QoS Overview
20 Quality of Service Guide
With a strict PHB at each network hop, service latency is mainly affected by the amount of
high-priority traffic at each hop. These classes are intended to be used for network control
traffic or for delay or jitter-sensitive services.
If the service core network is over-subscribed, a mechanism to traffic engineer a path through
the core network and reserve bandwidth must be used to apply strict control over the delay
and bandwidth requirements of high-priority traffic. In the router, RSVP-TE can be used to
create a path defined by an MPLS LSP through the core. Premium services are then mapped
to the LSP with care exercised to not oversubscribe the reserved bandwidth.
If the core network has sufficient bandwidth, it is possible to effectively support the delay and
jitter characteristics of high-priority traffic without utilizing traffic engineered paths, as long
as the core treats high-priority traffic with the proper PHB.
Assured Classes
The assured forwarding classes are Assured (af) and Low 1 (l1). Assured forwarding classes
provide services with a committed rate and a peak rate much like Frame Relay. Packets
transmitted through the queue at or below the committed transmission rate are marked in-
profile. If the core service network has sufficient bandwidth along the path for the assured
traffic, all aggregate in-profile service packets will reach the service destination. Packets
transmitted out the service queue that are above the committed rate will be marked out-of-
profile or exceed-profile (if exceeding the PIR is enabled). When an assured out-of-profile or
exceed-profile service packet is received at a congestion point in the network, it will be
discarded before in-profile assured service packets.
Multiple assured classes are supported with relative weighting between them. In DiffServ, the
code points for the various Assured classes are AF4, AF3, AF2 and AF1. Typically, AF4 has
the highest weight of the four and AF1 the lowest. The Assured and Low 1 classes are
differentiated based on the default DSCP mappings. All DSCP and EXP mappings can be
modified by the user.
Best-Effort Classes
The best-effort classes are Low 2 (l2) and Best-Effort (be). The best-effort forwarding classes
have no delivery guarantees. All packets within this class are treated by default as out-of-
profile assured service packets.
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