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

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.

  • $\begingroup$ 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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