Last time I sorted out all the key concepts in the computer network, many friends think it is very helpful, but there is a point that needs to be optimized is that these concepts do not know where they come from, so it seems to understand the shuttle in the cloud, while talking about the concept of application layer, and then ran to the network layer protocol. In view of this situation, I reorganize and summarize according to different chapters. This article should be much more comfortable to understand.

Basic concepts of computer networks

  1. Host: Any device that can connect to the network on the computer network is called host or end system, such as mobile phone, tablet computer, TV, game console, car, etc. With the arrival of 5G, there will be more and more terminal devices connected to the network.

  2. Communication link: A communication link is a physical path consisting of physical links (coaxial cables, twisted-pair cables, and optical fibers) connected together.

  3. Transmission rate: the unit is bit/s. It measures the data transmission rate of different links from one end system to another.

  4. Grouping: When one end system sends data to another, it is common to fragment the data and add the first byte to each segment to form the computer networking term grouping. These packets are sent over the network to the end system, where the data is then processed.

  5. Forwarding table: Internal route records the mapping of packet paths.

  6. Router: A device that connects local area networks (Lans) and wide area networks (Wans) on the Internet. A router maintains a routing table. Before sending data, the router queries the routing table and selects the best transmission path according to the information recorded in the routing table. It is a device at the network layer.

  7. Switch: The English term switch, is an optical signal forwarding device, it can access any two network nodes to provide a unique electrical signal path, it is a data link layer device.

  8. Hub: THE term hub. It is a device that can connect multiple Pairs of Ethernet cables or collections of optical fibers in the same physical medium. It happens in the physical.

The function of a switch is very similar to that of a hub. The switch has a memory function. After broadcasting, the switch can cache the target Mac, and the subsequent packets will be directly sent through the cached path. When the hub works, if one computer in the LAN wants to send a message, all computers in the LAN can receive the message. The security is poor, and the hub is in a half-duplex mode. So now most are using switches, hubs are slowly being phased out.

  1. Half-duplex mode: the end system connected to the hub can send only one packet at a time. Only after this packet is sent can other computers send it again. This is called half-duplex mode.

  2. Full-duplex mode: The end systems connected to the switch can communicate with each other and send messages without affecting each other.

  3. Path: THE path through which a packet travels through a series of communication links and packet switches called the path through the network.

  4. Internet service provider: ISP, not LSP (Lao SE PI). This is easy to understand, is the network operator, China’s three major operators: mobile, telecom, Unicom.

  5. Network protocol: Network protocols are rules, standards, or conventions established for data exchange in computer networks.

  6. IP: The Internet protocol that specifies the format of packets sent and received between the router and the end system.

  7. TCP/IP protocol cluster: not only TCP and IP, but a series of protocols based on TCP and IP, such as ICMP, ARP, UDP, DNS laundry, and SMTP.

  8. Packet loss: In computer networks, the loss of packets.

  9. Throughput: Throughput in computer networks is the number of successful data transfers per unit of time.

  10. Packet: Usually refers to packets at the application layer.

  11. Packet segment: Packets at the transport layer are usually referred to as packet segments.

  12. Datagrams: Packets at the network layer are commonly referred to as datagrams.

  13. Frames: Link layer groups are generally called frames.

  14. Circuit switching: it is one of the earliest switching modes in the communication network and is generally used in the telephone network. In the process of circuit switching, the data exchange is exclusive to the channel. The advantage of circuit switching is that the data transmission is reliable and fast, and the data will not be lost.

  15. Message switching: Message switching is to first transmit the entire message to the critical point, store all the messages, and then forward to the next node.

  16. Packet switching: Packet switching uses the store-and-forward mechanism to implement data exchange in packets. Packet switching does not monopolize channels, resulting in high resource utilization. Disadvantages are time delay jitter, relatively large overhead.

  17. Bandwidth: The bandwidth refers to the amount of data that can pass through a link per unit of time. It is usually expressed in terms of BPS, the number of bits that can be transferred per second.

  18. Frequency division multiplexing: mostly used for analog signals, frequency division multiplexing signals are parallel.

