Computer network terminology explanation and summary of short answer questions

Glossary

CSMA/CD (Carrier Sense Multipoint Access/Collision Detection Protocol)

CSMA/CD is a carrier sense multiple access technology based on collision detection. The CSMA/CD protocol requires stations to listen to the channel before sending data. If the channel is idle, the station can send data; if the channel is busy, the station cannot send data. However, if both stations detect that the channel is free and start transmitting data at the same time, this can cause a collision almost immediately

ARP (Address Resolution Protocol)

Resolve the IP address into a hardware address, solving the problemof hosts or routers on the same LANMapping problem between IP address and hardware address

SAP（service acces point）

Where services are provided at the interface between layers

RARP (Reverse Address Resolution Protocol)

Reverse Address Resolution Protocol enables a host that only knows its own hardware address to know its IP address

ICMP Internet Control Message Protocol

Allows a host or router to report error conditions and provide reports on any anomalies.
IP layer protocol
Message type: ICMP error message, ICMP query message
Two common applications: a> Traceroute, tracert (working at the network layer)
PING (working at the application layer, using ICMP directly)

IGMP Internet Group Management Protocol

IGMP is called the Internet Group Management Protocol and is the protocol responsible for the management of IPV4 multicast members in the TCP/IP protocol suite. Used to establish and maintain the relationship of multicast group members between the receiver host and the directly adjacent multicast router, IGMP uses IP datagrams to deliver its messages

CIDR (Classless Inter-Domain Routing)

CIDR (eliminates the traditional concepts of Class A, Class B, and Class C addresses and subnets, thereby allocating IPv4 address space more efficiently. It can combine several IP networks together, using Aclassless inter-domain routing algorithm that merges them into one route so that they are less in the routing table The routing entries reduce the burden on the Internet router and improve the forwarding speed of the router

RIP (Routing Information Protocol, Routing Information Protocol)

Interior Gateway Protocol, RIP is a protocol based on the Distance-Vector algorithm. It uses Hop Count as a metric to measure the distance to the destination network. distance.

OSPF (Open Shortest Path First, open shortest path first)

A type of GP that uses Dijkstra's algorithm
LSA (Link State Advertisement) to communicate between routers in the network Exchange network topology information

SMTP (Simple Mail Transfer Protocol)

Achieving effective and reliable email transmission
When using SMTP protocol:
①The sender’s user agent sends the message to the sender’s mail server Mail
②The sender’s mail server sends mail to the recipient’s mail server

POP3 (Post Office Protocol)

It is a kind of mail reading protocol. It uses the client/server working mode. The UA sends a request to the mail server to "pull" the mail in the user's mailbox.

IMAP (Internet Mail Access Protocol)

IMAP (Internet Message Access Protocol) is a protocol for receiving and managing email messages, similar to POP (Post Office Protocol), but with more features and flexibility.

The workflow of IMAP is as follows:

Connection server: The email client sends a connection request to the mail server through the IMAP protocol.
Authentication: The client needs to provide the correct username and password to log in to the email account.
Synchronization of emails: The client sends a synchronization email request to the server and obtains a list of all emails on the server.
Reading mail: The client can browse the list of mails on the server and select the mails to be viewed.
Download attachment: If the email contains an attachment, the client can choose to download the attachment.
Mark and move messages: The client can mark messages as read or unread on the server and move messages to different folders.
Deleting emails: The client can delete emails, but in the IMAP protocol, the emails will not be deleted immediately, but will be moved to "Deleted Items"

FTP (File Transfer Protocol)

can transfer files between any computers in a heterogeneous network. Provide interactive access, allow customers to specify the type and format of files, and allow files to have access permissions
Functions provided:
Communication between different types of host systems File transfer capabilities
Provide users with the ability to manage files on remote FTP servers in the form of user permissions
Provide public file sharing capabilities in the form of anonymous FTP

short answer questions

Introduction

Computer network topology and characteristics

The topology of a network refers to the interconnection pattern between nodes in the network, that is, the geometric distribution of network links and nodes, which defines the physical and logical locations of each node.

