Chapter 1 Overview
1.1 The role of computer networks in the information age
Some important features of the 21st century are digitalization, networking and informatization. It is an information age with the network as the core.
Three types of networks: telecommunications networks, wired networks, and computer networks
The Internet is the greatest revolution in the field of storing and exchanging information since the invention of printing
The Chinese translation of Internet; (two types) (1) Internet (2) Internet, currently the most popular and de facto standard translation.
The Internet is interconnected by a huge number of various computer networks. The Internet is meant to cover the whole world.
Two important fundamental features of the Internet: connectivity and sharing
Connectivity: The Internet allows Internet users to exchange various information very conveniently and economically, no matter how far apart they are.
Sharing: refers to the sharing of resources. The meaning of resource sharing is multifaceted, which can be information sharing, software sharing, or hardware sharing.
1.2 Internet overview
1.2.1 Network of networks
The Internet that originated in the United States has grown into the world's largest computer network with global coverage
A computer network consists of several nodes (nodes) and links (links) connecting these nodes. The nodes in a computer network can be computers, hubs, switches, or routers.
Internet (internetwork or internet): There are multiple computer networks connected to each other through some routers to form a computer network with a larger coverage, so the Internet is a "network of networks".
A computer connected to a computer network is often called a host, and an indispensable router in the Internet is a special computer.
A network connects many computers together, and the Internet connects many networks together through some routers. Computers connected to a network are often called hosts.
1.2.2 Three Stages of Internet Infrastructure Development
The first stage is the process of developing from a single network ARPANET to the Internet (packet switching network). In 1983, the TCP/CP protocol became the standard protocol on the ARPANET, so that all computers using the TCP/CP protocol can use the Internet to communicate with each other, so People regard 1983 as the birth event of the Internet
Distinguish between the nouns internet and Internet which have very different meanings
1. Internet (Internet) starting with the lowercase letter i is a general term, which generally refers to a computer network formed by interconnecting multiple computer networks, and does not necessarily use the TCP/CP protocol.
2. The Internet (Internet) starting with the capital letter I is a special term. It refers to the world's largest, open, and specific Internet formed by interconnecting many networks. It uses the TCP/CP protocol family as the communication rule , whose predecessor was ARPANET in the United States.
The second stage is to build a three-level structure of the Internet. National Science Foundation Network NSFNET, a three-level computer network, is divided into backbone network, regional network and campus network (or enterprise network).
The third stage is characterized by the gradual formation of the Internet with a multi-level ISP structure on a global scale. ISP (Internet Service Provider).
IXP (Internet Exchange Point) in order to use network resources more effectively and economically. A typical IXP consists of one or more network switches, and many ISPs are connected to the relevant ports of these network switches. IXPs often use network switches that work at the data link layer.
1.2.3 Internet standardization work
In 1992, the Internet Society (ISOC) was established. There is a technical organization under ISOC called the Internet Architecture Board IAB (Internet Architecture Board) responsible for managing the development of Internet-related protocols. There are two engineering departments under it.
(1) IETF (Internet Engineering Task Force): a forum composed of many working groups WG (Working Group). The specific work is under the responsibility of the Internet Engineering Steering Group (IESG). Mainly for protocol development and standardization.
(2) IRTF (Internet Research Task Force); there are forums composed of some research groups (RG) (Research Group). The work is managed by the Internet Research Steering Group (IRSG). The task is to study some issues that require long-term consideration, including some protocols, applications, and architecture of the Internet.
All of the Internet is published on the Internet in the form of RFCs. RFC (Request For Comments (request for comments)) RFC documents are numbered in order of publication time (that is, RFCxxxx, where xxxx is an Arabic numeral).
Formal standards for the Internet go through the following three stages
(1) Internet Draft (internet Draft): Valid for six months, it cannot be regarded as an RFC document
(2) Proposed standard (Propdsed Stangard); it is an RFC document.
(3) Internet Standard (Internet Standard): After long-term testing, it has been proved that a proposed standard can become an Internet standard.
1.3 Composition of the Internet
Edge part: It is composed of all hosts connected to the Internet. This part is directly used by users for connection communication and resource sharing.
