Computer Network (Eighth Edition) Chapter 1 Knowledge Summary (final review available)

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Chapter 2 Physical Layer

Chapter 3 Data Link Layer

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Chapter 1 Computer Network Architecture

1. The role of computer network

1.1 Basic Features of the Internet

• Connectivity: A computer network enables users on the Internet to exchange information as if their computers were directly connected to each other.

• Sharing: that is, resource sharing

2. Internet

2.1 Network of networks

• A network is composed of several nodes and links connecting these nodes

• The Internet is a "network of networks"

• Computers connected to the Internet are called hosts

• Commonly referred to as the "Internet" "Internet" refers to the "Internet"

2.2 Three Stages of Internet Development

• The first stage: the process of developing from a single network ARPANET to the Internet

• The second stage: building an Internet with three structures

• The third stage: the Internet with a global multi-level ISP structure gradually formed

  • ISP: Internet Service Provider (Internet Service Provider)

  • ISP is also divided into different levels of ISP: backbone ISP, regional ISP, local ISP

  • IXP: Internet Exchange Point; its function is to allow two networks to connect directly and exchange packets without needing to forward packets through a third network

2.3 Internet standardization work

• ISOC: Internet Society

  • IAB: Internet Architecture Research Board

    • IRTF: Internet Research Division

    • IETF: Internet Engineering Division

• Three stages of formal standards development for the Internet: RFC documents

  • internet draft

  • Recommended Standard

  • internet standards

3. Composition of the Internet

1. Edge part: Consists of all hosts connected to the Internet, this part is used directly by users for communication (transmitting data, audio and video) and resource sharing

2. Core part: Consists of a large number of networks and routers connecting these networks, this part provides services for the edge part (improves connectivity and switching)

3.1 Edge part

Edge part: consists of all hosts connected to the Internet, these hosts are also called end systems

Host A communicates with host B", actually means "a program running on host A communicates with another program running on host B", that is, "a process on host A communicates with another program on host B A process to communicate " referred to as "communication between computers"

Communication between end systems at the edge of the network is usually divided into two categories: client-server mode (C/S mode) and peer-to-peer mode (P2P mode)

3.1.1 Client-server mode (Client/Server mode)

• Client A requests service from server B, and server B provides service to client A

• The client runs the client program, and the server runs the server program

  The server can handle requests from multiple remote or local clients at the same time

3.1.2 P2P mode (peer-to-peer)

• Does not distinguish between service requesters and service providers

• As long as both hosts are running peer-to-peer connection software (P2P software), they can communicate on an equal, peer-to-peer connection.

• Both parties can download the shared documents that the other party has stored in the hard disk.

• The peer-to-peer connection method is still using the client-server method in essence, but each host in the peer-to-peer connection is both a client and a server at the same time

• Each host runs a P2P program.

3.2 Core part

• The core part of the network plays a special role: the router

• The router is the key construction to realize packet switching , and its task is to forward the received packets , which is the most important function of the core part of the network

3.2.1 Main features of circuit switching

Circuit switching (circuit switching) technology means that between the devices at the two ends of the communication, a dedicated physical line is actually established by switching the connection of the line in the device . Before the connection is removed, the devices at both ends occupy the line alone for data transmission.

• The telephone system uses circuit switching technology. A physical circuit is established between the calling phone and the receiving phone by physically connecting the incoming and outgoing lines in each switch.

• Before the call ends, the two parties in the call always occupy the end-to-end communication resources

• After the call is over, the input wires in these switches are disconnected from the output wires and the physical wires are severed.

1. Advantages

• After the connection is established, the data is transmitted at a fixed transmission rate, and the transmission delay is small.

• Since the physical lines are individually occupied, conflicts are not possible.

• It is suitable for real-time large-volume continuous data transmission.

2. Disadvantages

• Establishing a connection across multiple devices or cables can take a long time.

• After the connection is established, since the line is dedicated, even if it is idle, it cannot be used by other devices, resulting in a certain waste.

• For both parties in the communication, the communication must be completed only when the sending and receiving speed, encoding method, information format and transmission control of the two parties are consistent.

3.2.2 Main features of packet switching

Packet switching uses store-and-forward technology

• Usually we send the entire block of data into a message (message)

• Divide a message into small data segments of equal length. After each small data segment is preceded by a header composed of necessary control information , a packet is formed.

• The packet is also called "packet", and the header of the packet is also called "packet header". A packet is a unit of data that is transmitted on the Internet.

