Sequence and Acknowlegement number is very import in TCP protocol. They are used to detect if the packet is out of order or if there are any missing pakcets. The Seq in the SYN packet is called ISN(Initial Sequence Number)
Wireshark will calculate relative sequence nuber = real Seq - ISN
1. 36724 > 9999 [SYN] Seq=0 Win=43690 Len=0
36724 send a TCP header with SYN flag to 9999, ISN = 0. try to establish a TCP connection
2. 9999 > 36724 [SYN, ACK] Seq=0 Ack=1 Win=43690 Len=0
9999 reply with SYN flag and Seq = ISN, Ack = (ISN + 1)
3. 36724 > 9999 [ACK] Seq=1 Ack=1 Win=43776 Len=0
36724 send with Seq = ISN + 1, Ack = ISN + 1.
TCP connection is estableshed
4. 36724 > 9999 [PSH, ACK] Seq=1 Ack=1 Win=43776 Len=3
Seq = ISN = 1
Ack = Packet(3).Ack = 1
5. 9999 > 36724 [ACK] Seq=1 Ack=4 Win=43776 Len=0
Seq = ISN = 1
Ack = Packet(4).Seq + Packet(4).Len = 1 + 3 = 4
6. 9999 > 36724 [PSH, ACK] Seq=1 Ack=4 Win=43776 Len=3
Seq = Packet(5).Seq + Packet(5).Len = 1 + 0 = 1 Packet(5) is last packet sent from 9999 to 36724
Ack = Packet(5).Ack = Packet(4).Seq + Packet(4).Len = 4 Packet(4) is latest packet received from 36724
7. 36724 > 9999 [ACK] Seq=4 Ack=4 Win=43776 Len=0
Seq = Packet(4).Seq + Packet(4).Len = 1 + 3 = 4 Packet(4) is last packet sent from 36723 to 9999
Ack = Packet(6).Seq + Packet(6).Len = 1 + 3 = 4
8. 36724 > 9999 [PSH, ACK] Seq=4 Ack=4 Win=43776 Len=4
Seq = Packet(7).Seq + Packet(7).Len = 4 + 0 = 4 Packet(7) is last packet sent from 36724 to 9999
Ack = Packet(7).Ack = Packet(6).Seq + Packet(6).Len = 1 + 3 = 4 Packet(6) is latest packet received from 9999
9. 9999 > 36724 [PSH, ACK] Seq=? Ack=? Win=43776 Len=4
So do you know here the Seq and Ack ?
Seq is related with packet 6 and Ack is related with pakcet 8
So, the relative Seq can reflect how many bytes is already sent, and relative Ack could reflect how many bytes is already received.