Core part: It consists of a large number of networks and routers connecting these networks. This part serves the edge part of the craftsman (provides connectivity and switching)
1.3.1 The Edge of the Internet
The part at the edge of the Internet is all the hosts connected to the Internet. These hosts are also known as end systems.
Communication between computers: A process on host A communicates with another process on host B.
Communication methods can be divided into two categories: client-server mode (C/S mode) and peer-to-peer mode (P2P mode).
1. Client-server approach
Both client and server are two application processes involved in value communication. The client-server approach describes the relationship between the service and the serviced process.
Features: The client is the service requester, and the server is the service provider.
Both client and server originally refer to computer processes (software)
2. The peer-to-peer connection mode (peer-to-peer) is referred to as P2P.
When two hosts communicate with each other, it does not distinguish which one is the service requester and which one is the service provider. As long as the two hosts are running peer-to-peer connection software (P2P software), they can perform equal peer-to-peer connections. communication.
1.3.2 The Core Parts of the Internet
Playing a special role at the core of the network is the router, which is a special-purpose computer. The router is a key component to realize packet switching, and its task is to forward the received packets.
1. Main features of circuit switching
From the perspective of allocation of communication resources; exchange is to dynamically allocate transmission line resources in a certain way.
Circuit switching: It must go through the three-step switching method of "establishing a connection (occupying communication resources)" --- talking (always occupying communication resources) --- releasing the connection (returning communication resources).
An important feature of circuit switching is that during the entire call, two users always occupy end-to-end communication resources during a call.
The transmission efficiency of the line is often very low.
2. Main features of packet switching
Packet switching uses error storage and forwarding technology. Usually, the entire block of data sent is called a message. Before sending the message, divide the longer message into smaller equal-length data segments. Before each data segment, add some necessary The control information forms a header, which constitutes a packet. A packet is also called a packet, and the header of a packet can also be called a "header". A packet is a data unit transmitted on the Internet. The header of the packet contains important control information such as destination address and source address.
The host is used to process information for the user, and the router is used to forward packets, that is, to switch packets.
Routers must frequently exchange routing information mastered by each other. In order to create and dynamically maintain the forwarding table in the router, so that the forwarding table can be updated in time when the entire network topology changes.
Short packets are stored temporarily in the router's memory (memory) rather than on disk. Guaranteed a high exchange rate.
When some nodes or links in the network fail suddenly, the routing protocol running in each router can automatically find the most suitable path for forwarding packets.
The storage and forwarding of packets in each router needs to be queued, which will cause a certain delay.
1.4 The development of computer network in my country
1.5 Categories of Computer Networks
1.5.1 Definition of Computer Network
A computer network is primarily an interconnection of general-purpose, programmable hardware that is not dedicated to a specific purpose (for example, to transmit data or video signals).
1.5.2 Several different types of computer networks
1. Classify according to the scope of the network
(1) WAN (Wide Area Network) (remote network): range from tens to thousands of kilometers
(2) MAN (Metropolitan Area Network); range 5-10km
(3) LAN (Local Area Network): the range is about 1km, and the rate is above 10Mbit/s
(4) Personal Area Network PAN (Personal Area Network); the range is about 10 meters
2. Classify according to network users
(1) Public network; all people who are willing to pay the fee according to the regulations of the telecommunications company can use this network.
(2) Private network: a network built by a certain department to meet the needs of the unit's special business work.
3. The network used to connect users to the Internet
Access Network AN (Access Network), also known as local access network or resident access network
1.6 Indicators of Computer Networks
1.6.1 Performance indicators of computer network
1. Speed
The signals sent by the computer are all in digital form. Bit (binary digit) (binary digit) A bit is a unit of information quantity used in information theory.
The rate in network technology refers to the transmission rate of data, also known as data rate or bit rate. Unit: bit/s (bits per second). Two counts:
(1) Add a letter in front of bit/s:
k (kilo)=10^3=thousand, M (Mega)=10^6=mega, G(Giga)=10^9=ji, T(Tera)=10^12=too, P(Peta)=10 ^15=Pat, E(Exa)=10^18=Ai, Z(Zetta)=10^21=Ze, Y(Yotta)=10^24=Yao.
