In TCP 3-way handshake, 3 segments will be sent (SYN, SYN ACK, ACK). What if loss of syn+ack from the server will not establish a connection? What if the third segment(ACK) is lost? Is the sender going to resend the segment or give up establishing the connection? And how do the two hosts know the segment is lost?

  • could you elaborate more? do you mean third ACK in handshake? and from where is this information?
    – Effie
    Sep 23, 2021 at 16:30

1 Answer 1


Loss of SYN/ACK (2nd step) means no side has received an ACK = there's no way to send a segment.

Loss of last ACK (3rd step) means that the listener (server) isn't fully synchronized but the initiator (client) is. If the initiator then sends some kind of L7 request and includes the ACK flag, the situation is healed.

  • what if the client sends absolutely no data?
    – Effie
    Sep 23, 2021 at 16:48
  • Then the connection cannot be used. The server won't send any prompt or similar, so application protocols like SMTP won't work.
    – Zac67
    Sep 23, 2021 at 17:15