• The "header" in the packet is very important . Since the header of the packet contains important control information such as the destination address and source address , each packet can independently select the transmission path in the network and be correctly delivered to the packet transmission end point.

1. The communication process of packet switching

• Divide the message to be sent into shorter data blocks at the sender

• Each block adds a header with control information to form a packet (packet)

• Send each packet to the receiver in turn

• The receiving end strips the header, extracts the data part, and restores it to a message

2. The process of router processing packets:

• Put the received packet into the cache first (temporary storage)

• Look up the forwarding table to find out from which port a certain destination address should be forwarded

• Send the packet to the appropriate port for forwarding

3. Advantages

• Efficient: Dynamically allocate transmission bandwidth and occupy the communication link segment by segment.

• Flexible: use grouping as the unit of transmission and lookup routing.

• Fast: can send packets to other hosts without first establishing a connection

• Reliable: A distributed routing protocol that guarantees reliability makes the network highly survivable

4. Disadvantages

• Delay: Packets need to be queued when storing and forwarding at each node, which will cause a certain delay

• Overhead: The header (control information) that the packet must carry causes a certain amount of overhead

• Bandwidth not guaranteed: dynamically allocated

3.2.3 Message exchange

The entire message is transmitted to the adjacent node first, and after all is stored, the forwarding table is searched and forwarded to the next node.

3.2.4 Comparison of three exchange methods

• If a large amount of data is to be continuously transmitted, and the transmission time is much longer than the connection establishment time, the transmission rate of circuit switching is faster.

• Message switching and packet switching do not need to pre-allocate transmission bandwidth, which can improve the channel utilization of the entire network when transmitting burst data

• Since the length of a packet is often much smaller than the length of the entire message, packet switching has a shorter delay than message switching and has better flexibility.

4. Classification of computer networks

4.1 Definition of Computer Network

most common definition

• Computer network refers to the connection of multiple computers with independent functions in different geographical locations and their external devices through communication lines. Under the management and coordination of network operating systems, network management software and network communication protocols, resource sharing and communication are realized. A computer system for information transfer. (The exam writes what is a computer network and writes this definition)

better definition

• Computer networks are mainly interconnected by some general-purpose, programmable hardware , and these hardware and computer networks are mainly interconnected by some general-purpose, programmable hardware. Purpose (for example, to transmit data or video signals)

• These programmable hardware can be used to transfer many different types of data, and can support a wide range and growing applications.

4.2 Classification of computer networks

4.2.1 Classification according to the scope of the network

• WAN (Wide Area Network)

• LAN (Local Area Network)

• MAN (Metropolitan Area Network)

• Personal Area Network PAN (Personal Area Network)

4.2.2 Classification according to network users

• public network

• private network

4.2.3 Networks used to connect users to the Internet

• Access Network AN (Access Network)

5. Performance analysis of computer network

5.1 Computer performance indicators

5.1.1. Rate

• The most important performance indicator

• Refers to the transmission rate of data, also known as data rate (data rate) or bit rate (bit rate)

• Unit: bit/s, kbit/s, Mbit/s, Gbit/s

• Speed ​​often refers to rated or nominal speed , not actual operating speed

5.1.2. Bandwidth

• frequency domain

  • The frequency bandwidth of a signal

  • The unit is Hertz

  • The range of signal frequency bands allowed by a channel is called the bandwidth of the channel

• Time Domain

  • The ability of a channel in the network to transmit data means the "highest data rate" that a channel in the network can pass within a unit of time

  • The unit is bit/s

The essence of the two is the same, the wider the "bandwidth" of a communication link, the higher the "maximum data rate" it can transmit.

5.1.3. Throughput

• Throughput (throughput) refers to the actual amount of data passing through a certain network (or channel, interface) per unit time .

• Throughput is more often used as a measure of real-world networks to know how much data is actually going through the network.

• Limited by the bandwidth of the network or the rated rate of the network.

  • Rated speeds are absolute upper limits.

  • may be much less than the rated rate, or even drop to zero

• Sometimes expressed in bytes or frames per second

• Units: kBps,MBps.GBps,TBps

5.1.4. Latency

• Delay (delay or latency) refers to the time required for data (a message or packet, or bit) to be transmitted from one end of the network (or link) to the other end.

  • Latency is a very important performance indicator.

  • Latency is called delay or delay in some scenarios.

