Chapter One
1.1 Overview of Computer Networks
1.1.1 Concept of computer network
A computer network is a decentralized, independently functional computer system that communicates through is connected to lines by fully functional software A system that realizesresource sharingandinformation transfer. A computer network is an interconnected, autonomous collection of computers (interconnected ——Interconnection through communication links; autonomy-no master-slave relationship).
1.1.2 Composition of computer network
①From the component point of view, a computer network mainly consists of three major parts: hardware, software, and protocols.
② In terms of working methods, the computer network is divided intoedge part (users can use it directly, through C/S or P2P way) andthe core part (serving the edge part, through routers and networks).
③From the functional composition point of view, the computer network is divided into communication subnet and resource subnet .
Communication subnet consists of various transmission media, communication equipment, and corresponding network protocols to realize data communication, specifically the lower three layers:Physical layer(hub, repeater), Data link layer(switch, bridge), Network layer (router);
Resource subnet is a collection of equipment and software that implements resource sharing functions to achieve resource sharing or data processing, specifically the upper three layers:Session layer, Presentation layer, Application layer .
1.1.3 Functions of computer networks
①Data communication;
② Resource sharing: The behavior of using the computer resources of a certain computer for other computers on the same computer network, which can sharehardware , Software, Data;
③Distributed processing: Multiple computers each undertake different parts of the same task (Hadoop);
④Improve reliability;
⑤ Load balancing: Arrange and configure resources more rationally to make computers closer.
1.1.4 Classification of computer networks
1. Classification by distribution scope: ① Wide Area Network WAN (switching technology), ② Metropolitan Area Network MAN, ③ Local Area Network LAN (broadcast technology), ④ Personal Area Network PAN.
2. Classification by transmission technology: ①Broadcast network (shared public communication channel), ②Point-to-point network (using packet store-and-forward and routing mechanisms).
3. Classification according to topology: ① bus type, ② star type, ③ ring type, ④ mesh type.
4, user classification:publicuser account专Use page.
5. Classification by switching technology: ① circuit switching network (calls are busy with each other), ② message switching network (storing and checking the next hop before forwarding), ③ packet switching network (same message).
6. Classification by transmission medium: ① Wired network (twisted pair network, coaxial cable network), ② Wireless network (Bluetooth, microwave, radio, etc.).
*1.1.5 Development and standardization of computer networks
1. Development
Phase 1: The Defense Advanced Research Projects Agency (ARPA) designs a decentralized command system called ARPANET( ARPAnet); hoping to realize the interconnection of different networks, the internet (interconnected network) was establishedInternet; in 1983, ARPAnet accepted the TCP/IP protocol, The internet was selected as the main computer communication system and the Internet was established.
The second stage - three-level structure: campus network - regional network - backbone network.
The third stage - multi-level ISP (Internet service provider) structure: campus network, company - local ISP - regional ISP - backbone ISP. IXP can be used for exchange between regional ISPs.
2. Standardization work and related organizations
Standardization work: branchstatutory standardJapaneseactual standardization.
Legal standards are formal and legal standards (OSI) formulated by authoritative organizations; de facto standards are products of certain companies that have become mainstream in the competition. Over time, the protocols and technologies in the products have become standards (TCP /IP).
Relevant organizations for standardization work:
International Organization for Standardization ISO: OSI model, HDLC protocol;
International Telecommunications Union ITU: Develops communication rules;
Institute of Electrical and Electronics Engineers IEEE: academic institutions, IEEE802 series standards, 5G;
Internet Engineering Task Force IETF: Responsible for the formulation of Internet-related standards, RFC XXXX.
1.1.6 Performance indicators
①Rate
The rate is thedata rate or also known asdata transfer rateOrBit rate, the rate at which a host connected to a computer network transmits data bits on a digital channel, the unit is b/s, kb/s wait. Rate 1Tb/s=103Gb/s=106Mb/s=109kb/s=1012b/s, storage capacity1Byte (byte) =8bit (bit), 1GB=210MB=1024MB, 1MB=210KB=1024KB, 1KB=210B=1024B=1024*8b .