  19. Time division multiplexing: mostly used for digital signals, time division multiplexing signals are serial.

  20. Delay: The delay refers to the time required for a packet or group to be transmitted from one end of the network to the other end. The delay is classified into sending delay, propagation delay, processing delay, and queuing delay. The calculation method of total delay is as follows: Total delay = sending delay + propagation delay + processing delay + queuing delay.

  21. Processing delay: The delay required to check the packet head and determine the packet transmission path is called processing delay.

  22. Queuing delay: The time a packet waits on a link is called the processing delay.

  23. Transmission delay: In the actual link, the time spent from one end to the network from the beginning to the end of sending packets is called transmission delay, which can be understood as the time required to push packets.

  24. Propagation delay: The time it takes for a packet to propagate from one router to another.

  25. Unicast: The biggest feature of unicast is 1-to-1. The early fixed-line telephone is an example of unicast

  26. Broadcast: We used to broadcast gymnastics when we were little. This is an example of broadcast. The host is connected to all the end systems connected to it.

  1. multicastMulticast is similar to broadcast in that messages are sent to multiple receiving hosts, except that multicast is limited to a single group of hosts.

  1. As a broadcastAnycast is a communication method in which a receiver is selected from a specified number of hosts. Although similar to multicast, but different from multicast in behavior, anycast selects a host that best meets network conditions as the target host to send messages. The selected particular host will then return a unicast signal before communicating with the target host.

Computer network application layer

  1. Application architecture: is actually the application layer of the two organizational structure, divided into CS and P2P.
  2. Customer service system: It is a network application oriented architecture. The different systems in the system are divided into client and server. The client sends a service request to the server, and the server completes the requested service, and sends the processing result back to the client. In client-server architecture, there is a host that is always open calledServer (Server)It provides information fromCustomer (client)The service. Our most common server isThe Web server, Web server services fromThe browserThe request.

  1. P2P system,: Peer-to-peer architecture, where there is no server, everyone is a client, and each client can both send and respond to requests.

  1. Process: process is actually run in the end system of the program, the application of the communication of the most basic unit is the process.

  2. Distributed applications: An end system that exchanges data between multiple end systems is called a distributed application.

  3. Socket interface: Refers to the socket interface, which defines the way data is exchanged between end systems over the Internet.

  1. Client: Acts as a requester in a client-server architecture, usually a PC, smart phone, etc.

  2. Server: Plays the role of server in a client-server architecture, usually with a large cluster of servers playing the role of server.

  3. IP address: An IP address is an Internet protocol address that uniquely identifies a host on the Internet. Each device connected to the network has an IP address, which is divided into internal IP address and public IP address.

  4. Port number: A port number identifies different application processes on the same host.

  5. A Uniform Resource Identifier (URI) is a Uniform Resource Identifier (URI) that uniquely identifies resources on the Internet.

  6. URL: The full name of URL is Uniform Resource Locator. It is actually a subset of URI.

  1. HTML: HTML, known as hypertext Markup Language (HTML), is an identifier language. It includes a series of tags. These tags unify the format of documents on the network and connect scattered Internet resources into a logical whole. HTML text is descriptive text composed of HTML commands that describe text, graphics, animations, sounds, tables, links, etc.

  2. Web pages: A Web Page, also known as a Web Page, is composed of objects. An object is simply a file. This file can be an HTML file, an image, a Java application, etc., all of which can be found through a URI. A Web page contains many objects. A Web page can be said to be a collection of objects.

  3. Web Server: The official name of the Web Server is called the Web Server. The Web Server can provide documents to Web clients such as browsers, and can also place website files for the world to browse. You can place data files for the world to download. The three most popular Web servers are Apache, Nginx and IIS.

  4. CDN: The full name of CDN is Content Delivery Network. It applies caching and proxy technologies in HTTP protocol to respond to client requests instead of source sites. CDN is a network built on the basis of the existing network. It relies on the edge servers deployed in various places, through the central platform of load balancing, content distribution, scheduling and other functional modules, users can obtain the required content nearby, reduce network congestion, and improve user access response speed and hit ratio.

