Computer Network (Xie Xiren Edition) Knowledge Summary

computer network

Computer Network Overview

Internet overview

• network of networks

• Three stages of Internet standardization

  • internet draft
  • Recommended standards
  • formal standard

• The difference between Internet and internet

Internet composition

• edge part

  • Composed of hosts on the Internet and used directly by users

  • way of communication

    • client-server model
    • P2P mode

• core part

  • It consists of a large number of networks and routers that provide services for edge networks.

  • Data transmission method

    • circuit switching

      • Data is streamed from source to destination

    • packet switching

      • Messages are divided into many small packets and transmitted across the network in a store-and-forward manner.

    • message exchange

      • In store-and-forward mode, but the entire message is forwarded

Categories of computer networks

• Classification according to the scope of the network

  • Wide Area NetworkWAN
  • Metropolitan Area Network MAN
  • LAN
  • Personal Area Network PAN

• Sort by user

  • public network
  • private network

• local access network

  • The network between users and the first router in the Internet

computer network performance

• rate

  • data transfer rate

• bandwidth

  • The frequency bandwidth of the signal

• Throughput

  • Depends on bottleneck circuit

• Delay

  • Send delay
  • propagation delay
  • processing delay
  • Queuing delay

• Delay bandwidth product

• Round trip time RTT

  • After sending data once, the receiver will respond to the received data. The difference between the time when the sender receives the response and the sending time point

• Utilization

computer network architecture

• Five-layer protocol architecture

  • Application layer
  • transport layer
  • Network layer
  • data link layer
  • physical layer

• Advantages of layering

  • Each layer is independent
  • Good flexibility
  • structurally separable
  • Easy to implement and maintain
  • Can promote standardization work

• protocol

• Serve

• service access point

• entity

  • Software or hardware that sends or receives information

physical layer

Basic concepts of the physical layer

• The physical layer is the only layer that directly transmits data

• Characteristics of the physical layer related to the transport coal interface

  • Mechanical properties
  • Electrical characteristics
  • Features
  • process characteristics

Data communication knowledge

• Transmission and exchange of data between two entities

• data communication system

  • source system
  • Transmission system
  • target system

• conceptual terms

  • data

    • The entity that delivers the message

  • Signal

    • Electrical or electromagnetic representation of data

      • analog signal
      • Digital signal

  • code element

    • When representing digital signals using waveforms in the time domain (or simply time domain), the basic waveforms that represent different discrete values

  • channel

    • Used to represent a coal body that transmits information in a certain direction.

  • baseband signal

    • signal from source

  • modulation

    • baseband modulation

      • The converted signal is still a baseband signal

    • bandpass modulation

      • Use carrier wave for modulation to move the frequency range of the baseband signal to a higher frequency band and convert it into an analog signal

    • band pass signal

      • The signal after carrier modulation

• The ultimate capacity of the channel

  • In any channel, there is an upper limit to the code element transmission rate. If the transmission rate exceeds this upper limit, serious inter-symbol crosstalk will occur, making it impossible for the receiving end to identify the symbols.

  • Nye's criterion

    • The ultimate data transmission rate under an ideal low-pass channel = 2Wlog2 (V) b/s

  • signal-to-noise ratio

    • The ratio of the average power of the signal to the average power of the noise

  • Shannon formula

    • The ultimate data transmission rate of the channel = Wlog2 (1+S/N) b/s
    • Shannon's formula shows that the greater the bandwidth of the channel or the signal-to-noise ratio in the channel, the higher the ultimate transmission rate of information.

• Channel multiplexing technology

  • Frequency Division Multiplexing (FDM)

    • All users of frequency division multiplexing occupy different bandwidth resources at the same time

  • Time division multiplexing (TDM)

    • All users of time division multiplexing occupy the same frequency bandwidth at different times

  • Wavelength Division Multiplexing (WDM)

    • optical frequency division multiplexing

  • Code Division Multiplexing (CDM)

data link layer

Data link layer using point-to-point channels

• basic concept

  • link

    • A passive point-to-point physical line segment without any other switching nodes in between

      • physical link

        • That is equal to the link

      • logical link

        • That is, adding necessary communication protocols to the physical link

  • data link

    • In addition to the physical wires, there must be communication protocols to control the transmission of this data. If the hardware and software that implement these protocols are added to the link, a data link is formed.

