Why does a zero-window probe segment use a previous sequence number?

Issue

• Why does a zero-window probe segment use a previous sequence number?

Sample of zero-window probe segments:

# tcpdump -nnvvS -r /tmp/0probe.cap
...skip
15:51:38.821359 IP (tos 0x0, ttl 64, id 25797, offset 0, flags [DF], proto TCP (6), length 1500)
    10.0.0.5.52543 > 10.0.0.6.7777: Flags [.], cksum 0x7b6b (incorrect -> 0xc4e9), seq 449541968:449543416, ack 814785370, win 115, options [nop,nop,TS val 92400718 ecr 18778655], length 1448
15:51:38.861373 IP (tos 0x0, ttl 64, id 10910, offset 0, flags [DF], proto TCP (6), length 52)
    10.0.0.6.7777 > 10.0.0.5.52543: Flags [.], cksum 0x75c3 (incorrect -> 0x2c95), seq 814785370, ack 449543416, win 7, options [nop,nop,TS val 18778695 ecr 92400718], length 0
15:51:39.073437 IP (tos 0x0, ttl 64, id 25798, offset 0, flags [DF], proto TCP (6), length 948)
    10.0.0.5.52543 > 10.0.0.6.7777: Flags [P.], cksum 0x7943 (incorrect -> 0xbd39), seq 449543416:449544312, ack 814785370, win 115, options [nop,nop,TS val 92400971 ecr 18778695], length 896
15:51:39.073693 IP (tos 0x0, ttl 64, id 10911, offset 0, flags [DF], proto TCP (6), length 52)
    10.0.0.6.7777 > 10.0.0.5.52543: Flags [.], cksum 0x75c3 (incorrect -> 0x274b), seq 814785370, ack 449544312, win 0, options [nop,nop,TS val 18778907 ecr 92400971], length 0
15:51:39.285437 IP (tos 0x0, ttl 64, id 25799, offset 0, flags [DF], proto TCP (6), length 52)
    10.0.0.5.52543 > 10.0.0.6.7777: Flags [.], cksum 0x2605 (correct), seq 449544311, ack 814785370, win 115, options [nop,nop,TS val 92401183 ecr 18778907], length 0
15:51:39.285668 IP (tos 0x0, ttl 64, id 10912, offset 0, flags [DF], proto TCP (6), length 52)
    10.0.0.6.7777 > 10.0.0.5.52543: Flags [.], cksum 0x75c3 (incorrect -> 0x2677), seq 814785370, ack 449544312, win 0, options [nop,nop,TS val 18779119 ecr 92400971], length 0
15:51:39.709469 IP (tos 0x0, ttl 64, id 25800, offset 0, flags [DF], proto TCP (6), length 52)
    10.0.0.5.52543 > 10.0.0.6.7777: Flags [.], cksum 0x2389 (correct), seq 449544311, ack 814785370, win 115, options [nop,nop,TS val 92401607 ecr 18779119], length 0
15:51:39.709852 IP (tos 0x0, ttl 64, id 10913, offset 0, flags [DF], proto TCP (6), length 52)
    10.0.0.6.7777 > 10.0.0.5.52543: Flags [.], cksum 0x75c3 (incorrect -> 0x24cf), seq 814785370, ack 449544312, win 0, options [nop,nop,TS val 18779543 ecr 92400971], length 0
15:51:40.557432 IP (tos 0x0, ttl 64, id 25801, offset 0, flags [DF], proto TCP (6), length 52)
    10.0.0.5.52543 > 10.0.0.6.7777: Flags [.], cksum 0x1e91 (correct), seq 449544311, ack 814785370, win 115, options [nop,nop,TS val 92402455 ecr 18779543], length 0


Note:
15:51:39.073437 seq 449543416:449544312 <== Usual data segment
15:51:39.285437 seq 449544311       <== Probe segment using a previous sequence number
15:51:39.709469 seq 449544311       <== Probe segment using a previous sequence number
15:51:40.557432 seq 449544311       <== Probe segment using a previous sequence number

• Why we see TCP Keep-Alive before TCP ZeroWindow?

20  0.212616000 10.0.0.1  10.0.0.2 [TCP Window Full] 32516 → 8000 [PSH, ACK] Seq=115025 Ack=1 Win=26880 Len=4864 TSval=655997846 TSecr=2971487238
21  0.000138000 10.0.0.2 10.0.0.1  [TCP ZeroWindow] 8000 → 32516 [ACK] Seq=1 Ack=119889 Win=0 Len=0 TSval=2971487450 TSecr=655997846
22  0.206869000 10.0.0.1  10.0.0.2 [TCP Keep-Alive] 32516 → 8000 [ACK] Seq=119888 Ack=1 Win=26880 Len=0 TSval=655998053 TSecr=2971487450
23  0.000129000 10.0.0.2 10.0.0.1  [TCP ZeroWindow] 8000 → 32516 [ACK] Seq=1 Ack=119889 Win=0 Len=0 TSval=2971487657 TSecr=655997846
24  0.415860000 10.0.0.1  10.0.0.2 [TCP Keep-Alive] 32516 → 8000 [ACK] Seq=119888 Ack=1 Win=26880 Len=0 TSval=655998469 TSecr=2971487657
25  0.857001000 10.0.0.1  10.0.0.2 [TCP Keep-Alive] 32516 → 8000 [ACK] Seq=119888 Ack=1 Win=26880 Len=0 TSval=655999326 TSecr=2971487657
26  0.000120000 10.0.0.2 10.0.0.1  [TCP ZeroWindow] 8000 → 32516 [ACK] Seq=1 Ack=119889 Win=0 Len=0 TSval=2971488930 TSecr=655997846
27  1.664734000 10.0.0.1  10.0.0.2 [TCP Keep-Alive] 32516 → 8000 [ACK] Seq=119888 Ack=1 Win=26880 Len=0 TSval=656000990 TSecr=2971488930
28  0.000130000 10.0.0.2 10.0.0.1  [TCP ZeroWindow] 8000 → 32516 [ACK] Seq=1 Ack=119889 Win=0 Len=0 TSval=2971490595 TSecr=655997846
29  3.326014000 10.0.0.1  10.0.0.2 [TCP Keep-Alive] 32516 → 8000 [ACK] Seq=119888 Ack=1 Win=26880 Len=0 TSval=656004317 TSecr=2971490595
30  0.000114000 10.0.0.2 10.0.0.1  [TCP ZeroWindow] 8000 → 32516 [ACK] Seq=1 Ack=119889 Win=0 Len=0 TSval=2971493921 TSecr=655997846