Key Points of Computer Network review

Chapter 1 Overview

Circuit Switching: The bit stream of the entire packet is continuously transferred from the source point to the end point, as if in a pipe.

Packet Exchange: The entire packet is first transmitted to the adjacent node, all of which are stored and then searched for the forwarding table and forwarded to the next node.

Group exchange: a single group (which is only a part of the entire packet) is sent to adjacent nodes. After the group is stored, it finds the forwarding table and forwards it to the next node.

Computer Network: a set of interconnected and autonomous computers.

Different scopes can be divided into: Wan, man, lan, and pan)

Computer network performance indicators: speed, bandwidth, latency, utilization (channel utilization, network utilization) (here may be a short answer, calculation)

Network Protocol: rules, standards, or conventions established for data exchange in the Network Center. It consists of three elements: syntax, semantics, and synchronization.

Architecture: a set of layers of computer networks and their Protocols

OSI Layer-7 protocol: physical layer, data link layer, network layer, transport layer, Session Layer, presentation layer, and application layer.

TCP/IP layer-4 Protocol: network interface layer, Internet layer IP, transport layer, and application layer.

Five-Layer Protocol: physical layer, data link layer, network layer, transport layer, and application layer.

Chapter 2 Physical Layer

Main tasks of the physical layer: determine the characteristics related to the interface of the transmission media, that is, mechanical, electrical, functional, and process characteristics.

Three communication methods: one-way communication, two-way alternate communication, two-way simultaneous communication

Baseband signal: signal from the source.

Band-pass signal: Signal after Carrier Modulation

The most basic method of band-pass modulation: frequency modulation, amplitude modulation, and phase modulation.

Type of twisted pair wires: unshielded twisted pair wires and shielded twisted pair wires

Multi-Mode Optical Fiber: There can be a lot of Optical Fiber transmitted in a single optical fiber with different degrees of incident light

Single-Mode Optical Fiber: optical fiber that transmits light forward without multiple reflections

Frequency Division Multiplexing (OFDM): After a user allocates a certain frequency band, the user occupies this frequency band throughout the communication process. All users use different bandwidth resources at the same time.

Time division multiplexing (TDM): divides the time into long time division multiplexing frames (TDM frames) such as segment. Each time division multiplexing user occupies a time slot with a fixed serial number in each TDM frame. The time slot occupied by the user is cyclical. All users use the same band width at different time points. It is more conducive to digital signal transmission.

Statistics time division multiplexing STDM: dynamically allocates time slots as needed

Wavelength division multiplexing (WDM): optical frequency division multiplexing. Multiplexing of two optical carrier signals on one optical fiber.

Dense Wavelength Division Multiplexing (DWDM): Optical carrier signals that use multiple channels on an optical fiber.

Code Division Multiplexing (CDM): the shared channel method. Different users use different code types that have been specially selected, so they will not interfere with each other.

Code Division Multiple Access (CDMA): it has strong anti-interference capability. The spectrum is similar to white noise and is not easy to be detected by enemies. (After-school exercises, short answer may be provided)

Chapter 3 data link layer

Link: it is a passive point-to-point physical line segment with no other switching nodes in the middle.

Data Link: in addition to physical lines, there must also be a communication protocol to control the data transmission. Adding hardware and software implementing these protocols to the link constitutes a data link.

Three basic problems: encapsulation into frames, transparent transmission, and error detection. (CRC
Cyclic Redundancy detection and zero-bit filling short answer)

LAN topology: star chart, ring chart, total line chart, and tree chart

CSMA/CD Protocol: Multi-Point Access/collision detection for carrier listeners. "Multi-Point Access" indicates that many computers are connected to a bus through multi-point access; "carrier listening" means that each station must first check whether other computers on the bus are sending data before sending data. If so, do not send data temporarily to avoid collision; "collision detection" means that the computer sends data while detecting the signal voltage on the channel.

