Part 3 - Differentiated Services

The differentiated service code point (DSCP), also called the traffic class, provides an indicator for how a packet should be prioritized and handled. DSCP contains 6 bits. The first three bits tell routing queue priority. The second three bits are called the drop probability and effectively tell whether speed or accuracy is more important. These allow a router to manage the packet queue both by reordering the line based on priority and by dropping some packets if the queue becomes too long. This helps to ensure quality of service and is particularly helpful in routing gateway traffic or high bandwidth server applications in a local area network (LAN). Most routers recognize seven different precedence levels and four different drop probability levels in addition to being able to flag a packet as 000 000 for best effort. Some networks also use two additional bits for explicit congestion notification (ECN) to further manage endpoint to endpoint traffic.

Here, you will consider just three levels of precedence (expedited, regular, or delayed) and two levels of drop probability (high or low) according to the following table.

	Low Drop Probability (all packets maintained)	High Drop Probability (packets will be systematically dropped when queue is too long)
Expedited (packet prioritized ahead of regular traffic in queue)	Not Feasible	101 110
Regular	010 010	010 110
Delayed (packet waits for regular traffic to be sent before being sent)	001 010	001 110

Given these choice, a live stream video packet would likely be classified as 101 110. The media stream needs to be expedited in order to avoid latency or jitter; however, an occasional dropped packet is not an issue as even a full missing frame does not significantly disrupt video.

In contrast, an e-mail would likely be classified as 001 010. A minor delay in receiving an e-mail is unimportant; so, other traffic can be prioritized ahead of it, but it is essential to receive all information from all packets.

3a) Using this framework, what classification would you propose for voice over IP (VoIP)? Why?

3b) Using this framework, what classification would you propose for a peer-to-peer file sharing transfer? Why?

3c) Using this framework, what classification would you propose for a general web page transaction? Why?

3d) Although prioritization based on DSCP classification is common on LANs, prioritization of packets on the Internet is often a contentious issue particularly as it may be based on originating IP address or similar rather than just DSCP. What are arguments in support of net neutrality? What are arguments in support of offering an Internet fast lane? You may identify your personal position, but be sure you give arguments in support of both positions.