From TCP Connection Establishment Sequence Number Synchronization and Parameter Exchange :
Once each device has chosen its ISN,
it sends this value to the other
device in the Sequence Number field in
its initial SYN message. The device
receiving the SYN responds with an ACK
message acknowledging the SYN (which
may also contain its own SYN, as in
step #2 of the three-way handshake).
In the ACK message, the Acknowledgment
Number field is set to the value of
the ISN received from the other device
plus one. This represents the next
sequence number the device expects to
receive from its peer; the ISN
actually thus represents the sequence
number of the last byte received
(fictitious in this case, since the
connection is new and nothing yet has
been received).
