Computer Network review, computer network review materials

Computer Network review, computer network review materials

 

During the interview, if the interviewer wants to examine how your computer network has learned, how can he ask about the TCP three-way handshake process?

So, I started to review the computer network in October April 24. I had to review the computer network for three days.

 

ISP: Internet Service Provider

Eg: China Telecom, China Unicom, and China Mobile

 

A vro is a dedicated computer. The key component of the router for packet switching. The task is to forward the received group.

• 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 part of the entire packet) is sent to adjacent nodes, stored and searched for forwarding tables, and forwarded to the next node.

 

Network Type

• WAN-WAN (Wide Area Network)
• MAN (Metropolitan Area Network): City
• LAN-LAN (Local Area Network) eg: Campus Network or Enterprise Network
• Personal Area Network (Personal Area Network) (eg): The Network connected by wireless technology on a computer (wifi)

 

OSI, TCP/IP, architecture of layer-5 protocols, and Protocols at different layers

OSI Layer (Layer 7): physical layer, data link layer, network layer, transmission layer, Session Layer, presentation layer, and application layer.

TCP/IP layer (Layer 4): network interface layer, Internet layer, transport layer, and application layer.

Five-layer protocol (layer 5): physical layer, data link layer, network layer, transport layer, and application layer.

The combination of OSI and TCP/IP generates a five-layer structure: physical layer, data link layer, network layer, transport layer, and application layer. The Internet is the TCP/IP protocol used.

Each layer has the following functions:

• Physical Layer: transmits bits through media to determine mechanical and electrical specifications (BITs)
• Data link layer: Assemble bits into Frame and point-to-point transmission (Frame)
• Network Layer: responsible for transmitting data packets from the source to the destination and connecting to the Internet (PackeT)
• Transport Layer: Provides end-to-end reliable message transmission and error recovery (Segment)
• Session Layer: establishes, manages, and terminates sessions (Session Protocol Data Unit SPDU)
• Presentation Layer: translate, encrypt, and compress data (PPDU)
• Application Layer: a means to allow access to the OSI environment (application protocol data unit APDU)

 

Frequency Division Multiplexing: All users use different frequency bandwidth at the same time.

Time division multiplexing: All Users occupy the same band width at different times.

 

Protocol Data Unit of the data link layer of point-to-point channels-Frame

Three basic issues on the data link layer:

Maximum part length of the data frame-Maximum transmission Unit MTU (Maximum Transfer Unit)

CRC is an error detection method, and FCS is the verification code added after the data.

 

PPP protocol: the data link layer protocol used by your computer to communicate with ISP.

PPP frame format: 75 pages

 

Adapter (NIC): responsible for sending frames to the LAN and receiving frames from the LAN

The adapter receives and sends various frames without using the computer's CPU. The adapter is equipped with a processor and memory (including RAM and ROM)

 

Broadcast Communication: when a computer sends data, all computers on the bus can detect the data.

CSMA/CD Protocol: A station cannot send and receive at the same time (but must send and listen to the channel at the same time)

 

Uncertainty of sending: there is a possibility of collision in a short period of time after each station sends data. This short period of time is uncertain, and it depends on the distance from the station where another data is sent to this site.

Contention period (collision window): end-to-end round-trip time of Ethernet (2i)

After the competition period, the collision has not been detected, so that it is certain that this sending will not have a collision. At this time, you can rest assured that this frame of data is successfully sent.

 

Ethernet contention period is determined to be 51.2us

Ethernet specifies a minimum frame length of 64 bytes, that is, bit. If the number of sent data is very small, you must add some padding bytes so that the frame length is no less than 64 bytes.

When the Ethernet sends data, if there is no collision during the contention period (a total of 64 bytes are sent), the data sent in the future will not conflict.
Any frame with a length less than 64 bytes is an invalid frame aborted due to a conflict.

If a collision occurs during the competition period, it will be postponed for a while before the frame and retransmission that are temporarily retained.

 

The adapter provides the filter function: each time a MAC frame is received from the network, the target address in the MAC frame is checked by hardware. If the frame is sent to this site, it is accepted; otherwise, the frame is discarded.

"Frames sent to this site" includes the following three types of frames:

• Unicast (unicast) frame (one-to-one): the MAC address of the received frame is the same as the hardware address on this site.
• Broadcast frame (one to all): frames sent to all sites on the local area network (all 1 address)
• Multicast frame (one-to-multiple): frames sent to some sites on the Local Area Network

 

Hardware address (or physical address/MAC address ).
During adapter production, the 6-byte MAC address has been solidified in the ROM of the adapter.

MAC frame format: 92 pages

 

Hub: the Ethernet using the hub is still a common network logically. Each station shares a logical bus and uses the CSMA/CD protocol. A hub has many interfaces.

 

Bridge is required for the data link layer Ethernet extension ). The bridge works on the data link layer and forwards and filters received Frames Based on the target address of the MAC frame.

The bridge forwards frames through forwarding tables. The forwarding table is also called the routing directory or forwarding database. Page 95

 

Transparent bridge (transparent bridge): currently the most used. The bridge can work without manual Forwarding Table configuration. Is a plug-and-play device.

The bridge uses the self-learning algorithm (self-learning) to process the received frames (and gradually establishes a forwarding table) and forwards the frames according to the forwarding table. 98-important

 

An Ethernet switch is actually a multi-interface bridge.

