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Necessary conditions for forming a network: multiple computer systems with independent functions, communication subnets , network software
Production and development of computer network: terminal-oriented computer network stage, computer network stage with the main purpose of sharing resources , standard, open computer network stage, high-speed, intelligent computer network stage
Composition of computer network: physical structure : network software and hardware subsystems

Logical structure : resource subnet , communication subnet

Computer network architecture: a collection of systems , entities , layers , and protocols , and a precise definition of the functions that computer networks and their components should complete
System: A system composed of a computer network is usually a physical whole including one or more entities with information processing and communication functions.
Entities: In network layered architecture, each layer is composed of some entities. In a computer system, a process or program that can complete a specific function can be a logical entity. An entity can be either a software entity or a hardware entity.
Hierarchy : It is a way for people to deal with complex problems. Usually, the logical structure that can provide a certain type of service function in the system is called a layer.
Agreement : Refers to the rules and conventions that must be followed to complete communication or services between two entities.
Interface (Interface) is the connection point for exchanging information between adjacent layers in the same node or node.
Peer tier: The same tier of different systems.
Peer Entity (Peer Entity): Two communicating entities on peers of different systems.
Peer communication: Communication that exists between peer entities of different systems.
Peer-to-peer (peer-to-peer) protocol: The rules followed by communication between peer-to-peer entities. The protocol of each layer is only binding on the operation of the layer it belongs to, and does not involve other layers.
Service: Each layer in the hierarchy supports the work of the layer above it, and this support is a service.
Service Access Point (SAP) : The place where two adjacent entities on the interface exchange information. It is the logical interface of the two adjacent entities. For example, the N-tier SAP is where the N+1 layer can access the N-tier.
Service primitive : refers to a layer entity reporting the occurrence of an event to another layer entity.
There are four main types of service primitives: request , indication , response , and confirmation .
Data unit: Usually, each unit of data transfer is called a data unit .
Protocol data unit: Usually, each part of a data unit exchanged between peer entities of different computer systems is called a protocol data unit .
Service data unit: The data unit to be delivered when layer N requests layer N-1 to provide services.
Interface data unit: In an interaction between adjacent two-layer entities of the same system, the parameters to be passed and the results returned.
OSI is the abbreviation of Open System Interconnection, which means Open System Interconnection Reference Model
Any communication system can be regarded as composed of three parts : sending equipment , transmission channel and receiving equipment . We call the end that generates and sends information the source , the end that receives the information the sink , and the communication line that transmits the information from the source to the sink is called the channel .
Information generally refers to the news, intelligence or knowledge of a specific thing represented by sounds, words, images, symbols, etc. that can be transmitted in various ways.
Data is an entity that carries information and is a symbolic representation of objective things. In computer science, it refers to the general term for all symbols that are input into a computer and processed by a computer program. There are two types of data: analog data (continuous) and digital data (discrete).
A signal is a specific physical form of data, which has a definite physical description, such as an electrical signal, an optical signal, or a magnetic field strength. There are two types of signals: analog (continuous) and digital (discrete).
Analog signal : It is a continuously changing electrical signal , such as telephone voice signal, TV signal, etc. It is a function curve that changes with time
Digital signal : It is a discrete and discontinuous electrical pulse sequence , usually represented by a code composed of "high" and "low" level pulse sequences .
The baseband signal (Baseband Signal) directly uses two different voltages to represent digital signals 1 and 0, and then sends them to the line for transmission . Therefore, the natural frequency corresponding to the rectangular electrical pulse signal is generally called "baseband", and the corresponding signal is called baseband signal
The wideband signal (Wideband Signal) uses multiple groups of baseband signals and 0 to modulate carriers of different frequencies , and is composed of these modulated carriers occupying different frequency bands. In other words, a wideband signal refers to a frequency division multiplexed analog signal formed by modulating a baseband signal
Analog communication system : ordinary telephone, radio, television and other signals are all analog signals , and the communication system composed of analog signals is an analog communication system. An analog communication system usually consists of a source , modulator , channel , demodulator , sink , and noise source
Digital communication system : Signals such as computer communication, digital telephone, and digital TV are all digital signals , and communication systems composed of digital signals are digital communication systems. A digital communication system usually consists of a source , an encoder , a channel , a decoder , a sink , and a noise source
Data transmission rate : refers to the number of binary bits transmitted per second, also known as data rate or bit rate, the unit of rate is b/s, or kb/s, Mb/s, Gb/s, etc.
