TCP ACKed unseen segment

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Hi experts,

For the No.6 [TCP ACKed unseen segment], is the value of maxseqtobeacked 21? No.4 NextSeqNum？

No. Source  Destination Protocol   Seq  NextSeq Ack Length  Info
1   client  server  TCP 0   1   0   64  37353  >  8080 [SYN] Seq=0 Win=16000 Len=0 MSS=1460
2   server  client  TCP 0   1   1   64  8080  >  37353 [SYN, ACK] Seq=0 Ack=1 Win=64240 Len=0 MSS=1460
3   client  server  TCP 1   1   1   60  37353  >  8080 [ACK] Seq=1 Ack=1 Win=16000 Len=0
4   client  server  TCP 1   21  1   80  37353  >  8080 [PSH, ACK] Seq=1 Ack=1 Win=15000 Len=20
5   server  client  TCP 1   1   21  60  8080  >  37353 [ACK] Seq=1 Ack=21 Win=64220 Len=0
6   server  client  TCP 1   1   41  60  [TCP ACKed unseen segment] 8080  >  37353 [ACK] Seq=1 Ack=41 Win=64200 Len=0
7   client  server  TCP 41  61  1   80  [TCP Previous segment not captured] 37353  >  8080 [PSH, ACK] Seq=41 Ack=1 Win=15000 Len=20 [TCP segment of a reassembled PDU]
8   server  client  TCP 1   1   61  60  8080  >  37353 [ACK] Seq=1 Ack=61 Win=64180 Len=0


/* ACKED LOST PACKET*/
if( tcpd->rev->tcp_analyze_seq_info->maxseqtobeacked
&&  GT_SEQ(ack, tcpd->rev->tcp_analyze_seq_info->maxseqtobeacked )
&&  (flags&(TH_ACK))!=0 )

Regards, 7ACE