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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.

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    $\begingroup$ 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. $\endgroup$ – Yuval Filmus Jan 29 '17 at 17:13
  • $\begingroup$ Yes I know, but many examples don't take Transmission time into account. @Yuval $\endgroup$ – Mr. Sigma. Jan 30 '17 at 13:31
  • $\begingroup$ @YuvalFilmus the question is whether the round trip time also includes the transmission delay of the packet? when should we include / not include $\endgroup$ – void Jan 25 '18 at 5:34
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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.

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Ideally, TRTT should be 2*Tpropogation + Ttransmission.

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  • $\begingroup$ 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. $\endgroup$ – J_H Oct 25 '17 at 1:58

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