Which one should I report as total network I/O bytes among Sum(InBound) or Sum(InBound + OutBound)? I have a set of experiments with distributed environments. 

I am planning to present a total network I/O bytes usage of the system for each run.

Which one should I report as total network I/O bytes among Sum(InBound Traffic) or Sum(InBound Traffic + OutBound Traffic)?

For example, say there are two machines A and B in my cluster server.

A sends 10 bytes to B.

B sends 15 bytes to A.

What is the total network I/O bytes? Is it 25 or 50 bytes?
I would suggest reporting the sum of both.

As a general principle, the best solution is to define your metrics in the report or paper where you write up your results.  You should describe what you measured and how you measured it.  This includes clarifying whether "total network I/O" includes both inbound+outbound traffic or only inbound traffic.