364 ◾ Networking for Big Data
•� QoS technology both in individual switches (CoS/QoS) as well as e2e , where AF  and EF  modes of packet forwarding are specifically important and are well defined both at Layer 2 (CoS header) and Layer 3 (DSCP header) ; note that penetration rate of these standards in practical use is fairly low, which results in poor overall e2e QoS support in networks today [6,26–28].
•� Practical DC-specific technology applied to both intra- and inter-DC networking , which today mostly ignores the above two topics and mostly focuses on FCoE [21,22].
FIgURE 17.1 A simple way to visualize switches in respect to the congestion problem. The right side can be generalized as several overlayed many-to-one switches (left).