Diffserv model - IPP and DSCP

ToS field can be used for IP precedence, or DSCP (8 bytes)

IP Precedence is old, only uses 3 bits to define priority, 8 possible values, only 6 user-definable.
DSCP is new, backwards compatible with IPP, uses 6-bits

Default Per-hop-behaviour (PHB) is '000' for three most significant bits, queuing is best-effort service.

Assured Forwarding (AF) -three most significant bits, guaranteed bandwidth

Expidited Forwarding (EF) - three most significant bits, provides low delay service


Expidited Forwarding - Ensures low delay, low jitter, designed for real-time applications. It is important to limit the bandwidth of EF so that it doesn't starve other classes, ideally EF is used with admission control. During congestion EF polices bandwidth.

Non DSCP complient voice applications mark IPP as 101 (5-critical), the most significant bits of EF are 101, so enables backwards compatability.

Assured Forwarding - AF provides four queues for four classes of traffic (AF1x/AF2x/AF3x/AF4x), the 'x' value defines drop probability. There is no priority among the queues.

AF11 is less likely to be dropped than AF13. There are 3 drop priorities, low1, medium2, high3.

Each AF class is backwards compatible with IPP values, e.g. AF21 is equal to IPP2

EF class is backwards compatible with IPP value 5 (binary 101).


When implementing QoS it is important to ensure that enough bandwidth is reserved for each queue to avoid delay or drops. Providing an AF queue is not policed, it can consume more bandwidth providing it is available.