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Alteon OS Application Guide
134
Chapter 7: Quality of Service 42C4911, January 2007
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).
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.
7 6 5 4 3 2 1 0
Differentiated Services Code Point (DSCP)
unused
Version
Length
ToS
Length
ID
Offset TTL
Proto
FCS
DIP
SIP
Data