IP Precedence and DSCP
IP Precedence
Standard IPP markings are similar to PCP:
6 and 7 are reserved for 'network control' traffic (ie. OSPF messages between routers).
5 = voice
4 = video
3 = voice signaling
0 = best effort
With 6 and 7 reserved, 6 possible values remain.
Although 6 values is sufficient for many networks, the QoS requirements of some networks demand more flexibility.
DSCP
RFC 2474 (1998) defines the DSCP field, and other 'DiffServ' RFCs elaborate on its use.
With IPP updated to DSCP, new standard markings had to be decided upon.
By having generally agreed upon standard markings for different kinds of traffic, QoS design & implementation is simplified, QoS works better between ISPs and enterprises, among other benefits.
You should be aware of the following standard markings:
Default Forwarding (DF) - best effort traffic.
Expedited Forwarding (EF) - low loss/latency/jitter traffic (usually voice).
Assured Forwarding (AF) - A set of 12 standard values.
Class Selector (CS) - A set of 8 standard values, provides backward compatibility with IPP.
DF (Default Forwarding)
DF is used for best-effort traffic.
The DSCP marking for DF is 0.
EF (Expedited Forwarding)
EF is used for traffic that requires low loss/latency/jitter.
The DSCP marking for EF is 46.
AF (Assured Forwarding)
AF (Assured Forwarding) defines four traffic classes. All packets in a class have the same priority.
Within each class there are three levels of drop precedence.
High drop precedence = more likely to drop the packet during congestion
CS (Class Selector)
CS defines eight DSCP values for backward compatibility with IPP.
The three bits that were added for DSCP are set to 0, and the original IPP bits are used to make 8 values.
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