13 pages, 5 figures.-- ArXiv pre-print version available at: http://arxiv.org/abs/1007.2327
Contributed to: The 6th International Conference on emerging Networking EXperiments and Technologies (ACM CoNEXT 2010, Philadelphia, PA, US, Nov 30th-Dec 3rd, 2010).
A talk on this paper was held at IMDEA Institute Madrid on Nov 23rd, 2010, http://bit.ly/f4epai