  • frame

    • Protocol data unit of the data link layer for point-to-point channels

• Three basic questions about the data link layer

  • framed

    • Concept: Add a header and a trailer before and after a piece of data to form a frame. Determine frame boundaries.

    • frame delimiter

      • SOH、EOT

  • Transparent transmission

    • Workaround: Byte padding or character padding (ESC)

  • error detection

    • Cyclic Redundancy Check CRC
    • Frame check sequence FCS (remainder)

*Point-to-Point Protocol PPP (understand)

• The data link layer protocol used by user computers to communicate with ISPs

• Requirements that PPP Agreements Should Meet

  • Simple

  • framed

    • Special characters must be specified as frame delimiters

  • Transparency

    • Transparency of data transmission must be ensured

  • Multiple network layer protocols

    • Able to support multiple network layer protocols simultaneously on the same physical link

  • Various types of links

  • error detection

    • Ability to detect frames received by the receiving end and immediately discard erroneous frames

  • Check connection status

  • maximum transmission unit

  • Network layer address negotiation

  • Data compression negotiation

• Unnecessary features in PPP protocol

  • No correction required
  • No need to set serial number
  • No flow control required
  • Does not support multipoint lines, only supports point-to-point link communication
  • Does not support half-duplex or slingshot links, only full-duplex links

• Components of the PPP Agreement

  • A method of encapsulating IP datagrams onto a serial link
  • Link Control Protocol LCP (Link Control Protocol)
  • Network Control Protocol NCP (Network Control Protocol)

• PPP protocol frame format

  • The header and tail of the PPP frame are 4 fields and 2 fields respectively.

  • Transparent transmission issues in PPP protocol

    • When PPP is used on a synchronous transmission link, the protocol specifies the use of hardware to complete bit stuffing (the same as HDLC)
    • When PPP is used for asynchronous transmission, a special character padding method is used
    • Byte padding, zero bit padding

• Reasons why the PPP protocol does not provide reliable transmission using sequence numbers and acknowledgments

  • When the probability of errors in the data link layer is low, it is more reasonable to use the simpler PPP protocol.
  • In the Internet environment, the data put in the PPP information field is IP datagrams. Reliable transmission at the data link layer does not guarantee that transmission at the network layer is also reliable.
  • The Frame Check Sequence (FCS) field ensures error-free acceptance

Data link layer using broadcast channels

• local area network

  • main feature

    • The network is owned by one organization and has limited geographical scope and number of sites.

  • The main advantage

    • With broadcast function, the entire network can be easily accessed from one site. Hosts on the LAN can share various hardware and software resources connected to the LAN
    • Facilitates system expansion and gradual evolution, and the location of each device can be flexibly adjusted and changed
    • Improved system reliability, availability and survivability

• Coal body sharing technology

  • Static channel allocation

    • frequency division multiplexing
    • time division multiplexing
    • Wavelength division multiplexing
    • code division multiplexing

  • Dynamic media access control (multipoint access)

    • random access
    • Controlled access, such as multipoint line polling, or polling

• Two sublayers of the data link layer

  • Logical Link Control LLC (Logical Link Control) sublayer
  • Media Access Control MAC (Medium Access Control) sublayer

• adapter

  • Important functions of the adapter

    • Perform serial/parallel conversion
    • cache data
    • Install device drivers in your computer's operating system
    • Implement the Ethernet protocol

  • The computer communicates with the LAN through the adapter

• CSMA/CD protocol

  • Two important measures taken by Ethernet to make communication simple

    • Adopt a more flexible connectionless working method
    • Data sent over Ethernet uses Manchester encoding

  • Services provided by Ethernet

    • The services provided by Ethernet are unreliable delivery, that is, best-effort delivery
    • When the destination station receives an erroneous data frame, it discards the frame and does nothing else. Correction of errors is decided by senior management
    • If the upper layer finds that some data is lost and retransmits it, the Ethernet does not know that this is a retransmitted frame, but sends it as a new data frame.

  • CSMA/CD meaning

    • Carrier Sense Multipoint Access/Collision Detection

  • Why do collision detection?

