packet sizes and inter-packet timings. This paper might peak your interests http://cacr.uwaterloo.ca/techreports/2012/cacr2012-08.pdf . It tries to obfuscate the network traffics by morphing them so they statistically look like Skype Traffic.

They even open sourced their code at http://crysp.uwaterloo.ca/software/CodeTalkerTunnel.html