Using Dscp Values To Provide Qos; Differentiated Services Concepts; Figure 7-2:Layer 3 Ipv4 Packet - Nortel Alteon OS 42C4911 Application Manual

Alteon OS Application Guide

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 Dif-
ferentiated 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).
Version
Length
Figure 7-2 Layer 3 IPv4 packet
The GbESM 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 GbESM 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 for-
warding 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.
134 Chapter 7: Quality of Service
Length
ToS
Differentiated Services Code Point (DSCP)
7 6 5
ID Offset TTL Proto
unused
4 3 2 1
FCS SIP DIP
0
42C4911, January 2007
Data