    • Since the propagation rate of electromagnetic waves on the bus is limited, when a station detects that the bus is idle, the bus may not really be idle.
    • The information sent by A to B can only be transmitted to B after a certain period of time.
    • If B sends its own frame before the information sent by A reaches B (because B's carrier monitoring cannot detect the information sent by A at this time), it will inevitably collide with the frame sent by A at a certain time.
    • The result of the collision is that both frames become useless
    • So collision detection needs to be done during sending to detect conflicts

  • CSMA/CD important features

    • Ethernet using the CSMA/CD protocol cannot perform full-duplex communication but can only perform two-way alternating communication (half-duplex communication)
    • Within a short period of time after each station sends data, there is the possibility of encountering a collision.
    • This transmission uncertainty makes the average traffic volume of the entire Ethernet network much less than the maximum data rate of the Ethernet network.

  • Binary exponential type backoff algorithm

  • Minimum valid frame length

    • If a collision occurs, it must be within the first 64 bytes sent. Since transmission is immediately terminated as soon as a conflict is detected, the data sent at this time must be less than 64 bytes. Ethernet stipulates that the shortest valid frame length is 64 bytes. Any frame with a length less than 64 bytes is an invalid frame that is aborted abnormally due to conflict.

  • Strengthen collision

    • When the station sending data discovers that a collision has occurred:
      (1) Immediately stop sending data;
      (2) Continue to send a number of bits of jamming signal to let all users know that a collision has occurred.

  • Key Points of the CSMA/CD Agreement

    • ready to send
    • detection channel
    • Check for collisions

• Star topology using hubs

  • Hub Features

    • Hubs use electronics to simulate the working of actual cables, so the entire system still operates like a traditional Ethernet network
    • Ethernet using a hub is still a logical bus network, and each workstation still uses the CSMA/CD protocol and shares the logical bus.
    • A hub is much like a multi-interface repeater, working at the physical layer
    • The hub uses a specialized chip to perform adaptive crosstalk echo cancellation, reducing near-end crosstalk.

• Ethernet channel utilization

  • To improve the channel utilization of Ethernet, it is necessary to reduce the ratio of  to T0

• Ethernet MAC layer

  • Ethernet frame format

  • Invalid MAC frame

    • The length of the data field is inconsistent with the value of the length field
    • The length of the frame is not an integer number of bytes
    • Use the received frame check sequence FCS to detect errors.
    • The length of the data field is not between 46 ~ 1500 bytes
    • The valid MAC frame length is between 64 ~ 1518 bytes

• Extended Ethernet

  • Extending Ethernet at the physical layer

    • Extension using hub

      • advantage

        • Enable computers on the Ethernet that originally belong to different collision domains to communicate across collision domains
        • Expanded geographic coverage of Ethernet

      • shortcoming

        • The collision domain is increased, but the overall throughput is not improved
        • If different collision domains use different data rates, then you cannot interconnect them with a hub

  • Extending Ethernet at the data link layer

    • bridge

      • The bridge works at the data link layer and forwards and filters received frames based on the destination address of the MAC frame.
      • When the bridge receives a frame, it does not forward the frame to all interfaces. Instead, it first checks the destination MAC address of the frame, and then determines which interface to forward the frame to, or discards it.

    • Characteristics of Ethernet switches

      • The interface of the Ethernet switch has memory that can buffer incoming frames when the output port is busy.
      • An Ethernet switch is a plug-and-play device, and its internal frame exchange table (also called an address table) is automatically and gradually established through a self-learning algorithm.
      • Ethernet switches use dedicated switching fabric chips and use hardware forwarding, whose forwarding rate is much faster than bridges using software forwarding.

    • Self-learning function of Ethernet switch

Network layer

Two services provided

• Connection-oriented (virtual circuit), telecommunications network
• Connectionless (datagram service), the current mainstream service

Used in conjunction with IP protocol

• Address Resolution Protocol ARP (downward compatible, used by the data link layer)
• Internet Control Message Protocol ICMP (Service Upward)
• Internet Group Management Protocol IGMP (Service Upward)

Virtual interconnection network (IP network)

• Direct delivery: no need to go through any routers
• Indirect delivery: IP datagrams are forwarded to the router first
• Emphasis: The Internet can be composed of a variety of heterogeneous network interconnections

IP address (32 bits)

• IP address addressing method

  • (1) Classified IP address

    • Class A 0 (8 bits + 24 bits / 1 byte + 3 bytes)
    • Category B 10 (16 bits + 16 bits / 2 bytes + 2 bytes)
    • Class C 110 (24 bits + 8 bits / 3 bytes + 1 byte)
    • Class D 1110+ multicast address
    • Class E 1111 reserved for future use
    • Features: hierarchical address structure, equal network numbers
      (dotted decimal notation)

  • (2) Division of subnets

    • Basic ideas for dividing subnets

      • Things within the unit
      • Borrow several bits from the network's host number as the subnet number
      • Still based on the destination network number net-id of the IP datagram, first find the router connected to the unit's network.
      • Then after receiving the IP datagram, the router finds the destination subnet based on the destination network number net-id and subnet number subnet-id.