• The delay of network communication consists of four parts, which are sending delay, propagation delay, processing delay and queuing delay.

  • Total delay = sending delay + propagation delay + processing delay + queuing delay

• transmission delay : transmission delay

  • is the time required for a host or router to send a data frame

  • That is, the time required to send the last bit of the frame from the time when the first bit of the data frame is sent.

  • Sending delay = data frame length / sending rate

• Propagation delay

  • It is the time it takes for electromagnetic waves to travel a certain distance in the channel

  • Propagation delay = channel length / signal propagation rate on the channel

Send delay is fundamentally different from propagation delay

• The 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).

• 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.

• processing delay

  • It is the time it takes for a host or router to process a packet when it receives it.

  • For example, it takes a certain amount of time for a host or a router to analyze the header of a packet, extract the data part from the packet, perform error checking, and search for a route for forwarding.

• queuing delay

  • is the time the packet is queued for processing at the router.

  • When the packet is transmitted to the router, it needs to enter the router input queue for queuing. After the router determines the forwarding interface, it needs to enter the router output queue to wait for forwarding. During this process, the packets need to be queued for processing.

5.1.5. Delay-bandwidth product

• Delay-bandwidth product: multiply propagation delay and bandwidth together

  • The delay-bandwidth product of a link is also called the link length in bits

  • The number of bits in the pipeline represents the number of bits sent from the sender that have not yet reached the receiver.

  • A link is only fully utilized when the pipes representing it are all full of bits

5.1.6. Round trip time RTT

• Round trip time RTT (Round-Trip Time)

  • Indicates the total time elapsed from when the sender sends data to when the sender receives confirmation from the receiver (the receiver sends a confirmation immediately after receiving the data)

5.1.7. Utilization

• channel utilization

  • What percentage of the time a certain channel is utilized (that is, data passes through)

  • A completely idle channel has a utilization of 0

• network utilization

  • The weighted average of the channel utilization of the whole network

• When the utilization rate of a channel increases, the time delay caused by the channel increases rapidly.

  • If Ling D. Indicates the delay when the network is idle, and D indicates the current delay of the network

  • Then under appropriate assumptions, the following simple formula can be used to express the relationship between D and D: D=D0/1-U (U is the utilization rate of the network)

6. Computer network architecture

In the concept of computer network, hierarchical architecture is the most basic.

6.1 Formation of computer network architecture

In 1974, IBM announced the system network architecture SNA (System Network Architecture) based on the layered method.

6.1.1. Currently, there are two common network architectures:

• The legal (dejure) international standard: OSI/RM (OpenSystems Interconnection Reference Model), the basic reference model for Open Systems Interconnection, has not been recognized by the market.

• The de facto international standard: Transmission Control Protocol/Internet Interconnection Protocol, also known as network communication protocol, TCP/IP (Transmission Control Protocol/Internet Protocol) , is now the most widely used.

6.2. Protocol and division level

• The data exchange in the computer network must abide by the pre-agreed rules.

• These rules clearly stipulate the format of the data exchanged and the related synchronization issues (synchronization means timing)

6.2.1. Network Protocol

Abbreviated as a protocol, it is a rule, standard or agreement established for data exchange in the network

• Three elements:

  • Syntax: The structure or format of data and control information

  • Semantics: what kind of control information needs to be sent, what actions to complete, and what responses to make

  • Synchronization: A detailed description of the order in which events are implemented

• Network protocols are an integral part of computer networks.

• Any network communication must have a protocol.

  • For a computer on the network to do any work, there must be an agreement.

  • When performing stand-alone operations, such as document writing and video browsing on the local computer, no network protocol is required.

6.2.2. Agreement

There are two parts:

• Human-readable and comprehensible text description, RFC document.

• Program code that allows computers to understand, network protocol software, also known as network operating system

The protocol adopts a layered design, which is derived from the research experience of ARPANET

• The research experience of ARPANET shows that for a very complex computer network protocol, its structure should be hierarchical

6.2.3. File transfer between host A and host B

• The specific work consists of three parts

  • file transfer

  • Communication service

  • Network Interface

6.2.4. Advantages of protocol layering:

• Each layer is independent.

• good flexibility

• Structurally separable.

• Ease of implementation and maintenance

• Can promote standardization work

6.2.5. Architecture

The architecture of a computer network is a collection of layers of a computer network and their protocols.

