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If we have ack number so from there we can trace that how many bytes are received and which byte is need next so then what is the use of Ack flag there?

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The ACK flag indicates that the Acknowledgment Number field is significant, ie. containing a meaningful value. When a socket connection has already been established that is nearly always the case, but it isn't while a connection is being established.

Acknowledgment Number: 32 bits

If the ACK control bit is set this field contains the value of the
next sequence number the sender of the segment is expecting to
receive.  Once a connection is established this is always sent.

RFC 793

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  • a small note for reference: first SYN packet has ACK bit unset
    – Effie
    Apr 21 at 13:29
  • @Effie Yes, of course. The first SYN segment carries no meaningful value in the ACK number field. ;-)
    – Zac67
    Apr 21 at 14:52

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