    • Three-level structure after dividing subnets

      • advantage

        • Reduces waste of IP addresses
        • Make network organization more flexible
        • Easier to maintain and manage

      • The division of subnets is purely an internal matter of an organization and is transparent to the external network. To the outside world, it still appears as a network without subnets.

    • Subnet mask (to resolve whether subnet division has been performed)

      • (IP address) AND (subnet mask) = network address
      • Different subnet masks result in the same network address.
        But the effects of different masks are different.

    • Three items included in the routing table after subnetting

      • destination network address
      • subnet mask
      • Next hop address

  • (3) Forming a supernet (unclassified addressing CIDR)

    • Two main features of CIDR

      • CIDR eliminates the traditional concepts of Class A, Class B, and Class C addresses and subnetting
      • CIDR combines consecutive IP addresses with the same network prefix into a "CIDR address block"

    • Slash notation (CIDR notation)

    • route aggregation

      • A CIDR address block can represent many addresses. This aggregation of addresses is often called route aggregation.
      • Route aggregation is also called supernetting
      • Although CIDR no longer uses subnets, it still uses the term "mask" (but it is not called a subnet mask)

    • Benefits of CIDR

      • IPv4 address space can be allocated more efficiently, and appropriately sized CIDR address blocks can be allocated according to customer needs.

• IP address characteristics

  • (1) IP address is a hierarchical address structure
  • (2) In fact, an IP address is an interface that identifies a host (or router) and a link.
  • (3) Several LANs connected by repeaters or bridges are still one network
  • (4) All networks assigned to the network number net-id are equal

ARP protocol

• effect

  • From the IP address used at the network layer, the hardware address used at the data link layer is resolved.

• core

  • Store IP in host ARP cache ——> Hardware address mapping table

• process

  • If A sends a datagram to B, parsing is performed automatically.

• ARP issues to note

  • ARP solves the mapping problem between the IP address and the hardware address of the host or router on the same LAN.

• Four typical situations of ARP

  • Host——>Host (in the same network)
  • Host --> Host (other network), through router
  • Router——>Host (same network)
  • Router——>Host (other networks)

IP datagram format

• Fixed part (20 bytes, 12 specific fields)
• Variable part (variable length, an option field to support debugging, measurement, security, etc.

IP layer forwarding packet process

• Router packet forwarding algorithm

  • (1) Extract the IP address D of the destination host from the header of the datagram, and obtain the destination network address N.
  • (2) If network N is directly connected to this router, the datagram will be delivered directly to the destination host D; otherwise, it will be delivered indirectly and execute (3).
  • (3) If there is a route to a specific host with the destination address D in the routing table, send the datagram to the next-hop router specified in the routing table; otherwise, execute (4).
  • (4) If there is a route to network N in the routing table, transmit the datagram to the next-hop router specified in the routing table; otherwise, execute (5).
  • (5) If there is a default route in the routing table, transmit the datagram to the default router specified in the routing table; otherwise, execute (6).
  • (6) Report forwarding packet error.

• About routing tables

  • The routing table does not specify the complete path for the packet to a network.
  • The routing table indicates that to reach a certain network, you should first go to a certain router (that is, the next-hop router).
  • After reaching the next hop router, continue to search its routing table to know which router should be reached next.
  • Search step by step in this way until you finally reach the destination network.

ICMP protocol

• Purpose

  • To forward IP datagrams more efficiently and improve the chances of successful delivery

• ICMP messages

  • ICMP message types

    • ICMP error report message

      • When ICMP error messages are not sent

    • ICMP query message

  • There are 4 types of ICMP error report messages.

