"barcaroller" <barcaroller@music.net> wrote:
> Client Server
> -------- SYN J -------->
> <--- SYN K, ACK J+1 ----
> ------- ACK K+1 ------->
>
> During TCP connection establishment (above), the third segment is an
> ACK from the client to the server. What is the sequence number in
> that segment? Is it J or J+1 or 0 or some other value?

Figure 7 in RFC0793 shows the same handshake, but with the additional
detail needed to answer your question. The sequence number in the third
segment is shown as J+1.

In general, any TCP segment containing a set FIN flag, a set SYN flag,
and/or data must advance the sequence number. Any segment void of all three
of those sequence-number-consuming aspects should contain the current
sequence number.