In a situation where both client and server sets their respective sequence number to 0, I read that the following is true:
C-->S: SYN=1, SEQ=0 (No data bytes)
C<--S: SYN=1, SEQ=0, ACK=1 (No data bytes)
C-->S: SEQ=1, ACK=1 (Data bytes optional)
In the third part, I understand the server is expecting the next sequence number to be 1, but aren't sequence numbers supposed to be set to initial_seq_num + sent_data_bytes_num? Since there was no data bytes sent in the first part of the handshake shouldn't the seq # be 0?
Is this just an exception during the handshake or are segments sent to with no data bytes supposed to increment the sequence number by 1 if they can be sent at all?
(There is a similar Q & A but the answer doesn't explain if this is an exception during the handshake phase OR if this happens after a TCP connection has been establish. I'm not even sure if a segment with no data bytes can even be sent. I'm assuming you can't)
ADDED It seems TCP keep-alive packets have no payload either. RFC 1122 says in these packets, SEG.SEQ = SND.NXT-1, and because this sequence number will be an already ACKed number, and a duplicate ACK will be sent, so as to keep the sequence number of the server the same.
Otherwise, I couldn't find any indications of what needs to be done when the sequence number is correct but there is no payload. I might be wrong since I only briefly scanned the document, but there is also no statement of rules of sequence numbering during the handshake except for examples.
In RFC 1122, it says
Unfortunately, some misbehaved TCP implementations fail to respond to a segment with SEG.SEQ = SND.NXT-1 unless the segment contains data.
So I'm assuming it depends on each implementation, but if there is any statement of a) the sequence numbering during handshake, and b) how to behave when the sequence # is correct but there is no payload, I would really appreciate it if someone could point me to that part.
Thanks!
The first ACK (that occurs as part of Handshake) acknowledges the reception of SYN from the other end. The SYN segment does not carry any data. But to allow the provision for acknowledging the reception of SYN, the first ACK is incremented though no payload is present.