    • The destination is unreachable
    • time exceeds
    • Parameter problem
    • Change routing (Redirect)

  • ICMP query message

    • Return request and reply messages
    • Timestamp request and reply messages

• Application examples

  • Use PING to test connectivity between two hosts

routing protocol

• divide

  • Static routing policy (non-adaptive routing)
  • Dynamic routing strategy (adaptive routing)

• Autonomous System AS

  • An AS exhibits a single and consistent routing policy to other ASs

• Divide by level

  • Interior Gateway Protocol IGP

    • Routing Information Protocol RIP

      • RIP is a distributed, distance vector-based routing protocol

      • Definition of "distance"

        • The distance from a router to a directly connected network is defined as 1
        • The "distance" in the RIP protocol is also called the "hop count" because each time it passes through a router, the hop count increases by 1.

      • Three characteristics of the RIP protocol

        • (1) Only exchange information with neighboring routers
        • (2) The information exchanged is all the information currently known by this router, that is, its own routing table.
        • (3) Exchange routing information at fixed time intervals

      • RIP protocol advantages and disadvantages

        • advantage

          • The implementation is simple and the overhead is small.

        • shortcoming

          • RIP limits the size of the network, and the maximum distance it can use is 15 (16 means unreachable).
          • The routing information exchanged between routers is the complete routing table in the router, so as the network scale expands, the overhead also increases.
          • "Bad news travels slowly", making the update process take too long to converge.

      • RIP2 protocol message format

    • Open Shortest Path First OSPF

      • Three main points

        • Send information to all routers in this autonomous system. The method used here is the flooding method.
        • The information sent is the link status of all routers adjacent to this router, but this is only part of the information the router knows.
        • Only when the link status changes, the router uses the flooding method to send this information to all routers.

  • Exterior Gateway Protocol BGP

    • BGP is a protocol for exchanging routing information between routers in different autonomous systems

IPv6

• IPv6 datagram consists of two parts: basic header and payload

• Transition from IPv4 to IPv6

  • Two strategies for the transition to IPv6

    • Use dual protocol stack

    • Use tunneling technology

      • When the IPv6 datagram enters the IPv4 network, the IPv6 datagram is encapsulated into an IPv4 datagram, and the entire IPv6 datagram becomes the data part of the IPv4 datagram.
      • When the IPv4 datagram leaves the tunnel in the IPv4 network, the data part (that is, the original IPv6 datagram) is handed over to the host's IPv6 protocol stack.

IP multicast

Virtual Private Network VPN and Network Address Translation NAT

transport layer

Function

• Provide logical communication between application processes
• Reuse: All application processes can be transmitted to the network layer through the transport layer
• Divide: After the transport layer receives the data, it is delivered to the specified application process respectively.

Transport layer ports (implementing multiplexing and demultiplexing)

• The port number used by the server

  • Well-known ports (0~1023)
  • Register port number (1024~49151)

• The port number used by the client

  • Also known as ephemeral port number (49152~65535)

Two protocols at the transport layer

• User Datagram Protocol UDP (no connection)

  • Main features of UDP

    • (1) UDP is connectionless
    • (2) UDP uses best-effort delivery
    • (3) UDP is message-oriented
    • (4) UDP has no congestion control
    • (5) UDP supports one-to-one, one-to-many, many-to-one and many-to-many interactive communications
    • (6) UDP header overhead is small

  • UDP header format

    • UDP user datagram header and pseudo header

  • Checksum calculation

• Transmission Control Protocol TCP (connection-oriented)

  • The main characteristics of TCP

    • (1) TCP is a connection-oriented transport layer protocol

    • (2) Each TCP connection can only have two endpoints, and each TCP connection can only be point-to-point (one-to-one).

    • (3) TCP provides reliable delivery services

    • (4) TCP provides full-duplex communication

    • (5) Oriented to byte stream

      • TCP stream-oriented concepts

        • TCP stream-oriented concepts

  • TCP connection

    • Each TCP connection has two endpoints

    • The endpoint of a TCP connection is called a socket or socket

      • (The port number is spliced ​​to the IP address to form a socket)

    • socket

      • socket socket = (IP address: port number)

      • Each TCP connection is uniquely identified by the two endpoints (i.e., two sockets) at both ends of the communication.

        • TCP connection::= {socket1, socket2}
          = {(IP1: port1), (IP2: port2)}

  • How reliable transmission works

    • Two Characteristics of Ideal Transmission Conditions

      • (1) The transmission channel does not produce errors
      • (2) No matter how fast the sender sends data, the receiver always has time to process the received data.