Bandwidth (bandwidth): Originally refers to the frequency bandwidth of the signal , the unit is hertz (or kilohertz, megahertz, gigahertz, etc.), now "bandwidth" is a synonym for the "highest data rate" that digital channels can transmit, The unit is "bits per second", or b/s (bit/s)
Throughput (throughout) refers to the actual measurement value of the amount of information transmitted by a specific set of data through a specific path in a specific period of time .
Code unit transmission rate : In the information transmission channel, the signal unit carrying data information is called a code unit, and the number of code units transmitted through the channel per second is called the code unit transmission rate, or baud rate for short .
Baud rate : refers to the modulation rate of the data signal to the carrier , expressed by the number of carrier modulation state changes per unit time (that is, the number of symbols modulated per second), and its unit is Baud (symbol/s).
Delay (delay or Latency): refers to the time required for a message or packet to be transmitted from one end of a network (or a link) to the other end. Generally speaking, the delay is composed of the following different parts.
The total delay (delay or latency) experienced by data is the sum of sending delay , propagation delay , processing delay and queuing delay
Sending delay : the time required for a node to make a data block enter the transmission medium from the node when sending data , that is, the time required for the data block to be sent from the first bit of the data block to the end of the last bit. Also known as transmission delay.
Propagation delay : The time it takes for an electromagnetic wave to propagate a certain distance on a channel.
Processing delay : the time it takes for data to perform some necessary processing for store-and-forward at the switching node .
Queuing delay : When packets are transmitted through the network , they have to pass through many routers . However, after the packet enters the router, it must first be queued in the input queue for processing. After the router determines the forwarding interface, it must queue up in the output queue and wait for forwarding. The resulting delay.
Delay-bandwidth product : refers to the number of bits that a link can accommodate . The delay-bandwidth product of a link is also called the link length in bits .
Channel Utilization : Indicates how many percent of a certain channel is used (data passes through). A completely idle channel has zero utilization. Channel utilization is not as high as possible.
The network utilization is the weighted average of the channel utilization of the whole network .
Bit error rate : The ratio of the number of errored bits in the digital signal received within a certain period of time to the total number of bits of the digital signal received at the same time is called "bit error rate", or "bit error rate". ".
Bit error rate (BER: bit error rate) is an index to measure the accuracy of data transmission within a specified time .
Channel : refers to the signal path based on the transmission medium , which can be subdivided into narrow channel and generalized channel.
Narrow channel : A channel with a signal transmission medium is called a narrow channel, which is divided according to different types of specific media, and can be subdivided into: wired channel , wireless channel
Generalized channel : According to the different research objects and concerns , the generalized channel can also be divided into two types according to the functions it contains: modulation channel and coding channel .
Analog channel: A channel that transmits analog signals.
Digital Channel: A channel that transmits digital signals
Channel capacity : refers to the maximum amount of signals that a channel can transmit in a unit of time , and is usually used to indicate the limit transmission capacity of a channel .
Shannon's theorem : In the related fields of signal processing and information theory , a famous formula is obtained by studying how the signal decays after a certain distance and how much data a given signal can load, called Shannon's theorem
Parallel transmission means that data is transmitted in groups on multiple parallel channels .
Serial transmission refers to the transmission of data on a channel in a serial manner .
Synchronous transmission (Synchronous): bit synchronization (including external synchronization method, internal synchronization method) character synchronization
Asynchronous transmission (asynchronous): Asynchronous transmission only transmits one character at a time, and each character is guided by a start bit and terminated by a stop bit .
Baseband transmission : It is a transmission method that does not move the spectrum of the baseband signal . The signal to be transmitted without carrier modulation is called the baseband signal , and the basic frequency band it occupies is called the baseband. The ratio of the upper limit frequency to the lower limit frequency of the baseband is usually much greater than 1. Baseband transmission is widely used in audio cables and coaxial cables to transmit digital telephone signals
Frequency band transmission : It is a transmission technology in which digital information is modulated into an analog signal in the long-distance communication of the computer network system before being sent and transmitted, and when it reaches the receiving end, the analog signal is demodulated into the original digital signal .
