IP Precedence, TOS & DSCP

The IP Type of Service Header Field

The Type of Service field in the IP header was originally defined in RFC 791. It defined a mechanism
for assigning a priority to each IP packet as well as a mechanism to request specific treatment such as
high throughput, high reliability or low latency.

In practice, only the IP Precedence part of the field was ever used. At its
simplest, the higher the value of the IP Precedence field, the higher the
priority of the IP packet. Simple…

In RFC 2474 the definition of this entire field was changed. It is now called the “DS” (Differentiated
Services) field and the upper 6 bits contain a value called the “DSCP” (Differentiated Services Code
Point). Since RFC 3168, the remaining two bits (the two least siginficant bits) are used for Explicit
Congestion Notification.

The diagram below illustrates the relationship between the bits in the Type of Services/Differeniated
Services field in the IP header:-

Interpretation of the TOS/DS Field

The DSCP

The most signficant three bits of the DSCP (corresponding directly to the IP Precedence) are
referred to as the Class Selector.

DSCP <=> IP Precedence Conversion Table

References

• RFC 791 – Internet Protocol Protocol Specification
• RFC 2474 – Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers
• RFC 2475 – An Architecture for Differentiated Service
• RFC 2597 – Assured Forwarding PHB Group
• RFC 3246 – An Expedited Forwarding PHB (Per-Hop Behavior)
• RFC 3168 – The Addition of Explicit Congestion Notification (ECN) to IP
• Chapter from “Traffic Engineering with MPLS”