Mac hardware address: An address consisting of 48-bit, hexadecimal numbers. That is, the physical address of the NIC. It is actually an adapter address or an adapter identifier EUI-48. A mac address or hardware address is used to define the location of a network device. In the OSI model, the layer 3 network layer is responsible for IP addresses, while the Layer 2 data link layer is responsible for MAC addresses. Therefore, a host has an IP address, and each network location has a MAC address dedicated to it.

Collision domain: it can also be called a conflict domain. In Ethernet, if two computers on a CSMA/CD Network Conflict during simultaneous communication, the CSMA/CD network is a conflict domain. If the network segments in the Ethernet are connected by relay, they are still a conflicting domain because conflicts cannot be avoided.

(After-school questions) Transparent Bridge:

Broadcast storm: The Bridge is only suitable for LAN with less than a few hundred users and less traffic. Otherwise, network congestion may occur due to the spread of too much broadcast information. This is the so-called broadcast storm.

Chapter 4 Network Layer

The network layer provides two services:Virtual circuit ServiceAndDatagram ServerServices

Application Layer: various application layer protocols (HTTP, FTP, SMTP)

Transport Layer: TCP and UDP. InNetwork Layer or aboveUsedIntermediate DeviceYesGateway. To connect two incompatible systems with a gateway, protocol conversion must be performed at the high level.

Network Layer: ICMP, IGMP, IP, RARP, ARP.Intermediate DeviceYesVro

Network Interface Layer: It is used with various network interfaces.

Data Link Layer: UsedIntermediate DeviceYesBridge or bridge

Physical Layer: Physical hardware. UsedThe intermediate device is a forwarder.

Classified IP addresses: (short answer)

Arp, The Address Resolution Protocol, enables you to know the physical address through the IP address.

Workflow: 1. the ARP process broadcasts an ARP request group on the local LAN;

2. All ARP processes running on hosts on the local LAN receive the ARP request group;

3. When host B sees its IP address in the ARP group, it sends an ARP response group to host a and writes its own hardware address;

4. After host a receives the ARP response group from host B, it writes the IP address ing from host B to the hardware address in its ARP cache.

RARP: The Reverse Address Resolution Protocol allows physical machines on the LAN to request IP addresses from the ARP table or cache of the gateway server.

IP addresses are converted to physical addresses through ARP, and physical addresses are converted to IP addresses through RARP.

Subnet mask, mask (IP address range): questions after class

Rip: the routing information protocol is a distributed routing selection protocol based on distance vectors. It is a standard protocol for the Internet and has the greatest advantage of being simple. It is a standard for exchanging routing selection information between the gateway and the host.

Work Principle required

IGP Internal Gateway Protocol: the routing selection protocol used inside an autonomous system.

EGP external Gateway Protocol: if the source host and target host are in different autonomous systems, when the datagram is transmitted to the boundary of an autonomous system, you need to use one protocol to transmit route selection information to another autonomous system.

Distance Vector Algorithm

Router composition: the routing selection part (control part, core component is the routing selection processor) and grouping forwarding part (exchange structure, a group of input ports, a group of output ports ).

IGMP Internet Group Management Protocol: Let multicast routers connected to the local lan know whether a host participates in or exits a multicast group.

VPN Virtual Private Network: a temporary and secure connection established through a public network is a secure and stable tunnel that passes through a chaotic public network.

Nat Network Address Translation: This technology converts private (retained) addresses into valid IP addresses. It is widely used in various Internet access methods and networks. The reason is simple. Nat not only perfectly solves the problem of insufficient LP addresses, but also effectively avoids attacks from outside the network and hides and protects computers inside the network.

Chapter 5 Transport Layer

Flag process: because the transport layer directly provides services for the upper-layer calling process, it requires an address to uniquely mark inter-process communication. Or the process uses the TSAP (transportservice access point) to call the services provided by the transport layer.

