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Each time RTO doubled in the “Delta” column. The screenshot is below.Īs you see, 4 times retransmission happened before it gave up. In this scenario, we will use a Windows 10 client and try to reach 8.8.8.8 (dns.google) through port 125, which we know it is filtered by a firewall. I will create 3 scenarios in which we will observe RTOs and retransmissions.ĪLSO READ: Detect Rogue DHCP Server with Wireshark Scenario 1 The number of retransmissions depends on what type of operating system (OS) you use or what kind of configuration they have. When RTO is calculated, RTT parameter is taken into account. This timer is called RTO which is doubled after each retransmission. If the packet gets lost or the sender does not receive the acknowledgment in the time, the timer expires and the sender retransmits the packet again.

When a packet is sent over a network, the sender starts a timer and expects the packet to be acknowledged before the timer expire.

We will measure RTT for the first packet (SYN) in the flow. Let’s get our hands dirty and capture a TCP flow. Wireshark is capable of calculating and displaying TCP RTT in the header. RTT is how long it takes to receive an ACK for data that has been sent. The similar concept can be applied to TCP as well. Minimum = 11ms, Maximum = 37ms, Average = 18ms Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),Īpproximate round trip times in milli-seconds:

My ping command displays the RTT for each packet sent and returned below. After that, the copy gets deleted and resources are released for future use.ĪLSO READ: Steps to perform Remote Packet Capture with Wireshark

Whenever a sender transmits a packet, it saves a copy of the packet int its buffer and it stays there until receiving side acknowledges that it has received the packet. The lost packets are recovered by retransmissions and acknowledgments while sequence numbers are used to determine correct packet reordering in the receiving side. The protocol offers packet delivery guarantees, even if some of the packets have been lost during the transmission. TCP is one of the reliable protocols working in the transport layer, in terms of Open System Interconnect (OSI) model.