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Link Scheduler Configuration
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conditioning the marked packets at network boundaries in accordance with the
requirements or rules of each service.
The requirements or rules of each service must be set through administrative policy mechanisms,
which are outside the scope of this user guide. A differentiated services-compliant network node
includes a classifier that selects packets based on the value of the DS field, along with buffer
management and packet scheduling mechanisms capable of delivering the specific packet
forwarding treatment indicated by the DS field value. Setting of the DS field and conditioning of the
temporal behavior of marked packets need only be performed at network boundaries and may vary
in complexity.
In the IP header field, called the DS (for differentiated services) field is illustrated. Six bits of the DS
field are used as a codepoint (DSCP) to select the per-hop behavior (PHB) a packet experiences at
each node. A two-bit currently unused (CU) field is reserved and its definition and interpretation are
outside the scope of this document. Differentiated services-compliant nodes when determining the
per-hop behavior to apply to a received packet ignore the value of the CU bits.
Figure
Field Structure
In a DSCP value notation 'xxxxxx' (where 'x' may equal '0' or '1') used in this user guide, the left-most
bit signifies bit 0 of the DS field (as shown above), and the right-most bit signifies bit 5.
The DSCP field within the DS field is capable of conveying 64 distinct codepoints. The codepoint
space is divided into three pools for the purpose of codepoint assignment and management: a pool of
32 recommended codepoints (Pool 1) to be assigned by Standards Action, a pool of 16 codepoints
(Pool 2) to be reserved for experimental or local use (EXP/LU), and a pool of 16 codepoints (Pool 3)
which are initially available for experimental or local use, but which should be preferentially utilized
for standardized assignments if Pool 1 is ever exhausted. The pools are defined in the Table
(where 'x' refers to either '0' or '1'):
15-8: DS
15-3
Software Configuration Guide, Revision 1.03