Using DSCP values to provide QoS
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.
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
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 an 802.1p priority
Once the DSCP value is marked, the switch can use it to direct traffic prioritization.
Per Hop Behavior
The DSCP value determines the Per Hop Behavior (PHB) of each packet. The PHB is the forwarding
treatment given to packets at each hop. QoS policies are built by applying a set of rules to packets,
based on the DSCP value, as they hop through the network.
The HP 1:10GbE switch default settings are based on the following standard PHBs, as defined in the IEEE
Expedited Forwarding (EF)—This PHB has the highest egress priority and lowest drop precedence
level. EF traffic is forwarded ahead of all other traffic. EF PHB is described in RFC 2598.
Assured Forwarding (AF)—This PHB contains four service levels, each with a different drop
precedence, as shown below. Routers use drop precedence to determine which packets to discard
last when the network becomes congested. AF PHB is described in RFC 2597.
Assured forwarding drop-down precedence
Drop Precedence Class 1
AF11 (DSCP 10)
AF21 (DSCP 18)
AF12 (DSCP 12)
AF22 (DSCP 20)
AF13 (DSCP 14)
AF23 (DSCP 22)
Quality of Service
AF31 (DSCP 26)
AF41 (DSCP 34)
AF32 (DSCP 28)
AF42 (DSCP 36)
AF33 (DSCP 30)
AF43 (DSCP 38)