocols serve processes in hosts

    • The network layer encapsulates the packet segments or user datagrams passed down from the transport layer into packets

  • Data link layer: The network layer aims at data transmission services between hosts, and hosts can use many links. The link layer protocol provides data transmission services for hosts on the same link, and the data link layer encapsulates the packets transmitted from the network layer into frames

  • Physical layer: Consider how to transmit bit streams over transport media, not the specific transport media. The role of the physical layer is to mask as much as possible the differences in transmission media and communication means, while the data link layer does not feel these differences

5.2 the OSI

  • Presentation layer: data compression, encryption, and data description, which frees applications from having to worry about the internal formatting of data on different hosts

  • Session layer: establishes and manages sessions

  • The five-tier protocol leaves out the presentation and session layers, leaving these capabilities to the application developer

5.3 the TCP/IP

There are only four layers, which is equivalent to merging the data link layer and physical layer into the network interface layer in layer 5 protocol

The TCP/IP architecture does not strictly follow the OSI layering concept, and the application layer may directly use the IP layer or network interface layer

5.4 Data transfer process between layers

  • On the way down, the header or tail required by the underlying protocol needs to be added, and on the way up, the header and tail are separated
  • The router has only the following three layers of protocol, because the router is located in the network core and does not need to provide services to processes or applications, thus eliminating the need for transport and application layers