In a mesh topology, each device has a dedicated line connection to every other device.
Star topology, each device is only connected to the central controller
Tree topology. Most devices are first connected to a secondary controller, which is then connected to the central controller
Bus topology consists of a network of long cables and network devices connected to it.
Ring topology, a closed ring structure network in which each device in the network is connected through a communication link connected end to end.
Hybrid topology, different topologies connected together

Why computer networks need to be layered? Explain the functions and smallest units of each layer of TCP/IP

Advantages of layering: Each layer is independent of each other, has good flexibility, can be separated structurally, is easy to implement and maintain, and is conducive to standardization work.

Physical layer: Specifies the mechanical, electrical, functional and process aspects. The transmission unit isbit
Data link layer: Data transmission of adjacent nodes, link establishment, maintenance and teardown, frame synchronization, data encapsulation and teardown, error control and flow control, unit Yesframe
Network layer: Data transmission, routing, blocking control, and Internet interconnection of any node. The unit isgroup
. The relevant protocols are: IP, ICMP, ARP, RARP
Transport layer: End-to-end communication, flow control and error control, the unit is message
Relevant protocols are: TCP UDP
Application layer: Provides applications with the ability to access underlying services, in units ofdata
Related protocols There are: FTP SMTP HTTP POP DNS SNMP

Data communication basics

What is circuit switching, what is time division multiplexing, frequency division multiplexing

Circuit switching: Creates a temporary physical connection between two devices.
Time division multiplexing: TDM divides the channel by dividing time.
Frequency division multiplexing: By dividing the bandwidth of the communication line, the shared communication line is divided into several independent communication channels.

Computer network performance indicators

Channel throughput: the amount of information sent per unit time, expressed in bps (bits per second)
Channel utilization: the ratio of data rate to channel throughput after removing all control information. The greater the control overhead, the lower the channel utilization.
Delay-bandwidth product: How many bits have been sent by the sender when the first bit of the sender is about to reach the key point. Delay-bandwidth product = propagation delay * channel bandwidth
Delay time
Queuing delay, access delay, sending time, propagation delay

data link layer

Briefly describe the CSMA/CD binary exponential backoff algorithm (high-frequency test points)

CSMA/CD (Carrier Sense Multipoint Access/Collision Detection)

For each frame, when a collision occurs for the first time, set parameter L=2
The backoff interval is a random number ranging from 1 to L time slices. One time slice is equal to 2 times the maximum transmission delay on the link.
When frames collide again, the parameter L is doubled. The maximum value of L is 1024, that is, when L increases to 1024, L will no longer increase.
The maximum number of retransmissions of a frame is 16. If this number is exceeded, the frame will not be retransmitted and an error will be reported.

The execution process of spanning tree algorithm

Determine the root bridge
1. The bridge with the lowest address (or identification ID) is the root node of the spanning tree
2. The selection of the root bridge is determined by sending special frames called Bridge Protocol Data Units. When a bridge receives a BPDU, it compares the source bridge ID with its own ID. If its ID is larger, it is not the root bridge. If its ID is smaller, it does not forward the received bridge ID. Bridge protocol data unit. Finally, only the bridge with the lowest ID can send BPDUs, and other bridges will not send BPDUs.
Determine root port
1. Since each bridge has previously recorded the path cost on each port for each received root bridge protocol data unit, it can know which port number is the root port by simply looking up which one has the smallest cost.
Designate a root bridge for each LAN
1. When a bridge receives a bridge protocol data unit from a certain LAN, it compares the cost of the bridge's protocol data unit with its own cost to the root bridge. If its own cost is high, it will not be called the designated bridge of the LAN. If there are two or more bridges with the same cost, the bridge with the smallest ID is called the designated bridge.
Build spanning tree

The functions and characteristics of network bridges and their similarities and differences with switches

The bridge stores and forwards information frames at the data link layer. The bridge has data link functions such as isolation communication, error detection, frame format conversion, and frame routing.
Similarities and differences between bridges and switches:

Ethernet switches and bridges both work at the data link layer;
Ethernet switches usually have more interfaces, and bridges generally only have 2-4 interfaces. An Ethernet switch is essentially a multi-interface bridge;
The port of the bridge is generally connected to the LAN, and each interface of the Ethernet switch is connected to the host;
Switches allow simultaneous communication between multiple pairs of computers, and bridges allow computers on each network segment to communicate at the same time;
The bridge uses the store-and-forward method for forwarding, while the Ethernet switch can also use the cut-through method for forwarding;
Ethernet switches use dedicated switching fabric chips and have faster forwarding speeds than bridges.