  5. Dedicated CDN: The CDN is unique to the content provider.

  6. Third-party CDN: It provides services on behalf of multiple content providers.

  7. WAF: WAF is an application protection system that implements HTTP/HTTPS security policies to protect Web applications. It is an application-layer firewall that detects HTTP traffic and protects Web applications.

  8. WebService: WebService is a Web application. WebService is a remote call technology across programming languages and operating system platforms.

  9. HTTP: ONE of the TCP/IP protocols. It is a protocol and specification for the transfer of hypertext data, such as text, pictures, audio, and video, between two points in the computer world.

  10. Session: A Session is essentially a cache of client sessions, designed to compensate for the stateless nature of HTTP. The server can use the Session to store records of the client’s operations during the same Session. When a client requests a server, the server allocates a memory space for the request. This object is the Session object, and the storage structure is ConcurrentHashMap.

  11. Cookie: HTTP Cookie includes Web Cookie and browser Cookie. It is a small piece of data sent by the server to the Web browser. Cookies sent by the server to the browser are stored by the browser and sent to the server with the next request. Typically, it is used to determine whether two requests are coming from the same browser, for example when the user remains logged in.

  12. SMTP: The protocol that provides the email service is called SMTP. SMTP also uses TCP at the transport layer. SMTP protocol is mainly used for mail information transfer between systems, and provide notification about letters.

  13. POP3: a simple mail access protocol with limited functions.

  14. DNS protocol: Because THE IP address is the address that the computer can recognize, and we humans are not convenient to remember this address, so for the convenience of human memory, DNS protocol is used to map the network address that we can easily remember as the IP address that the host can recognize.

  1. Root DNS Server: The top-level DNS server. There are over 400 root DNS servers around the world, managed by 13 different organizations. The root DNS server provides the IP address of the TLD server.
  2. Top-level domain DNS serverThe TLD server provides the IP address of the authoritative DNS server, like the common top-level domains (such as com, org, net, edu and gov) and all the national top-level domains (UK, FR, CA and JP).
  3. Authoritative DNS ServerThis server is the one on the Internet that has DNS records for publicly accessible hosts.
  4. Local DNS Server: Generally, each ISP has a local DNS server, which is adjacent to the host.

  1. TELNET protocolA remote login protocol that allows a user (Telnet client) to communicate with a remote device through a negotiation process. It provides the user with the ability to perform remote host work on the local computer.

  1. SSH: SSH is a secure encryption protocol based on the application layer. Because TELNET has a very obvious shortcomings, that is on the host and the remote host to send data packets is clear in the process of transmission, without any security encryption, the consequences of this is likely to be Internet criminals to the packet sniffer to do bad things, for the safety of data, we usually use SSH for remote login.

  2. FTP: a file transfer protocol. It is an application-layer protocol. The FTP protocol consists of two parts: an FTP server and an FTP client. The FTP server is used to store files. Users can use the FTP client to access resources on the FTP server through FTP. FTP is used to transfer large files with high transmission efficiency.

  1. The MIME typeWhich represents the types of resources on the Internet, Common types include hypertext Markup language text. HTML text/ HTML, XML document.xml text/ XML, plain text. TXT text/plain, PNG image.png image/ PNG, and GIF graph.gif Image/JPEG,.jpg image/ JPEG, AVI file.avi video/ X-MSVideo etc.
  2. demultiplexing: At the receiving end, the transport layer checks fields such as the source port number and destination port number, and then identifies the received socket. The process of delivering the data of the transport layer packet segment to the correct socket is called multiplexing.
  3. multiplexing: At the sender, the process of collecting data blocks from different sockets, encapsulating header information into the blocks to generate message segments, and then passing the message segments to the network layer is called multiplexing.
  4. Zhou indicates the port number: In host applications, port numbers from 0 to 1023 are restricted and are called peri-known port numbers. These port numbers generally cannot be occupied.

Computer network transport layer

  1. Reliable data transfer: Ensuring that data can be transferred accurately from one end of the application to the other.

  2. Lost-tolerant applications: Applications may lose data when sending data.

  3. Non-persistent connection: Each request/response goes through a different connection, and each connection is established, held, and destroyed. And the connection is disconnected after each request/response.

