Differentiated Services Concepts; Per Hop Behavior - IBM RackSwitch G8000 Application Manual

Differentiated Services Concepts

Per Hop Behavior

© Copyright IBM Corp. 2011
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.
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 16. Layer 3 IPv4 packet
Version
Length
ToS
Length
Differentiated Services Code Point (DSCP)
7
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.
Trusted/Untrusted Ports
By default, all ports on the G8000 are trusted. To configure untrusted ports, re-mark
the DSCP value of the incoming packet to a lower DSCP value using the following
commands:
RS G8000(config)# interface port 1
RS G8000(config-if)# dscp-marking
RS G8000(config-if)# exit
RS G8000(config)# qos dscp dscp-mapping <DSCP value (0-63)> <new value>
RS G8000(config)# qos dscp re-marking
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.
ID Offset
6 5 4 3 2
TTL Proto FCS SIP
unused
1 0
Chapter 11. Quality of Service
DIP Data
135