Why do I need to wait for 2msl?

1.5.5.5 TCP Four waves （ Connection release ）
name effect
SYN Synchronization sequence number , Used to establish the connection process , In the connection request ,SYN=1 and ACK=0 Indicates that the data segment does not use the acknowledgement field of the piggyback , And the connection response comes with a confirmation , namely SYN=1 and ACK=1
FIN For releasing connections , by 1 Indicates that the sender has not sent
ACK by 1 Indicates that the confirmation number is valid , by 0 Indicates that the message does not contain confirmation information , Ignore the confirmation number field , The validity of the above confirmation number is controlled by this identification
ack Is the sequence number of the first data byte that is expected to be received from the other party's next segment
seq TCP Each byte in the byte stream passed in the connection is sequentially numbered
notes ：
The first paragraph means :
FIN=1：(A) To release the connection ！
seq=u： Sent a lot of data , I'm just using u Refer to , here u There is a definite value

The second paragraph means :
ACK=1： The connection is established , After that ACK Must be set to 1
seq=v： Sent a lot of data , I'm just using v Refer to , here v There is a definite value
ack=u+1： Previous sender (A) It is said that the number sent is u Bit data , So I (B) What I want is u+1 Bit data （ Even though at this time A It has been decided to release the connection ）

The third paragraph means :
FIN=1：(B) To release the connection ！
ACK=1： The connection is established , After that ACK Must be set to 1
seq=w： Sent a lot of data , I'm just using w Refer to , here w There is a definite value
ack=u+1： Previous sender (A) It is said that the number sent is u Bit data , So I (B) What I want is u+1 Bit data （ because A Don't send data directly , So in the second and third paragraph ack All are u+1）

The fourth paragraph means :
ACK=1： The connection is established , After that ACK Must be set to 1
seq=u+1： The data sent before is u Bit data ,B Also the first u+1 Bit data , So I send the first u+1 Bit data
ack=w+1： Previous sender (B) It is said that the number sent is w Bit data , So I (A) What I want is w+1 Bit data

Why wait for the time 2MSL？
Because it can guarantee B You can receive A And then enter the closed state
For example, if A The fourth segment of the message is lost , Then wait for one MSL after B It will retransmit the third message , The cost is less than 1MSL after A You will receive the third message , Then you can go to B Send the fourth message prompt B Close the connection
1.5.8.1.2 Fast retransmission and fast recovery
This is similar to the first steps of slow start and congestion avoidance above , It is first exponential growth and then linear growth .
The difference is that fast retransmission and fast recovery algorithms are received continuously ack Execute after confirmation , there ack It's redundancy ack, redundancy ack The feature is that if the data requested for a certain section is not received many times , When a certain number is reached, retransmission will be performed immediately . But it's only down to now cwnd Half of , Re linear growth . Instead of starting from scratch like slow start and congestion avoidance
1.6.2.2 Domain name server （DNS The server ）
DNS There are many servers , It is divided into three layers according to the hierarchy , Root domain server , Top-level domain server , Domain name server
The local domain name server is not a hierarchy , The feature is that the main engine is relatively close , When the host communicates with another host that is closer , You don't have to go to any more advanced servers
The root domain name server is not a domain name, but only one , Instead, one domain name corresponds to multiple domain name servers , There is a total of 13 Such a domain name , Namely a.rootservers.net,b.rootservers.net,c.rootservers.net,~m.rootservers.net
In the permission domain name server , Although it seems that abc.com Than y.abc.com One less , But their status is still equal , The corresponding two domain name servers