  4. Continuous connection: Each request/response passes through the same connection, which means that each request/response can reuse the connection and does not disconnect after each request/response.

  5. Transmission control protocol (TCP) : The English name is TCP. The name indicates that TCP has the function of controlling transmission, which is mainly reflected in its controllability and reliability. TCP provides the application layer with a reliable, connection-oriented service that can reliably transport packets to other hosts.

  6. User packet protocol (UDP) provides the application layer with a way to send datagrams directly without establishing a connection.

  7. Three-way handshake: To establish a TCP connection, three packet segments are sent. This process is called three-way handshake.

  8. Maximum packet segment length: MSS, which refers to the maximum value taken from the cache and put into the packet segment.

  9. Maximum transmission unit (MTU), which refers to the size of the payload that the communication parties can receive. MSS is usually set according to the MTU.

  10. Redundant ACK: Indicates the ACK of a packet segment. The loss of a packet segment results in redundant ACKS.

  11. Fast retransmission: Retransmits the lost packet segment before the packet segment timer expires.

  12. Select confirmation: In the case of a packet segment loss, TCP can select a packet segment that is out of order. This mechanism is usually used in conjunction with retransmission.

  13. Congestion control, congestion control to say is, when a certain period of time in the network packet is overmuch, too late to make the receiving end, causing some or the entire network performance degradation phenomenon of a kind of inhibiting the sender to send data, such as after a period of time or network improvements continue to send a message and then a method.

  14. Quadruple wave: TCP disconnects through four packet segments. This disconnection process is quadruple wave.

  15. Send buffer: When TCP sends a packet, it stores the packet in the kernel’s send buffer and waits for an appropriate time to send the packet.

  16. Receive buffer: When receiving a packet, the host does not process the packet immediately, but stores the packet in the kernel’s receive buffer and waits for an appropriate time to process it.

  17. SYN: Synchronize Sequence Numbers SYN: Synchronize Sequence Numbers is a data packet sent when a TCP/IP connection is established. The data packet is a synchronization Sequence number that identifies the request sent by the client.

  18. ACK: Acknowledge character. An ACK is a packet that responds to a request.

  19. FIN: Finish. A packet with the FIN flag bit indicates that the client wants to disconnect.

  20. State changes in three handshakes

    • LISTEN: indicates to wait for any connection requests from remote TCP and ports.
    • SYN-SEND: indicates that a connection request is sent and a matching connection request is awaited.
    • SYN-RECEIVED: indicates the status of the server after the second step in the TCP three-way handshake
    • ESTABLISHED: indicates that a connection has been established and application data can be sent to other hosts

  21. State changes during the four waves

    • FIN-WAIT-1: indicates waiting for a connection termination request from remote TCP or for confirmation of a previously sent connection termination request.
    • FIN-WAIT-2: indicates to wait for a connection termination request from remote TCP.
    • CLOSE-WAIT: indicates waiting for the connection termination request from the local user.
    • CLOSING: indicates waiting for confirmation of connection termination request from remote TCP.
    • LAST-ACK: represents waiting for confirmation of connection termination requests previously sent to remote TCP (including its connection termination request).
    • TIME-WAIT: indicates to wait enough time to ensure that remote TCP receives an acknowledgement of its connection termination request.
    • CLOSED: Indicates that the connection is closed.

  22. Sliding window: Sliding window is a kind of flow control technology. In the early days of the Internet, the communication parties usually do not consider the network situation, and generally communicate directly and send data at the same time, which is easy to cause congestion and no one can send data. In view of this phenomenon, sliding window is proposed. The receiver tells the sender how much data it can receive.

  23. Window length: The window length refers to the group range that has been sent but has not been confirmed. The structure of the sending window in the figure below is the window length.

  24. Cumulative acknowledgement: TCP provides that the sender only needs to receive the last acknowledgement ACK from the recipient within a certain period of time without retransmitting the packet segment.

  25. Redundant ACK: TCP uses the cumulative ACK mechanism. That is, when receiving a packet segment with a larger sequence number than expected, the receiving end repeatedly sends the acknowledgement signal of the latest acknowledged packet segment. This is called redundant ACK.

