To make sure the delay and RTT metrics are independent of the size of the packet

Question

To make sure the delay and RTT metrics are independent of the size of the packets, it is measured using a given bit in the packet. E.g., you can measure delay by the time from the moment the sender sends the first bit to the moment the first bit arrives to the receiver; the RTT would be measured as time from the moment the sender sends first bit of its packet to the moment the sender receives the first bit of the response. Consider a sender and a receiver connected with a sequence of 2 routers in-between. Let the each link has propagation delay of 10ms, and transmission rate of 10Mbps. Assume no competing traffic (so no queuing delays anywhere) and negligible nodal processing delay at the routers. Let sender measure this path's RTT by sending a packet to the receiver that elicit an immediate response packet back. Let sender's packet be 1500 bytes in size, and, for simplicity, assume the response packet is negligible in size (i.e., takes negligible time to transmit compared to any reasonable RTT). What RTT will the sender observe in this network using the timing of the first-bit departure/arrival measurements? Will the RTT observed by the sender in problem (a) change if the sender used the 2d bit for its measurement? The last bit? What is the maximum amount of data (in bytes) could be in transit on the way from the sender to the receiver in this network? How would the observed RTT change if the sender used 500-byte packet instead of 1500 packet?

Explanation / Answer

a)

RTT = 2* propagation delay

RTT = 20ms

c)

maximum data = 20ms*10Mbps

=20*10^-3 * 10* 10^6 bits

=200K bits

=25K Bytes

d)

Packet size doesn't influence RTT

so RTT = 20ms

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