TCP Transport Protocol Layer4 Checksum – How to Calculate TCP Checksum

I recently started with networking and tcp/ip.
The RFC 793 document tells the checksum is made of:
pseudo header+tcp header+tcp payload+zeroes(if not divisible by 16).
So I have to take 16bit fragments and sum them. And a final ones-complement onto the result.
I have captured a simple-communication, the server pings the client several times.

Device: lo
Number of packets: 900
Filter expression: port 8080

Packet number 1:
IP Header length : 20
       From: 127.0.0.1
         To: 127.0.0.1
   Protocol: TCP
Len : 101 CLen: 101
Seq: 1812084491
Ack: 818336762
   Src port: 8080
   Dst port: 45336
   Payload (35 bytes):
00000   81 21 34 32 5b 22 6d 65  73 73 61 67 65 22 2c 7b    .!42["message",{
00016   22 67 72 65 65 74 69 6e  67 22 3a 22 70 69 6e 67    "greeting":"ping
00032   22 7d 5d                                            "}]
CRC: 0100101111111110

Packet number 2:
IP Header length : 20
       From: 127.0.0.1
         To: 127.0.0.1
   Protocol: TCP
Len : 66 CLen: 66
Seq: 818336762
Ack: 2399287051
   Src port: 45336
   Dst port: 8080
Payload empty
CRC: 0010100011111110

Packet number 3:
IP Header length : 20
       From: 127.0.0.1
         To: 127.0.0.1
   Protocol: TCP
Len : 101 CLen: 101
Seq: 2399287051
Ack: 818336762
   Src port: 8080
   Dst port: 45336
   Payload (35 bytes):
00000   81 21 34 32 5b 22 6d 65  73 73 61 67 65 22 2c 7b    .!42["message",{
00016   22 67 72 65 65 74 69 6e  67 22 3a 22 70 69 6e 67    "greeting":"ping
00032   22 7d 5d                                            "}]
CRC: 0100101111111110

Packet number 4:
IP Header length : 20
       From: 127.0.0.1
         To: 127.0.0.1
   Protocol: TCP
Len : 66 CLen: 66
Seq: 818336762
Ack: 2986489611
   Src port: 45336
   Dst port: 8080
Payload empty
CRC: 0010100011111110

Packet number 5:
IP Header length : 20
       From: 127.0.0.1
         To: 127.0.0.1
   Protocol: TCP
Len : 101 CLen: 101
Seq: 2986489611
Ack: 818336762
   Src port: 8080
   Dst port: 45336
   Payload (35 bytes):
00000   81 21 34 32 5b 22 6d 65  73 73 61 67 65 22 2c 7b    .!42["message",{
00016   22 67 72 65 65 74 69 6e  67 22 3a 22 70 69 6e 67    "greeting":"ping
00032   22 7d 5d                                            "}]
CRC: 0100101111111110

I noticed the CRC of packets 1,3,5 are the same. Why so? I thought the sequence number differ, thus the crc must differ too.

If a TCP packet is sent, when exactly is the checksum calculated, and how do you trigger the layers to do a calculation? (Setting checksum to 0 before sending the packet trough network?)