In Chapter 1 of this book, we presented the OSI and TCP/IP networking models. As we saw there, even if the TCP/IP model has the widest usage, the reference model is OSI.
Let's have a look at the TCP/IP and OSI models again:
At Layer 7 of the OSI model, we find Application (HTTP, FTP, SSH, etc.). As you can see from the picture above, TCP/IP compacted OSI Layers 7, 6, and 5 into one Layer, TCP/IP Layer 4 (Application), which has the same name, but different functionality.
Filtering and prioritizing traffic from some applications can be very easy and very hard at the same time. Normally, we would filter/prioritize web traffic by matching TCP packets with source or destination port 80, which is the standard HTTP port. However, web servers can be configured to use any port; so our filters/prioritizations won't work for that particular traffic.
Another big problem network administrators have is filtering traffic belonging to P2P (peer to peer) applications like Kazaa...