Marking at Layer 3
The concept behind DiffServ (DS) is to group traffic into classes and mark it
once at the edge of the network. DiffServ was created to be highly scalable
by separating classification from policy creation and by servicing aggregate
classes of traffic rather than individual flows.
DiffServ uses Layer 3 markings, setting the eight-bit ToS field in the IP
header. Unlike the Layer 2 header, this marking remains with the packet as it
traverses the network, and changes only if some device overwrites the value
of these bits. You can set either IP Precedence, using the top three bits, or
Differentiated Services Code Points (DSCP), using the top six bits of the
field. The bottom two bits can be used for congestion notification. The
default DSCP value is zero, which corresponds to best-effort delivery. When
properly configured, DSCP is backward compatible with IP Precedence.
Each hop in the network is provisioned to treat traffic differently based on its
markings; this is called “per-hop behavior” (PHB). RFC 2475 defines PHB as
“the externally observable forwarding behavior applied at a DS-compliant
node to a DS behavior aggregate.” A behavior aggregate is a logical grouping
of traffic that needs similar service levels. It is also referred to as a service
class. Four PHBs are defined:
■ Default
■ Assured forwarding
■ Class selector
■ Expedited forwarding