The computer network of the school recruit interview Knowledge Point review

the computer network of the school recruit interview Knowledge Point review1 circuit switching and packet switching 2 hierarchical model of the Internet 3UDP and TCP 4 Routing and forwarding 5IP routing algorithm 7 multi-Access Protocol 8 Address Resolution Protocol ARP 9 switch vs. router 10 short answer part of the RIP works illustrates the main purpose of the media Access control protocol for the local area network and outlines CSMA The working process of the CD protocol the MAC address and IP address have any essential differences as to what is the difference between the intermediate system transponder Bridge router and gateway try to explain the difference between the following protocols and the IPARPRARP and ICMP CS mode and peer mode Is it possible to use a large switching switch in an internet instead of a lot of routers in the Internet? The routing table only gives the IP address of the next-hop router to the destination network and then gives the IP address of the next hop router in the next router's routing table to reach the destination network for direct delivery. What are the benefits of the IP protocol having sharding but the groupings in the WAN do not have to be fragmented why TCP Three-time handshake brief introduction of transition technology from IPV4 to IPV6

1, circuit switching and packet switching circuit Exchange characteristics: The resources will be reserved circuit exchange divided into: Frequency Division multiplexing and time division Multiplexing advantages: can guarantee a specific number of end-to-end bandwidth for a call disadvantage: low efficiency

Packet switching features: Resources are not reserved advantages: high Efficiency

Today's Internet is a model of packet switching.

Delay in packet switching

Processing delay, queue delay, transmission delay, propagation delay, transmission delay

Special attention should be paid to the difference between transmission delay and propagation delay!!!

Transmission delay:

r= link bandwidth (bps)
L= packet Length (bits)
Time the send bit enters the link = L/R

Propagation delay:

D = Length of physical link
s = speed of propagation in media (~2x108 m/sec)
Propagation delay = d/s

Packet switching vs. circuit switching

Circuit Exchange before the data transmission, must establish an end-to-end connection once a node failure, must reestablish the connection after the establishment of data transmission no additional delay data does not have to include the address domain, only the short virtual circuit number of the data in sequential transmission, but the low channel utilization for long-term transmission of large quantities of data, streaming data

Packet switching before data transfer, do not have to establish an end-to-end connection as long as the next node is idle, the transmission channel utilization of high data transmission using storage and forwarding, the delay can not be estimated in the data must contain the Address field received the packet is not necessarily ordered, may also need to reorganize suitable for the transmission of text data 2, Hierarchical model of the Internet:

Application tier: Support for network applications
FTP, HTTP, DNS, SMTP, DHCP, RIP, etc.

Transport Layer: host-to-host data transfer
TCP, UDP

Network layer: Routing from source to destination datagram
ARP, IP, ICMP, OSPF, BGP, etc.

Data Link layer: Transferring data between neighboring network elements
ALOHA, Csma or CSMA/CD, TDMA, FDMA, CDMA

Physical layer: "On-line" bits
Any kind of modulation and demodulation system

Application beginning: The application layer protocol is distributed on multiple end systems. Applications in one end system are grouped using protocol exchange information. We refer to this grouping of information in the application layer as messages.

Transport beginning section: Grouping of transport layers

Network Layer datagram: Network layer grouping in the Internet

Data Link Layer frames: Link Layer grouping

Physical layer bit: Physical layer grouping 3, UDP and TCP

UDP Features: is a non-connected Transport layer protocol

Advantage: The application layer can better control the data to be sent and send time without establishing a connection no connection status packet header overhead is small

Applications: often used in streaming multimedia applications
Packet loss tolerance rate sensitive other
DNS SNMP

TCP Features: Point-to-order full-duplex control congestion control for connected traffic

When TCP starts to establish a connection, the syn=1,ack=seq+1 is transmitted in the middle, and the confirmation number is only relevant for transmitting data.

Question: What mechanisms are used by the TCP protocol to ensure reliable data transmission. Three handshake at the time of connection and four handshake confirmation mechanism (timeout, slight acknowledgement) during disconnection congestion control, flow control

Method of congestion control in TCP: additive increase, multiplicative slow start response to timeout events

Additive increase: When no congestion is detected, the congestion window value is increased by one point (linear) after receiving a confirmation

Multiplicative subtraction: Half of the current congestion window value is halved each time a packet loss event occurs

Pay special attention to the difference between congestion control and flow control.

Congestion control: Prevents excessive data from being injected into the network, so that routers in the network are not overloaded. The premise of congestion control is that the network can withstand the existing network load.

Flow control: Control of Point-to-point traffic. Controls the rate at which sending data is sent.

Here, pay special attention to the difference between the choice of retransmission, fallback n step

Select retransmission: Only retransmission of the data frame of the error. However, the receiving window must be enlarged so that the data frames that are not contiguous but still in the receiving window are received. Wait until the data frame of the missing serial number is received and handed over to the host.

Fallback n Step: When the receiver detects out-of-order data frames, requires the sender to resend the last properly received information after all unacknowledged frames, or when the sender has sent n frames, found that the previous frame of n frame after the timer timeout has not returned its confirmation information, the frame is sentenced to time-out or error, At this point the sender has to resend the error frame and the frame after it. 4. Route selection and forwarding

Forwarding: When a packet arrives at an input link to a router, the router must move the packet to the appropriate output link.

Routing: When grouping flows from sender to receiver, the network layer must determine the route or path that these groupings take.

Routing refers to a process that determines the network scope of an end-to-end path when grouping from source to destination.

Forwarding refers to a router local action that transfers packets from one input link interface to the appropriate output link interface. 5. IP

The message format for IP is shown in the following figure:

The address assignment policy of the Internet is called Classless Domain Routing (cird). Prior to the adoption of Cird, IP addresses were divided into a,b,c,d,e five categories, namely:

However, in between, there are several private addresses that are not able to be used, including: Class A 10.0.0.0–10.255.255.255 class B 172.16.0.0–172.31.255.255 Class C 192.168.0.0–192.168.255.255 6. Routing algorithm link state routing algorithm (LS algorithm)