Class Selector Codepoints
The DS field values
of xxx000 and CU field unspecified are reserved as a set of Class Selector
Codepoints. The PHBs which are mapped to by these codepoints must satisfy
the Class Selector PHB requirements in addition to preserving the
Default PHB requirements on codepoint 000000.
Hop Behaviour Standardization Guidelines
This topic is important
with the point of view that the behaviours are one of the important parameters
with respect to aggregated flows:
A PHB group usually
consists of a set of two or more PHBs that can only be meaningfully specified
and implemented simultaneously, due to a common constraint applying to
each PHB within the group.
A PHB group specification
should describe conditions under which a packet might be remarked to select
another PHB within the group.
A PHB standard must specify
a recommended DS codepoint selected from the codepoint space reserved for
In cases where a
new PHB group is proposed, it's specification should have an overview of
the behavior and it's purpose.
The abovementioned few
points are only a chosen few in a long list of PHB guidelines, for more
information, there is a complete and comprehensive list of PHB guidelines
It can be possible for
a packet exhibiting a certain PHB within a PHB group that can be remarked
to select another PHB within the same group. This can be happening within
a specified domain or across a domain
boundary. In such cases the circumstances should be explicitly stated.