Selection of sliding serial port protocol window size

In the fallback n protocol: if the frame number range is 0~n-1, the sending window size is n-1 and the receiving window is 1
The number range of the frame is 0~n-1, and the size of the sending window and the receiving window are required to be equal. The sizes of the sending window and the receiving window are both n/2.

Network layer

In IPv4, what are the categories of IP addresses? The value range of each category of addresses is given.

IP addresses are divided into 5 categories, namely A, B, C, D, and E. The address range of classless addresses is:

Class A: 0.0.0.0 to 127.255.255.255
Class B: 128.0.0.0 to 191.255.255.255
Class C: 192.0.0.0 to 223.255.255.255
Class D: 224.0.0.0 to 239.255.255.255
Class E: 240.0.0.0 to 247.255.255.255 For Internet experimentation and development only

What is IPV6, what is the difference from IPV4, explain the unicast, multicast and anycast of IPV6

IPv6 is a new type of network protocol and the next generation standard of IP protocol.

Compared with IPv4, IPv6 mainly has the following changes:

Larger address space. Increased from the original 32 bits of IPv4 to 128 bits;
Extended address hierarchy. The IPv6 address space is large and can be divided into more levels;
Flexible header format, IPv6 defines many optional extension headers;
Improved options. IPv6 allows datagrams to contain option control information and therefore can contain some new options;
Allow the protocol to continue to expand.
Support plug and play
Supports pre-allocation of resources

Unicast, multicast and anycast for IPv6

Unicast: Unicast is a point-to-point transmission method, that is, data packets are sent from a source IP address to a destination IP address. IPv6 unicast includes: aggregating global unicast addresses, link-local addresses, site-local addresses and other special unicast addresses;

Multicast: Multicast is a many-to-many transmission method, that is, data packets are sent from a source IP address to a group of destination IP addresses. Multicast can reduce network congestion and bandwidth usage. Members in the group are dynamic and can join or leave at any time.

Anycast: Anycast address is a unique address type of IPv6. It is used to identify a group of network interfaces. Messages sent to anycast will only be sent to the nearest interface. . Anycast addresses and unicast addresses use the same address space, so there is no difference in the expressions of anycast and unicast; the anycast address must be clearly indicated during configuration to distinguish unicast and anycast.

transport layer

Draw the TCP header and explain which fields in the TCP header are related to congestion control, flow control and error control.

TCP header
TCP is a transport layer protocol with error control and congestion control. To implement retransmission in the future, error detection and loss detection mechanisms must be available; in order to achieve congestion control, a congestion notification mechanism and a traffic adjustment mechanism must be available;

TCP flow control manages the traffic at both ends to avoid sending too fast, causing the receiving end to overflow, or the receiving end processing too fast, resulting in a waste of time. The sliding window method is used, and the transmission unit is bytes.
TCP Congestion Control, if the network load (the number of packets sent to the network) is greater than the network traffic (the number of packets that the network can process at the same time), congestion will occur and the network will be judged The two factors of congestion are delay and throughput.
There are three mechanisms to deal with congestion:
1. Slow start algorithm (exponential increase)
2. Congestion avoidance (additive increase)
3. Congestion detection (reduce by 2)
TCP error control, TCP must ensure that data is sent in order, without errors, without partial loss, and without duplication to the application layer, using methods: checksum, confirmation , timeout retransmission
TCP uses flow control to achieve congestion control. In the TCP protocol, each transmitted byte has a sequence number. Each TCP message has three fields in its message header: sequence number, confirmation sequence number and window size. These three fields are related to flow control and errors. Control is related to congestion control.

Explain the three-way handshake and its functions

The three-way handshake is used to establish a communication device connection to ensure reliable transmission of data. There are three main actions, called the three-way handshake:

Request the connection party to send a connection request packet to the expected receiver; CR(seq=x)
The receiver sends back a confirmation packet to the requester; CC(seq=x+1,ack=y)
The requesting party sends back a packet to the receiving party to confirm the confirmation packet. AK(seq=x+1,ack=y+1)

The sliding serial port flow control used in both TCP layer and MAC layer explains their working principles and differences.

Sliding Window Flow Control is a flow control mechanism in network transmission.