Digital signal encoding of digital data : It is how to express digital data with the waveform of physical signal , and it is a method of expressing binary with different combinations of high and low levels
decoding:
Analog signal encoding of digital data : The modulation of digital data is based on three modulation techniques: amplitude modulation , frequency modulation , and phase modulation , which are called amplitude shift keying , frequency shift keying, and phase shift keying .
Amplitude modulation (AM): The original analog data is used as the modulation signal to modulate the amplitude of the carrier according to the amplitude of the modulation signal . After modulation, the frequency and phase of the carrier signal remain unchanged, and the amplitude follows the amplitude of the modulation signal. Varies as the value changes
Frequency modulation (Frequency modulation, FM): The original analog data is used as the modulation signal, and the frequency of the carrier is modulated according to the frequency of the modulation signal. After modulation, the phase and amplitude of the carrier signal remain unchanged, and the frequency follows the amplitude of the modulation signal. change with change
Phase modulation (Phase modulation, PM): The original analog data is used as the modulation signal, and the phase of the carrier is modulated according to the phase of the modulation signal. After modulation, the frequency and amplitude of the carrier signal remain unchanged, and the phase follows the amplitude of the modulation signal. change with change
The most common method of digital encoding of analog data is pulse code modulation (Pulse Code Modulation, PCM): The working process of PCM includes 3 steps: sampling , quantization and encoding
Multiplexing : The bandwidth required to transmit signals is different from the bandwidth allowed by the transmission medium. In order to save overhead , the bandwidth of the transmission medium should be fully utilized. The transmission mode that transmits more than one signal at the same time on one medium is called multiplexing of the medium.
Frequency division multiplexing : When the available bandwidth of the physical channel exceeds the bandwidth required by a single original signal, the total bandwidth of the physical channel can be divided into several sub-channels with the same bandwidth as the transmission of a single signal , and each sub-channel transmits one signal, which is frequency division multiplexing

Time Division Multiplexing: Time Division Multiplexing (TDM) divides the transmission time of a physical channel into several time slices. full bandwidth.
Wavelength Division Multiplexing (WDM): It uses wavelength division multiplexing technology to transmit multiple optical signals of different wavelengths in the same transmission channel.
Code-division multiplexing (CDM): It is a new technology used in mobile communication systems. The network communication of mobile computers such as notebook computers and palmtop computers will use code-division multiplexing technology extensively.
Wired (guided) transmission media : twisted pair , coaxial cable , optical fiber
Coaxial cable (Coaxial Cable): There are baseband coaxial cable and broadband coaxial cable .
Functions of the physical layer: establishment , maintenance and release of physical connections , transmission of physical service data units ( transmission data), completion of some management work of the physical layer
The characteristics of the four major interfaces of the physical layer: mechanical characteristics , electrical characteristics , functional characteristics , and protocol characteristics
Hub (Hub): The main function is to regenerate , shape , and amplify the received signal to expand the transmission distance of the network , and at the same time concentrate all nodes on the node centered on it. Hubs work on the physical layer in the OSI reference model. The hub is the same as the network card , network cable and other transmission media, and belongs to the basic equipment in the local area network, and adopts the CSMA/CD access method.
Link : refers to a physical line from one node to an adjacent node, and there is no other switching node in the middle of this physical line.
Data link : When two nodes transmit data on a path, in addition to a physical line between them, there must be some essential communication protocols to control the data transmission process. A link to which hardware and software implementing these protocols are attached is called a data link.
Frame synchronization : also known as frame delimitation, means that the receiver can accurately extract data frames from the received bit stream , and accurately distinguish the start and end positions of a frame.
Error control : In data communication, affected by factors such as physical link performance and network communication environment , some transmission errors will inevitably occur. The data link layer mainly realizes this function through "error control" technology.
Link management: including the establishment , maintenance and release of data links
MAC addressing : data can be sent to the correct destination, and the receiver knows the address of the sender. Since the LAN has multiple media access controls, a unified data link layer cannot be used. The data link layer of the LAN is divided into logical chains There are two layers of link control and access link control .
Transparent transmission : Any combination of bit data can be transmitted on the link. When the bit combination of data is the same as the control character, corresponding processing is adopted.
