# Do we include Transmission delay in Round trip time?

Is RTT = $2*T_{Propagation delay}$ only. Or we include Transmission delay to send packets as well. Some examples include Transmission delay some not. I'm confused. Can anyone help.

• Round-trip time is the time it takes for a packet that you send to get back to you. Now you can forget about the formulas, and rederive them to fit the definition. Jan 29 '17 at 17:13
• Yes I know, but many examples don't take Transmission time into account. @Yuval Jan 30 '17 at 13:31
• @YuvalFilmus the question is whether the round trip time also includes the transmission delay of the packet? when should we include / not include
– void
Jan 25 '18 at 5:34

Suppose you have two computers (A and B) both connected to a switch with cut-through forwarding. When you are interested about RTT between computer A and computer B, you'll omit transmission delay, because the switch transmits the frame to B as it arrives from A, so transmission delay is not significant. Now, suppose that our switch uses store-forward, in this case, when the frame arrives, the switch will first store it completely, and then sends it to B, so we have to deal with transmission delay.

Ideally, TRTT should be 2*Tpropogation + Ttransmission.

• You wrote (2 * T_propagation) + T_transmission, which seems to account for 1 out of 4 serialization delays. Any intermediate routers may do cut-through routing independent of frame size, or may add their own serialization delays. Also, you are modeling scheduling / response time at the far end as zero, which is a fair approximation, but there may be local measurements available to flesh out that term, as well.
– J_H
Oct 25 '17 at 1:58

RTT is defined as the time it takes for a small packet to travel from client to server and back to client. It includes queueing delay, propagation delay and packet processing delay but doesn't include transmission delay. If we talk about TCP 3-way handshake or a request packet from client to server then the size of the packet involved isn't worth counting the transmission delay.

"To this end, we define the round-trip time (RTT), which is the time it takes for a small packet to travel from client to server and then back to the client. The RTT includes packet-propagation delays, packet queuing delays in intermediate routers and switches, and packet-processing delays"

Computer Networks: A Top Down Approach" by James F Kurose Above referenced book

• Please credit the source of the image. Also, please transcribe it to make the text searchable. Apr 15 '21 at 9:52