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2.TCP If the data is not sent successfully , It will notify the sender of the application layer error , At this time, the application layer knows that the data is lost

3.TCP It can ensure that the application layer of the receiver receives the data strictly in the order of sending

4.TCP Ensure that the data will not be inadvertently damaged (UDP It's fine too )

5.TCP Maintain network quality as much as possible

TCP The mechanism of

ack

ack The representative confirms the information , When the other party receives the message , Will be ack Send back to the sender , At this time, until the data is successfully transmitted .

Numbering mechanism

When we send data , It must be some data , At this time, you only need to pass the beginning of the serial number of the data to be transferred .

1. Number of data sent (SN)

SN That is, the beginning of the sent data number

2. Confirmed data number (ASN)

ASN Confirmed data number , This information is usually sent back by the receiver twice , It represents the next starting number of the sender , It is convenient for the next sending end to send data .

3. Numbering rules

Generally not from 0 Start , Because it's too easy to be hijacked by bad guys , Good guess

In general, our payload All are byte[] Array , So we basically use a number for each byte

4.SN Sending TCP Segment Of Header How to embody

TCP The complete data content sent and received is generally called Segment( paragraph ), That is to say header and payload The sum of

We send multiple data at a time , Again because byte[] Continuity of arrays , So we SN Directly fill in the data number of the first byte of the data sent this time .

stay Header in , We will save payload Data of the first byte in （SN Number ）, So the number becomes available , When we send , Just send this byte of data ,segent Will undertake payload The length of

5.ASN Filling rules

1. stay header in , Namely 32 Confirm the column of serial number

2.ASN Represents the next byte of data to be received , Send it back to the sender , The sender knows what to send

6.ISN

ISN Refers to the first byte number of data , Generally not from 0 Start , This is from a safety point of view , In case malicious users speculate on Legal SN Value to use .

TCP segment Play a role

segement That is, the complete content of the above figure is merged , It can not only serve send Role , When our flag switch is turned on , Also has the acknowledge segment The function of .

Switch in header Embodiment in

header There are flag bits in , Each flag bit is imagined as a switch , Switch on , It has the response function

Retransmission mechanism

Retransmission can be divided into the following two cases

1. No data received , So I didn't answer

1. It is possible that the data is still on the way

2. It is possible that the data has been lost

2. Receive the data , Also answered , But it hasn't come back yet

1. It may still be on the way

2. The loss of

We can set a reasonable timeout , Solve the problems on the way , We can wait a little longer , No more , It can only be lost .

3. When to choose to resend data

Whether the data has been lost , Or is the data halfway , As long as the timeout is not received ack, We can all choose to resend data without brain

4. Resend failure processing

After trying to resend several times, I still can't receive ack, We will stop sending . here TCP I have tried my best

Next , You can do this ：

1. Notification application layer , fail in send ( Generally in write() Exception will be thrown in )

2. Try your best to contact again (reset segment) ： The last attempt , If you don't receive it, you won't receive it

TCP buffer

Send buffer

TCP There is a send buffer , Because I don't know if I can send it successfully , So you need to keep the data temporarily , In case of a one thousand

Once the data is put into the local TCP Send buffer , It proves that the transmission is successful

Receive buffer

TCP There is also a receive buffer , Usage and UDP One meaning

TCP Connect

Connection is an abstract concept , To put it bluntly, it is the interconnection of two hosts , It can exchange information

Connection management

When we connect , There are several stages ：

handshake ( Connection is established ) -> Use -> wave ( Connection is broken )

Understanding of handshake

Sync information

Handshake is a process of information synchronization between both sides

Handshake process

Generally, the sender sends the synchronization request first , After being received by the receiver , Then put it back to send synchronization requests and ack, such , The two sides are connected and can exchange data

syn Flag bit and carry data problems

Also in header In the flag bit , Turn on the switch , Sync now

When we first set up synchronization , You can't carry data , Because I don't know if I can connect

Although it doesn't carry data , but syn It will also consume serial numbers

Three handshakes

We can see from the above description , There are three interworking processes between the two ends , After that, it can be used normally , This is called three handshakes