Services provided by the data link layer: connectionless service without confirmation , connectionless service with confirmation , connection-oriented service with confirmation
Flow control : This function synchronizes the sending and receiving data, which refers to the adoption of certain technical measures to make the information flow on some or all links and nodes in the communication network not exceed a certain limit value , so as to ensure the smooth flow of information and avoid due to Excessive information flow causes information congestion , reduces information passing capacity, and even causes system " deadlock "
Stop waiting for the ARQ protocol : ARQ (Automatic Repeat reQuest), literal translation is automatic repeat request, but it means automatic request for retransmission .
The cause of the error: In data communication, the data received at the receiving end is inconsistent with the data at the sending end
Commonly used error detection codes are: parity check code , cyclic redundancy code , Hamming code and so on.
Error correction code: Each transmitted packet carries sufficient redundant information ; the receiving end can detect and automatically correct transmission errors.
Common methods of error control: feedback error correction , forward error correction , hybrid error correction
Common codes for error control: parity check code , cyclic redundancy code CRC
Bridge meaning: A bridge is a layer 2 device whose function is to connect two or more LAN segments . Each of these segments is an independent collision domain .
Switch meaning: Switch (Switch), also known as switch or switching hub, is a packet switching device specially designed for computers to communicate with each other and share bandwidth exclusively .
The frame forwarding mode of the switch: storage forwarding , cut-through forwarding , non-fragmented forwarding
Switch switching technology : port switching technology , frame switching technology , cell switching technology
Local Area Network (LAN for short) usually refers to a computer group formed by interconnecting multiple computers in a certain area. A LAN can consist of two computers in one office, or thousands of computers in a company.
The LAN system is mainly composed of two parts: the LAN hardware system and the LAN software system .
Performance of LAN: topological structure , transmission medium (coaxial cable, twisted pair, optical fiber, electromagnetic wave, etc.), medium access control method
LAN topology: star network , ring network , bus network , tree network
Types of LAN: Shared Media LAN , Switched LAN
Network mode of LAN: client/server mode (Client/Server, C/S), browser/server mode , peer-to-peer server network mode
Local area network reference model: The architecture of the local area network only includes the data link layer and the physical layer . Among them, the data link layer is divided into two sublayers: logical link control and media access control .
802.1 : A defines the LAN architecture ; B defines network interconnection , network management and performance testing
802.2 : Defines the functions and services of the LAN logical link control LLC sublayer
802.3 : Defines the LAN CSMA/CD bus medium access control sublayer and physical layer specification
802.11 : defines the wireless local area network access method and physical layer specification
The concept of medium access control method: the method of effectively allocating the frequency band of the transmission medium to each node on the network is called the medium access control method
IEEE 802 stipulates a variety of LAN media medium access methods: Carrier Sense Multiple Access (CSMA/CD) method with collision detection suitable for bus structure , token bus (Token Bus) method suitable for ring structure, suitable for ring structure The Token Ring method
The ALOHA system was originally implemented over radio channels . Pure ALOHA can work in the wireless channel, also can work in the bus network
CSMA : Carrier Sense Multiple Access CSMA (Carrier Sense Multiple Access) is also called carrier sense multiple access . The main difference between the CSMA protocol and ALOHA is that there is an additional carrier sense device, whose function is to monitor before sending
Types of CSMA protocols: non-persistent CSMA , 1- persistent CSMA , p- persisting CSMA
Collision : There may be two or more devices sending frames at the same moment on the channel , so that errors occur on the channel due to frame overlap . This phenomenon is called collision
CSMA/CD receiving process: each site on the Internet is usually monitoring the bus , if there is an information frame coming, it will receive the information frame; then analyze and judge the receiving end address in the information frame , if the address is the address of the station , then copy Receive the frame; otherwise, simply discard the frame.
There are two commonly used Ethernet MAC frame formats: DIX Ethernet V2 standard , IEEE802.3 standard
Ethernet data encapsulation format : The encapsulation of Ethernet IP datagrams is defined in detail in RFC 894 , and the encapsulation of IP datagrams in IEEE 802 networks is defined in R.FC1042
Ethernet address : Unicast Address , Multicast Address , Broadcast Address
T in 10BASE- T refers to the twisted pair, and 10BASE-5 refers to the transmission distance of 500 meters (that is, thick cable)

There are several types of IEEE standards:

10BASE-5 : Thick cable . The maximum transmission distance is 500 meters , using the AUI connector to connect or using the transceiver cable and the transceiver (MAU) for connection.
10BASE-2 : thin cable . The actual transmission distance is 185 meters, using BNC connectors (T-type and N-type).