  26. Select confirm: Selects to confirm the out-of-order packet segment instead of retransmitting the last packet segment.

Computer network network layer

  1. Routing algorithm: An algorithm at the network layer that determines the routing of packets.

  2. Forwarding: This refers to the action of moving packets from an input link to an appropriate output link.

  3. Routing: The process of determining the selected path for sending packets from one end to the other end.

  4. Three routing switching technologies: memory switching, bus switching, Internet switching.

  5. Packet scheduling: Packet scheduling deals with how packets are transmitted through the output link. There are three main scheduling methods: first-in, first-out, priority queuing, and “circular and weighted fair queuing”.

  6. First in, first out: FIFO, or FCFS, the packets that arrive first are processed first.

  7. Priority queue: Priority queue: the group that reaches the output link is placed in the priority queue.

  8. Queuing rules: Round robin Queuing discipline Is a queuing machine that provides queuing services in turn.

  9. IPv4: The fourth and most widely used version of the Internet protocol. IPv4 is a connectionless protocol that does not ensure reliable data delivery. Use 32-bit addresses.

  10. IPv6: the sixth version of the Internet protocol. The address length of IPv6 is 128 bits. The biggest problem of IPv4 is the lack of network address resources, which seriously restricts the application and development of the Internet. The use of IPv6 can not only solve the problem of the number of network address resources, but also solve the obstacles of a variety of access devices connected to the Internet.

  11. Interface: Boundary between a host and a physical link.

  12. ARP: ARP is a protocol to solve the address problem. Based on the IP bit clue, it can locate the MAC address of the next network device that receives data. If the destination host and the host are on different links, you can use ARP to search for the IP address of the next hop route. However, ARP is only applicable to IPv4, not IPv6.

  13. RARP: RARP is a protocol that reverses ARP and locates IP addresses using MAC addresses.

  1. The proxy ARPProxy ARP is used to forward ARP requests to neighboring network segments.
  2. The ICMP protocol: Internet Packet control protocol. If an IP packet fails to reach the destination host for some reason during IP communication, an ICMP message is sent. ICMP is actually a part of IP.

  1. DHCP protocol: DHCP is a dynamic host configuration protocol, also known as plug and play protocol or zero configuration protocol. DHCP automatically sets IP addresses, allocates IP addresses in a unified manner, and implements plug and play.
  2. NAT protocol: Network Address Translation protocol (NAT), which translates all hosts with local addresses into global IP addresses on the NAT router before they can communicate with other hosts.
  3. NAT translation tableSimilar to a routing table, a NAT table records the translation between private IP addresses and public IP addresses.
  4. NAT throughNAT traversal is used to solve the problem of establishing connections between hosts on private TCP/IP networks that use NAT devices.
  5. The IP tunnelIP tunneling refers to the process by which a router encapsulates a network-layer protocol into another protocol for transmission across the network to another router.
  6. OSPF: IS a link-state protocol based on THE OSI IS-IS protocol. This protocol can also effectively solve the problem of network loop.
  7. BGPThe Border Gateway protocol, which connects thousands of ISPs on the Internet.
  8. IGP: Internal gateway protocol, which is used in autonomous routing systems (aS) built by enterprises.
  9. EGP: External gateway protocol, EGP is used to exchange routing information between network hosts.
  10. RIP: a distance vector routing protocol widely used in LAN networks.

Computer network data link layer and physical layer

  1. node: Refers to devices in the link layer protocol.
  2. link: Generally, the communication channel connecting adjacent nodes along the communication path is called a link.
  3. MAC protocols: media access control protocol, which specifies the rules for frames to be transmitted over a link.
  4. Parity bit: a method of error detection, mostly used in computer hardware error detection, parity check is usually used in data communication to ensure the validity of data.
  5. Forward error correction: The receiver’s ability to detect and correct errors is called forward error correction.
  6. EthernetEthernet is one of the most common local area network technologies today, which specifies the physical layer of wiring, electronic signals, and MAC protocol content.
  7. VLANVirtual local area network (VLAN) is a group of logical devices and users. These devices and users are not limited by physical location, but can be organized according to functions, departments, and applications. They communicate with each other as if they are in the same network segment.
  8. The base station: Wireless network infrastructure.
  9. Parity bit: A method of error detection.
  10. Forward error correction: The receiver’s ability to detect and correct errors is called forward error correction, or FEC.
  11. The checksum: checksum, in data processing and data communication, used to verify the sum of a set of data items at a destination.
  12. Cyclic redundancy detectionCRC, an error detection technique in use today, uses polynomials for error detection.
  13. CSMA/CDCSMA/CD requires each medium to check the link in advance for possible conflicts and release the channel as soon as possible.
  14. Shared media network: The name means a network in which multiple devices share the same communication medium.
  15. Unshared media network: In contrast to a shared media network, this network does not use the same communication media.
  16. Token ring: A shared medium type network transmission mode.