(2) Take byte B as the unit of measurement (B means byte) 1B=8bit
k (kilo)=2^10=thousand=1024, M (Mega)=2^20=mega, G(Giga)=2^30=ji, T(Tera)=2^40=too, P(Peta) =2^50=pat, E(Exa)=2^60=Ai, Z(Zetta)=2^70=Ze, Y(Yotta)=2^80=Yao
2. Bandwidth
(1) The bandwidth is originally the frequency bandwidth of a certain signal. Unit: Hertz (or kilohertz, megahertz, gigahertz), which means that the signal frequency band range allowed by a certain channel is called the bandwidth (or passband) of the channel.
(2) Bandwidth indicates the ability of a certain channel in the network to transmit data. Therefore, the network bandwidth represents the "highest data rate" that can pass through a certain channel in the network per unit time. The unit is the unit bit/s of the data rate, which is bits per second.
3. Throughput
Throughput represents the actual amount of data passing through a network (or channel, interface) per unit time
4. Delay
Latency refers to the time it takes for data (a message or packet, or even bits) to travel from one end of the network (or link) to the other.
(1) Sending delay
Transmit latency is the time required for a host or router to transmit a data frame.
Calculation formula: sending delay = data frame length (bit) / sending rate (bit/s)
(2) Propagation delay
Propagation delay is the time it takes for an electromagnetic wave to travel a certain distance in a channel.
Calculation formula: Propagation delay = channel length / electromagnetic wave propagation speed on the channel (m/s)
The propagation velocity in a copper cable is about 2.3*10^5km/s. The propagation speed in optical fiber is about 2.0*10^5km/s.
Transmission delay occurs in the transmitter inside the machine and has nothing to do with the length of the transmission channel (or the distance the signal travels). But the propagation delay occurs on the transmission channel medium outside the machine, and has nothing to do with the transmission rate of the signal. The farther the signal travels, the greater the propagation delay.
(3) Processing delay
(4) Queuing delay
Total delay = sending delay + propagation delay + processing delay + queuing delay
It cannot be generally considered that "the higher the data transmission rate, the smaller the total delay of its transmission"
For high-speed network links, what we increase is only the sending rate of data rather than the propagation rate of bits on the link. The propagation rate depends on the communication line dielectric material.
Increasing the data sending rate only reduces the data sending delay.
5. Delay-bandwidth product (link length in bits)
Calculation formula: delay-bandwidth product = propagation delay * bandwidth
The delay-bandwidth product represents the volume of the pipeline, indicating how many bits such a link can accommodate.
6. Round trip time RTT
Round trip time RTT
Sending time = data length / sending rate
Effective data rate = data length / (send time + RTT)
7. Utilization rate
There are two types of utilization rate: channel utilization rate and network utilization rate. The channel utilization rate value shows how many percent of a channel is used (data passes through). The network utilization is the weighted average of the channel utilization of the whole network.
D0 indicates the delay when the network is idle, D indicates the delay of the network at that time, and U indicates the utilization rate of the network again
Formula: D=D0/(1-U)
If the channel utilization or network utilization is too high, a very large delay will be generated.
1.6.2 Non-performance characteristics of computer networks
1. Fee
2. Quality
3. Standardization
4. Reliability
5. Scalability and upgradeability
6. Easy to manage and maintain
1.7 Computer Network Architecture
In computer network, hierarchical system network is the most basic.
1.7.1 Formation of computer network architecture
The computer initiating the communication must activate the data communication path.
A layered approach was proposed at the beginning of the design. Hierarchy can transform a large and complex problem into several smaller problems, and these smaller local problems are easier to study and deal with.
1977 Open System Interconnection Basic Reference Model OSI/RM, referred to as OSI
Today's largest, global, TCP/IP-based Internet does not use OSI standards.
The most widely used is not the de jure international standard OSI, but the non-international standard TCP/IP
1.7.2 Protocol and division level
Follow the rules, which clearly define the format of the data exchanged and the related synchronization issues.
