Key points of computer network foundation

Chapter 1 Overview

computer network

Computer networks are mainly interconnected by some general-purpose, programmable hardware, and these hardware are not specifically used to achieve a specific purpose.
Computer-provided services

Connectivity and sharing
Packet forwarding

The message is grouped, plus the header, stored and forwarded by the router, and merged at the destination.
The difference between circuit switching, message switching and packet forwarding
- Circuit switching: The end-to-end communication quality is reliably guaranteed due to the agreed communication resources, and the efficiency is high for continuous large amounts of data
- Message exchange: There is no need to make an appointment for transmission broadband, and the dynamic use of transmission broadband segment by segment is highly efficient for burst data
- Packet forwarding: high efficiency, speed, small packets, flexible routing, good network survivability
internet and internet
- internet: Internet, a network formed by multiple computer networks, without a specific protocol
- Internet: Internet, especially TCP/IP protocol
Types of computer networks
- Wide area network, metropolitan area network, local area network, personal network
- Public network
Core part and edge part
- Core part: composed of routers, providing services for the edge part
- Edge part: consists of hosts, users directly process and share information
Why layered network architecture
- Each layer is independent, and the upper layer can use the services provided by the next layer without worrying about how to implement it
- Good flexibility, when one layer changes, the interface relationship remains unchanged, and other layers are not affected
- The structure can be separated, and each layer can be realized by the most suitable technology
- Easy to implement and maintain
- Can promote standardization
Agreement and service
- The implementation of the protocol can ensure that services are provided to the upper layer, and the upper layer can see the services of the lower layer, but not the agreement
- The protocol is "horizontal", and the protocol is the rule that controls the communication between two peer entities; the service is "vertical", and the lower layer provides services to the upper layer
Network protocol
- Syntax: the structure or format of data and control information
- Semantics: what kind of control information needs to be sent, what kind of action and response to complete
- Synchronization: Description of the order in which events are implemented
Five-layer protocol architecture
- Physical layer: Transparent transmission of bit stream
- Data link layer: error-free transmission frame on the lines of two adjacent nodes
- Network layer: select the appropriate route, forward the packet
- Transport layer: provide a reliable end-to-end service for communication with the upper layer process
- Application layer: directly provide services for the user's application process

Chapter 2 Physical Layer

Glossary
- Data: The entity that carries the information
- Signal: electrical or electromagnetic representation of data
- Simplex communication: only one direction of communication and no reverse direction
- Half-duplex communication: the receiver cannot send and receive at the same time
- Full-duplex communication: the receiver can send and receive at the same time
- Baseband signal: from the source, the output data signal representing various text images
- Bandpass signal: baseband signal after carrier modulation
The characteristics of the physical layer interface
- Mechanical properties
- Electrical characteristics: voltage range
- Functional characteristics: the meaning of a certain level
- Procedure characteristics
Transmission media
- Twisted pair: STP shielded twisted pair, UTP unshielded twisted pair
- Coaxial cable
- Optical cable
- Wireless transmission
Channel multiplexing technology

In order to share channels, maximize channel utilization

Frequency division, time division, code division, wave division
ADSL technology, HFC, FTTx access technology

ADSL uses digital technology to transform existing analog phones, so that it can carry broadband services with low cost and easy implementation; HFC has a very wide frequency band; FTTx has a high cost

Chapter 3 Data Link Layer

The difference between data link and link

A link is a physical line from a node to an adjacent node;

The data link must also have some implementation procedures, hardware and software of the protocol;
What functions does the link control of the data link layer include? Discuss the advantages and disadvantages of making a reliable link at the data link layer
- Encapsulated into frames, transparent transmission, error checking

For channels with severe interference, the reliable link layer can restrict the retransmission range to the local link
- For high-quality channels, the use of reliable links will increase resource overhead and affect transmission efficiency
The data link layer is made into a reliable link layer, which means that the communication of each link in the entire communication path from the source host to the destination host is reliable. The advantage of this is that a node in the network can find an error in the transmission early, so the error can be corrected by retransmission at the data link layer. If the data link layer is not made as a reliable link layer, then when a node in the network finds that the received frame has errors (regardless of whether the data link layer is made to be reliable or not, this error checking step is always Yes), only the frames with errors are discarded, and the sending node is not notified to retransmit the frames with errors. Only when the high-level protocol of the destination host (for example, the transport layer protocol TCP) finds this error, it notifies the source host to retransmit the erroneous data. However, it is too late at this time, and more data (including data without errors) may be retransmitted, which is a waste of network resources.

