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Emulab FAQ: Troubleshooting: I decreased the bandwidth on a link and now the ping time between the nodes has increased

Emulab FAQ: Troubleshooting: I decreased the bandwidth on a link and now the ping time between the nodes has increased

Short answer: Decreasing the bandwidth of a link means that your bytes take longer to get where they are going!

Long Answer: A ping packet is 98 bytes of data; 56 bytes of data plus 8 bytes of ICMP header plus 20 bytes of IP header plus 14 bytes of ethernet header. At 100Mbs those 98 bytes takes .0078ms to traverse the wire, which is hardly noticeable! If you have set the delay of your link to 10ms, then your ping packets will incur 10ms+0.0078ms of delay in each direction, for a 20ms roundtrip time.

Say you set the bandwith of your link to 250Kbs. The wire time for those same 98 bytes is now 3ms. If your delay is 10ms like above, then your ping packets will incur 10ms+3ms of delay in each direction, for a 26ms roundtrip time! If you set the bandwith to 100Kbs, the wire time is now 7.8ms and your ping packets will incur 10ms+7.8ms of delay in each direction, for a 35.6ms roundtrip time!

Note: If you have a router connecting two nodes, then each of the two links will incur the same wire time (and delay of course). In the above 250Kbs example, each ping packet will incur 3ms of wire time to the router and another 3ms from the router to the destination. The ping reply packet will see the same 6ms of wire time. If your delay is again 10ms, then the ping roundtrip is 52ms.