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Chapter 9: Quality of Service
environments, DiffServ is used to support routed point-to-point WANs. DiffServ is
an abbreviation for Differentiated Services.
When a packet reaches the edge of the network, either from an endpoint or from a
remote network, DiffServ tags that packet’s ToS header based on the priority estab-
lished for that packet by policy. Once admitted into a DiffServ-equipped WAN,
however, all subsequent router hops must enforce the priority set by the edge router
that admitted the packet.
So, while DiffServ is considered a per-hop behavior CoS system like 802.1p, all Diff-
Serv routers in the network core must uphold the prioritization decision that occurs
at the edge. That is to say, core routers don’t reorder or prioritize the packets, but
they do forward them using a precedence policy that was established at the edge.
Since a majority of bottlenecks occur on the edge of the network, on
access links, rather than within the core network where there’s usu-
ally tons of bandwidth, DiffServ may be the only Quality-of-Service
standard necessary. Its policy decision points are always at the edge of
In a nutshell, DiffServ furthers the concept of 802.1p so that priority policy can be
established at the edge—once for the entire network—rather than on each individ-
ual hop. What neither of these protocols does, though, is ensure that the network is
never overrun—that’s a key difference between CoS (what 802.1p and DiffServ are)
Common Open Policy Service, or COPS, is a way of storing and querying central-
ized policy information on the network. DiffServ can use COPS to obtain its march-
ing orders for how to handle traffic coming into the network. In a COPS scheme, a
centralized server called the policy server contains a policy record of traffic shaping
and prioritization preferences that DiffServ or another CoS/QoS mechanism can
retrieve. COPS itself doesn’t enforce the prioritization—that’s the job of DiffServ,
which runs on routers; COPS just provides a way of maintaining a centralized record
of the traffic policy. COPS is a product of the IETF Resource Allocation Protocol
working group. Another IETF recommendation, LDAP (Lightweight Directory
Access Protocol), can also be used as the basis of a policy server.
DiffServ Code Points (DSCP) are IP packet headers DiffServ associates with different
levels of importance. Since they’re 6 bits in length, DSCPs can be used to define quite
a wide scale of possible service levels. Most implementations support only 3 bits,