Actually, it's the Wiretap Act (See 18 U.S.C.§2511), and the Pen Register and Trap and Trace Act (See 18 U.S.C. §3127) you have to ensure you follow when performing this type of research.
Equally interestingly, the same researchers who assisted with the paper on the legality of network monitoring for research purposes were later accused of wiretapping (although never charged) for monitoring Tor for research: http://www.cnet.com/news/researchers-could-face-legal-risks-...
Here's an interesting analysis: http://spot.colorado.edu/~sicker/publications/issues.pdf
Equally interestingly, the same researchers who assisted with the paper on the legality of network monitoring for research purposes were later accused of wiretapping (although never charged) for monitoring Tor for research: http://www.cnet.com/news/researchers-could-face-legal-risks-...