Diff Serv Per Hop Behaviors

Other than the general QoS strategies described in this chapter, DiffServ really provides two additional key features: the DSCP field, and some good suggestions on how to use the DSCP field. In fact, two of the DiffServ RFCs, 2597 and 2598, are devoted to describing a set of DSCP values, and some suggested PHBs that should be associated with each DSCP value.

IP defined a type of service (ToS) byte in RFC 791, which came out in September 1981. The IP protocol creators intended the ToS byte to be used as a field to mark a packet for treatment with QoS tools. Inside the ToS byte, the first 3 bits were defined as a field called IP Precedence, which can be marked for the purposes of implying a particular class of service. The Precedence field values imply that the larger the value, the more important the traffic. In fact, names were given to each value 0 from routine (precedence 0) to critical (precedence 5) and network control (precedence 7). The complete list of values from the ToS byte's original IP Precedence 3-bit field, and the corresponding names, are listed in Table 2-13.

Table 2-13 IP Precedence Values and Names

Field and Value (Decimal)

Binary Value


Precedence 0



Precedence 1



Precedence 2



Precedence 3



Precedence 4


Flash Override

Precedence 5



Precedence 6


Internetwork Control

Precedence 7


Network Control

In additon to the Precedence field, the ToS byte included other flag fields that were toggled on or off to imply a particular QoS service—for instance, low or high delay would be signaled by a 1 or a 0 in the delay bit. Bits 4 through 6 (RFC 795) comprised the ToS field inside the ToS byte, with flags for throughput, delay, and reliability. RFC 1349 expanded the ToS field to bits 4 through 7, adding a cost flag. For instance, the original ToS byte creators envisioned the ability to choose a different route, using a more reliable link, for packets with the reliability flag set.

The DS field redefines the ToS byte in the IP header. It removes the definition of the 4 ToS bits (bits 3 through 6). DiffServ creates a replacement for the Precedence field with a new 6-bit field called the Differentiated Services (DS) field. (The last 2 bits of the ToS bytes are used by another specification in RFC 3168.) Figure 2-16 shows the fields inside the ToS byte (per RFC 1349) and the DS field (per RFC 2474).

Changing a protocol that is used in production may result in compatibility issues. If the protocol has available unused fields in the header, and those can be added to the protocol specifications, then all is well. When changing the meaning of an already defined field, however, problems can occur. In this case, DiffServ took advantage of the fact that the ToS field (not ToS byte, but just bits 3 through 6 of the ToS byte) were seldom used. Therefore, DiffServ only had to build compatibility with the Precedence field.

Figure 2-16 IP ToS Byte and DS Field

0-2 3-6 7


TOS Field



IP Header, Before DiffServ

IP Header, After DiffServ

TOS Byte

DS Field



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