②Bandwidth
Bandwidth originally refers to the frequency bandwidth of a certain signal, that is, the difference between the highest frequency and the lowest frequency. The unit is Hertz (Hz). In computer networks, bandwidth is used to represent the ability of the network's communication lines to transmit data. Usually refers to the "highest data rate" that can pass from one point on the network to another point in a unit time, the unit is b/s, kb/ s etc.
③Throughput
Throughput representsunit timethrougha certain network (or channel, interface) The amount of data a>. The unit is b/s, kb/s, Mb/s, etc. The throughput is limited by the bandwidth of the network or the rated rate of the network.
④Delay-bandwidth product
The delay-bandwidth product refers to the total number of bits (capacity) accommodated in the propagation delay unit time. The delay-bandwidth product (bit) = propagation delay (s) ×Bandwidth (bit/s).
⑤Delay
Latency refers to the time required for data (message/packet/bit stream) to be transmitted from one end of the network (or link) to the other end. It is also called delay or delay. The unit is s. The delay is sending delay (transmission delay), propagation delay, queuing delay, and the sum of processing delay.
1. Transmission delay, calculated from the first bit of the packet sent to the time required for the last bit of the packet to be sent. Transmission delay (s) = data length (b)/channel bandwidth (b/s);
2. Propagation delay depends on the propagation speed of electromagnetic waves and the link length. Propagation delay = channel length/transmission rate of electromagnetic waves on the channel.
3. Queuing delay, waiting for the output/input link to be available.
4. Handle delays, detect errors, and find exits.
⑥Round trip delay
Round trip delay RTT is the total delay experienced from the sender sending data to the sender receiving data confirmation (immediately after reception). RTT includes ① round-trip propagation delay, that is, propagation delay ×2, ② terminal processing time.
⑦Utilization rate
Usage rate fractionShindo usage rateJapaneseNet usage rate.
Channel utilization=The time when data passes/total time. (Highway road utilization)
Network utilization is the weighted average of channel utilization. (The more cars there are, the slower the speed and the more traffic jams there are)
1.2 Computer network architecture and reference model
1.2.1 Computer network hierarchical structure
Service Data Unit (SDU): Data (user data) that should be transmitted to complete the functions required by the user. The service data unit of the nth layer is recorded as n-SDU.
Protocol control information (PCI): Information that controls protocol operations. The protocol control information at layer n is recorded as n-PCI.
Protocol Data Unit (PDU): The data unit transmitted between peer levels. The protocol data unit of the nth layer is recorded as n-PDU. In an actual network, the protocol data unit of each layer has a popular name. For example, the PDU of the physical layer is called bits, and the PDU of the data link layer is called The PDU of frame, the PDU of network layer is called packet, and the PDU of transport layer is called segment .
| send |
take over |
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| Line 5 |
5-SDU+5-PCI=5-PDU |
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| Line 4 |
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| Line 3 |
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| Line 2 |
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2-PDU—2-PCI=2-SDU |
| Line 1 |
011000110101010100110001101010101001100011010101010… |
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1.2.2 Concepts of computer network protocols, interfaces, and services
①Agreement
Protocol, the rules, standards or conventions established for data exchange betweenpeer entities in the network are called network protocols (horizontal , exists between peer entities), the elements of the agreement are syntax, semantics and are synchronized, and the syntax specifies the format of the transmitted data; Semantics stipulates the functions to be completed; synchronization stipulates the sequence of various operations.
②Interface
The interface is the entrance for theupper layer to use thelower layer service. The service is provided to the upper layer through the service access point SAP, which is actually a logical interface.
③Service
Service refers to the function call provided by the lower layer to the immediately upper layer (vertically).
When the upper layer uses the services provided by the lower layer, it must exchange some commands with the lower layer. These commands are called service primitives in the OSI reference model. The OSI reference model divides primitives into four categories:
1. Request: Sent by the user to the service provider to request the completion of a certain task.
2. Indication: Sent by the service provider to the user to instruct the user to do something.
3. Response: Sent by the user to the service provider as a response to the instruction.
4. Confirmation: Sent by the service provider to the user as confirmation of the request.
These four types of primitives are used for different functions, such as establishing connections, transmitting data, and disconnecting. Response services include all four types of primitives, while non-response services have only two types of primitives: request and indication.