  1. filter: A function at the link layer that determines whether a frame should be forwarded to an interface or to the switch that should discard it.
  2. forwardingForwarding A function of the switch that determines which interfaces a frame should be directed to and moves the frame to those interfaces.
  3. The switch table: The filtering and forwarding functions of the switch are implemented by the switch table.
  4. MPLS: It is a tag switching technology. Tag switching sets a tag for each IP packet and forwards it according to this tag.

Computer Network security

  1. The four elements of secure communication are confidentiality, insulation integrity, endpoint identification, and operational security.
  2. confidentiality: Packets must be encrypted to some extent to prevent eavesdroppers from intercepting packets.
  3. Message integrity: Ensure that the packet content does not change during packet transmission.
  4. The endpoint to identify: Both sender and receiver shall verify the identity of the other party during the communication.
  5. Operational safety: Indicates the capability of the device to protect packets from attack.
  6. clear: Unencrypted content is called plaintext.
  7. The encryption algorithm: The original plaintext file or data is processed according to an algorithm, which is the encryption algorithm.
  8. cipher: Encrypts the plaintext. The generated packets are called ciphertext.
  9. Decryption algorithm: indicates the algorithm for decrypting ciphertext.
  10. The key: Decryption algorithm A tool for decrypting ciphertext is called a key.
  11. Symmetric encryptionThe same key can be used to encrypt and decrypt information at the same time. This encryption method is called symmetric encryption, also known as single-key encryption.
  12. Block cipherBlock ciphers, also known as block ciphers, divide encryption and decryption sequences into groups, and finally merge each sequence together to form either plain or ciphertext.
  13. Stream cipher: Stream ciphers are also called sequence ciphers. Each encryption generates a key stream through a key stream. Decryption also uses the same key stream. The plaintext performs xOR operations with the same key stream to obtain ciphers.
  14. The public key: A public key is the non-secret half of a key pair used with a private key algorithm. Public keys are typically used to encrypt session keys, verify digital signatures, or encrypt data that can be decrypted with the corresponding private key
  15. The private key: A private key is usually the other half of a public key that is known only to itself and can be used for encryption or decryption.
  16. CAThe certification authority, CA, is responsible for legitimizing the identification and issuance of certificates.
  17. A firewallA combination of hardware and software that isolates an organization’s internal network from the Internet, allowing some groups to pass and preventing others from passing.
  18. Firewalls generally fall into three categories: group filters, state filters, and application gateways.
  19. Block filter: The organization isolates the internal network from the external network. All packets leaving and entering the internal network pass through this router, which checks the information of each group.
  20. State grouping filter: Filters the packets according to the TCP connection status.
  21. Application gatewayAn application gateway is a specific application through which all application data passes.
  22. Intrusion detection system: The device that generates a warning when observing potentially malicious traffic is called an intrusion detection system
  23. Intrusion prevention system: The device that filters malicious traffic is called an intrusion prevention system.

conclusion

Computer network within the scope of the key concepts, this article, I summarized the part I think should be the key to understand the concept of, maybe there are some concept explanation is not so clear, you can serve as a reference or index, such as when you meet the relevant concepts can be roughly understand its meaning, so to reach the purpose of this article, if the articles are helpful to you, Welcome everyone to like, look again, share, your every support is my biggest power!

The 166 core concepts of Computer Network welcome to follow the public account: Programmer Cxuan, there are more good articles waiting for you.