10BASE-T : Twisted pair . The transmission distance is 100 meters, using RJ45 connector.
10BASE-F : Optical fiber . Divided into FP, FL, FB three link types, FP uses a passive hub connection, the transmission distance is 500 meters, FB uses an active connector, the transmission distance is 3000 meters, FL can use multiple repeaters, and can further extend the transmission distance.
100BASE-TX : Twisted pair , use two pairs of unshielded twisted pair or two pairs of category 1 shielded twisted pair to connect, the transmission distance is 100 meters
100BASE-T4 : 4 pairs of category 3 unshielded twisted pair , transmission distance 100 meters
100BASE-F : single-mode or multi-mode optical fiber , the transmission distance is about 2000 meters
1000BASE-T : Category 5 unshielded twisted pair , transmission distance 100 meters
1000BASE-CX : shielded twisted pair , transmission distance 25-50 meters
1000BASE-LX : single-mode fiber , transmission distance 5000 meters / multi-mode fiber, transmission distance 316-550 meters
1000BASE-SX : Multi-mode optical fiber , SC type optical fiber connector is used to connect the optical fiber, the transmission distance is 275 meters to 550 meters

10Gbit/s 10 Gigabit Ethernet, the goal is to expand Ethernet so that it can go beyond LAN to enter WAN and MAN
The concept of VLAN : virtual local area network , namely VLAN (Virtual Local Area Network). It is a data exchange technology that logically divides LAN devices into smaller LANs to realize virtual workgroups .
802.1Q standard : general standard, based on IEEE 802.1Q additional VLAN information, like tags attached when delivering items. Therefore, it is also called " Tagging VLAN ( Tagging VLAN) ".
ISL (Inner Switch Link) : a dedicated standard , a Cisco-specific marking method, only Cisco equipment can understand ISL-encapsulated data frames
VLAN link types : access link (Access Link) , trunk link (Trunk Link) , hybrid (hybrid) link
Token ring network workflow: intercept token and send data frame , receive and forward data , cancel data frame and resend token
The working principle of the token bus network: interception of tokens , address reprinting , and token forwarding
Expansion of LAN: Expansion with hub/switch
The main features of switched Ethernet : compatibility , compatibility , ease of use , flexibility , support
Wireless Local-Area Network ( WLAN) is also called Radio LAN, just as its name describes: it breaks away from the shackles of traditional cables and provides the function of Ethernet or token network
Main types of WLAN : Infrared LAN , Spread Spectrum Wireless LAN , Narrowband Microwave Wireless LAN
IEEE802.11a expands the standard physical layer. It works in the 5.0GHz frequency band, with transmission rates of 5Mb/s, 11Mb/s and 54Mb/s, and adopts Orthogonal Frequency Division Multiplexing (OFDM) technology.
The IEEE802.11b standard works in the 2.4GHz frequency band, using DSSS spread spectrum technology and compensation code keying (CCK) modulation . The transmission rate is 1Mb/s, 2Mb/s, 5.5Mb/s and 11Mb/s.
The IEEE802.11g standard works in the 2.4GHz frequency band, adopts Orthogonal Frequency Division Multiplexing (OFDM) technology, and the rate can reach 54Mb/s
Composition of wireless local area network : wireless local area network with fixed infrastructure (Infrastructure working mode), wireless local area network without fixed infrastructure (Ad-hoc working mode)
WLAN architecture: Physical layer in IEEE802.11 standard , MAC sublayer in IEEE802.11 standard
The complexity problem in WLAN : It can be divided into the hidden station problem and the exposed station problem .
Media access control mode of WLAN : distributed control mode , central control mode
The way to build WLAN : wireless network card , wireless network hub , wireless network bridge
The function of the network layer: the main function is to provide routing , select the best path to the target host , and transmit data packets along this path . Other functions: Eliminate network congestion , flow control , congestion control , establish and tear down network connections , multiplexing , segmentation and blocking , service selection and transmission, etc.
Two services provided by the network layer: connection-oriented service and connectionless service
Connection-oriented services are like the telephone system while connectionless services are like the postal system
Addressing method of IP address: classified IP address , division of subnet , and formation of supernet .