Port Number: generally has two meanings: first, physical ports, such as ADSL modem, Hub, switch, router, and other interfaces used to connect to other network devices, such as RJ-45 port, SC port and so on. The second is the logical port, which generally refers to the port in the TCP/IP protocol. The port number ranges from 0 to 65535,

IP Address: A 32bit address assigned to each host connected to the Internet.

UDP User Datagram Protocol: a connectionless transport layer protocol in the OSI reference model. It provides a simple and unreliable information transmission service for transactions.

TCP transmission control protocol: it is a connection-oriented, reliable, and byte stream-based transport layer communication protocol.

Reuse: all processes at the application layer can be transferred to the IP layer through the transport layer.

Usage: After receiving data from the IP layer, the transport layer must deliver the data to the specified application process.

Implementation of TCP reliable transmission: Sliding Window in bytes; selection of timeout retransmission time; selection of validation sack

TCP traffic control: the transmission rate of the sender should not be too fast, so that the receiver can receive the message in time. The sliding window is used for traffic control. The transmission efficiency must be considered.

TCP congestion control: prevents excessive data from being injected into the network, so that the routers or links in the network are not overloaded.

Methods: slow start, congestion avoidance, fast retransmission, and fast recovery

Transport connection management of TCP: three phases of transport connection: Connection establishment; data transmission and connection release.

Chapter 6 Application Layer

DNS domain name system:It is used to name computers and network services organized into the domain hierarchy. Is a naming system used by the Internet to convert the names of machines that are easy to use to IP addresses.

Primary Domain Name Server: to improve the reliability of the Domain Name Server, the DNS domain name server copies data to several domain name servers for storage, one of which is the primary Domain Name Server, and the other is the secondary Domain Name Server.

Generally, recursive queries are used by the host to query the Local Domain Name Server. The Local Domain Name Server to query the Domain Name Server is usually an iterative query.

FTP File Transfer Protocol: it is the most widely used File Transfer Protocol on the Internet. It provides interactive access, allows the types and formats of well-known files, and allows users to access files.

Www: it is a large-scale online storage of information.

Uniform URL Resource Locator: it is a way to fully describe the addresses of web pages and other resources on the Internet.

HTTP Hypertext Transfer Protocol (Hypertext Transfer Protocol) is a standard for client and server requests and responses. It is a transaction-oriented application layer protocol and is an important foundation for reliable file exchange on the World Wide Web.

HTML hypertext markup language: a standard language for making web pages. It eliminates the barriers to information exchange between different computers. An important foundation for the World Wide Web.

Email system: User Agent; email server; and mail sending protocol (stmp) and mail reading protocol (POP3)

Chapter 7 Network Security

Security threats to computer networks: interception, interruption, tampering, and forgery.

Interception: attackers intercept others' communication content from the Network (passive)

Interruption: attackers intentionally interrupt others' network communication.

Tampering: attackers intentionally tamper with packets transmitted over the network.

Forgery: attackers send forged information over the network.

Traffic Analysis: Understand the addresses and identities of protocol entities in communication, study the PDU length and transmission frequency, so as to understand the nature of the exchanged data passive attacks.

CA Certification Center: it is a trustworthy institution that can bind a public key with its corresponding body.

Chapter 10 next-generation Internet

IPv6 address length: To solve the IPv4 address depletion problem, IPv6 is proposed. Increase the address size from 32-bit IPv4 to 4 times, that is, to 128 bits, and increase the address space by 2 ^ 96 times.

Policies for transition from IPv4 to IPv6: Dual-protocol stack and tunneling technology.

Dual-Protocol Stack: before the full transition to IPv6, some hosts are equipped with two protocol stacks, one IPv4 and one IPv6

Tunneling Technology: When an IPv6 datagram enters an IPv4 network, it encapsulates the IPv6 datagram into an IPv4 datagram, and then the IPv6 datagram is transmitted in an IPv4 network tunnel. When an IPv4 datagram leaves the tunnel in an IPv4 network, the data part is handed over to the IPv6 protocol stack of the host.