Some network apps e-mail ÷voice over IP(e.g., web Skype) text messaging real-time video remote login conferencing P2P file sharing social networking multi-user network search games streaming stored video (YouTube,Hulu,Netflix) Application Layer 2-4 Application Layer 2-4 Some network apps  e-mail  web  text messaging  remote login  P2P file sharing  multi-user network games  streaming stored video (YouTube, Hulu, Netflix)  voice over IP (e.g., Skype)  real-time video conferencing  social networking  search  …  …