RCA THOMSON SpeedTouchTM (Wireless) Business DSL Router - Manual (Page 24)

Chapter 3
Basic QoS Concepts
E-NIT-CTC-20041213-0013 v0.5
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3.4 Differentiated Services Code Point (DSCP)
Introduction A small bit-pattern, called the DS field, in each IP packet is used to mark the packets
that should receive a particular forwarding treatment. The DS field uses the space of
the former ToS byte in the IPv4 IP header and the traffic class byte in the IPv6
header. All network traffic inside of a domain receives a service that depends on the
traffic class that is specified in the DS field.
The structure of the DS field is shown below:
A six-bit field, known as the Differentiated Services Code Point (DSCP), in the DS
field specifies the PHB for a given flow of packets. The DSCP is composed of the six
most significant bits of the DS field. The two least significant bits of the DS field are
used for Explicit Congestion Notification (ECN) by DiffServ-capable nodes that
support ECN. The ECN field contains 2 bits, the ECT bit and the CE bit.
The ECT bit is set to 1 to advertise to the network that the node is an ECN capable
node.
The CE bit is set to 1 incase the node experiences congestion.
Per Hop Behaviour Routers look at the DSCP to select a per-hop behaviour, such as a queueing
algorithm and its parameters.
A PHB defines a DiffServ router’s externally observable forwarding behaviour (in
terms of buffer/bandwidth resource allocation) related to a BA. This is essentially
defined by the queuing/scheduling/buffer management in the forwarding path.
PHBs are implemented in DiffServ nodes by means of some buffer management and
packet scheduling mechanism. The PHB definition is not depending on the
mechanism that offers the service but in terms of behaviour characteristics relevant
to service provisioning policy.
For example, "voice" traffic might select a "strict" queuing algorithm with a
parameter of "place in top priority queue".
Standardized PHBs The following specific PHBs and recommended DSCPs for each PHB have been
standardized by the IETF:
 Default PHB.
 Expedited Forwarding PHB.
 Class Selector (CS) PHB.
 Assured Forwarding PHB.
Refer to
RFC2474
for more information on the definition of the DS field.
7 6 5 4 3 2 1 0
DSCP ECN
Refer to
RFC2475
for more information.