Special IP addresses and dedicated IP addresses: broadcast address , limited broadcast address , "0" address , loopback address
Reasons for subnetting: The utilization rate of IP address space is sometimes very low. Assigning a network number to each physical network will make the number of hosts in each network too large, thus deteriorating network performance. Two levels of IP addresses are not flexible enough.
Mask: NetMask has two types : network mask and subnet mask
The flow of IP layer forwarding packets: direct delivery and indirect delivery , packet forwarding using subnet mask
Network address translation type: static NAT , dynamic address NAT , NAPT
Address Resolution Protocol (ARP) solves the mapping problem between the IP address and hardware address of a host or router on the same LAN
Reverse Address Resolution Protocol RARP : Enables hosts that only know their own hardware addresses to know their IP addresses
DHCP (Dynamic Host Configuration Protocol): Dynamic Host Configuration Protocol ,  Working in client/server (C/S) mode, including two roles of DHCP server and DHCP client, mainly providing IP address , default gateway , subnet mask code information
The working process of DHCP is divided into four steps, namely, the client requests IP , the server responds , the client selects IP , and the server confirms the IP lease .
The role of ICMP (Internet Control Message Protocol ) : In order to improve the chance of successful delivery of IP datagrams , ICMP (Internet Control Message Protocol) is used in the Internet layer .
ICMP allows hosts or routers to report error conditions and provide reports about abnormal conditions.
ICMP is not a high-level protocol, but an IP layer protocol.
The ICMP message is used as the data of the IP layer datagram, and the header of the datagram is added to form an IP datagram and sent out.
Different strategies for routing selection: Static routing strategy —that is, non-adaptive routing selection, which is characterized by simplicity and low overhead , but cannot adapt to changes in network status in time; dynamic routing strategy —that is, adaptive routing selection, whose The feature is that it can better adapt to changes in the network state , but it is more complicated to implement and the overhead is relatively large .
The distance vector routing algorithm is to let each router maintain a table (ie vector), which gives the best known distance and route to each destination. The information in the table is updated by exchanging information with neighboring routers. The distance vector routing algorithm is also known as the Bellman-Ford routing algorithm and the Ford-Fulkerson algorithm, namely the RIP algorithm.
The most common distance vector routing protocols mainly include: Routing Information Protocol (Routing Information Protocol, RIP) , Interior Gateway Routing Protocol (Interior Gateway Routing Protocol, IGRP) and so on.
Definition of Autonomous System AS (Autonomous System) : a group of routers under a single technical management, and these routers use an AS internal routing protocol and common metrics to determine the routing of packets within the AS, while also using An inter-AS routing protocol used to determine the routing of packets between ASs.
IGP (Interior Gateway Protocol) is a routing protocol used within an autonomous system . Currently, such routing protocols are used most, such as RIP and OSPF.
External Gateway Protocol EGP (External Gateway Protocol) If the source station and the destination station are in different autonomous systems , when a datagram is transmitted to the boundary of an autonomous system, it is necessary to use a protocol to transfer routing information to another autonomous system. system. Such a protocol is the Exterior Gateway Protocol EGP. BGP-4 is currently the most used exterior gateway protocol .
Internet Routing Protocol

Internal gateway protocol IGP : There are many specific protocols, such as RIP and OSPF .
Exterior Gateway Protocol EGP : The currently used protocol is BGP .

RIP (Routing Information Protocol) : RIP is a distributed routing protocol based on distance vector . The RIP protocol requires each router in the network to maintain a record of the distance from itself to every other destination network .
Interior Gateway Protocol OSPF (Open Shortest Path First ) is a distributed link state protocol .
OSPF area: In order to make OSPF applicable to a large-scale network, OSPF divides an autonomous system into several smaller areas called areas.
Three main points of OSPF

Send information to all routers in the autonomous system , the method used here is the flooding method .
The information sent is the link status of all routers adjacent to this router , but this is only part of the information that the router knows.
Only when the link state changes , the router sends this information to all routers by flooding .

Interior Gateway Routing Protocol ( IGRP) is a Cisco proprietary routing protocol that provides routing functionality in autonomous systems .
IGRP is a distance vector Interior Gateway Protocol (IGP). Enhanced Interior Gateway Routing Protocol (Enhanced Interior Gateway Routing Protocol, EIGRP) is an enhanced version of IGRP.