    • stop waiting for agreement

      • (1) No error situation

      • (2) An error occurs

      • (3) Confirmation lost and late confirmation

      • (4) Channel utilization

        • Channel utilization

    • Continuous ARQ protocol

  • TCP message header format

    • A TCP message segment is divided into two parts: header and data.
    • The meaning of each field in the header

  • Implementation of TCP reliable transmission

    • Sliding window in bytes

      • send buffer
      • receive cache

    • Timeout retransmission time selection

      • Message segment round trip time RTT
      • Weighted average round trip time RTTS
      • Timeout retransmission time RTO
      • Weighted average of RTT deviations RTTD
      • Karn algorithm

    • Select SACK

  • TCP flow control

    • Utilize sliding windows

      • Flow control is to prevent the sender's sending rate from being too fast, so that the receiver has time to receive, and does not cause network congestion.
      • continuous counter

    • TCP transmission efficiency

      • Sender Confused Window Syndrome

        • Nagle algorithm

  • TCP congestion control

    • Congestion: Demand for resources > Available resources

    • Retransmissions caused by congestion will not alleviate network congestion, but will intensify it.

    • The role of congestion control

      • The role of congestion control

    • Metrics for monitoring network congestion

      • Percentage of packets dropped due to lack of buffer space
      • average queue length
      • Number of packets retransmitted after timeout
      • average packet delay
      • Standard deviation of packet delay

    • TCP congestion control algorithm

      • slow-start

        • Gradually increase the congestion window value from small to large.

          • Each increase in congestion window cwnd = min (N, SMSS)
          • After each transmission round, the congestion window cwnd is doubled.

      • congestion avoidance

        • Grows slowly and linearly

          • Implementation examples of slow start and congestion avoidance algorithms
          • "Congestion avoidance" does not mean that congestion can be completely avoided. It is still impossible to completely avoid network congestion using the above measures.
          • "Congestion avoidance" means controlling the congestion window to grow linearly during the congestion avoidance phase, making the network less prone to congestion.

      • fast retransmit (fast retransmit)

        • Using the Fast Retransmission FR (Fast Retransmission) algorithm allows the sender to know as early as possible that individual segments are lost.

      • fast recovery

  • TCP transport connection management

    • Three stages of transport connections

      • Connection established
      • data transmission
      • connection release

    • TCP connection establishment - three-packet handshake

      • Various states of TCP connection established using three-message handshake

    • TCP release connection - four message handshake

      • The TCP connection must elapse for 2MSL before it is actually released.

    • Finite state machine for TCP

      • Thick solid arrows indicate normal transitions to client processes
      • Thick dashed arrows indicate normal transitions to server processes
      • Thin arrows indicate abnormal changes

Application layer

Domain Name System DNS

• Domain Name System Overview

  • Domain Name SystemDNS (Domain Name System)

• Internet domain name structure

  • The structure of a domain name consists of a sequence of labels. Each label is separated by a dot. Each label represents a different level of domain name.

  •           … . 三级域名 . 二级域名 . 顶级域名
    

  • Top Level Domain TLD (Top Level Domain)

    • (1) National top-level domain name nTLD
    • (2) Generic top-level domain name gTLD
    • (3) infrastructure domain

  • Internet domain name space

    • Internet domain name space

• domain name server

  • basic concept

    • district

      • The range that a server is responsible for (or has authority over) is called a zone.

    • Authoritative domain name server

      • Each zone is set up with a corresponding authority domain name server to save the mapping from domain names to IP addresses of all hosts in the zone.

    • Examples of different division methods of districts

      • Examples of different division methods of districts

  • Four types of domain name servers

    • Root name server

      • There are 13 sets of root domain name servers, not 13 machines.

    • top-level domain name server

      • The top-level domain name server (ie TLD server) is responsible for managing all second-level domain names registered in the top-level domain name server.

    • Authoritative domain name server

      • Responsible for the domain name server of a zone

    • local nameserver

  • Domain name resolution process

    • recursive query

      • The host's query to the local domain name server generally uses recursive query

    • Iterative query

      • The query from the local domain name server to the root domain name server usually uses an iterative query.

File Transfer Protocol FTP

• FTP features

  • The main function of FTP is to reduce or eliminate incompatibilities in processing files under different operating systems

  • FTP uses client server method

  • FTP server process

    • a main process

      • Responsible for accepting new requests

    • several slave processes

      • Responsible for handling individual requests

• The basic working principle of FTP

  • Two TCP connections used by FTP
  • FTP uses two different port numbers, so there is no confusion between the data connection and the control connection.

