TCP Sequencing and Sliding Windows

Sequence Number In + Bytes of Data Received = Acknowledgment Number Out

Note. During the TCP startup and teardown sequence, a "phantom byte" causes the sequence number and acknowledgment number fields to increment by 1 even though no data is exchanged. This phantom byte can be confusing when you have just learned that the sequence number field increments only when data is sent.

Ref:http://support.novell.com/techcenter/articles/nc2001_05e.html
http://packetlife.net/blog/2010/jun/7/understanding-tcp-sequence-acknowledgment-numbers/