Exterior Gateway Protocol BGP: It is a protocol for exchanging routing information between routers in different autonomous systems , and it only transmits path information . A newer version of BGP is BGP-4 (BGP version 4)
Type of internet connection
The router mainly has the following functions: network interconnection function , data processing function , network management function
Direct delivery and indirect delivery: The physical transmission process and datagram forwarding delivery mechanism of IP packets forwarded by routers in the Internet are called packet delivery . Packet delivery can be divided into direct delivery and indirect delivery . Direct delivery occurs when the packet's source and destination hosts are on the same network, or the forwarding is between the last router and the destination host. If the destination host is not on the same network as the source host, the packet will be delivered indirectly
The structure of the router: the routing selection part and the packet forwarding part ; the packet forwarding part consists of three parts: a switching structure , a set of input ports and a set of output ports
Router switching fabric: switching via bus , switching via crossbar switching fabric , shared memory
How Routing Works: p227
Routing table type: static routing table , dynamic routing table
Routing table items include: destination network prefix , subnet mask , next hop field
Types of routing table entries: network route , main route , default route
Routing table structure: network ID , forwarding address , interface , hop count
There are mainly three types of IPv6 addresses: unicast , multicast , and anycast . They are also called unicast addresses , multicast addresses , and anycast addresses, respectively .
A unicast address is a single interface identifier . Packets sent to a unicast address will be delivered to the interface identified by the address.
An anycast address is an identifier for a set of interfaces . A packet sent to an anycast address will be delivered to the interface that is closest (measured according to the routing protocol used) to the source node among the set of interfaces identified by the address.
A multicast address is an identifier for a group of interfaces . Packets sent to a multicast address will be delivered to all interfaces identified by that address .
Transition from IPv4 to IPv6: dual stack (dual stack) , tunneling (Tunneling) technology , NAT-PT (Network Address Translation-Protocol Translation , network address translation with protocol translation)
Port : When an application (such as a user) process wants to establish a connection with a remote application process, it must specify which application process it is connected to. The normal approach to take is to define transport addresses on which the process can listen for connection requests . In the Internet, these endpoints are called ports.
TSAP (Transport Service Access Point , transmission service access point ) . Similar endpoints in the network layer (ie, network layer addresses) are called NSAPs. An IP address is an example of an NSAP
The method by which the transport entity establishes a connection: a three-way handshake method .
Release the transport connection: three-way handshake to release the connection
Ways to terminate connections: asymmetric release and symmetric release
Asymmetric release is the way the phone system works: when one party hangs up, the connection is broken.
Symmetric release treats the connection as two separate unidirectional connections , requiring each side to release the connection individually.
Flow control and buffering : In summary, for low-bandwidth traffic , caching at the sender is better; for stable transmission of high-bandwidth traffic , caching at the receiver is better
The transport layer of TCP/IP has two different protocols: User Datagram Protocol (UDP) ; Transmission Control Protocol (TCP) .

the difference

UDP does not need to establish a connection before transmitting data, that is, it provides a connectionless service . In addition, UDP does not provide broadcast or multicast services.
TCP provides connection-oriented services , and a connection must be established before transmitting data. TCP also does not provide broadcast or multicast services.

Overview of User Datagram Protocol (UDP): The transport layer protocol needs to have the following functions: one is to create process -to-process communication; the other is to provide a control mechanism at the transport layer
UDP port allocation method: port number allocation of client process , port number allocation of server process
Socket address: UDP requires two identifiers , an IP address and a port number , each used at one end to establish a connection. The combination of an IP address and a port number is called a socket address or socket address.
To use UDP services , you need a pair of socket addresses: client socket address and server socket address .
TCP is a connection-oriented , reliable transport layer protocol. The TCP protocol provides reliable , full-duplex data stream transmission to the application layer user process
TCP data numbering and confirmation: TCP is byte-oriented ; when the connection is established, the two parties must agree on the initial sequence number ; TCP confirmation is the confirmation of the highest sequence number of the received data
TCP flow control: The TCP protocol is byte-oriented. TCP regards the segment to be transmitted as a data stream composed of bytes
Send buffer and receive buffer: TCP needs buffer to store data. There are two buffers in each direction, a send buffer and a receive buffer
Confused Window Syndrome: Symptoms generated by the sender The solution is to prevent the sender TCP from sending data byte by byte . The sender TCP must be forced to wait for it to collect data in order to send large chunks of data; a solution to the symptoms produced by the receiver . (1) Clark's solution . (2) Delayed Confirmation
TCP congestion control: acceleration decrement strategy , slow start
According to whether the status of both parties is equal in the communication process, the network application mode can be divided into: client/server (C/S , Client/Server) mode, peer-to-peer mode (P2P , Peer to Peer) mode
P2P is the abbreviation of Peer-to-Peer, called peer-to-peer network, which emphasizes the peer-to-peer relationship between nodes in the system, and P2P network emphasizes the peer-to-peer status of nodes. The positional relationship between peers in P2P is logical.