• Advantages of using two different port numbers

  • Make protocols simpler and easier to implement
  • Control connections can also be utilized when transferring files (for example, the client sends a request to terminate the transfer)

• Simple File Transfer Protocol TFTP

Remote Terminal Protocol TELNET

• TELNET uses the network virtual terminal NVT format

World Wide Web and HTTP Protocol

• World Wide Web Overview

  • World Wide Web WWW (World Wide Web) is a large-scale, online information repository

  • Hypermedia and Hypertext

    • The World Wide Web is a distributed hypermedia system and an extension of the hypertext system.
    • A hypertext consists of multiple information sources linked into
    • The difference between hypermedia and hypertext is that the document content is different

  • The working state of the World Wide Web

    • The World Wide Web works as a clientserver

• Uniform Resource Locator URL

  • General form of URL

    • <protocol>://<host>:<port>/<path>

  • URL using HTTP

    • http://<host>:<port>/<path>

• Hypertext Transfer Protocol HTTP

  • HTTP is a transaction-oriented application layer protocol. It is an important foundation for the reliable exchange of files (including various multimedia files such as text, sound, images, etc.) on the World Wide Web.

    • How the World Wide Web works

  • Main features of HTTP

    • HTTP is a transaction-oriented client-server protocol
    • The HTTP 1.0 protocol is stateless
    • The HTTP protocol itself is also connectionless, although it uses the services provided upward by connection-oriented TCP

  • proxy server

    • Proxy server, also known as Web cache, makes HTTP requests on behalf of the browser.

  • HTTP message structure

    • The message consists of three parts, namely the start line, the header line and the entity body.
      In the request message, the start line is the request line.

    • HTTP has two types of messages

      • request message
      • response message

• World Wide Web Documentation

  • Hypertext Markup Language HTML

  • Dynamic Web Documents

    • The main difference between dynamic documents and static documents is on the server side

  • Common Gateway Interface CGI

    • is a standard that defines how dynamic documents should be created, how input data should be provided to applications, and how the output results should be used

  • Active Web Documents

    • Active documents are created on the client side

Email transmission process

• Some standards for email

  • Protocol for sending emails: SMTP
  • Protocols for reading mail: POP3 and IMAP

• The main components of email

• Email composition

  • envelope
  • content

• Email address format

  • Recipient email name@domain name of the host where the email is located
  • "@" is pronounced as "at", which means "at"

• Simple Mail Transfer Protocol SMTP

  • SMTP uses client server method

  • Three stages of SMTP communication

    • 1. Connection established
    • 2. Mail delivery
    • 3. connection release

• Mail reading protocols POP3 and IMAP

  • POP uses a client-server approach
  • IMAP (Internet Message Access Protocol) also works in a client-server manner
  • IMAP is an online protocol

Dynamic Master Configuration Protocol DHCP

• Protocol configuration

  • The action of assigning values ​​to these parameters in the protocol software is called protocol configuration.

• DHCP uses clientserver method

• The DHCP relay agent forwards discovery messages in unicast mode

Simple Network Management Protocol SNMP

• General model of network management
• The application process connects to the network through a socket
• Call socket to create a socket
• Servers that work concurrently

P2P file system

cyber security

Cyber ​​security threats

• passive aggressive

  • (1) Tampering
  • (2) Malicious programs
  • (3) Denial of Service Dos

• Active attack

Data encryption model

• Data encryption model

Two types of cryptosystems

• Symmetric key cryptography

  • The encryption key is the same as the decryption key

    • symmetric key system

  • The data encryption standard DES is a symmetric key system

  • The confidentiality of DES only depends on keeping the key secret, and the algorithm is public

• public key cryptography

  • Use different encryption and decryption keys

    • public key cryptography

  • The encryption key (public key) PK is open to the public, while the decryption key (secret key) SK needs to be kept secret.

• digital signature

  • Implementation of digital signature based on public key

  • Confidential digital signature

  • Public key cryptography, encryption is a private key, decryption is a public key

  • necessary functions

    • Message identification
    • Message integrity
    • undeniable

identify

• Message identification

  • MD5 algorithm (Message Digest)

• Entity authentication

  • Verification code

(This summary is derived from a mind map. Friends who need a mind map can leave their email in the comment area)