Rules, standards or conventions established for data exchange in a network are called network protocols. It mainly consists of the following three elements.
(1) Syntax, that is, the structure or format of data and control information
(2) Semantics, that is, what kind of control information needs to be sent, what kind of action to complete and what kind of response.
(3) Synchronization, that is, a detailed description of the sequence of events
Whenever we want another computer connected to the network to do something, we need a protocol. When we often save files on our personal computers, we don't need any network protocols.
Benefits of layering:
(1) Each layer is independent
(2) Flexible and good
(3) Structurally separable
(4) Easy to implement and maintain
(5) Can promote standardization work
1. Error control
2. Flow control
3. Fragmentation and reassembly
4. Multiplexing and demultiplexing
5. Connection establishment and release
The collection of layers and protocols of a computer network is the architecture of the network. The architecture of a computer network is the precise definition of the functions that the computer network and its components should complete.
Architecture is abstract, while implementation is concrete, the actual running computer hardware and software.
1.7.3 Architecture with five-layer protocols
The four-layer architecture of TCP/IP: application layer, transport layer, Internet layer and link layer
OSI architecture: application layer, presentation layer, session layer, transport layer, network layer, data link layer, physical layer
The architecture of the five-layer protocol: application layer, transport layer, network layer, data link layer, physical layer
(1) Application layer
The application layer is the highest layer in the architecture, and the task of the application layer is to complete specific network applications through the interaction between application processes. The application layer protocol defines the rules for communication and interaction between application processes.
Application layer · The data unit of interaction is called a message
(2) Transport layer
The task of the transport layer is to provide general data transmission services for the communication between the processes in the two hosts. Multiplexing means that multiple application layer processes can use the services of the lower transport layer at the same time. In contrast to multiplexing, the transport layer delivers the received information to the corresponding processes in the upper application layer.
The protocols mainly used by the transport layer:
1. Transmission Control Protocol TCP-----provides connection-oriented and reliable data transmission services, and the unit of data transmission is the segment
2. User Datagram Protocol UDP----provides connectionless best-effort data transmission services (the reliability of data transmission is not guaranteed), and the unit of data transmission is user datagram.
(3) Network layer
The network layer is responsible for providing communication services for different hosts on the packet switching network. The data transmission unit datagram, in the TCP/IP system, because the network layer uses the IP protocol, the packet is also called an IP datagram, or simply a datagram.
The Internet is interconnected by a large number of heterogeneous networks through routers. The network layer protocol used by the Internet is the connectionless Internet Protocol IP and many routing protocols, so the network layer of the Internet is also called the Internet layer or IP layer.
(4) Data link layer
Referred to as the link layer. The data link layer assembles the IP datagram handed over by the network layer into a frame, and transmits the frame on the link between two adjacent nodes. Each frame includes data and necessary control information.
(5) Physical layer
The unit of transmitting data on the physical layer is a bit. The physical layer has to consider how much voltage to use to represent "1" or "0", and how the receiver recognizes the sender's bit. The physical layer also determines how many pins the plug of the connecting cable should have and how the pins should be connected.
The OSI reference model refers to the data unit transmitted between the peer layers as the protocol data unit PDU of the layer
1.7.4 Entities, protocols, services and service access points
When studying information exchange in open systems, the more abstract term entity is often used to refer to any hardware or software process that can send or receive information.
A protocol is a collection of rules governing the communication between two peer entities. The rules of the syntactic aspect of the protocol define the format of the information exchanged, while the rules of the semantic aspect of the protocol define the operation to be completed by the sender or receiver.
The implementation of the protocol guarantees that services can be provided upwards. Entities using the services of this layer can only see the services but not the underlying protocols. The protocol below is transparent to the entities above.
Protocols are the rules governing communication between peer entities. Services are provided from the lower layer to the upper layer through the interlayer interface
The agreement must estimate all unfavorable conditions in advance, and cannot assume that everything is normal and ideal. It must be very carefully checked whether the agreement can cope with any abnormal situation with a very small probability of occurrence.