Domain name service: The way to identify a computer on the network is to use IP, but a set of IP numbers is not easy to remember, and has no meaning of association. Therefore, it is necessary to choose a meaningful and easy-to-remember name for the server on the network. This name is called the domain name (Domain Name) .
Domain name: This hierarchical naming management mechanism implemented in TCP/IP is called Domain Name System
The TCP/IP domain name system consists of two conceptually independent layers. Among them, one level is the abstract level , which specifies the naming syntax and the rules for assigning domain name management privileges ; the other level is the concrete level , which describes how to implement efficient mapping from domain names to IP addresses .
Understand the two query methods of DNS server: iterative query and recursive query
The file transfer protocol FTP only provides some basic services for file transfer, it uses TCP reliable transfer service
FTP works using a client/server model
An FTP server process can provide services for multiple client processes at the same time . The server process of FTP consists of two parts: a main process , which is responsible for accepting new requests; and several slave processes , which are responsible for processing individual requests.
Types of FTP user login: anonymous access , username and password
Trivial File Transfer Protocol (TFTP) was originally intended to be used to boot diskless systems ( usually workstations or X terminals), so it uses UDP instead of TCP for file transfers to keep the algorithm simple .
The TFTP message does not provide the user name and password, because TFTP is designed for the system boot process , it is impossible to provide the user name and password.
Port number: TFTP server uses UDP port 69
Remote login protocol Telnet purpose: Simply put, remote login refers to the process in which the user uses the Telnet command to temporarily make his computer an emulated terminal of the remote host.
The Telnet remote login service actually regards the computer used by the user to log in as an input terminal, and the remote computer or remote host to be logged in as a host with fast processing capabilities
An email system has three main components: the user agent , the mail server , and the protocol used by email
E-mail format: It consists of two parts: envelope (envelope) and content (content).
E-mail related protocols: mail reading protocol POP or IMAP, mail transfer protocol SMTP
Browser/server mode (B/S mode): The browser is the World Wide Web client program on the user's computer. The computer where the WWW document resides runs a server program, so this computer is also called a WWW server; the client program sends a request to the server program, and the server program sends back the WWW document requested by the client to the client program.
Page: A World Wide Web document displayed on the main window of a client program is called a page
Hyperlink
hypertext document
Uniform Resource Locator URL (Uniform Resource Locator): Use the Uniform Resource Locator URL to identify various documents on the World Wide Web
Hypertext Transfer Protocol HTTP (HyperText Transfer Protocol) : the protocol used to interact between the World Wide Web client program and the World Wide Web server program
HTTP is an application layer protocol that uses TCP connections for reliable delivery.
Hypertext Markup Language HTML
Dynamic Host Configuration Protocol (DHCP) meaning: Dynamic Host Configuration Protocol (Dynamic Host Configuration Protocol, DHCP) provides a plug-and- play networking (plug-and-play networking) mechanism.
DHCP has the following characteristics: safe and reliable configuration , centralized management , saving IP
Scope : It is a continuous range of all assignable IP addresses in a network . It is mainly used to define the IP address range of a single physical subnet in the network.
Super scope : It is a set of scopes , which is used to implement multiple logical IP subnets in the same physical subnet .
Excluded ranges : are sequences of IP addresses that are not used for allocation .
Address pool : After the user defines the DHCP scope and exclusion range, the remaining addresses constitute an address pool, and the addresses in the address pool can be dynamically assigned to clients in the network.
Lease period (lease period) : The IP address assigned by the DHCP server to the DHCP client is temporary, so the DHCP client can only use the assigned IP address for a limited period of time. The DHCP protocol calls this period of time the lease period.
Reserved address : Users can use reserved address to create a permanent address lease period . Reserved addresses ensure that specified hardware devices in the subnet use the same IP address .
Use client/server mode when DHCP works

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