The “initial round trip time” (IRTT) is the time taken for the initial SYN packet in the TCP handshake to the final ACK packet in the initial three-way handshake. Most TCP implementations will initially wait for up to 0.5 seconds until retransmitting a packet, but will dynamically adjust this to 3x – 4x the IRTT after the initial handshake. Wireshark will report the IRTT value in the final ACK packet of the three-way handshake.