Using Dscp Values To Provide Qos; Differentiated Services Concepts - Lenovo RackSwitch G8264CS Application Manual

Using DSCP Values to Provide QoS

Differentiated Services Concepts

© Copyright Lenovo 2017
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.
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 23. 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.
ID Offset
6 5 4 3 2
TTL Proto FCS SIP
unused
1 0
Chapter 12: Quality of Service
DIP Data
217