notes: Qos Packet Headers

ToS byte - 1-byte field.  The ToS byte was intended to be used as a field to mark a packet for treatment with QoS

IP Precedence – 3 bit field in ToS Byte of the IP header originally used to classify & prioritize types of traffic.

DiffServ – maintains the interoperability w non-Diffserv compliant devices  (IP Precedence)

           -  standardized a redefinition of the ToS byte.

The ToS byte itself was renamed the Differentiated Services (DS) field, and IPP was replaced with a 6-bit field (high-order bits 0–5) called the Differentiated Services Code Point (DSCP) field. Later, RFC 3168 defined the low-order 2 bits of the DS field for use with the QoS Explicit Congestion Notification (ECN) feature.

DSCP Settings and Terminology


1.  Expedited Forwarding (EF) - defines a DSCP of decimal 46, with a name . According to that RFC, packets marked as EF should be given queuing preference so that they experience minimal latency, but the packets should be policed to prevent
them from taking over a link and preventing any other types of traffic from exiting an interface during periods when this high-priority traffic reaches or exceeds the interface bandwidth. These suggested settings, and the associated QoS behavior recommended when using each setting, are called Per-Hop Behaviors (PHBs) by DiffServ. (The particular example listed in this paragraph is called the Expedited Forwarding PHB.)
- The EF traffic class is given strict priority queueing above all other traffic classes.
- The design aim of EF is to provide a low loss, low latency, low jitter, end-to-end expedited service through the network.
- The EF traffic class is suitable for voice, video and other real-time services.
2.  Class Selector PHB - Class Selector (CS) PHBs, that provide backward compatibility
with IPP. A C&M feature can set a CS DSCP value, and if another router or switch just looks at the IPP field, the value will make sense from an IPP perspective.
- Each IP precedence value gets mapped to a DiffServ value known as Class-Selector code-points.
- The CS code-points above are in the form ‘xxx000'.
- The first three bits ‘xxx’ are the IP precedence bits for backwards compatibility, while the last 3 bits are set to zero.
- If a packet is received from a non-DiffServ aware router that used IP precedence markings, the DiffServ router can still understand the encoding as a Class-Selector code-point.

3.  Assured Forwarding
- The AF behaviour allows the operator to provide assurance of delivery as long as the traffic does not exceed the subscribed rate.
- Traffic that exceeds the subscription rate faces a higher probability of being dropped during times of congestion.
- The DiffServ architecture defines 4 separate classes in the AF PHB (Per Hop Behaviour).
- Within each class (1 to 4), packets are given a drop precedence (1 to 3) (low=1, medium=2 or high=3).
- The 1st three bits of the six-bit DSCP field define the class, the next two bits define the drop-probability, and the last bit is reserved (= zero).
- AF is presented in the format AFxy, where ‘x’ represents the AF-class (HIGHER class value is more PREFERRED) and ‘y’ represents the drop-probability (HIGHER value is more likely to be DROPPED).

- AF23, for example, denotes class 2 and a high drop preference of 3.
- If AF23 was competing with AF21, AF23 will be dropped before AF21, since they in the same class and AF23 has higher drop value.
- But if AF33 and AF21 was competing, AF33 is a more important class, therefore AF21 will be dropped first.
- A nice formula to work out the decimal value of the AF bits, will be 8x+2y. Example AF31 = (8*3) + (2*1), thus AF31 = 26.
- Alternatively if the predefined DiffServ values are not used, any of the 64 DSCP values (0-63) can be used, by configuring just
that decimal value. (The higher the decimal value the more preferred)


Ethernet LAN Class of Service
Ethernet supports a 3-bit QoS marking field, but the field only exists when the Ethernet header includes either an 802.1Q or ISL trunking header. IEEE 802.1Q defines its QoS field as the 3 mostsignificant bits of the 2-byte Tag Control field, calling the field the user-priority bits. ISL defines the 3 least-significant bits from the 1-byte User field, calling this field the Class of Service (CoS).
Generally speaking, most people (and most IOS commands) refer to these fields as CoS, regardless of the type of trunking. Figure 12-2 shows the general location of the CoS field inside ISL and 802.1P headers.