The role of network adapters
- The communication between the adapter and the local area network is carried out in serial transmission through cables or twisted pairs, while the communication between the adapter and the computer is in parallel transmission through the IO bus on the computer motherboard. Therefore, an important function of the adapter is to perform the conversion of data serial transmission and parallel transmission.
- Working at the data link layer and physical layer
Features of PPP protocol
- Simple (CRC check, accept if correct, discard if error), provide unreliable datagram service, error detection, no error correction, no serial number, no flow control, support for multiple types of links
- Full duplex link
- Suitable for situations where the line quality is not too bad
Features of LAN
- Shared transmission channel
- Limited geographic area and limited number of users
- broadcast
- Type: star network, bus network, ring network
Advantages and disadvantages of CSMA/CD protocol and time division multiplexing

Carrier sense multi-point access / collision detection
- Time division multiplexing is a static time slot allocation, uniform, high channel utilization under high load, low load waste of resources
- CSMA/CD is used dynamically, the channel utilization is high when the load is low, and the conflict is large when the load is high
- When the load on the network is light, the CSMA/CD protocol is very flexible. You can send it whenever you want, and the probability of collision is very small. If time division multiplexing TDM is used, the efficiency is relatively low. When many stations have no information to send, the allocated time slots are also wasted. However, when the network load is heavy, there are many collisions caused by the CSMA/CD protocol, and retransmissions often occur, so the efficiency is greatly reduced. But at this time, the efficiency of TDM is very high.
Features of Ethernet switches
- Link layer equipment
- VLAN is a logical group that is composed of some local area network segments and has nothing to do with physical location, insert a tag (specify which virtual local area network)
- An Ethernet switch is essentially a multi-interface bridge, which is very different from the repeater and hub that work at the physical layer. In addition, each interface of the Ethernet switch is directly connected to a single host or another hub, and generally works in full duplex mode. When the host needs to communicate, the switch can connect to many pairs of interfaces at the same time, so that each pair of hosts that communicate with each other can transmit data without collision like an exclusive transmission medium.
- For a common 10Mbit/s shared Ethernet, since a user monopolizes communication instead of sharing the bandwidth of the transmission media with other network users, the total capacity of a switch with N pairs of interfaces is N×10 Mbit/s
Ethernet switches form a virtual local area network

A virtual local area network VLAN is a logical group composed of some local area network segments and has nothing to do with physical location, and these network segments have certain common requirements. Each VLAN frame has a clear identifier, which indicates which VLAN the workstation sending the frame belongs to. The virtual local area network protocol allows a 4-byte identifier to be inserted into the Ethernet frame format, called a VLAN tag, to indicate which virtual local area network the workstation sending the frame belongs to.

Chapter 4 Network Layer

Services provided by the network layer upward
- Virtual circuit service
  - Communication is guaranteed by the network
  - Connection-oriented
  - Use destination address only when connecting
  - Packets are forwarded according to the same route
  - A fault occurs, and the virtual circuit cannot work normally
  - Reach the end in the order of sending
- Datagram service
  - Communication is guaranteed by the host
  - No connection
  - Each packet is independently routed and forwarded
  - Failure, packet loss may occur
  - Arriving at the end is not necessarily in order
Intermediate equipment
- Physical layer: repeater
- Data link layer: bridge or bridge
- Network layer: router
- Above the network layer: gateway
IP address classification, characteristics
- ABCDE five types
- Classification (network number, host number)
- A router should have at least two different IP addresses
- The network connected by the repeater and the bridge is still the same network
- IP addresses are treated equally
IP address and MAC address
- IP address is to assign a unique identifier in the world to each host connected to the Internet
- MAC address is based on physics and can identify specific link communication objects
Maximum transmission unit MTU
- The maximum length of the data field in the frame format defined in the data link layer
- Related to the total length field in the IP datagram header

The main difference between IGP and EGP two types of routing protocols

IGP（RIP和OSPF）
- The routing protocol used within the autonomous system to strive for the best routing
- The internal gateway protocol only cares about how the autonomous system transmits datagrams, and does not care about other systems
EGP（BGP）
- Use routing protocols in different autonomous systems and strive for better routing (not going around in circles)
- Other factors, bandwidth, time, etc. must be considered
- External gateway protocol, does not care about the interior of the autonomous system

RIP,OSPF,BGP

	RIP	OSPF	BGP
Gateway protocol	internal	internal	External
Routing table content	Destination network, next stop, distance	Destination network, next stop, distance	Destination network, full path
Basis of Optimal Path	Hop count	cost	Multiple strategies
algorithm	Distance vector	Link status	Distance vector
Delivery method	Transport layer UDP	IP datagram	TCP connection
other	Simple, low efficiency, good news spreads quickly, bad news spreads slowly	High efficiency, frequent exchange of information by routing, difficult to maintain consistency	Large scale, unified measurement is reachability

