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Transport layer selective ACK

2022-06-29 14:21:00 【Foxconn quality inspector zhangquandan】

Confirmation of improvement — Choose to confirm （SACK）

It can be seen from above that you have received 1 2 3 4 Give it a confirmation ,ack=401 You want it to send the 5 A the ,1,2,3 There is no need to send a confirmation , The statement has been received , Just send the last one .

Then I received 5,6,7 lost ,8 I also received , Return to confirm and tell it to send 601 The bytes , If selective confirmation is not supported , It will be 7,8,9,10 Send it all over again , Also is to 601 Send all the back ones .

And with selective confirmation , When returning the confirmation , Will tell the boundary , That is, head and tail , Which paragraphs received , that 8 There is no need to resend this paragraph .

continuity ARQ Both protocol and sliding window protocol adopt cumulative confirmation

The receiver received it 1-49640 These bytes , And then from 51101 Byte to 51453 These bytes are also received , The rest of the white part is the part that has not been received .

You can see the confirmation number 49641, Explain that I have received all the above ,49641 It's time to send it to me after the start , But I also received the following paragraph , At this time, I am confirming that , stay TCP There is an option in the first part , There are... In the options 12 Bytes , The left and right boundaries are specified in the options , The left boundary is 51101, Right border 51454, That is, the number of bytes received plus 1. In this way, the sender will know which data has been lost , Then resend the lost bytes .

A border will occupy 4 In bytes , Length takes up 1 Bytes , Selectively confirm occupancy 1 Bytes .TCP The head is fixed 20 The length of bytes , The longest is 60 Bytes , Then the options section can occupy 40 Bytes , Remove length and selectively confirm occupied 2 Bytes , So there is still 38 Bytes , A boundary occupies 4 Bytes .