[Click] Unable to understand output of Click

[John Adams]

Hi,

I was looking at the output of Click using tcpdump and a seeing something
that I cannot understand (I am using a sender to send a file of 1 MB to a
receiver):

22:01:13.442111 IP 10.0.0.1.12345 > 10.0.0.2.54321: S 1234:1234(0) win 65535
22:01:13.471705 IP 10.0.0.2.54321 > 10.0.0.1.12345: S 4321:4321(0) ack 1235
win 65535
22:01:13.497422 IP 10.0.0.1.12345 > 10.0.0.2.54321: . 1235:2682(1447) win
65535
22:01:14.573322 IP 10.0.0.2.54321 > 10.0.0.1.12345: . ack 5981 win 65535

I'm just wondering where the ack for 5981 came from. I mean, in Line 3, the
sender sent from 1235 to 2682 so the ack should've been for 2683 but instead
it is for 5981. Can someone explain what I could be missing here?

Thanks,
John