Chapter 5 Transport Layer

What is the difference between communication at the transport layer and communication at the network layer
- The transport layer provides end-to-end logical communication between application processes (logical communication is horizontal virtual communication)
- The network layer provides logical communication between hosts
When the application process uses connection-oriented TCP and connectionless IP, whether this transmission is connection-oriented or connectionless
- From different levels, the transport layer is connection-oriented, while the network layer is connectionless
UDP is message-oriented, TCP is byte-oriented
- UDP delivers the packets handed down by the application to the IP layer after adding the header, without merging or splitting
- TCP treats the message data block handed over by the application as an unstructured byte stream
The role of the port
- Uniformly mark the application processes of the TCP/IP system, so that the application processes of computers running different operating systems can communicate with each other
- Server side-well-known port, registered port
- Client-ephemeral port
Why put the TCP port number in the first 4 bytes in the TCP header?
- The ICMP error message should contain the contents of 8 bytes after the P header, and there are the source port and the destination port in the TCP header. When TCP receives an ICMP error message, it needs to use these two ports to determine which connection has an error.
In addition to the fixed length of the TCP header, there are options, so the length of the TCP header is variable

*UDP header length is fixed
Multiplexing and demultiplexing at the transport layer
- Multiplexing: different application processes of the sender can use the same transport layer protocol to transmit data
- Sharing: The transport layer of the receiver can deliver the data to the destination process correctly after stripping off the header of the message
Characteristics of TCP and UDP
- TCP features
  - Connection-oriented protocol (three-way handshake)
  - reliable
  - Transfer large amounts of data
  - slow
  - The connection is point to point (one to one)
  - Duplex communication
  - Byte stream oriented
- UDP
  - Send data directly without establishing a connection
  - Unreliable, best effort delivery
  - Small amount of data
  - fast
  - One to one, one to many, many to one, many to many
  - Message-oriented

Chapter 6 Application Layer

DNS server type
- Root name server
- Top-level domain server (TLD)
- Authority name server
- Local domain name server
Simple Mail Transfer Protocol SMTP
- Connection establishment After the
  sender's mail is sent to the mail cache of the sender's mail server, the SMTP client scans the mail cache at regular intervals (for example, 30 minutes). If any mail is found, use the well-known SMTP port number (25) to establish a TCP connection with the SMTP server of the recipient's mail server.
- Mail transmission The transmission of
  mail starts with the MAIL command. There is the sender's address after the MAIL command. One or more RCPT commands follow, depending on sending the same message to one or more recipients. The function of the RCPT command is to find out whether the recipient's system is ready to receive mail before sending the mail. The purpose of this is to avoid wasting communication resources and not to discover the address error after sending a long email.
  The following is the DATA command, which means to start sending the contents of the mail.
- After the connection is released
  , the SMTP client should send the QUIT command. If the SMTP server agrees to release the TCP connection, the entire process of mail transmission ends.
What is the main difference between TFTP and FTP?
- Trivial File Transfer Protocol TFTP, it is a small and easy to implement file transfer protocol. Although TFTP also uses a client-server method, it uses UDP datagrams, so TFTP needs its own error correction measures. FTP uses TCP to transfer data, so it is very reliable. But because of this, FTP is much more complicated than TFTP. TFTP only supports file transfer and does not support interaction. TFTP does not have a huge command set, does not have the function of listing directories, and cannot authenticate users.
- There are two main advantages of TFTP: first, TFTP can be used in a UDP environment; second, the memory occupied by TFTP code is small.

Glossary

FTP (File Transfer Protocol)

Suitable for transferring files between any computers in a heterogeneous network
Including client and server
TCP

TFTP (Trivial File Transfer Protocol)

Use UDP datagram, you must have your own error correction measures
Only support file transfer but not interaction

TELNET (Remote Terminal Protocol)

Connect to another host remotely via TCP

BGP (Exterior Gateway Protocol)

HTTP (Hypertext Transfer Protocol)

Defines how the browser requests a document from the World Wide Web server, and how the server transmits the document to the browser

SMTP (Simple Mail Transfer Protocol)

How to exchange information between two SMTP processes communicating with each other

DHCP (Dynamic Host Configuration Protocol)

SNMP (Simple Network Management Protocol)

ARP (Address Resolution Protocol)

OSPF (Open Shortest Path First)

RIP (Routing Information Protocol)

DNS (Domain Name System)