Specifications

Chapter 2. IBM System Networking Switch 10Gb Ethernet switch features 103
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 6 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 2-23 IPv4 packet with DSCP field
The switch can perform the following actions on 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.
After 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 default settings are based on the following standard PHBs, as defined in the
IEEE standards:
򐂰 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, found at:
http://www.ietf.org/rfc/rfc2598.txt
򐂰 Assured Forwarding (AF): This PHB contains four service levels, each with a different
drop precedence, as shown in Table 2-7. Routers use drop precedence to determine
which packets to discard last when the network becomes congested. AF PHB is described
in RFC 2597, found at:
http://www.ietf.org/rfc/rfc2597.txt
Table 2-7 Assured Forwarding PHB
Drop
precedence
Class 1 Class 2 Class 3 Class 4
Low AF11 (DSCP 10) AF21 (DSCP 18) AF31 (DSCP 26) AF41 (DSCP 34)
Medium AF12 (DSCP 12) AF22 (DSCP 20) AF32 (DSCP 28) AF42 (DSCP 36)
High AF13 (DSCP 14) AF23 (DSCP 22) AF33 (DSCP 30) AF43 (DSCP 38)