Chapter 10 Quality of Service (QoS)
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IEEE 802.1Q Tag
The IEEE 802.1Q standard defines an explicit VLAN tag in the MAC header to identify the VLAN
membership of a frame across bridges. A VLAN tag includes the 12-bit VLAN ID and 3-bit user priority.
The VLAN ID associates a frame with a specific VLAN and provides the information that devices need to
process the frame across the network.
IEEE 802.1p specifies the user priority field and defines up to eight separate traffic types. The following
table describes the traffic types defined in the IEEE 802.1d standard (which incorporates the 802.1p).
DiffServ
QoS is used to prioritize source-to-destination traffic flows. All packets in the flow are given the same
priority. You can use CoS (class of service) to give different priorities to different packet types.
DiffServ (Differentiated Services) is a class of service (CoS) model that marks packets so that they
receive specific per-hop treatment at DiffServ-compliant network devices along the route based on the
application types and traffic flow. Packets are marked with DiffServ Code Points (DSCPs) indicating the
level of service desired. This allows the intermediary DiffServ-compliant network devices to handle the
packets differently depending on the code points without the need to negotiate paths or remember
state information for every flow. In addition, applications do not have to request a particular service or
give advanced notice of where the traffic is going.
DSCP and Per-Hop Behavior
DiffServ defines a new Differentiated Services (DS) field to replace the Type of Service (TOS) field in the IP
header. The DS field contains a 2-bit unused field and a 6-bit DSCP field which can define up to 64
service levels. The following figure illustrates the DS field.
DSCP is backward compatible with the three precedence bits in the ToS octet so that non-DiffServ
compliant, ToS-enabled network device will not conflict with the DSCP mapping.
The DSCP value determines the forwarding behavior, the PHB (Per-Hop Behavior), that each packet
gets across the DiffServ network. Based on the marking rule, different kinds of traffic can be marked for
Table 56 IEEE 802.1p Priority Level and Traffic Type
PRIORITY LEVEL
TRAFFIC TYPE
Level 7
Typically used for network control traffic such as router configuration messages.
Level 6
Typically used for voice traffic that is especially sensitive to jitter (jitter is the variations in delay).
Level 5
Typically used for video that consumes high bandwidth and is sensitive to jitter.
Level 4
Typically used for controlled load, latency-sensitive traffic such as SNA (Systems Network
Architecture) transactions.
Level 3
Typically used for “excellent effort” or better than best effort and would include important
business traffic that can tolerate some delay.
Level 2
This is for “spare bandwidth”.
Level 1
This is typically used for non-critical “background” traffic such as bulk transfers that are allowed
but that should not affect other applications and users.
Level 0
Typically used for best-effort traffic.
DSCP (6 bits)
Unused (2 bits)
