Differentiated Services | Quality of Service on Wired Networks

So how do wireline networks get quality of service? They do so through the use of prioritization. Instead of asking for, and accounting for, resources and reservations and policing, the network becomes very simple. Traffic is divided up into classes. Some classes are better than others, and will get special treatment. Most likely, this treatment is just to cut to the head of the line. Each packet, not flow, is independently marked with the priority or class it belongs to. Every router and switch along the way that understands the tags will provide that differentiation, and the ones that do not simply ignore the tags and treat the packet as best effort.

This is the concept of differentiated services. For IP networks, the TOS/DSCP field in IPv4 and Traffic Class field in IPv6 is expected to hold the specific class or priority that the packet belongs to. The sender self-marks the packet, and the network takes it from there.

Here, the two conflicting concepts of the IPv4 Type of Service (TOS) come in contact with the Differentiated Services Code Point (DSCP) definition, for the same byte in the header. Each is a mechanism that was created to try to classify packets on a per-packet basis. TOS is the older mechanism, and is now considered to have fallen out of use. However, for the purposes of voice mobility, a lot is similar about TOS and DSCP. TOS defined, among other things, eight priority levels.

The format of the now formally deprecated TOS field is shown in Table 1.

Table 1: The TOS Field in IPv4 

	Precedence	Delay	Throughput	Reliability	Reserved
Bit:	0-2	3	4	5	6-7

The precedence value is a prioritization that is used within the network to determine its handling. The values run from 0 to 7, with 0 being the lower end of the range. The definitions originally conceived for this value is given in Table 2.

Table 2: The TOS Precedence 

Value	Old Meaning	802.1 p Meaning	WMM Meaning
7	Network Control	Network Management	Voice
6	Internetwork Control	Voice	Voice
5	CRITIC/ECP	Video	Video
4	Flash Override	Controlled Load	Video
3	Flash	Excellent Effort	Best Effort
2	Immediate	Undefined	Background
1	Priority	Background	Background
0	Routine	Best Effort	Best Effort

The table suggests a gradual rise in priority from 0 to 7. The problem with this definition is that different technologies use the 0-7 range for priorities. Most equipment endeavors tomaintain a consistent mapping for the number to a priority level, no matter how the priority got to the packet. The three different meanings are shown in the columns. The second column is from IEEE 802. 1p, which is a per-frame prioritization extension to Ethernet, and uses a special header to advertise the priority. The third column contains the meaning of the same eight values in WMM, the Wi-Fi prioritization standard. In general, it is best to assume the meaning of the final two columns. Note that the priority for values 1 and 2 are actually less than best effort in that case. When in doubt, do not use those priorities.

The remaining three flags in Table 3 represent extra information that may have been useful for the packet. Setting the delay bit meant to ask for low delays, whereas setting the throughput or reliability bit was meant to signal that throughput or reliability was a greater concern to the application.

TOS is considered to be replaced, and yet many modern devices in the world of IP telephones use the TOS meanings, and not the later DSCP meanings, in order to support older network configurations that may still be in use.

DSCP requires that the TOS meanings for the top three bits still be preserved, as long as the remaining bits are zero. However, DSCP looks at the one byte a different way. Table 3 shows the new meaning.

Table 3: The DSCP Field in IPv4 (Same Byte as TOS; Different Meaning 

	Code Selector	ECN
Bit:	0-5	6-7

There are a couple of RFCs that define what the code selector maps to. The goal of the DSCP is to interpret the selector as a somewhat arbitrary code, mapping into a specific quality of service type.

RFC 2597 defines the concept of Assured Forwarding (AF), the purpose of which is to allow a service provider to accept markings of packets and apply a certain amount of guaranteed bandwidth, as well as allowing more bandwidth to be given. Each class is named AFxx, where the first x is a number from one to four, representing the class of traffic, and the second x is a number from one to three, representing the drop probability from low to high (see Table 4).

Table 4: Assured Forwarding DSCP Values 

Drop Probability	Class 1	Class 2	Class 3	Class 4
Low	AF11 = 10	AF21 = 18	AF31 = 26	AF41 = 34
Medium	AF12 = 12	AF22 = 20	AF 32 = 28	AF42 = 36
High	AF13 = 14	AF23 = 22	AF33 = 30	AF43 = 38

The network administrator is expected to assign meanings to the four classes, in terms of assured, set-aside bandwidth that these codes can eat into. The drop probabilities are meant to be sent by the traffic originator to make sure that, if resources are getting exhausted, some packets get more protection than others.

A different concept is defined in RFC 2598. Expedited Forwarding (EF) sets up a specific codepoint, 46, to allow packets to be marked as belonging to a "virtual lease line," a high-performing point-to-point measure of quality of service. (There is a wrinkle with this DSCP code as it applies to Wi-Fi: All EF tagged packets get transmitted in the class of service designated for video because of the way the EF tag is coded.)

In total, there are 21 commonly seen DSCPs: the twelve AFs, the EF codepoint, and the eight original precedence values, now known default and CS1 to CS7.

Nothing in DSCP or differentiated services defines just what the qualities of the differentiated services are to be. This is the advantage of differentiated services: the differentiation is up to the administrator, and can grow as the network grows.