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BLADEOS 6.5.2 Application Guide
BMD00220, October 2010 Chapter 9: Quality of Service 137
Using DSCP Values to Provide QoS
The switch uses the Differentiated Services (DiffServ) architecture to provide QoS functions.
DiffServ is described in IETF RFCs 2474 and 2475.
The six most significant bits in the TOS byte of the IP header are defined as DiffServ Code Points
(DSCP). Packets are marked with a certain value depending on the type of treatment the packet
must receive in the network device. DSCP is a measure of the Quality of Service (QoS) level of the
packet.
The switch can classify traffic by reading the DiffServ Code Point (DSCP) or IEEE 802.1p priority
value, or by using filters to match specific criteria. When network traffic attributes match those
specified in a traffic pattern, the policy instructs the switch to perform specified actions on each
packet that passes through it. The packets are assigned to different Class of Service (COS) queues
and scheduled for transmission.
Differentiated Services Concepts
To differentiate between traffic flows, packets can be classified by their DSCP value. The
Differentiated Services (DS) field in the IP header is an octet, and the first six bits, called the DS
Code Point (DSCP), can provide QoS functions. Each packet carries its own QoS state in the DSCP.
There are 64 possible DSCP values (0-63).
Figure 15 Layer 3 IPv4 packet
The switch can perform the following actions to the DSCP:
Read the DSCP value of ingress packets.
Re-mark the DSCP value to a new value
Map the DSCP value to a Class of Service queue (COSq).
The switch can use the DSCP value to direct traffic prioritization.
With DiffServ, you can establish policies to direct traffic. A policy is a traffic-controlling
mechanism that monitors the characteristics of the traffic, (for example, its source, destination, and
protocol) and performs a controlling action on the traffic when certain characteristics are matched.
7 6 5 4 3 2 1 0
Differentiated Services Code Point (DSCP)
unused
Version
Length
ToS
Length
ID
Offset TTL
Proto
FCS
DIP
SIP
Data