By Jimmy Ray Purser
Courtesy of Cisco


As your smaller business' network traffic grows, getting faster switches isn't your only option. Jimmy Ray Purser says that paying attention to a network's quality of service -- by classifying packets as they enter your network -- can be just as important


The one thing that any network admin can tell you (besides a massive amount of Star Trek trivia) is the increase in network traffic and users over the years. Networks are like kids: they start small and grow big, then don't listen to what you have to say.



To keep up with the demand, vendors have been building faster switches with deeper buffers every single year. This is an example of Moore's Law at its finest. But is that enough? Of course, any account manager will say yes! Buy more Cisco, problem solved. But a realistic network engineer needs to consider the role quality of service will play in the network. Most people understand how 802.1p works. Diffserv seems to be the big unknown in the world of QoS.

The real term is differentiated services, but the alpha geek crowd just says diffserv. Diffserv is concerned with classifying packets as they enter the local network. This classification then applies to a flow of traffic where a flow is defined by five elements:

Source IP address
Destination IP
Source port
Destination port
Transport Protocol
A flow that has been classified or marked can then be acted upon by other QoS mechanisms. Multiple flows can therefore be dealt with in a multitude of ways, depending on the requirements of each flow: voice this way and data that way and so on.

Diffserv uses a code point (the cool way to say this is DSCP) to determine the per-hop behavior of the flow. There are 64 total DSCP values. They range from 0 through 63. Remember that a high DSCP value is not equal to a high QoS queue. However, understand that in the assured forwarded (more to come) there are three drop probabilities. The DSCP determines the per-hop behavior (PHB) of a flow.