At the TCP layer, sliding serial port flow control isimplemented through the TCP window in the TCP protocol. For the sender, the size of the TCP window represents the maximum amount of data that can be sent, while for the receiver, the size of the TCP window represents how much data can be received. Sliding serial port flow control will dynamically adjust the amount of data transmitted according to the window size of the sender and receiver, to ensure the stability of transmission and avoid data leakage. Congestion and packet loss during transmission.

At the MAC layer, sliding window flow control is implemented through the ACK mechanism of the frame. The sender sends a data frame, and the receiver needs to send an ACK frame to indicate receipt after receiving the data frame. In the sending window set by the sender, only data frames that have received ACK frames can continue to be sent. When the sender receives the ACK frame, it will adjust the window size accordingly to facilitate the next transmission.

The difference in working principle is thatThe sliding window of the TCP layer is implemented on the transport layer and is mainly used to control the transmission of application layer data, while the MAC layer is implemented on the data link. Implemented on the road layer, it is mainly used to control frame flow control. In addition, the sliding window of the TCP layer is maintained by the transport layer protocol, while the sliding window of the MAC layer is maintained by the data link layer protocol.

The difference between TCP and UDP

TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are two common protocols in network transmission.

TCP is aconnection-oriented protocol, which means that a network connection must be established before data can be transferred. It uses a three-way handshake to establish a connection anduses reliable, ordered data transfer. TCP willcheck for errors during data transmission and retransmit lost data packets to ensure the reliability of the transmitted data. TCP is suitablefor application environments that require high reliability of data transmission and ensuring data integrity, such asfile transmission or electronic Mail.

UDP is aconnectionless protocol. There is no need to establish a connection before data transmission. UDP does not require reliable, orderly data transmission, nor does check for errors or perform retransmissions a>. Live video or live audio, and is suitable for application environments with high real-time requirements, such asfaster, simpler and lightweight. This makes UDP

In general, TCP is suitable for transmission scenarios that require high data reliability and large amounts of data, while UDP is suitable for scenarios that require high real-time performance and transmission efficiency.

Application layer

DNS vs. ARP comparison

Different functions: DNS (Domain Name System) analysisThe mapping relationship between domain names and IP addresses, equivalent to the phone book on the Internet ; ARP (Address Resolution Protocol) parsesthe mapping relationship between MAC addresses and IP addresses.
Data processing methods are different:DNS is an application layer protocol, using UDP or TCP to transmit data; ARP isData link layer protocol sends data frames in broadcast form.
Information storage methods are different: DNS stores the mapping relationship between IP address and domain name inDNS server; ARP stores MAC address and IP address The mapping relationship is stored inlocal ARP cache.
Different usage scenarios: DNS is often usedto access websites through browsers and in emails. With the promotion of new technologies, many applications Rely on DNS to resolve domain names; ARP is generally used in LAN to find the target MAC address and determine which host should send data.

The process of sending and receiving emails

Mail system consists of: user agent, mail server, and email protocol.

The sender calls the user agent to edit the message to be sent;
The sending mail server puts the mail into the mail cache queue and waits for sending;
The SMTP client process running on the sending mail server finds that there are emails to be sent in the mail cache, so it establishes a TCP request;
After the TCP connection is established, the SMTP client process performs a handshake interaction with the remote SMTP server;
The SMTP process running on the receiving mail server receives the email and puts the email into the recipient's user mailbox;
The recipient calls the user agent, uses mail acceptance protocols such as POP3 and IMAP, and retrieves it from the user's mailbox on the receiving server.

cyber security

Use words and pictures to describe the workflow of digital signatures

The workflow of digital signature is as follows:

Creating a message: A user creates a message or document using some computer application or tool, such as Adobe Acrobat or Microsoft Word.

Sign a message: The signer selects a digital signature application or tool and uses that tool to create their digital signature. The digital signature algorithm generates a set of encryption keys, with the private key known only to the signer and the public key publicly available. The signer uses the private key to encrypt the message's digest, generate a signature, and append it to the message.

Send a message: The signer sends the message along with a digital signature to the recipient.

Verify signature: The recipient verifies the signature using the same signing application or tool. The recipient decrypts the signature using the public key and creates a digest using the message's digest and the digest algorithm published by the signer. The recipient then compares the two to verify the digital signature.