• The architecture of a computer network is the precise definition of the functions that a computer network and its components should perform

• Implementation (implementation) is a question of what kind of hardware or software is used to complete these functions under the premise of following this architecture.

• Architecture is abstract, while implementation is concrete, the actual running computer hardware and software.

6.3 Architecture with five-layer protocol

The concept of OSI's seven-layer protocol architecture is clear and theoretically complete, but it is complex and impractical

The four-layer protocol architecture of TCP/IP is widely used, and has been supported and implemented by network manufacturers, but the lowest layer of the network interface layer has no specific content

• The four-layer protocol architecture of TCP/IP is the application layer, transport layer, Internet layer and network interface layer

• Take a compromise approach, that is, integrate the advantages of OSI and TCP/IP, and adopt a system structure with only five layers of protocols . The architecture of this five-layer protocol is concise and can explain the concept clearly.

6.3.1 Application layer

The application layer is the highest layer of the architecture. 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.

6.3.2 Transport layer (transport layer)

• The task of the transport layer is to be responsible for providing a common data transmission service for the communication between the processes in the two hosts. The application process uses this service to transmit the application layer message.

• The transport layer mainly uses two protocols:

  • Transmission Control Protocol TCP (Transmission Control Protocol) - provides connection-oriented, reliable data transmission services, and its data transmission unit is a segment

  • User Datagram Protocol UDP (User Datagram Protocol) - provides a connectionless best-effort data transmission service (the reliability of data transmission is not guaranteed), and the unit of data transmission is the user datagram

6.3.3 Network layer (network layer)

• The network layer is responsible for providing communication services for different hosts on the packet switching network. When sending data, the network layer encapsulates the segment or user datagram groups or packets for transmission

• The network layer is responsible for providing routing for each message, so that the packets transmitted by the transport layer of the source host can find the destination host through the routers in the network.

6.3.4 Data link layer (data link layer)

• The data link layer is often referred to simply as the link layer. The data between two hosts is transmitted on a segment-by-segment link.

• The data link layer encapsulates the data message delivered by the network layer into a frame (framing), and transmits the frame on the link between two adjacent nodes.

6.3.5 Physical layer

The unit of data transmitted on the physical layer is bit

• The commonly used protocols are TCP and IP.

  • The protocol TCP/IP that people often use now is not necessarily the two protocols of TCP and IP. The TCP/IP protocol often means the entire TCP/IP protocol suite (protocolsuite) used by the Internet

6.4 Entities, protocols, services, and service access points (understanding)

1. Entity

Indicates any hardware or software process that can send or receive information

2. Agreement

• is a collection of rules governing the communication between two peer entities

  • Under the control of the protocol, the communication between two peer entities enables this layer to provide services to the upper layer. To realize the protocol of this layer, it is also necessary to use the services provided by the lower layer.

  • Where entities of two adjacent layers in the same system interact (exchange information), it is usually called a service access point.

• The agreement must estimate all unfavorable conditions in advance, and cannot assume that everything is normal and ideal.

  • To see whether a computer network protocol is correct, we should not only check whether it is correct under normal conditions, but also carefully check whether the protocol can cope with various abnormal conditions.

  • It seems that it is a very simple protocol, but there are still many issues to be considered in design, so the protocol is very complicated.

Glossary:

1. Internet: Internet is a general term, which generally refers to a computer network formed by the interconnection of multiple computer networks (multiple networks are connected to each other through some routers to form a computer network with a larger coverage)

2. Internet: The Internet or the Internet is a proper term. It refers to the world's largest, open, and specific Internet connected by many networks. It uses the TCP/IP protocol family as the communication rule, and its predecessor is ARPANET in the United States

3. WWW: world wide web, Chinese translation of World Wide Web, developed by CERN, the European Nuclear Research Organization, is widely used on the Internet, which greatly facilitates the use of the Internet by non-network professionals and has become the main driving force for the exponential growth of the Internet

4. OSI: Open Systems Interconnection Reference Model (OpenSystems Interconnection Reference Model). The computer network is divided into seven layers, which are physical layer, data link layer, network layer, transport layer, session layer, presentation layer, and application layer. The OSI protocol is a legal international standard, and its theory is complete, but due to its complexity and impracticality, it is rarely used in actual use.

5. RTT: Round-trip time, also known as round-trip delay (Round-Trip Time), is an important performance indicator in computer networks. Indicates the total delay experienced from the time the sender sends data to the time when the sender receives the confirmation from the receiver.