Services provided by computer networks can be classified in three ways.
①Connection-oriented services (TCP) and connectionless services (IP, UDP)
②Reliable services and unreliable services
③With response service (FTP) and no response service (WWW)
1.2.3 OSI reference model and TCP/IP reference model
1. OSI reference model
| 7 |
Application layer |
resource subnet (data processing) |
DATA+H7 |
| 6 |
session layer |
7-PDU+H6 |
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| 5 |
presentation layer |
6-PDU+H5 |
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| 4 |
transport layer |
5-PDU+H4 |
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| 3 |
Network layer |
Communication subnet (data communication) |
4-PDU+H3 |
| 2 |
data link layer |
T2+3-PDU+H2 |
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| 1 |
physical layer |
01010100111111100000000000 |
The packing process is 7-1, and the unpacking process is 1-7.
Application layer: All programs that can interact with users and generate network traffic. Typical application layer services include: file transfer (FTP), email (SMTP), World Wide Web (HTTP), etc.
Presentation layer: used to handle the presentation of information exchanged between two communication systems (syntax and semantics). Function ①: Data format Conversion (translation); Function ②: Data format Encryption< a i=6>; Function ③: Data compression and recovery.
Session layer: Provides to the entity/user process of the presentation layer to establish a connection and transmit data in an orderly manner on the connection. This is Session, alsoEstablish synchronization (SYN). Function ①: Establish, manage, and terminate sessions; Function ②: Use checkpoint to enable the session to continue from the checkpoint/synchronization point when communication fails. Restore communication and achieve data synchronization (suitable for transferring large files). Main protocols: ADSP, ASP.
Transport layer: Responsible for the communication between two processes in the host, that is, end-to-end For communication of a>, the transmission unit is a message segment or user datagram. Function ①: Reliable and unreliable transmission; Function ②: Error control ; Function ③: Flow control; Function ④: Multiplexing and demultiplexing . Main protocols: TCP, UDP.
Network layer: The main task is to transmit packets from the source to the destination, and provide communication services for different hosts on the packet switching network. The transmission unit of the network layer is datagram. Function ①: Routing selection (Best path); Function ②: Flow control; Function ③: Error control (check according to the protocol); Function ④: Congestion control. Main protocols: IP, IPX, ICMP, IGMP, ARP, RARP, OSPF.
Data link layer: The main task is to assemble datagrams from the network layer into Frame (transmission unit). Function ①: Framing (defining the start and end of the frame); Function ②: Error control (frame errors and bit errors); Function ③: Flow control; Function ④: Access (access) control (Control access to the channel). Main protocols: SDLC, HDLC, PPP, STP.
Physical layer: The main task is to realize the transparent transmission of bit streamphysical media a> (meaning that no matter what bit combination the received data is, it can be transmitted on the link). The transmission unit isbit. Function ①: Define interface characteristics; Function ②: Define transmission mode (simplex, half-duplex, duplex); Function ③: Define transmission speed; Function ④: Bit synchronization; Function ⑤: Bit encoding. Main protocols: Rj45, 802.3.
2. TCP/IP reference model
| OSI reference model |
TCP/IP reference model |
TCP/IP protocol stack |
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| 7 |
Application layer |
Application layer |
HTTP |
FTP |
DNS |
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| 6 |
session layer |
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| 5 |
presentation layer |
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| 4 |
transport layer |
transport layer |
TCP |
UDP |
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| 3 |
Network layer |
Internet layer |
IP |
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| 2 |
data link layer |
network interface layer |
Ethernet |
ATM |
Frame Relay |
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| 1 |
physical layer |
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Combining the advantages of OSI and TCP/IP reference models - 5-layer reference model
| Application layer |
Supports various network applications, FTP, SMTP, HTTP. |
| transport layer |
Process - data transmission between processes, TCP, UDP. |
| Network layer |
Source host - data packet routing and forwarding of the target host, IP, ICMP, OSPF, etc. |
| data link layer |
Assemble the datagrams transmitted from the network layer into frames, Ethernet, PPP. |
| physical layer |
bit transmission. |