The DS Field Definition
The DS field is used
to replace the ToS octet in IPv4 and IPv6 Traffic class octet. Now
six bits out of the 8 bitsof the DS field are used as a code point i.e.
the DSCP is used to select the PHB a packet experiences at each node. The
remaining two bits called Currently Unused (CU) bits are reserved
for future use.
The DS field structure
is given below:
Differentiated Services CodePoint.
CU: Currently Unused.
Within the six available
bits, the following mappings are defined:
DE--- stands for
Default, it is a Best-effort type of service.
EF--- stands for
Expedited Forwarding, this type of PHB is a forwarding treatment
where the departure rate of the traffic from any Diff-Serv node must equal
a configurable rate independent of the intensity of any other traffic attempting
to transit the node.
It is imperative that
the above mentioned mappings and that too EF and AF are to be comprehensively
explained. The code points for the different PHB's are defined in the next
AF--- stands for
Assured Forwarding, this type of PHB is intended for customers
that need reliable services from their service providers , even in time
of network congestion.