How to set/ get the time spent on packets fragmentation in ns-3 models?

Question

If I transfer a packets through multiple subnets, which have different MTU on the routers, it may be fragmented. How can I get or set the time spent on each operation of fragmentations in ns-3 models? I need to know this to calculate the speed.

Answer 1

What you are asking is unclear to me but let me try to answer.

If you want to measure the CPU time it takes for ns-3 to create fragments and reassemble them, you can run a simple 2-node experiment and change the mtu of the outbound network interface of the sending node to see how much wall-clock time is spent fragmenting vs non-fragmenting.

If, on the other hand, you want to measure the effect in terms of simulation time of splitting a packet in multiple packets and perform the MAC-level access function for each fragment, it's just a function of

• the access function used at the MAC level. If you want to model switched ethernet, it's easy. make it zero.
• the transmission delay through your medium. If ethernet, it's easy again: it's the length of the cable modulo the speed of the electromagnetic waves in your cable which depends on the quality of the cable.
• the size of the fragment and the throughput of your medium.

Basically, if you know how many times the packet will be fragmented (it is possible for multiple routers to fragment the packet consecutively in smaller fragments) and which mtu will be used every time, you can trivially create an analytical model of that process and predict the actual transport-level transmission delay of your packets through the simulation.