Maximum advantage of a vswitch: The bandwidth from each interface to the host is 10 Mb/s. The total capacity of vswitches with N pairs of interfaces is N * 10 Mb/s.

Ethernet frame format: page 102

 

Virtual lan vlan is a logical group composed of some lan cidr blocks that are not related to physical locations. These CIDR blocks have common requirements. The VLAN protocol allows you to insert a 4-byte identifier into the frame format of an Ethernet network, called a VLAN tag ). It is used to specify which Lan the workstation that sends the frame belongs.

 

Class A address network number: 126, that is, 2 ^ 7-2 = 126;

Cause of 2nd reduction: 0 indicates "current network" in the IP address ". The network number 127 (0111 1111) is reserved for communication between processes on the local software loopback test.

 

Host numbers of Class A addresses account for 3 bytes. Therefore, the maximum number of hosts in each class A network is 2 ^ 24-2;
Cause of 2nd reduction: the host number field of all 0 indicates that the IP address is a single network address connected to "local host;
(If the IP address of a host is 5.6.7.8, the network address of the host is 5.0.0.0)
The value 1 indicates "all". Therefore, the value 1 indicates all hosts on the network.

A vro always has two or more IP addresses. That is, each router interface has an IP address with different network numbers. Page 1

 

ARP is an Address Resolution Protocol. A simple language explains how ARP works.

1. First, each host creates an ARP list in its own ARP buffer to indicate the correspondence between the IP address and the MAC address.

2. When the source host needs to send data, first check whether the IP address of the target host exists in the ARP list. If yes, find the corresponding hardware address in the ARP cache, write the hardware address to the MAC frame, and then send the MAC frame to the hardware address through the LAN.

If no, an ARP packet is sent to all hosts in this segment. The packet includes the source host IP address, the source host MAC address, and the target host IP address.

3. When all hosts in the network receive the ARP packet, first check whether the IP address in the packet is its own IP address. If not, ignore the packet. If yes, first, the IP address and MAC address of the source host are retrieved from the data packet and written into the ARP list. If the source host already exists, the IP address is overwritten and then the MAC address is written into the ARP response packet, tell the source host that it is the MAC address it wants.

4. After the source host receives the ARP response packet. Write the IP address and MAC address of the target host to the ARP list and use this information to send data. If the source host has not received the ARP response packet, ARP query fails.

Broadcast sends ARP requests and unicast sends ARP responses.

 

 

IP address and subnet mask phase and obtain host number

Internet Control Message Protocol (ICMP)
There are two types of ICMP packets: ICMP error messages and ICMP inquiry messages.

ICMP query message:

• Send back requests and answers: requests sent from the host or router to a specific target host. The host that receives this message must send an ICMP response packet to the source host or router.
• Timestamp Request and answer: Ask a host or router to answer the current date and time.

PING: used to test the connectivity between two hosts.

Ping is an example of using network layer ICMP directly at the application layer. It does not pass the transport layer's TCP or UDP

 

RIP and OSPF

 

Virtual Private Network

For these computers only used within the organization, the Organization can allocate its own IP address. That is to say, to allow these computers to use only valid IP addresses (called local addresses) in the Organization, without applying for a globally unique IP address (referred to as a Global Address) from the Internet management organization ). This can greatly save valuable global IP Address resources.

 

Private address: it can only be used for internal communication of one organization, but not for communication with hosts on the Internet. That is, a private address can only be used as a local address but not a global address.

All routers on the Internet do not forward data packets whose destination address is a dedicated address.

Private address:

• 10.0.0.0 to 10.20.255.255
• 172.16.0.0 to 172.31.255.255
• 192.168.0.0 to 192.168.255.255

 

What is the difference between TCP and UDP?

TCP provides connection-oriented and reliable data stream transmission, while UDP provides non-connection-oriented and unreliable data stream transmission.

The unit of TCP transmission is called the TCP packet segment, and the unit of UDP transmission is called the user datagram.

TCP focuses on data security and UDP data transmission is fast, because there is no need to wait for a connection and many operations are missing, but its security is average.

 

The DNS domain name system briefly describes how it works.

A Domain Name Server is a server that saves all the host domain names and corresponding IP addresses in the network and converts the domain name to an IP address.

 

What are the characteristics of connection-oriented and non-connection-oriented services?

Connection-oriented services. Before communication, both parties must establish a complete channel for communication. During the communication process, the entire connection can be monitored and managed in real time.

For non-connection-oriented services, you do not need to establish a connection with two communication nodes in advance. When communication is required, the sending node can send information to the network, information is automatically transmitted over the network, which is generally not monitored during transmission.

 

 

Port and corresponding service?

 

TCP and UDP protocols

TCP protocol:

• FTP: defines the file transfer protocol. Port 21 is used.
• Telnet: A port used for remote login. port 23 allows you to remotely connect to a computer as your identity and provide communication services in DOS mode.
• SMTP: Mail Transfer Protocol, used to send mail. The server opens port 25.
• POP3: it corresponds to SMTP, and POP3 is used to receive emails. The POP3 protocol uses port 110.
• HTTP: A transfer protocol that transfers hypertext from a Web server to a local browser.

UDP protocol:

• DNS: used for domain name resolution service to convert the domain name address to an IP address. DNS uses port 53.
• SNMP: Simple Network Management Protocol. port 161 is used to manage network devices. Because of the large number of network devices, the unconnected services show their advantages.
• TFTP (Trival File Transfer Protocal), simple File Transfer protocol, which uses UDP Service on well-known port 69.